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The strongly basic 1,8-bis(tetramethylguanidino)naphthalene (TMGN) has been mono- and di-protonated by pentachlorophenol (PCP) and HClO 4 acid. Formation of hydrogen bonds in the 1:1 and 1:2 mixtures in acetonitrile has been studied by FT-IR, 1 H NMR; 13 C NMR spectroscopy as well as in the gas phase by ESI MS and PM5 semiempirical methods. The spectroscopic data show that in the 1:1 mixtures of TMGN with PCP or perchloric acid the base becomes protonated forming an intramolecular [N–H⋯N] + hydrogen bond. This hydrogen bond is strongly asymmetrical and is at most of medium strength. In the 1:2 mixtures, the intramolecular [N–H⋯N] + hydrogen bond is conserved irrespective of the kind of the acid and, in the case of the TMGN complex with PCP, additionally an intermolecular [O⋯H⋯O] − hydrogen bond is formed whereas in the TMGN complex with HClO 4 another N atom of the guanidine groups is protonated. The ESI MS measurements indicate the formation of protonated TMGN and deprotonated carboxylic acids and phenols as separated ions instead of a complex in the gas phase. The ClO 4 - anion forms, however, complexes with the protonated TMGN molecules. We show that the anion has a strong influence on the fragmentation pathway. The structures of TMGN and its selected protonated species are visualized using the semiempirical PM5 method. The calculated hydrogen bond parameters are in good agreement with the X-ray data.

Infrared and Raman spectra of the Mg2+ salt of poly(U) in D2O were recorded in the 1600-1800 cm−1 region and between 1 and 20C. The ir spectra showed a melting curve similar to the uv melting curves with a temperature of transition of about 6.5°C. This spectral change is assumed to be associated with the formation of the secondary structure of Mg2+-poly(U) in D2O at this temperature. Three double-helical and two triple-helical structures were used as inputs to compute the normal modes of vibration. A double-helical structure was found to give the best agreement with the observations. Knowledge of the C=0 eigenvectors, and of the expression for transition probability from quantum mechanics, was used to explain the so far unanswered question of H. T. Miles [(1964) Proc. Natl. Acad. Sci. USA 51, 1104–1109; (1980) Biomolecular Structure, Conformation, Function and Evolution, Pergamon, Oxford, pp. 251–264] as to why there is an increase in the ir vibrational wave number of a carbonyl band when that group is H-bonded to another polynucleotide chain in a helix. Such considerations also explain why a predicted band at about 1648 cm−1 is not to be seen in the ir spectra but is present in the Raman spectra. The model incorporating the CO transition dipole-dipole coupling interaction is able to explain also the observed higher intensity of the higher wave-number ir band. The experimental results demonstrate that the complete picture of vibrational dynamics of Mg2+-poly(U) in D2O is obtained only by looking simultaneously at ir and Raman spectra and not at only one of them. Weak ir bands were found to be as useful as the strong ones in understanding structure and vibrational dynamics. On the bases of our ir and Raman spectra, of the normal-mode analyses, and of the literature data, it is concluded that Mg2+-poly(U) in D2O is present in a double-helical structure at temperatures below the temperature of transition, whereby the uracil residues are paired according to arrangement (a) (see Fig. 1). This structure is rodlike and arises by refolding of one poly(U) chain. The computations show that no normal mode is associated with a single CO group vibration; all CO group vibrations are heavily mixed motions of various CO groups.

The ability to complex formation of oligomycin A (OLA) with monovalent cations was studied by the ESI mass spectrometry, 1 H, 13 C, 7 Li and 23 Na NMR spectroscopic and PM5 semiempirical methods. The ESI MS spectra indicated that OLA formed stable 1:1 complexes with all monovalent cations irrespective of the stoichiometry and cone voltage values used. With increasing cone voltages only the degradations of the OLA–Li + and OLA–Na + complexes with formation of fragmentary metal cation complexes were found. For OLA complexes with K + , Rb + and Cs + cations, with increasing cone voltage the fragmentation of these complexes occurred without the formation of fragmentary metal cation complexes. On the basis of the NMR studies the assignment of the signals of hydroxyl groups in the spectrum of OLA was made and the spectra of its complexes evidenced the involvement of certain oxygen atoms in the complexation process. The PM5 semiempirical calculations allowed visualizations of all structures of the OLA—monovalent cation complexes as well as the fragmentary cations. The most interesting structure was found for OLA–Li + complex, in which the Li + cation is fully isolated from the interactions with the environment.

The formation of hydrogen-bonded complexes in the mixtures of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) with 4-tert-butylphenol and 4-cyanophenol has been studied as a function of the phenol concentration in chloroform and acetonitrile by FTIR spectroscopy. In chloroform, the formation of cyclic hydrogen-bonded structures between the TBD and phenol molecules was observed. These structures show large proton polarizability due to collective proton motion within these hydrogen bonds indicated by continuous absorptions in the FTIR spectra. In acetonitrile, the cyclic hydrogen-bonded structure is relatively stable in the 1:1 mixtures. With increasing concentration of phenol molecules the formation of non-cyclic hydrogen-bonded chains is much more favorable.

Complexes of gramicidin S (gS) with proton and monovalent cations were studied by FT-IR spectroscopy, and their structures were calculated by the PM5 semiempirical method. The far infrared spectra of the gS complexes with metal cations revealed intense continua indicating large polarizabilities of Li + , Na + , K + , Rb + and Cs + in the gS circular arrangement. These cation polarizabilities appear, similarly as in the case of the cation complexes of gramicidin A, as a result of the fast fluctuation of the cations between four or six CO groups of the gS backbone.

Nine NH acids with pK a values between 0 and 8.57 were studied by FTIR spectroscopy in the middle and far-infrared region. In addition, NMR spectra were taken and a semiempirical AMI analysis of the systems performed. Theproton potential in the NHN - - NHN hydrogen bonds is almost the same. The proton polarizability of these hydrogen bonds is almost independent of the pK a of the studied N acids. There is no relation between the position of the hydrogen bond vibration in the far-infrared region and the pK a of the N acids. Only with the pyrrol, the pyrazol, and the 4,(5)-methylimidazol systems these hydrogen bonds are linear. With all other systems these hydrogen bonds are more or less strongly bent. The position of the hydrogen bond vibrations v o calculated with the AMI procedure agrees well with the experimentally obtained values. The chemical shift of the hydrogen-bonded proton increases in proportion to the pK a values of the N acids.

The influence of the entropy on the proton transfer equilibria AH···B [Formula: see text]A–···H+B in the dimethylphosphinic acid (DMP)-N-base and in the methanesulfonic acid (MSA)-N-oxide hydrogen bonds is studied by 1H NMR spectroscopy as a function of the ΔpKa, i.e., the pKa of the base minus the pKa of the acid. It is shown that with increasing ΔpKa the proton transfers to the N-oxide acceptor. In the DMP + N-base family of systems, first the double-minimum proton potentials (ΔH = 0) occurs and only with further increasing ΔpKa do the POH···N [Formula: see text] PO–···H+N equilibria become symmetrical (ΔG = 0). ΔG and ΔH are connected by the Gibbs–Helmholtz equation, ΔG = ΔH –TΔS. Hence, the reason of this effect is the large negative interaction entropy term (ΔSI) arising from the large order around the polar structure, which shifts the equilibria strongly to the left-hand side. With the MSA-N-oxide hydrogen bonds a single-minimum proton potential was found. The minimum shifts with increasing ΔpKa from the donor to the acceptor of the O···H+···N hydrogen bonds. It is shown that with increasing ΔpKa the proton potential becomes on average symmetrical, whereas the proton is still largely on the left-hand side. This result is also explained by the large negative interaction entropy due to the order of the environment if the proton is at the acceptor B.Key words: entropy, hydrogen bonds, infrared, spectroscopy, proton transfer.

Hydrated complexes of gramicidin A (gA) with K + , Rb + and Cs + cations were studied by FT-IR spectroscopy. The water bands observed in the FT-IR spectrum are identical as those obtained for the hydrated gA–Li + and gA–Na + complexes studied previously. In the far infrared spectrum of the gA–K + and gA–Rb + complexes an intense continuum and a strongly broadened band were found indicating large K + and Rb + polarizability in the gA channels. These K + and Rb + polarizabilities appear, similarly as in the case of the Li + and Na + complexes, as a result of the fast fluctuation of the K + and Rb + ions between two water molecules and four CO groups of the gA backbone. The much smaller barriers in the six minima Na + potential explain the much larger mobility of Na + when compared with the Li + and other cations.

The kinetic parameters for the hydrolysis of the heptapeptide Pro–Thr–Glu–Phe-(4-NO2)Phe–Arg–Leu by the pepsin model compound tetrabutylammonium monosalt of m-aminobenzoic acid diamide of fumaric acid (TBA m-FUM) and porcine pepsin were determined using a spectrophotometric technique. According to the DS values obtained, in the transition state the inner motion in the TBA m-FUM–heptapeptide complex is more restricted than that in the pepsin–heptapeptide complex. The model compound TBA m-FUM can cause a cleavage of the Phe— (4-NO2)Phe bond in the substrate molecules following a mechanism similar as that suggested for pepsin, but its catalytic activity is much lower. Copyright  2001 John Wiley & Sons, Ltd.

2,2′-Biphenol mixtures with 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) were studied by FTIR spectroscopy. In chloroform, a proton transfer from 2,2′-biphenol to MTBD occurs. In this solution the protonated MTBD molecules are hydrogen-bonded to the 2,2′-biphenol–2,2′-biphenolate chains. In acetonitrile, after the proton transfer, the complexes dissociate and hence protonated MTBD molecules and hydrogen-bonded 2,2′-biphenol–2,2′-biphenolate chains are present. The hydrogen bonds and the hydrogen-bonded chains show large proton polarizability. In the systems intra- as well as inter-molecular hydrogen bonds are formed.

In crystals with hydrogen bonds infrared continua are sometimes observed. They demonstrate that in crystals hydrogen bonds may show large proton polarizability. Heteroconjugated OH⋯N⇌ − O⋯H + N bonds present in phenol+amine crystals as well as long, weak N + H⋯N⇌N⋯H + N bonds are discussed. Furthermore, results with short, strong − O⋯H + ⋯ − O bonds formed in carboxylic acid–carboyxlate crystals are considered. It is shown that in crystals a continuum of energy levels arises due to the interaction of the polarizable hydrogen bonds via proton dispersion forces. The nature of these forces is explained; they are attractive forces.

A new compound 3-(hydroxymethyl)-2,2‘-biphenol (HMBP) was synthesized. Complexes of this compound with three N-bases were studied by FTIR and 1H NMR spectroscopy. In the 1:1 mixture of HMBP and triethylamine (TEA) in chloroform, the complexes are formed completely. The hydrogen-bonded chain of these complexes shows large proton polarizability due to collective proton fluctuation. In acetonitrile the alcoholic group of HMBP is no longer bonded to the hydrogen-bonded chain. The rest of the hydrogen-bonded chain still shows proton polarizability. In the 1:1 mixtures of HMBP with a stronger base 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) the phenolic proton transfers to MTBD and is localized there. In chloroform the protonated MTBD is weakly bonded to HMBP via two other hydrogen bonds. In acetonitrile the complex dissociates. The complex between HMBP and urotropine is not formed completely and all hydrogen bonds within this complex are asymmetrical and weak.

B+H⋯B⇌B⋯H+B or AH ⋯ B ⇌ − A ⋯ H + B hydrogen bonds, respectively, with double minimum proton potential well or broad flat potential well cause continuous absorptions in the infrared spectra. The intensity of these infrared continua is often very large, i.e. their integrated intensity is much larger than that of infrared bands. Short strong hydrogen bonds with broad flat in the average largely symmetrical proton potentials in which the protons fluctuate, show so-called proton polarizabilities. With such systems, the infrared continua extend in the far infrared region, i.e. in the region below 600 cm−1. It is, however known that with such bonds no proton transitions occur in this region. It is demonstrated that the intensity of these infrared continuous absorptions is strongly increased by the coupling of the hydrogen bonds with large proton polarizability with the phonons of the thermal bath around these hydrogen bonds. Further, it is demonstrated that the pronounced absorption in the far infrared region, i.e. the region below 600 cm−1, occurs due to the strong coupling of these hydrogen bonds to the polaritons. The polaritons are the quanta of the thermal bath coupled to the phonons of the thermal bath.

Three tris(oxaalkyl) borates and their complexes with HAuCl 4 were studied by FT-IR, 1 H and 13 C NMR spectroscopy. Continuous absorption in the FT-IR spectra of the complexes testifies the formation of proton channels showing large proton polarizability. The shape of the proton potential is discussed on the basis of the NMR data.

The complexes formed by tetrazole with MTBD, an N-base of guanidin-like character, were studied by FTIR spectroscopy. In the chloroform solution of 1:1 ratio, a −N⋯H+MTBD asymmetrical complex is formed and no dissociation of this complex is visible. With increasing tetrazole–MTBD ratio, the formation of chains of tetrazole molecules is observed. In acetonitrile solution of tetrazole with MTBD, the amount of protonated and dissociated H+MTBD species increases with increasing concentration of tetrazole. In these mixtures, negatively charged chains are observed to be formed with tetrazole molecules only; however, some amount of free tetrazole molecules is also detected in this case. All these chains show large proton polarizability

Low temperature studies of UV and IR spectra have been performed for 2-(N,N-dimethylaminomethyl)-4,6-dichlorophenol (Cl2MB), 2-(N,N-diethylaminomethyl)-3,4,6-trichlorophenol (Cl3MB) and 2-(N,N-diethylaminomethyl)-tetrachlorophenol (Cl4MB) in n-butylchloride, which undergoes transition to the glassy state at 150 K. The proton transfer (PT) equilibrium in the liquid state is clearly seen only for Cl4MB. In the case of Cl3MB, negligible amounts of the PT species are observed in liquid phase above 150 K, and complete PT takes place below the glassy state transition. No PT was detected for Cl2MB down to 90 K. The temperature effects on the high frequency region and background IR absorption extended down to 650 cm−1 are analyzed. The broad absorption below 1600 cm−1 vanishes for Cl4MB and Cl3MB on cooling when they transform to the PT ion pairs. The potential energy curve for ion-pairs is characterized by a single minimum.

Maltodextrinphosphorylase (MDP) was studied in the pH range 5.4-8.4 by Fourier transform infrared (FT-IR) spectroscopy. The pK(a) value of the cofactor pyridoxalphosphate (PLP) was found between 6.5 and 7.0, which closely resembles the second pK(a) of free PLP.FT-IR difference spectra of the binary complex of MDP + alpha-D-glucose-1-methylenephosphonate (Glc-1-MeP) minus native MDP were taken at pH 6.9. Following binary complex formation, two Lys residues, tentatively assigned to the active site residues Lys533 and Lys539, became deprotonated, and PLP as well as a carboxyl group, most likely of Glu637, protonated. A system of hydrogen bonds which shows large proton polarizability due to collective proton tunneling was observed connecting Lys533, PLP, and Glc-1-MeP. A comparison with model systems shows, furthermore, that this hydrogen bonded chain is highly sensitive to local electrical fields and specific interactions, respectively. In the binary complex the proton limiting structure with by far the highest probability is the one in which Glc-1-MeP is singly protonated. In a second hydrogen bonded chain the proton of Lys539 is shifted to Glu637. In the binary complex the proton remains located at Glu637. In the ternary complex composed of phosphorylase, glucose-1-phosphate (Glc-1-P), and the nonreducing end of a polysaccharide chain (primer), a second proton may be shifted to the phosphate group of Glc-1-P. In the doubly protonated phosphate group the loss of mesomeric stabilization of the phosphate ester makes the C1-O1 bond of Glc-1-P susceptible to bond cleavage. The arising glucosyl carbonium ion will be a substrate for nucleophilic attack by the nonreducing terminal glucose residue of the polysaccharide chain.

Li + , Na + and K + 1:1 complexes of bis[3,3′-(2,2′-dihydroxybiphenyl)]methane were studied by FT-IR spectroscopy. Far infrared continua in the spectra of these complexes demonstrate that these complexes show large cation polarizabilities. These polarizabilities decrease in the series of the above-mentioned series of cations since the amplitude of the cation motion in the cation bonds decreases.

FTIR and NMR of 1H, 13C, 11B, 7Li and 23Na nuclei were used for the study of the complexation of Li+ and Na+ cations by three tris(oxaalkyl) borates. The NMR techniques proved the formation of complexes and the fluctuation of Li+ and Na+ cations in the respective channels formed by three oxaalkyl chains. In the FTIR spectra of Li+ complexes with three tris(oxaalkyl) borates, continuous absorption in the far-infrared region indicates the fast fluctuations of Li+ ion between O atoms of the channels. The dependence of the continua shape on the length of the channels, i.e., the number of minima in the multiminima potentials, demonstrates that the fluctuation of Li+ ion occurs along the channel. Owing to the larger mass and diameter of the Na+ cation in the Na+–tris(oxaalkyl) borate complexes, only systems with longer channels show a large Na+ polarizability due to the fast fluctuation of Na+ cations in a multiminima potential.

Homoconjugated B + H … B ↭ B … H+B as well as heteroconjugated AH … B ↭ -A … H+B bonds show proton polarizability due to fluctuation and shifts of the proton within these hydrogen bonds. These proton polarizabilities are about two orders of magnitude larger than polarizabilities of electron systems. The catalytic mechanism of serine proteinases which is based on a shift of the positive charge in a hydrogen bond with large proton polarizability is discussed. Particularly large proton polarizabilities caused by collective proton motion in hydrogen-bonded chains are observed. This result is obtained with poly-α-amino acid + dihydrogen phosphate systems and studied with intramolecular hydrogen-bonded chains. A theoretical treatment has been performed with the formic acid-water-formate and the formic acid-water-water-formate systems. It is shown that the proton pathway in the L550 intermediate of bacteriorhodopsin as well as the pathway in the F0 subunit of the ATP synthase are hydrogen-bonded systems with lar...

The mono- as well as the di-Mannich bases of 5,5′-dimethoxy-2,2′-biphenol are studied by FT-IR spectroscopy. It is shown that in these bases the protons remain localized at the phenol O atom. This result is in contrast to those obtained earlier for more acidic biphenols in which the protons are not localized but fluctuate and the relevant hydrogen bonds show large polarizability. An intense infrared continuum in the FT-IR spectrum of the mono-protonated di-Mannich base of 5,5′-dimethoxy-2,2′-biphenol demonstrates that the intramolecular hydrogen bond in this compound shows large proton polarizability.

Sixteen 1:1 dimethylphosphinic acid + N-base systems were studied in acetonitrile−chloroform (1:2) solutions as a function of the basicity of the N-bases. The complexes were measured in the middle infrared (MIR) and far-infrared (FIR) region at 20 °C and at −40 °C. The observed IR continua demonstrate that in the systems with the weaker bases an asymmetrical double minimum proton potential with the deeper well at the acid site is present. With increasing basicity the well at the base site becomes deeper and deeper and the proton potentials obtain a more symmetrical shape. The largest proton polarizability is attained with the system that shows the (on the average) most symmetrical proton potential, as indicated by the maximum bathochromic shift of the IR continuum. This shift toward lower wavenumbers is largest with the dimethylphosphinic acid + triallylamine complex. The most intense integrated absorbance of the IR continuum is also observed in this system. The characteristic intensity distribution at th...

Gramicidin A and C as well as their proton and Li+ complexes were studied by FT-IR spectroscopy. With an increasing number of hydrated and protonated channels, infrared continuous absorption is observed. It proves large proton polarizability due to very fast collective fluctuations and shifts of the excess proton in hydrogen-bonded single file water chains in the gramicidin channels. The infrared spectrum of the hydrated Li+ cation in the channels shows a continuum in the far-infrared region. This continuum is explained in terms of Li+ ion fluctuations and shifts between the electron lone pairs of two water molecules and at least three carbonyl O atoms of the backbone. The continuum indicates large Li+ polarizability of these channels. If the Li+-bonded system is polarized by an external electrical field, the diffusion of the Li+−water complex in field direction is favored.

Mixtures of imidazole derivatives with the strong base 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) were studied by FTIR spectroscopy as a function of the imidazole derivative:MTBD ratio. In the case of complexes of MTBD with the lowest acidic imidazole only molecular complexes are found. This is true in the chloroform solutions as well as in the acetonitrile solutions. With increasing acidity of the imidazoles in both solvents more and more polar structure of the complexes is found. In chloroform the polar structure predominates and the proton polarizability in the chains remains weak. In the acetonitrile solution the complexes dissociate. If more imidazole molecules are present one observes large proton polarizability of the negatively charged structurally symmetrical imidazole chains.

It is shown using the PM3 method that 1-methyladenine possesses four low-lying tautomeric forms, three of which are of the imino type. One of the imino forms, Ia is predominant and not energetically equivalent to the other ones. The effect of protonation on the spectra of these forms has been studied. By a polar environment modeled by Cramer-Truhlar SM3 solvation model, the amino form is favored. The assignment of vibrations of all low-lying tautomeric forms of 1-methyladenine in vacuo is elaborated.

Three di-Schiff bases of 2-hydroxy-5-methyl-isophthalaldehyde with 4-R-anilines (RH, CH3, OCH3) and their 1:1 complexes with HClO4 were studied by FT-IR, 1H, and 13C NMR spectroscopy in acetonitrile and [2H3]acetonitrile solutions, respectively. In di-Schiff bases intramolecular OH…N hydrogen bonds have been detected; however, they show no proton polarizability. Hydrogen-bonded systems with fast proton fluctuation and large proton polarizability have been found in the 1:1 complexes of di-Schiff bases with HClO4.

Complexes of tributyl phosphate (TBPO) and tris(10-chloro-3,6,9-trioxadecyl) phosphate (TODPO) with HAuCl4 and their hydrates with various amounts of water were studied by FT-IR spectroscopy. TBPO with HAuCl4 forms homoconjugated PO+H···OP ⇌ PO···H+OP hydrogen bonds with large proton polarizability. In hydrates of homoconjugated PO+H···OP ⇌ PO···H+OP complexes, H5O2+ is formed with a hydrogen bond having large proton polarizability. In the 1:1 mixture no H3O+, but both H5O2+ and PO+H···OP ⇌ PO···H+OP homoconjugated bonds are present in the a 1:1 ratio. In the protonated and hydrated mixtures of TODPO no homoconjugated PO+H···OP ⇌ PO···H+OP hydrogen bonds and no H5O2+ species are found. In these complexes the proton and water molecules are present in the channels which show large proton polarizability. The behaviour of these protonated and hydrated channels is discussed with respect to the behaviour of the gramicidin channel.

Sixteen 1:1 methanesulfonic acid–N-oxide systems were studied as a function of the basicity of the N-oxides. The observed IR continua demonstrate that a single minimum proton potential is shifted, with increasing basicity, from the acid to the N-oxide. The largest proton polarizability is attained with the system that shows the (on average) most symmetrical proton potential, as indicated by the maximum bathochromic shift of the IR continuum. This shift toward lower wavenumbers is largest with the methanesulfonic acid–3,5-dichloropyridine-N-oxide complex, for which the integrated absorbance of the IR continuum is also largest and the continuous absorption is nearly temperature independent. The significant intensity distribution at the symmetry point indicates very strong and short O···H···O hydrogen bonds. The proton transfer was studied via the SO-stretching vibration bands. The steady shift of the acid to the acid anion SO bands reflects a continuous proton transfer process within this family of systems. Study of the far-IR region shows that, especially in the most symmetrical cases, the IR continua extend down to 100 cm-1 or less. Semi-empirical calculations show that the observed hydrogen-bond vibrations differ largely from hydrogen-bond stretching vibrations (νσ) and have complicated vibrational character (νHB). In contrast to the methanesulfonic acid–sulfoxide (phosphine oxide, arsine oxide) family of systems no broadening of this transition occurs. The position of the bands indicates, however, a significant trend of the force constant. It is shown that the system with the strongest hydrogen bonds is that with the largest bathochromic shift of the IR continuum, i.e. the system with the largest proton polarizability.

We synthesized systems with NH·N − ⇌ − N·HN hydrogen bonds by the reaction of five NH acids with the phosphazene (P 2 Et) base and studied them by FT-IR spectroscopy. The intensity of the observed IR continua caused by the NH·N − ⇌ − N·HN hydrogen bonds is comparable with all systems. Thus, the proton polarizabilities and the proton potentials are independent of the p K a of the NH acids. The band structure observed in the region 3000-1700 cm −1 is caused by Fermi resonance of ν(NH) and the overtone of the ring breathing vibration.

5,5'-Dinitro-3-diethylaminomethyl-2,2'-biphenol ( 1 ) and 5,5'-dinitro-3,3' bis(diethylaminomethyl)-2,2'-biphenol ( 2 ) as well as 5,5'-dinitro-2,2'-biphenol ( 3 ) were synthesized and studied by FT-IR and 1 H NMR spectroscopy in acetonitrile or acetonitrile-d 3 solutions, respectively. With compound 1 a hydrogen-bonded system with large proton polarizability is found. In the hydrogen bonds in compound 2 the protons are localized at the N atoms. These hydrogen bonds show no proton polarizability. In the protonated compound 2 a very strong homoconjugated − O⋯H + ⋯O − hydrogen bond with large proton polarizability is found, whereas two other protons are localized at the N atoms. The deviation of the results obtained with other derivatives of 2,2'-biphenols are caused by the larger acidity of the nitro groups.

Complexes of tris(2-ethylhexyl) phosphate (EHPO) with HAuCl4 and its hydrates with various amounts of water were studied by FT-IR spectroscopy. In the water free solution of HAuCl4 in EHPO homoconjugated PO···H+···OP hydrogen bonds with proton polarizability are formed. We demonstrate that in the 1:1 and 1:2 mixture with water only the H5O2+ species is present and no H3O+ species is formed. In the 1:1 mixture with water in addition to the H5O2+ species, 50% homoconjugated hydrogen bonds of protonated EHPO are still present. This homoconjugated hydrogen bond, as well as the central bond in H5O2+ shows large proton polarizability, as indicated by IR continua. In the 1:3 mixture of the homoconjugated complex with water a cyclic hydrogen-bonded system with a three-minima proton potential is present. The proton polarizability of this system is the largest, as indicated by a particularly intense IR continuum. In the case of the 1:4 mixture, a cyclic hydrogen-bonded arrangement with a four-minima proton potentia...

We synthesized a triamide of Kemp's acid with two cysteine groups and one histidine group (compound 1), and a triamide of 1,3,5-pentane tricarboxylic acid with tyrosine, histidine, and arginine molecules (compound 2). From compound 1 we obtained the hydrated Zn 2+ complex, compound 3. The FT-IR spectra of various complexes of compounds 1–3 with NAD + show no IR continua and hence, no hydrogen-bonded chains with proton polarizability are present. In the case of the complex (compounds 2 and 3 and NAD + ) an intense continuum demonstrates that a hydrogen-bonded chain is formed with large proton polarizability due to collective proton motion. This proton pathway is discussed. The O atom of the nicotinamide group of NAD + is a strong hydrogen bond acceptor. This result is discussed with regard to the catalytic mechanism.

Maltodextrinphosphorylase (MDP) catalyses the degradation and in vitro elongation of polysaccharides. This enzyme is a pyridoxal-5′-phosphate-dependent (PLP-dependent) protein. The phosphate group of PLP plays a decisive role in the catalytic mechanism. Two hydrogen-bonded systems are present at the active site of MDP. Using Fourier transform infrared (FTIR) difference spectroscopy, the pH dependence of these hydrogen-bonded systems are studied. On the basis of these results, the protonation changes in the active centre of MDP when the substrate phosphate glucose-1-methylenephosphonate is added are discussed. Two hydrogen-bonded chains are formed: (1) a chain connecting Lys 533-PLP and glucose-1-methylenephosphonate; this chain shows large proton polarizability due to collective proton motion, but is polarized in such a way that the substrate phosphate remains protonated; (2) a chain connecting Lys 539, Tyr 538 and Glu 637; via this chain the excess proton of Lys 539 is shifted to Glu 637 and remains there; thus, in the binary complex, the excess proton of Lys 539 cannot be transferred to the phosphate group of the substrate. Finally, we discuss how, in the ternary complex, the C 1 O 1 bond between the glycosyl residue and the phosphate group can be destabilized and split, and how the glycosyloxocarbonium ion so formed can be added to the polysaccharide chain.

Five very strong N bases, 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), pK a =23.4; 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),pK a =23.9; tetramethylguanidine (TMG), pK a =23.3; 2-phenyl-tetramethylguanidine (PhTMG), pK a =20.6; and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), pK a =24.97; have been studied by osmometric measurements which showed that they are monomeric in acetonitrile solutions. The constants of the formation of homoconjugated complexes were determined by potentiometric measurements. In the IR spectra of the semi-protonated complexes of DBN, DBU and TMG, the homoconjugated N + –H···NN ···H– + N hydrogen bonds cause broad band complexes in the region 3200–2500 cm -1 instead of the expected continua. This spectral peculiarity is discussed.

The following FTIR difference spectra were studied: (pepsin) minus (Asp 215 or Asp 32 modified pepsin), (pepsin + pepstatin) minus (the modified pepsin + pepstatin), (at 40°C incubated pepsin + substrate) minus (pepsin + substrate at 4°C), and (at 40°C incubated pepsin + substrate) minus (EPNP modified pepsin + substrate). From these spectra, it is concluded that in native pepsin Asp 215 is protonated and Asp 32 deprotonated. A water molecule is present between these Asp residues. When substrate is added, Asp 215 is deprotonated and Asp 32 becomes protonated. This is performed by the hydrogen-bonded system Asp 215-water-Asp 32. This system shows very large proton polarizability due to collective proton motion. Asp 32 binds to the O atom of the peptide group. The catalytic mechanism is a base catalysis performed by the water molecule that is strongly polarized by the negatively charged Asp 215 residue. With their lone pairs, the water molecules attack the electrophilic carbon atom of the peptide group. © 1997 John Wiley & Sons, Inc. Biospectroscopy 3: 291–297, 1997

We synthesized and studied by Fourier transform infrared spectroscopy nine monosalts of diamides as models for the active side of aspartic proteinases. One compound, the monosalt of meta-aminobenzoic acid diamide of fumaric acid (m-FUM), shows the same biological activity as pepsin with regard to the splitting of peptide bonds of the Pro-Thi-Glu-Phe-Phe(4-NO2)-Arg-Leu heptapeptide. The monosalt of m-FUM forms with oxindole a complex in which the carboxylic acid group of the monosalt of m-FUM is strongly hydrogen bonded with the O atom of the peptide bond of oxindole. When one water molecule is added to this complex, the strong field of the carboxylate group destabilizes an O-H bond of the water molecule. The distorted water molecule attacks the carbon atom of the peptide group, and the water proton transfers to the peptide N atom. Simultaneously, the C-N bond of the amide group is broken. Hence it is demonstrated that the catalytic mechanism of aspartic acid proteinases is a base catalysis. The results show that for this catalytic mechanism there are sufficient carboxylic and carboxylate groups, as well as a water molecule in the correct arrangement. It was also demonstrated with other monosalts of dicarboxylic acids that well-defined steric conditions of the carboxylic acid and the carboxylate group must be fulfilled to show hydrolytic activity with regard to oxindole molecules.

Earlier RAMAN studies have shown that a strong interaction of NAD + with alcohol dehydrogenases occurs. In this paper nicotinamide adenine dinucleotide (NAD + ) is studied by FT-IR difference spectroscopy. The pH dependence of NAD + is investigated, as well as the influence of conformational changes and hydrogen bonding on the spectra. With the protonation of NAD + four bands disappear. These bands are characteristic for the protonation of the N 1 atom of the adenine residue. They are not influenced by a change from a closed to an expanded conformation of NAD + . The formation of hydrogen bonds shifts these bands only slightly toward higher wavenumbers. These results are the basis of a better understanding of the NAD + -alcohol dehydrogenase interaction.

The complexes formed by phenols with 1,3,4,6,7,8-hexahydro-l-methyl-2 H -pyrimido[1,2- a ]pyrimidine (mTBD), an N-base with guanidine-like character, were studied as a function of the p K a of the phenols by FT-IR spectroscopy. The following phenols were used: 4-cyanophenol (4-CNPh), pentachlorophenol (PCP) and 2,6-dichloro-4-nitrophenol (DNPh). In the case of chloroform solutions of 1: 1 mixtures of the phenols with MTBD the corresponding complexes are formed completely. With increasing acidity of the phenols the hydrogen bonds become increasingly asymmetrical. The O … N ⇋ − O … H + N hydrogen bond in the 4-CNPh-MTBD complex shows large proton polarizability. In the other cases only the polar structure is realized. With increasing phenol MTBD ratio, the formation of chains with two phenol molecules is observed. With decreasing p K a of the phenols the fluctuation is limited to the phenol-phenolate bond and finally, the phenol-protonated MTBD bond begins to dissociate. In acetonitrile solutions, N + H …O − hydrogen bonds are observed in the case of the 1:1 mixture of 4-CNPh with MTBD. A weak continuum indicates the presence of homoconjugated phenol-phenolate bonds with large proton polarizability. In the case of 2:1 mixtures only protonated MTBD and homoconjugated phenol-phenolate bonds are observed, independent of the p K a of the phenols. The results are discussed with regard to the proton pathway in bacteriorhodopsin.

The far infrared hydrogen bond vibration and the infrared continua observed when hydrogen bonds show large proton polarizability are studied as a function of the p K a of the donor or acceptor molecules with three families of systems: (1) substituted phenols-trimethylamine N -oxide, (2) pyridinium-pyridine, and (3) methanesulfonic acid-S, P, As oxide. With the first family of systems the hydrogen bond vibration depends only on the p K a of the phenols and is independent of changes in the mass of the phenols. With the second family of systems the continua and hence the proton polarizability are independent of the p K a of the protonated pyridines. With the third family, i.e., the methanesulfonic acid-S, P, As oxide systems, single minimum proton potentials are present. In these systems with largely symmetrical O − ⋯ H + ⋯ O bonds the proton potentials are broad and flat. These bonds also show proton polarizability, and the continua extend into the far infrared region. With the most symmetrical systems the hydrogen bond vibration is strongly broadened and merges into the continuum. All these experimental results are explained by semiempirical and by SCF ab initio treatments.

Model systems for the hydrogen bonded chain in the active centre of maltodextrinphosphorylase are synthesized and studied by FTIR spectroscopy. A hydrogen bonded system between an amine, the phosphate group of pyridoxalphosphate (PLP) and the phosphate group of glucose-1-phosphoric acid (GLC-1-P) causes an intense continuum in the IR spectrum, indicating that this hydrogen bonded chain shows large proton polarizability owing to collective proton tunnelling in multi-minima proton potentials. If the sodium salt of stearic acid is added to this hydrogen bonded chain, a very intense continuum is again observed. In addition, the carboxylate group of the stearic acid sodium salt becomes partially protonated since a v(CO) vibration of the carboxyl group arises at 1678 cm−1. The protons are located in this additional part of the chain.

We synthesized a 2-N-methylaminoethyl-tetramethylquanidine amide of Kemp's triacid (CP2). Furthermore, we added to CP2 tetrabutylammonium 4- tert -butylpenolate. The pK a value of 4- tert -butylphenol is comparable to that of tyrosine. We studied by FT-IR spectroscopy CP2 and the complex of CP2 with 4- tert -butylphenolate. This complex is a model for the hydrogen-bonded chain in the active centre of the bacteriorhodopsin molecule. An intense continuum in the FT-IR spectra demonstrates that this hydrogen-bonded chain shows large proton polarizability due to collective proton motion. As earlier demonstrated also Schiff base carboxylic acid bonds show large proton polarizability. Thus, in all these hydrogen bonds protons can easily be shifted by collective proton motion due to changes of the local electrical fields and by changes of specific interactions arising from conformational changes.

Binary solutions of chlorophenols in tributylamine (TBA) have been studied by UV and IR spectroscopy at temperatures down to the glassy transition state of TBA (123 K). The pentachlorophenol–TBA system has also been studied at 13–123 K in TBA matrices prepared by co-deposition of the gaseous components on a cooled window. 2,4-Dichlorophenol, 2,4,5-trichlorophenol, 2,6-dichlorophenol and 2,4,6-trichlorophenol in solution show proton-transfer (PT) equilibria which shift sharply toward the PT species on cooling. No PT is detected in the case of 2-chlorophenol. Pentachlorophenol appears entirely as a PT ion pair across the whole temperature range both in solution and in a matrix. Significant influence of ion-pair self-association on the IR spectra of solutions has been found by comparison with spectra obtained in rigid matrices. All systems studied reveal broad, intense, temperature-dependent bands corresponding to protonic vibrations. An extremely intense and broad absorption, with a maximum shifted to 700–500 cm–1, is observed when the PT equilibrium constant is close to unity. The contribution of different processes, e.g. hydrogen-bond strengthening, proton transfer and association of ion pairs, to the temperature transformations of IR spectra is discussed.

The conditions under which hydrogen-bonded chains show large proton polarizability due to collective proton motion are discussed on the basis of theoretical as well as experimental results. These chains are very suitable pathways for the conduction of protons, since they can easily be regulated by electric fields. Three such pathways present in biological systems are discussed in which water molecules are believed to play an important role: (1) the proton-conducting system in the bacteriorhodopsin molecule; (2) the proton pathway in the F0 subunit of ATP synthase; (3) the proton pathway in the enzyme alcohol dehydrogenase.

From the tetrabutylammonium salt of the Mannich base N-oxide and 4,4′-bis-(tetramethylquanidine) azobenzene (BTA) 1:1 complexes with KClO 4 were prepared. All these substances were studied by osmometric measurements and by FTIR spectroscopy. The 1:1 complex of the Mannich base with KClO 4 is a monomer. The intramolecular K + bond shows still weak K + polarizability. The properties of these bonds are compared with Li + and particularly with Na + bonds showing larger cation polarizabilities. The 1:1 complex of BTA with KClO 4 exists in the acetonitrile-chloroform solution as dimer. The K + bonds in the complex show, however, no K + polarizability since the K + motions are strongly compensated to the π-electron motion in a conjugated system.

5,5′-Dibromo-3-diethylaminomethyl-2,2′-biphenol ( 1 ) and 5,5′-dibromo-3,3′-bis(diethylaminomethyl)-2,2′-biphenol ( 2 ) were synthesized and studied by FTIR and 1 H NMR spectroscopy. In dichloromethane solution, 1 forms a hydrogen-bonded system with large proton polarizability due to collective tunnelling of protons in the two intramolecular hydrogen bonds. In acetonitrile the system is polarized to a greater extent. If this system is protonated, the HO⋯HO⋯bond is broken and hence the collective system destroyed. The FTIR spectrum of protonated 2 shows an intense continuum indicating a collective hydrogen-bonded system with large proton polarizability due to collective proton motion. The butyronitrile solution of 1 shows an increase of the Fermi resonance bands with decreasing temperature (above the glass transition). The hydrogen bonds become stronger. Furthermore, the proton polarizability increases. With the transition in the glass state the continuum remains unchanged since the local fields are frozen. Only the Fermi resonance effect becomes slightly stronger. With the butyronitrile solution of 2 the IR continuum caused by the polarizable OH⋯N ⇌ O − ⋯H + N bonds increases above the glass transition with decreasing temperature. The equilibrium is shifted in favour of the polar structure. In the glass state no change of the continuum and thus of the proton polarizability is observed, since the local fields are frozen.