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Pyridoxal-5'-phosphate (PLP) is introduced to a biomimetic indicator displacement assay for simultaneous determination of the absolute configuration, enantiomeric composition and concentration of unprotected amino acids, amino alcohols and amines. The chiroptical assay is based on fast imine metathesis with a PLP aryl imine probe to capture the target compound for circular dichroism and fluorescence sensing analysis. The substrate binding yields characteristic Cotton effects that provide information about the target compound ee and the synchronous release of the indicator results in a nonenantioselective off-on fluorescence response that is independent of the enantiomeric sample composition and readily correlated to the total analyte concentration. [ABSTRACT FROM AUTHOR]

A novel enantioselective copper-catalyzed intermolecular cyanotrifluoromethylation of alkenes has been developed, in which a variety of CF3-containing alkylnitriles are furnished with excellent enantiomeric excess. Preliminary mechanistic studies revealed (1) the reaction was initiated by a SET process between activated Togni's CF3+ reagent and a Cu(I) catalyst; (2) the released CF3 radical readily added to styrene to provide a benzylic radical, which was then trapped by a chiral Cu(II) cyanide species to deliver the desired alkylnitriles; (3) a low concentration of the CN anion was crucial to obtain high enantioselectivity. [ABSTRACT FROM AUTHOR]

The function of a parity generator/checker, which is an essential operation for detecting errors in data transmission, has been realized with multiphotochromic switches by taking advantage of a neuron-like fluorescence response and reversible light-induced transformations between the implicated isomers. [ABSTRACT FROM AUTHOR]

Single-molecule fluorescence photoswitching plays an essential role in ultrahigh-density (Tbits/inch2) optical memories and super-high-resolution fluorescence imaging. Although several fluorescent photochromic molecules and fluorescent proteins have been applied, so far, to optical memories and super-high-resolution imaging, their performance is unsatisfactory because of the absence of "non-destructive fluorescence readout capability". Here we report on a new molecular design principle of a molecule having non-destructive readout capability. The molecule is composed of acceptor photochromic diarylethene and donor fluorescent perylenebisimide units. The fluorescence is reversibly quenched when the diarylethene unit converts between the open- and the closed-ring isomers upon irradiation with visible and UV light. The fluorescence quenching is based on an electron transfer from the donor to the acceptor units. The fluorescence photoswitching and non-destructive readout capability were demonstrated in solution (an ensemble state) and at the single-molecule level. Femtosecond time-resolved transient and fluorescent lifetime measurements confirmed that the fluorescence quenching is attributed to the intramolecular electron transfer. [ABSTRACT FROM AUTHOR]

C78 is one of the most widely investigated higher fullerenes. Among its huge isomer family, only one non-IPA (IPR = isolated pentagon ring) cage, the C2-symmetric #22010C78, was previously stabilized by endohedral derivatization. Here we report a new C1-symmetric non-IPR hollow isomer, #23863C78, which was captured as #23863C78 Cl8 and then subjected to a regioselective substitution reaction with benzyl hydroperoxide to form #23863C78(OOCH2C6H5)Cl7. The structural connectivity of #23863C78, which contains a pair of fused pentagons, was confirmed by single-crystal X-ray diffraction analysis of the #23863C78(OOCH2C6H5)Cl7 molecule, which shares the same fullerene core with #23863C78Cl8 support for the structure is provided by comparable IR measurements and computation. Theoretical studies suggest that the differences in C-Cl bond length, intermediate stability, and steric effects of the involved molecules account for the chemical regioselectivity of the substitution reaction. [ABSTRACT FROM AUTHOR]

It is observed in this study that the chirality of cysteine stabilizers has a distinct effect on both the growth kinetics and the optical properties of CdTe nanocrystals synthesized in aqueous solution. The effect was studied by circular dichroism spectroscopy, temporal UV--vis spectroscopy, photoluminescence spectroscopy, and several other microscopy and spectroscopic techniques including atomic modeling. Detailed analysis of the entirety of experimental and theoretical data led to the hypothesis that the atomic origin of chiral sites in nanocrystals is topologically similar to that in organic compounds. Since atoms in CdTe nanocrystals are arranged as tetrahedrons, chirality can occur when all four atomic positions have chemical differences. This can happen in apexes of nanocrystals, which are the most susceptible to chemical modification and substitution. Quantum mechanical calculations reveal that the thermodynamically preferred configuration of CdTe nanocrystals is S type when the stabilizer is D-cystaine and R type when L-cysteine is used as a stabilizer, which correlates well with the experimental kinetics of particle growth. These findings help clarify the nature of chirality in inorganic nanomaterials, the methods of selective production of optical isomers of nanocrystals, the influence of chiral biomolecules on the nanoscale crystallization, and practical perspectives of chiral nanomaterials for optics and medicine. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) predicts a detailed mechanism for the reported potential photocatalytic system for solar hydrogen production from water, (P-da-PNN)RuH(CO) (1, P-da = dearomatized at the phosphorus side arm, PNN = (2-(di- tert-butylphosphinomethyl)-6-diethylaminometh- yl)pyridine) (Science 2009, 324, 74). In the initial thermal reaction, the coordination of a water molecule is followed by cleavage of an 0-H bond and aromatization of the PNN ligand to form (PNN)RuH(CO)(OH) (3'), the most stable complex in the reaction. This low-barrier step is followed by the rate-determining dearomatization and formation of H2. Next, a second water molecule is activated, resulting in the formation of the cis-dihydroxo complex (PNN)Ru(CO)(OH)2 (7), which photolytically eliminates H2O2. Time-dependent DFT calculations predict that the breaking of the two strong Ru-O bonds and the formation of the O-O bond in this photolytic reaction involve low-energy triplet states and singlet-triplet crossings. Rather than regeneration of initial complex 1 after the light-induced H2O2 evolution in the catalytic cycle, the DFT calculations predict a new route with a lower energy barrier via the regeneration of 1', an isomer of 1 with the unsaturated carbon at the nitrogen side arm of the PNN ligand. This new route involves hydride transfer from the methylene group at the nitrogen side, rather than the previously proposed regeneration of 1 through hydride transfer from the phosphorus side arm of the PNN ligand. [ABSTRACT FROM AUTHOR]

Diisopropyl fluorophosphatase (DFPase) from Loligo vulgar/s is an efficient and robust biocatalyst for the hydrolysis of a range of highly toxic organophosphorus compounds including the nerve agents sarin, soman, and cyclosarin. In contrast to the substrate diisopropyl fluorophosphate (DFP) the nerve agents possess an asymmetric phosphorus atom, which leads to pairs of enantiomers that display markedly different toxicities. Wild-type DFPase prefers the less toxic stereoisomers of the substrates which leads to slower detoxification despite rapid hydrolysis. Enzyme engineering efforts based on rational design yielded two quadruple enzyme mutants with reversed enantioselectivity and overall enhanced activity against tested nerve agents. The reversed stereochemical preference is explained through modeling studies and the crystal structures of the two mutants. Using the engineered mutants in combination with wild-type DFPase leads to significantly enhanced activity and detoxification, which is especially important for personal decontamination. Our findings may also be of relevance for the structurally related enzyme human paraoxonase (PON), which is of considerable interest as a potential catalytic in vivo scavenger in case of organophosphorus poisoning. [ABSTRACT FROM AUTHOR]

While many biogenic and anthropogenic organic constituents in the atmosphere are surface-active and chiral, the role of stereochemistry in heterogeneous oxidation chemistry in the atmosphere has not yet been evaluated. Here, we present nonlinear vibrational surface spectra of fused silica substrates functionalized with quinuclidine diastereomers during exposure to 1011 to 1013 molecules of ozone per cm³ in 1 atm helium to model ozone-limited and ozone-rich tropospheric conditions. Kinetic studies show that diastereomers that orient their reactive C=C double bonds toward the gas phase exhibit heterogeneous ozonolysis rate constants that are 2 times faster than diastereomers that orient their C=C double bonds away from the gas phase. Insofar as our laboratory model studies are representative of real world environments, our studies suggest that the propensity of aerosol particles coated with chiral semivolatile organic compounds to react with ozone may depend on stereochemistry. We expect that the differences in chemical accessibility will lead to the enrichment of one oxidation product stereoisomer over the other. The oxidation products could be gaseous or surface-bound, indicating that kinetic resolution could lead to the stereochemical enrichment of the gas phase or the aerosol, which may have also been important in prebiotic chemistry. Implications of these results for chiral markers that would allow for source appointments of anthropogenic versus biogenic carbon emissions are discussed. [ABSTRACT FROM AUTHOR]

The thiolate-for-thiolate ligand exchange was performed on well-defined gold nanoparticles under an inert atmosphere without any modification of the core size. This reaction is faster than the well-known core etching. Surprisingly, if a chiral thiol is exchanged for its opposite enantiomer, the optical activity in the metal-based electronic transitions is reversed although the form of the CD spectra remains largely unchanged. The extent of inversion corresponds to the overall ee of the chiral ligand in the system. This shows that the chiral arrangement of metal atoms in the metal particle (surface) can not withstand the driving force imposed by the ligand of opposite absolute configuration. If the incoming thiol has a different structure, the electronic transitions in the metal core are slightly modified whereas the absorption onset remains unchanged. These results emphasize the influence of the thiols on the structure of the gold nanoparticles and give insight on the ligand exchange pathways. [ABSTRACT FROM AUTHOR]

The interconversion between helical diastereomers of nickel-salen-based foldamers can be observed on a NMR time scale. Such complexes provide quantitative information about the propensity of different elements of central chirality to control the absolute sense of folding. trans-Cyclohexane-1,2-diamine—a common component of chiral salen catalysts—is a surprisingly weak director of absolute helicity in nickel-salen foldamers. Implications for asymmetric catalysis are discussed. [ABSTRACT FROM AUTHOR]

Complementary to studies that provided the first yatakemycin total synthesis resulting in its structure revision and absolute stereochemistry assignment, a second-generation asymmetric total synthesis is disclosed herein. Since the individual yatakemycin subunits are identical to those of duocarmycin SA (alkylation subunit) or CC-1065 (central and right-hand subunits), the studies also provide an improvement in our earlier total synthesis of CC-1065 and, as detailed herein, have been extended to an asymmetric total synthesis of (+)-duocarmycin SA. Further extensions of the studies provided key yatakemycin partial structures and analogues for comparative assessments. This included the definition of the DNA selectivity (adenine central to a five-base-pair AT sequence, e.g., 5′-AAAAA), efficiency, relative rate, and reversibility of ent-(-)-yatakemycin and its comparison with the natural enantiomer (identical selectivity and efficiency), structural characterization of the adenine N3 adduct confirming the nature of the DNA reaction, and comparisons of the cytotoxic activity of the natural product (L1210, IC50 = 5 pM) with those of its unnatural enantiomer (IC50 = 5 pM) and a series of key partial structures including those that probe the role of the C-terminus thiomethyl ester. The only distinguishing features between the enantiomers is that ent-(-)-yatakemycin alkylates DNA at a slower rate (krel = 0.13) and is reversible, whereas (+)-yatakemycin is not. Nonetheless, even ent-(-)-yatakemycin alkylates DNA at a faster rate and with a greater thermodynamic stability than (+)-duocarmycin SA, illustrating the unique characteristics of such ‘sandwiched’ agents. [ABSTRACT FROM AUTHOR]

A concise synthesis of (+)-rishirilide B (2) is described. This is the first synthesis to be reported for the (+)-enantiomer of rishirilide B (2) as found in nature. The strategy accentuates the valuable combination of a method for o-quinone methide coupling with a method for enantioselective resorcinol dearomatization, which provides a densely functionalized chiral building block. The convergent synthesis illustrates several improvements and refinements to these methods and their supporting chemistries. Among these is the in situ generation of PhI[OTMS]OTf. Combination of this oxidant with phenol 31 constitutes the first example of a diastereoselective oxidative dearomatization of a resorcinol displaying a 2-alkyl substituent. In addition, the preparation of the cyclic sulfone 34 is reported. As a new dimethide precursor expressing a readily cleavable O-benzyl residue, sulfone 34 should prove useful in future endeavors. A protocol using the aluminum amide of dimethylhydrazine for opening and cleavage of a [1,4]-dioxan-2-one is also described. This procedure unmasks the hydroxy dione 36 by jettisoning the chiral directing group. Regioselective O-carbamylation of the 1,3-dione 36 enables the transformation of the remaining carbonyl into the α-hydroxy carboxylic acid found in 2. The total synthesis of (+)-rishirilide B (2) requires 15 pots from benzaldehyde 17 and 13 pots from benzaldehyde 32. The final product emerges in yields of 12.5% and 20.3% from compounds 17 and 32, respectively. The longest linear sequence requires eight chromatographies. Important observations leading to the development of the principle asymmetric method are described within the context of the total synthesis. [ABSTRACT FROM AUTHOR]

The isomerization dynamics of tris-catecholate complexes have been investigated by variable- temperature NMR methods, demonstrating that the intramolecular racemization of Δ and Λ enantiomers of d0 TiIV is facile and faster than that of d10 GaIII and GeIV analogues. Activation parameters for the racemization of K2[Ti23] (H22 = 2,3-dihydroxy-N,N'-diisopropylterephthalamide) were determined from line shape analysis of 1H NMR spectra {methanol-d4: ΔH‡ = 47(1) kJ/mol; ΔS‡ = -34(4) J/molK; ΔG‡298 = 57(3) kJ/mol; DMF-d7: ΔH‡ = 55(1) kJ/mol; ΔS‡ = -16(4) J/molK; ΔG‡298 = 59(3) kJ/mol; D2O (pD* = 8.6, 20% MeOD): ΔH‡ = 48(3) kJ/mol; ΔS‡ = -28(10) J/molK; ΔG‡298 = 56(3) kJ/mol}. The study of K2[Ti43] (H24 = 2,3-dihydroxy-N-tert-butyl-N'-benzylterephthalamide) reveals two distinct isomerization processes: faster racemization of mer-[Ti43]2- by way of a Bailar twist mechanism (D3h transition state) {Tc ≈ 242 K, methanol-d4}, and a slower mer⇌-fac [Ti43]2- isomerization by way of a Ray-Dun mechanism (C2v transition state) {Tc e≈ 281 K, methanol-d4}. The solution behavior of the TiIV complexes mirrors that reported previously for analogous GaIII complexes, while that of analogous GeIV complexes was too inert to be detected by ¹H NMR up to 400 K. These experimental findings are augmented by DFT calculations of the ML3 ground states and Bailar and Ray-Dun transition states, which correctly predict the relative kinetic barriers of complexes of the three metal ions, in addition to faithfully reproducing the ground-state structures. Orbital calculations support the conclusion that participation of the TiIV d orbitals in ligand bonding contributes to the greater stabilization of the prismatic TiIV transition states. [ABSTRACT FROM AUTHOR]

The structure and visible spectra of a large panel of thioindigo dyes and derivatives have been evaluated using a TD-PBEO/6-31 1 +G(2d,p)//PBEO/6-31 1 G(d,p) approach explicitly taking bulk solvent effects into account by means of the polarizable continuum model. The influence of the solvent characteristics, the trans-cis isomerization, and the chemical substitution on the benzene rings have been investigated. In addition, hemi-thioindigo dyes, thiazine-indigo, chromophore-like molecules, and selenoindigo have been considered. Though the relative oscillator strengths of the two allowed visible transitions in the nonplanar cis isomers are not always correctly reproduced by theory, the agreement between theoretical and experimental results is far above expectations. For the 170 cases studied, we obtained a mean unsigned error on the predicted λmax limited to 6.9 nm or 0.03 eV, with only 6 (4) cases for which the difference exceeds 20 nm (0.10 eV). These errors are 1 order of magnitude smaller than what has previously been reported for indigoids. A linear correlation between the central double bond length and the λmaxhas been established, while the bond length and vibrational frequency of the carbonyl groups do not correlate with the thioindigo color. The higher excitation energies of the cis conformers, compared to the trans structures, result from a less stabilized LUMO in the former case. Indeed, for cis thioindigo, the two electron-rich (in the excited state) carbonyl units lie close to each other. [ABSTRACT FROM AUTHOR]

Bao Gong Teng A, an optically active tropane alkaloid with hypotensive and miotic activity isolated from the Chinese herb Erycibe obtusifolia Benth, was synthesized by an "organometallic chiron" strategy in which single enantiomers of TpMo(CO)2(η3-pyridinyl) complexes are produced in quantity and then elaborated easily and efficiently to generate, after demetalation, highly enantiopure advanced synthetic intermediates possessing the tropane core. [ABSTRACT FROM AUTHOR]

The diastereomers of ketones 2 and 3 are shown to exhibit distinct photochemical reactivities due to conformational preferences; while the anti isomers of 2 and 3 undergo efficient Yang cyclization in 75-90% yields with a remarkable diastereoselectivity (> 90%), the syn isomers predominantly undergo Norrish Type II elimination. The differences in the product profiles of the diastereomers are consistent with a mechanistic picture involving the formation of precursor diastereomeric triplet 1,4-biradicals in which the substituents at α and β-positions stabilize the cisoid (cyclization) or transoid (elimination) geometry. The fact that such a diastereomeric relationship does indeed ensue at the triplet-excited-state itself is demonstrated via the nanosecond laser-flash photolysis of model ketones 1. The diastereomeric discrimination in the product profiles observed for ketones 2 and 3 as well as in the triplet lifetimes observed for ketones 1 can both be mechanistically traced back to different conformational preferences of the ground-state diastereomeric ketones and the intermediary 1,4-biradicals. Additionally, it emerges from the present study that the syn and anti diastereomers of ketones 2 and 3 represent two extremes of a broad range of widely examined butyrophenones, which lead to varying degrees of Yang photocyclization depending on the alkyl substitution pattern. [ABSTRACT FROM AUTHOR]

The structure elucidation of (+)-amphidinolide A, a cytotoxic macrolide, has been accomplished by employing a combination of total synthesis and NMR spectroscopic analysis. Amphidinolide A possesses two skipped 1,4-diene subunits which are accessible by ruthenium-catalyzed alkene-alkyne couplings. Previous total syntheses had revealed that the reported structure was incorrect; therefore, to incorporate maximum flexibility into the synthesis, with the ultimate goal of determining the correct structure, a highly convergent approach was chosen. Furthermore, liberal use was made of catalytic asymmetric transformations to set individual stereocenters. Three different strategies were envisioned for the end game, and due to the highly convergent nature of the synthesis, all three routes disconnect to the same three key intermediates, 5, 6, and 7. Diastereomers of 6 and 7 were easily prepared by modification of the synthetic routes to allow access to multiple diastereomers of 1 for structural determination. [ABSTRACT FROM AUTHOR]

Halo alcohol dehalogenase HheC catalyzes the highly enantioselective dehalogenation of vicinal halo alcohols to epoxides, as well as the reverse reaction, the enantioselective and β-regioselective nucleophilic ring opening of epoxides by pseudo-halides such as azide and cyanide. To investigate this latter reaction, we determined X-ray structures of complexes of HheC with the favored and unfavored enantiomers of para-nitrostyrene oxide. The aromatic parts of the two enantiomers bind in a very similar way, but the epoxide ring of the unfavored (S)-enantiomer binds in a nonproductive inverted manner, with the epoxide oxygen and Cβ atom positions interchanged with respect to those of the favored (R)-enantiomer. The calculated difference in relative Gibbs binding energy is in agreement with the observed loss of a single hydrogen bond in the S bound state with respect to the R bound state. Our results indicate that it is the nonproductive binding of the unfavored (S)-enantiomer, rather than the difference in affinity for the two enantiomers, that allows HheC to catalyze the azide-mediated ring opening of para-nitrostyrene oxide with high enantioselectivity. This work represents a rare opportunity to explain the enantioselectivity of an enzymatic reaction by comparison of crystallographic data on the binding of both the favored and unfavored enantiomers. [ABSTRACT FROM AUTHOR]

Chiral groups attached to the end of quinoline-derived oligoamide foldamers give rise to chiral helical induction in solution. Using various chiral groups, diastereomeric excesses ranging from 9% to 83% could be measured by NMR and circular dichroism. Despite these relatively weak values and the fact that diastereomeric helices coexist and interconvert in solution, the right-handed or left-handed helical sense favored by the terminal chiral group could be determined unambiguously using X-ray crystallography. Assignment of chiral induction was performed in an original way using the strong tendency of racemates to cocrystallize, and taking advantage of slow helix inversion rates, which allowed one to establish that the stereomers observed in the crystals do correspond to the major stereomers in solution. The sense of chiral helical induction was rationalized on the basis of sterics. Upon assigning an Rs or Ss chirality to the stereogenic center using a nomenclature where the four substituents are ranked according to decreasing sizes, it is observed that Rs chirality always favors left-handed helicity and Ss chirality favors right-handed helicity (P). X-ray structures shed some light on the role of sterics in the mechanism of chiral induction. The preferred conformation at the stereocenter is apparently one where the bulkiest group should preferentially point away from the helix, the second largest group should be aligned with the helix backbone, and the smallest should point to the helix. [ABSTRACT FROM AUTHOR]

The first total syntheses of new monoterpene alkaloids (-)-incarvilline, (+)-incarvine C, and (-)-incarvillateine, corresponding to the natural enantiomers, have been accomplished. The strategy for the synthesis of these natural products utilized 6-epi-incarvilline as a common precursor, which was assembled by a three-component coupling reaction using (4S)-4-siloxy-2-cyclopenten-1-one to construct an appropriately trisubstituted cyclopentanone, followed by ring closure to the cis-perhydro-2-pyrindine skeleton by means of a reductive Heck-type reaction. Furthermore, topochemically controlled [2 + 2] photodimerization of cinnamic acid derivatives in the solid state for the stereospecific construction of a 1,2 ,3,4-tetrasubstituted cyclobutane ring was also investigated as a means to access (-)-incarvillateine. [ABSTRACT FROM AUTHOR]

A simple procedure is reported for the catalytic asymmetric allylation of ketones, utilizing titanium tetraisopropoxide, BINOL, 2-propanol additive, and tetraallylstannane as allylating agent. A variety of ketone substrates, including acetophenone derivatives and αβ-unsaturated cyclic enones, reacted to form tertiary homoallylic alcohols in good yields (67-99%) and with high levels of enantioselectivity (generally >80%). A novel one-pot enantioselective allylation/diastereoselective epoxidation has also been introduced. Thus, upon completion of the allyl addition to conjugated cyclic enones, I equiv of tert-butyl hydroperoxide is added and the directed epoxidation of the allylic double bond ensues to afford the epoxy alcohol with high diastereoselectivity. [ABSTRACT FROM AUTHOR]

The aqueous, lyotropic liquid-crystalline phase behavior of the α-helical polypeptide, poly(Nepsi;- 2-[2-(2-methoxyethoxy)ethoxy]acetyl-lysine) (1), has been studied using optical microscopy and X-ray scattering. Solutions of optically pure 1 were found to form cholesteric liquid crystals at volume fractions that decreased with increasing average chain length. At very high volume fractions, the formation of a hexagonal mesophase was observed. The pitch of the cholesteric phase could be varied by a mixture of enantiomeric samples L-1 and D-1, where the pitch increased as the mixture approached equimolar. The cholesteric phases could be untwisted, using either magnetic field or shear flow, into nematic phases, which relaxed into cholesterics upon removal of field or shear. We have found that the phase diagram of 1 in aqueous solution parallels that of poly(γ-benzyl glutamate) in organic solvents, thus providing a useful system for liquid-crystal applications requiring water as solvent. [ABSTRACT FROM AUTHOR]

The mechanism of the optical resolution of γ-valerolactone (VAL) enantiomers by enclathration in cholic acid (CA) channels was investigated. 13C cross-polarization magic-angle spinning spectra of ON VAL inclusion compounds show four methyl 13C peaks of VAL with different intensities depending on the enantiomeric ratios. The four peaks were assigned to the inner and end (S)-(-)-enantiomers (5) in the S domain and the inner and end (R)-(+)-enantiomers (R). The relative intensities of the four methyl 13C peaks cannot be explained by the random process model for inclusion but are successfully reproduced by assuming the first-order Markov process, in which the inclusion probabilities of S and R depend on which enantiomer has precedingly entered the CA channel. The probability ps/s that two S enantiomers successively enter a channel is thus found to be 83%, and ps/s is 50%. The large probability of ps/s indicates that once an S enantiomer enters a channel, it become easy for other S enantiomers to successively enter the channel, and thus the large enantiomeric excess of S is obtained. The inclusion probabilities of S and R were confirmed by 1D 13C-13C polarization-transfer experiments among the four methyl carbons of VAL in the CA channel. Further, we found that the 13C line widths and peak positions of the CA tail group change depending on the enantiomeric ratio. We concluded that once S is included, it changes the conformation of the CA tail group so that other S enantiomers become easy to successively enter the channel. [ABSTRACT FROM AUTHOR]

Two diastereomers of a photoresponsive oligodeoxyribonucleotide tethering a trans-azobenzene, based on the chirality of the central carbon of a diol linker, were separated by reversed-phase HPLC. On the basis of 2D NMR analysis, absolute configurations of the diastereomers α and β (tentatively designated from differences in their retention time) were determined as R- and S-forms, respectively. For both diastereomers, their NMR-determined duplex structure showed that trans-azobenzene intercatates between base pairs, because distinct NOEs were observed between the protons of azobenzene and those of the adjacent base pairs, such as with the imino protons and methyl protons of thymine. The melting temperatures of both duptexes were higher than that of the corresponding native duplex, which contained no azobenzene residue, due to the intercalated trans-azobenzene stabilizing the duplex by a stacking interaction. Between these two diastereomers, differences in T[subm] were also found: the melting temperature of the R-form duplex (β-isomer) was higher than that of the S-form (β-isomer). On the basis of the NMR-determined structure, this difference was attributed to the fact that the S-form (β isomer) causes more stress forming the duplex than does the R-form (a isomer) due to disturbances of the right-hand helix. [ABSTRACT FROM AUTHOR]

Microwave rotational spectra of eleven isotopomers of the N[sube3]-NH[sub3] van der Waals tetramer were measured using a pulsed jet, Balle-Flygare type Fourier transform microwave spectrometer. The transitions measured fall between 4 and 17 GHz and correspond to the ground internal rotor state of the weakly bound complex. The [sup20]Ne[subs]- and [sup22]Ne[sub3]-containing species are symmetric top molecules while the mixed [sup20]Ne[sub2][sup22]Ne- and [sup20]Ne[sup22]Ne[sub2]-isotopomers are asymmetric tops. For each of the deuterium-containing isotopomers, a tunneling splitting was observed due to the inversion of NH[sub3] within the tetramer. The [sup14]N nuclear quadrupole hyperfine structures were resolved and included in the spectroscopic fits of the various isotopomers. The rotational constants obtained from the fits were used to estimate the van der Waals bond lengths of the tetramer while the [sup14]N nuclear quadrupole coupling contants and the observed inversion tunneling splittings provided information about the internal dynamics of the NH[sub3] moiety. The experimental results were complemented by the construction of three ab initio potential energy surfaces [CCSD(T)] for the Ne[sub3]-NH[sub3] complex, each corresponding to a different internal geometry of NH[sub3] (∠HNH = 106.67°, ∠HNH = 113.34°, and ZHNH = 120.00°). The topologies of the surfaces are related to the structures and dynamics of the tetramer. Extensive comparisons are made between the results obtained for the Ne[sub3]NH[sub3] tetramer in this work and previous experimental and ab initio studies of related Rg[subn]-NH[sub3] van der Waals clusters. [ABSTRACT FROM AUTHOR]

The 2-(buta-1,3-diynyl)cycloprop-2-yl-1 -ylidene radical, a new three-membered ring chain with C[subs] symmetry, has been detected by electronic spectroscopy in the gas phase. The experimental investigation used a mass selective resonant two color two photon ionization technique coupled to a supersonic plasma source. Structures and relative stability energies of eight isomers of the C[sub7]H[sub3] radical have been calculated. Based on the rotational analysis and the theoretical calculations, the observed spectrum is assigned as an [sup2]A" ← X[sup2]A' electronic transition of this exotic chemical species. This result shows that such a plasma source is a powerful tool to investigate intermediates involved in hydrocarbon chemistry as in flames. [ABSTRACT FROM AUTHOR]

A chemical synthesis of both diastereomers of the tetrahedral intermediate involved in 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) catalysis has been accomplished. Combination of methyl dibromopyruvate with a protected shikimic acid derivative, phosphorylation, and lactonization afforded the intermediates (S)-15 and (R)-15, whose configurations were assigned by NMR. After introduction of the 3-phosphate group and deprotection, photoinitiated radical debromination of the dibromo analogues (5')-5 and (R)-5 was accomplished with tributyltin hydride in mixed aqueous solvents in the presence of surfactant to give the pyruvate ketal phosphates (R)-TI and (S)-TI, respectively. These compounds are stable at high pH, but decompose at pH 7 with a half-life of ca. 10 min. (R)-TI proved to be inert to EPSPS, while (S)-TI was converted by the enzyme to a mixture of 5-enolpyruvylshikimate 3-phosphate, shikimate 3-phosphate, and phosphoenolpyruvate. The demonstration that the enzymatic intermediate possesses the S-configuration at the ketal center confirms the mechanism as an anti addition followed by a syn elimination. Furthermore, it appears that the syn stereochemistry of the second step requires the phosphate leaving group to serve as the base in catalyzing its own elimination. [ABSTRACT FROM AUTHOR]

Examines the structure on three isomers by nuclear magnetic resonance on divalent metallofullerences. Symmetry on the divalent metallofullerence; determination on the cage sculpture; Comparison between divalent metallofullerence; Consequences of congestion.

Shows that the Birch reduction gives a complex mixture of C[sub 60]H[sub 36] isomers. Transfer hydrogenation method of Ruchardt; Isolation of the isomers by preparative high-pressure liquid chromatography; Determination of nuclear magnetic resonance spectroscopic properties; Comparison of experimental chemical shifts against those computed using density functional theory.

Shows that the conformational changes in the A103 region of the protein may be triggered by optical excitation of a nonisomerizable molecule located in the retinal binding site of bacteriorhodopsin. Efficiency of the functional groups in the retinal oxime displacement; Conformational changes in the bacteriorhodopsin pigments; Protein-dye spin-labeled complexes.

Discusses the synthesis and alkali-binding properties of two isomers of calix[6]arene bis-crown-4. 1,2,3-alternate conformation of one of the isomers; Determination of the alkali metal complex stoichiometries, association constants and ion selectivities using nuclear magnetic resonance titration experiments.

Investigates the kinetics and mechanism of the isomerization of the 1-pentene by iron carbonyls in the gas phase. Production of cis- and trans-2-pentene isomers; Evaluation of the effects of chain length on the microscopic kinetics and thermodynamics of the system.

Discusses the synthesis of molecular triangles from ligand 2,2-bipyrazine. Formation of cis and trans conformers; Establishment of chelation modes; Observation of anion binding properties; Isolation and characterization of isomers.

investigates the gas-phase reactions of phenyl halides with FeO+. Comparison with structural isomers generated via independent routes; Predomination of the initial coordination of FeO+ at the functional group; Degradation of chlorobenzene.

Examines the preparation of optically active (E)-cycloheptene. Generation of (E)-cycloheptene by enantiodifferentiating Z-E photoisomerization; Sensitization of (Z)-isomer by chiral benzene-tetracarboxylates; Configuration of (E)-cycloheptene; Stability of (E)-cycloheptene; Characterization of cycloheptene.

Investigates the reversible photochromism reaction of a closed-ring form isomer in a single-crystalline phase. Dichroism upon irradiation with 366-nm light; Attribution of yellow and blue colors to two perpendicular electronic transitions; Use of x-ray crystallographic analysis.

Investigates the infrared multiphoton dissociation (IRMPD) of the isomers M(1,3-butadiene)+ and M(C2H2)-(C2H4)+ where M may be iron, copper, or nickel. Observation of an isotope effect; Performance of sustained off-resonance irradiation technique; Observation of multiple IRMPD products for the undeuterated iron and cobalt ion complexes.

Reports the synthesis and structural characterization of isomeric Bp'Mo(eta2-C(O)R)(CNR')(CO)(PMe3) complexes and isomeric Bp'Mo(eta2-C(N-t-Bu)R)(CO)2(PMe)3 complexes. Solid state structures; Kinetics of isomerization.

Reports the characterization of an aromatic monotrans (10)annulene configuration at the second order perturbation and density functional levels of theory. Estimated aromatic stabilization energy; Diamagnetic susceptibility exaltation; Upfield nuclear magnetic resonance chemical shift of the inner hydrogen.

Describes an optically active ruthenocenylbis(phosphines). Effectivity as chiral ligands; Chiral phosphines' ability to induce enantioselectivity; Preparation through stereoselective lithiation of ((R)-N,N-dimethyl-1-ruthenocenylalkyamines; Structure of PdCl2((R)-(S)-Et-BPPRA); Enantioselectivity of ruthenocenylphosphines.

Reports on the photochemistry and vibrational spectroscopy of three isomers of chlorine dioxide isolated in cryogenic matrices. Photolysis of chloryl chloride; Fourier transform infrared spectroscopy; Vibrational spectra and ab initio calculations; Dissociation channels; Photodecomposition in an argon matrix.

Reports on the gas-phase potential energy surface of isomers. Use of collisional activation; Ion/molecule reactions employing the conditions of ion cyclotron resonance mass spectrometry; Reaction mechanisms in the oxidation of benzene and ethylbenzene by ferrous oxide; Dissociation of the iron complex.

Investigates the role of isomers of planarly chiral nonbridged bis(indenyl)metal type complexes in propene polymerization. Synthesis of the chirally substituted metallocene complexes; X-ray crystal structure analyses; Propene polymerization reactions; Conformations of bis(1-hydrocarbylindenyl)- and -(tetrahydroindenyl)zirconium dichloride complexes.

It has been found that (E)-5-cyclononen-1-one (2a) exhibits marginal planar chirality owing to an insufficient topological constraint, whereas the enolates 3 derived from 2a show robust planar chirality. Enantioenriched enolates are easily prepared by enzymatic hydrolysis, and they show an ability to serve as chiral nucleophiles. [ABSTRACT FROM AUTHOR]

The article explores the synthesis of a T2 chemical agent that can modulate the relaxation time by aggregation of magnetic nanoparticles using magnetic resonance imaging (MRI). The agent consists of a spiropyran (SP) derivative that is said to change conformation from hydrophilic to hydrophobic isomers as response to light. The absorbance of the SP reduced to 563 nanometers (nm) in visible light irradiation. Reaction time in light irradiation was shortened.

The article focuses on the photoswitching activity of 5,6 dihydrodibenzo[c,g][1,2]diazocine. It mentions that dihydrodibenzo is derived from azobenzene (AB) but contains more favorable photochromic characteristics than ordinary AB. It cites that the product of the photoconversion of the isomers was examined using nuclear magnetic resonance (NMR) spectroscopy and thin layer chromatography. It notes that the switching of dihydrodibenzo can be attained with the help of visible light.

The article presents a study which examines the metal-directed synthesis of a supramolecular system such the enantiomerially pure dimetallic lanthanide luminescent triple-stranded helicates. It describes the function of the chiral ligands of the enantiomerically pure, dinuclear triple-stranded helicates. The study was able to demonstrate changes of chemical shift and gradual broadening of several resonances in the H NMR spectrum.

The article reports that enantiomeric excesses of up to 50 percent were achieved in the [2 + 2] cross photoaddition of fluoroanthenes within modified chiral cages. According to the authors, the present results confirm that, similar to enzymes in nature, indirect cavity-control will be an important and achievable strategy for chiral synthesis.

The article presents a study on the synthesis of caryophylloids, which depends of the accessibility of the yet unknown chiral dienone and its enantiomer, ent-4. It states that the results of this study implies several noteworthy developments including the implementation of a new technique involving the use of a chiral cyloolefin in the synthesis of caryophylloids, an uncomplicated synthesis of the chiral cyclononadienones 4 and ent-4, and 4-step conversion of ent-4 to coraxeniolide A.