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The dramatic rise in cancer incidence, alongside treatment deficiencies, has elevated cancer to the second-leading cause of death globally. The increasing morbidity and mortality of this disease can be traced back to a number of causes, including treatment-related side effects, drug resistance, inadequate curative treatment and tumor relapse. Recently, anti-cancer bioactive peptides (ACPs) have emerged as a potential therapeutic choice within the pharmaceutical arsenal due to their high penetration, specificity and fewer side effects. In this contribution, we present a general overview of the literature concerning the conformational structures, modes of action and membrane interaction mechanisms of ACPs, as well as provide recent examples of their successful employment as targeting ligands in cancer treatment. The use of ACPs as a diagnostic tool is summarized, and their advantages in these applications are highlighted. This review expounds on the main approaches for peptide synthesis along with their reconstruction and modification needed to enhance their therapeutic effect. Computational approaches that could predict therapeutic efficacy and suggest ACP candidates for experimental studies are discussed. Future research prospects in this rapidly expanding area are also offered.

For the synthesis and selection of active platinum-based anticancer drugs that perform better than cisplatin and its analogues, six-coordinate octahedral Pt(IV) complexes appear to be promising candidates as, being kinetically more inert and more resistant to ligand substitution than four-coordinate Pt(II) centers, they are able to minimize unwanted side reactions with biomolecules prior to DNA binding. Due to their kinetic inertness, Pt(IV) complexes have also been exploited to bypass inconvenient intravenous administration. The most prominent example is satraplatin (Sat.) which is the first platinum antineoplastic agent reported to have oral activity. The present paper deals with a theoretical DFT investigation of the influence that the acidity of the biological environment can have on the activity of satraplatin and analogous octahedral Pt(IV) complexes having two carboxylates as axial ligands. Moreover, here the outcomes of a joint electrospray ionization mass spectrometry and DFT investigation of the fragmentation pathways of the protonated satraplatin are reported. Calculations show that the simulated acidic environment has an important impact on the satraplatin reactivity causing a significant lowering of the barrier that is necessary to overcome for the hydrolysis of the first acetate ligand to occur. Data from electrospray ionization mass spectrometry

Pt-based anti-cancer drugs can form complexes with biological ligands other than their intended pharmacological targets. Such complexes are believed to be a major contributor to the reduction of the anti-cancer drugs’ therapeutic potential and can act as potential causes of toxicity. Recently, the cytoplasmic ligand β - alanyl-L-histidine dipeptide (carnosine) was shown to act as a sequestering agent for oxaliplatin (OxPt) inhibiting its cytotoxic action most likely through the formation of complexes that are less cytotoxic than OxPt alone. In this paper, the calculated binding modes and binding energies of protonated carnosine as well as canosine complexes with various biologically relevant metal cations and some common Pt-based anti-cancer drugs both in the gas phase and in solution are presented. Theoretical evidence for the existence of protonated carnosine in a zwitterionic form is offered. Six calculated geometrical arrangements comprising general structural motifs of metal ion-carnosine complexes are presented. In all but one of these six conformers the metal ions were shown to form three bonds to each of the carnosine ligands. These six general binding motifs were found to cover the lowest energy structures of all species investigated here whether in the gas phase or in solution. The data presented increases our knowledge of the interactions of carnosine with different metals, either those naturally found inside the body or those that are up taken via foods, supplements or drugs.

A detailed structural analysis of the complexes resulting from the association of the neutral, mono- and di-anionic forms cytoplasmic thiol containing tripeptide glutathione (GSH) to the post water substituted hydrolysed form of the anti-cancer drug cisplatin, [Pt(NH 3 ) 2 ] 2+ , in the gas phase and in solution is presented. These structures which are formed from a 1:1 molar ratio of the glutathione ligand and the Pt-drug, have been proposed as possible intermediates for several postulated disulfide bridged polymeric species. Ten gas phase structures on the [GSH − H + Pt(NH 3 ) 2 ] + potential energy surface are presented and the coordination of the Pt metal is discussed. The [GSH + Pt(NH 3 ) 2 ] 2+ potential energy surface was also explored and twelve gas phase structures on this surface are presented. In order to describe the complexes examined here in solution, water solvation effects were considered for eight and nine of the most energetically favored structures of the [GSH − H + Pt(NH 3 ) 2 ] + and [GSH + Pt(NH 3 ) 2 ] 2+ potential energy surfaces, respectively. The enthalpy and free energy, based on the most thermodynamically favored conformers of the reactants and products, for the addition reaction [Pt(NH 3 ) 2 ] 2+ + [GSH − H] − → [GSH − H + Pt(NH 3 ) 2 ] + in the gas phase at 298 K were determined to be −331.8 and −314.8 kcal mol −1 , respectively. Similarly, the enthalpy and free energy of the gas phase reaction [Pt(NH 3 ) 2 ] 2+ + GSH → [GSH + Pt(NH 3 ) 2 ] 2+ at 298 K were determined to be −185.0 and −167.4 kcal mol −1 .

Protein phosphorylation and de-phosphorylation, provide one of the most common signalling pathways within cells, being involved in regulating cellular processes, mediating enzyme inhibition, protein–protein recognition and protein degradation. Compared with normal proteomics, phosphoproteomics poses some additional challenges requiring more initial separation and additional sensitivity to detect and quantify potentially ultra-low abundance species. In this work, the selective detection of phosphopeptides is described based on the incorporation of a metal tag, gallium–N,N-biscarboxymethyl lysine (Ga-LysNTA), in solution before separation and detection by liquid chromatography coupled to inductively coupled plasma mass spectrometry (LC–ICP-MS). Experimental and theoretical characterisation of the resulting Ga–phosphopeptide complex is presented based on linear ion trap electrospray ionisation mass spectrometry (ESI-MS), Fourier transform mass spectrometry (FT-MS) and molecular modelling data. Linear ion trap electrospray ionisation mass spectrometry (ESI-MS) was employed to study the interaction of the gallium tag with platelet derived growth factor beta receptor (s-PDGF), a small phosphopeptide. In addition high resolution Fourier transform mass spectrometry (FT-MS) was used for accurate mass determination and multistage tandem mass spectrometry of the gallium–s-PDGF complex identified the fragmentation pathway. Finally, molecular modelling was used to investigate the energetically favoured structures of both the Ga-LysNTA material and the s-PDGF–Ga-LysNTA complex.

This paper describes methodologies, based on sector field inductively-coupled plasma mass spectrometry (SF-ICP-MS), and their application in the holistic study of the fate of Pt in human cell populations following treatment with cis- or oxaliplatin and combination treatments. Pt–DNA adduct formation data at several time points has been determined in the leukocytes from patients undergoing Pt-based chemotherapy demonstrating significant inter-patient variability and excellent reproducibility of the assay. The sensitivity of the technique enabled quantitation of as little as 0.2 Pt adducts per 10 6 nucleotides using 10 μg of patient DNA. Further, the first ever reported in vivo sub-cellular Pt fractionation data on a patient sample is presented indicating the feasibility of applying the methods presented here in a clinical environment. For in vitro studies, three cell models were used: A549 human lung adenocarcinoma epithelial cells were exposed to 50 μM cisplatin for 1 h; HCA7 human colorectal cancer cells were treated with either FOLFOX (200 μM 5-fluorouracil, 200 μM folinic acid and 50 μM oxaliplatin) or 50 μM oxaliplatin; and HT29 human colorectal cancer cells were treated with 50 μM oxaliplatin in combination with 20 μM methaneseleninic acid, CH 3 SeO 2 H (MSA). The cells were harvested and either the DNA extracted and/or a commercially available kit used to fractionate the treated cells into four sub-cellular compartments. Each of the sub-cellular fractions and extracted DNA were digested separately, evaporated to dryness and reconstituted in 2% nitric acid for analysis by SF-ICP-MS. The sub-cellular Pt distribution for cisplatin treated A549 cells was shown to be as follows: ∼70% localized in the cytosol, ∼17% in the membrane and membrane localized fraction, ∼9% in the nuclear fraction and ∼4% in the cytoskeletal fraction. Both FOLFOX and oxaliplatin treated HCA7 cells showed comparable sub-cellular Pt distributions, and Pt–DNA adduct formation was similar for the oxaliplatin and FOLFOX treatments with adduct yields of 5.6 and 5.5 adducts per 10 6 nucleotides respectively. It was found that the combination of oxaliplatin with 20 μM MSA did not change the distribution of Pt or significantly alter its accumulation in the cytosol of the HT29 cells. Mass balance experiments showed a >99% recovery of the total Pt in the sub-cellular fractions. These experiments are the first to provide such a detailed quantitation of Pt-drug partitioning and they show that the Pt broadly follows the total protein content of the individual compartments with the majority being scavenged in the cytosol compartment.

Positive mode electrospray mass spectrometry was used to investigate complexes of cysteine and selenocysteine with the metallo-drug Auranofin and the analogous chlorotriethylphosphine gold. Evidence for all the complexes, with the exception of the Auranofin–selenocysteine complex, was obtained. Several fragmentation products were examined and their proposed structures reported. Structural details of many experimentally observed ions including [Auranofin + H] + , [(CH 3 CH 2 ) 3 P–Au–cysteine] + and [(CH 3 CH 2 ) 3 P–Au–methylselenocysteine] + were examined by means of hybrid density functional theory at the B3LYP/DZVP level. Structural information and relative free energies are presented for several isomers of each ion considered.

The complexes of Ag + with the peptides MetGly, ProGly, GlyPro, GlyHis and GlyProAla were investigated using hybrid density functional theory at the B3LYP/DZVP level. The silver ion binding free energies at 298 K to each of these peptides was calculated to be 60.8, 52.0, 54.3, 71.2 and 63.3 kcal mol −1 , respectively. Structural information and relative free energies are presented for several isomers for each of the five complexes. Each of the global minima found for the five complexes is a charge-solvated ion. An important finding is that the Ag + –ProGly is the only complex where a salt bridge structure is energetically favored occurring at 4.0 kcal mol −1 higher in free energy than the global minimum. The Ag + ion in this salt bridge structure is attached to the carboxylate anion of zwitterionic ProGly in which the terminal amino nitrogen is protonated. For all the other complexes studied, the salt bridge structure occurs at much higher energies. All the dipeptide complexes with Ag + , but one, exhibit a di- or tri-coordinate metal where the sites of attachment are amino and carbonyl groups. However, the highest coordination numbers are not always the global minima due to steric costs. The global minimum of the Ag + –GlyProAla complex is the only structure found in this study where the metal is tetra-coordinated, binding to the terminal amino nitrogen and all three carbonyl oxygen atoms. Silver binding to sulphur and imidazole nitrogen atoms of MetGly and GlyHis, respectively, are present in the three most energetically favored species in each of these cases.

Platinum(II)-based anticancer drugs are square-planar d(8) complexes that, activated by hydrolysis, cause cancer cell death by binding to nuclear DNA and distorting its structure. For that reason, interactions of platinum anticancer drugs with DNA have been extensively investigated, aiming at disentangling the mechanism of action and toxicity. Less attention, however, has been devoted to the formation of adducts between platinum drugs with biological ligands other than DNA. These adducts can cause the loss and deactivation of the drug before it arrives at the ultimate target and are also thought to contribute to the drug's toxicity. Here are reported the outcomes of electrospray ionization mass spectrometry experiments and density functional theory (DFT) computations carried out to investigate the fragmentation pathways of the protonated carnosine-carboplatin complex, [Carnosine + CarbPt + H](+). DFT calculations at the B3LYP/LANL2DZ level employed to probe fragmentation mechanisms account for all experimental data. Because of the relative rigidity of the structure of the most stable 1A conformer, stabilized by three strong hydrogen bonds, the first step of all of the examined fragmentation pathways is the interconversion of the 1A conformer into the less stable structure 1B. Formation of the [Carnosine + H](+) fragment from the precursor ion, [Carnosine + CarbPt + H](+), is calculated to be the lowest-energy process. At slightly higher energies, the loss of two amino groups is observed to produce the [Carnosine + (CarbPt - NH3) + H](+) and [Carnosine + (CarbPt - 2NH3) + H](+) ions. At significantly higher energies, the loss of CO2 occurs, yielding the final [Carnosine + (CarbPt - NH3) - CO2 + H](+) and [Carnosine + (CarbPt - 2NH3) - CO2 + H](+) products. Formation of the [CarbPt + H](+) fragment from [Carnosine + CarbPt + H](+), even if not hampered by a high activation barrier, is calculated to be very unfavorable from a thermodynamic point of view.

Collision-induced dissociation (CID) experiments on protonated carnosine, [carnosine + H](+), with several collision energies were shown to yield eleven different fragment ions with the generation of product ions [carnosine-H2O + H](+) and [carnosine-NH3 + H](+) being the lowest energy processes. Energy-resolved CID showed that at slightly higher collision energies the ions [histidine + H](+) and [histidine-H2O-CO + H](+) are formed. At even higher energies four other product ions were observed, however, attained relatively lower abundances. Quantum chemistry calculations, carried out at different levels of theory, were employed to probe fragmentation mechanisms that account for all the experimental data. All the adopted computational protocols give similar energetic trends, and the range of the calculated free energy barrier values for the generation of all the observed product ions is in agreement with the fragmentation mechanisms offered here.

The gas-phase dissociation chemistries of protonated 3-iodo- l -tyrosine, 3,5-diiodo- l -tyrosine and 3,3′,5,5′-tetraiodo-thyronine (thyroxine) have been examined using a combination of tandem mass spectrometry and density functional theory (DFT) calculations. It was found that, at low collision energy, all protonated tyrosines exhibit common fragmentation pathways, including the competitive eliminations of NH3 and the concomitant loss of H2O and CO, but there are significant differences in relative abundances, depending on the combined electron-donating abilities of the substituents in the phenyl ring. The ions initially formed by loss of NH3 are phenonium ions, but subsequent fragmentation is most easily understood in terms of the isomeric benzyl cation structures. These [M + H − NH3]+ ions fragment at relatively low collision energies, mainly by loss of ketene; by contrast, the [M + H − H2O − CO]+ ions are more stable towards dissociation. At higher collision energies, losses of one, two and even three iodine atoms were observed. DFT calculations (at the B3LYP/DZVP level of theory) were performed on protonated 3-iodotyrosine to compare the reaction profiles for the fragmentation mechanisms. The iodo-substituent in the 3-position is weakly electron-withdrawing and this results in a barrier (27.5 kcal/mol at 0 K) that is slightly higher than that for protonated tyrosine (26.8 kcal/mol). The phenoxy group PhO– is a weaker electron-donor than HO– and protonated 3,5-diiodo-4-phenoxytyrosine has an even higher barrier (31.1 kcal/mol) to NH3 loss than protonated 3,5-diiodotyrosine (28.8 kcal/mol). Linear free energy plots for Δ H 0 ° ‡ and Δ G 298 ° ‡ against σ+ for the four protonated tyrosine derivatives show good correlations. More importantly, as the products of the dissociation are higher in energy than the transition states to their formation, the plots of Δ H 0 ° and Δ G 298 ° for the overall reaction for NH3 loss also correlate very well with σ+ (correlation coefficients of 0.99 and 0.98, respectively). The positive slopes of these Hammett plots show that the barriers to the loss of NH3 by the neighboring-group mechanism are increased by the presence of electron-withdrawing groups in the phenyl rings.

The room-temperature reactivities of complexes of Cu + and Ag + with glycine have been investigated using an inductively coupled plasma-selected ion flow tube (ICP-SIFT)/multicollision-induced dissociation (CID) tandem mass spectrometer. These complexes were produced in a flow tube, collisionally thermalized and then allowed to react with CO, D 2 O or NH 3 . The measured reactivities of the CuGly + and AgGly + complexes have been compared with those of the bare metal cations towards the same neutral reagents. All observed reactions resulted in adduct formation, with helium buffer gas presumably acting as a stabilizing agent. Reaction rate enhancements of up to three orders of magnitude and lower extents of ligation were the main characteristics of the reactions initiated by the metal cation–glycine adducts as compared with those initiated by the bare metal cations. The kinetic information combined with equilibrium analyses and CID results suggest that, irrespective of the reagent, ligation to CuGly + is stronger than to AgGly + and, in both instances, ligation of carbon monoxide and water has comparable strengths, while much stronger coordination is achieved with ammonia. The structures of the complexes have been investigated computationally using density functional theory (DFT) at the B3LYP level employing the DZVP basis set. Optimized structures and the free energy changes associated with ligation have been computed. A good correlation was obtained between the experimental reaction efficiencies and the calculated free energies of bonding. Also, results are presented for the potential energy surfaces for the conversion of the “charge solvated” forms into the “metal salt” forms of CuGly + and AgGly + and of their adducts with CO or NH 3 . The computations indicate a modest catalytic effect of the CO and NH 3 on this conversion.

The binding energies at 0 K of sodium and silver ions to ammonia, methylamine, ethylamine, acetonitrile, and benzonitrile were determined using threshold collision-induced dissociation (CID) and molecular orbital calculations at the ab initio and density functional theory levels. There is good agreement between experimental and calculated binding energies. For the five ligands, threshold CID/CCSD(t)(fu)/6-311++G(2df,p)//MP2(fu)/6-311++G(d,p) Na+ binding energies are the following: ammonia, 25.6 ± 2.8/24.8; methylamine, 27.0 ± 1.4/25.9; ethylamine, 27.7 ± 2.3/27.1; acetonitrile, 30.0 ± 2.3/30.3; and benzonitrile, 32.7 ± 1.4/35.0 (B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p)) kcal/mol. Threshold CID and B3LYP/DZVP Ag+ binding energies are the following: ammonia, 40.6 ± 3.0/38.9; methylamine, 41.5 ± 2.3/41.1; ethylamine, 42.9 ± 1.4/43.2; acetonitrile, 40.8 ± 2.0/39.3; and benzonitrile, 41.5 ± 2.8/43.1 kcal/mol. Wherever comparisons with literature data are possible, the Na+ binding energies determined in this ...

A hybrid quantum mechanical molecular mechanical (QMMM) approach is used to study H3O+, H2O, NH4+, NH3, Cl−, HCl, F−, HF, CH3COO−, CH3COOH, Ag+ and glycine in both zwitterionic and nonzwitterionic forms in water. The free energies of hydration of these species are presented and are shown to compare favorably with experimental values. The difference in water–glycine interaction energy between the zwitterionic and nonzwitterionic forms is calculated as a lower limit and is in line with previous findings. The first theoretical examination of the Ag+–glycine complex in solution is presented.

The complexes of silver ion, Ag+, with the twenty naturally occurring amino acids have been calculated using hybrid density functional theory at the B3LYP/DZVP level. For all of these silver complexes, several possible structures were examined, but as there are remarkable similarities between all the structures at the global minima, only summarized data are reported. All of the complexes, except that with proline, are solvated ions. Amino acids containing only hydrocarbon side chains are bidentate, coordinating through the amino and carbonyl groups and the remaining amino acids (with the exception of proline) are tricoordinate with the same two interactions as in the simpler amino acids and an additional interaction through the side chain. The proline complex contains zwitterionic proline with the Ag+ ion attached to the carboxylate anion. Enthalpies (at 298 K) for dissociation of Ag+ from the complexes range from 49.3 kcal mol-1 for glycine to 80.4 kcal mol-1 for arginine. Free energies for these reactio...

The binding enthalpies at 0 K of the silver ion to water, methanol, ethanol, diethyl ether, and acetone were calculated using density functional theory (DFT) using the hybrid B3LYP level of theory with the DZVP basis set; they were also measured using the threshold collision-induced dissociation (CID) method. There is good agreement between the two sets of data. For the five ligands, the DFT/threshold CID values are: water, 28.1/31.6 ± 2.5; methanol, 30.1/33.0 ± 3.7; ethanol, 32.0/33.9 ± 3.5; diethyl ether, 33.3/33.2 ± 1.5; and acetone, 36.2/38.0 ± 1.4 kcal/mol. The average of the absolute differences between the DFT and threshold CID results is 2.0 kcal/mol, a value smaller than the average experimental uncertainty of 2.5 kcal/mol. For identical ligands, the silver ion binding energies are lower than the lithium ion binding energies, but higher than the sodium ion binding energies.

Gas-phase collision-induced dissociation of the Ag+-proline complex shows six major product ions. Tandem mass spectrometry reveals that at least three of the fragment ions are formed directly from the complex. These are a cyclic immonium ion, formed after elimination of AgH from the Ag+-proline complex, another cyclic immonium ion formed after the elimination of both AgH and CO2, and finally an ion formed as the product of a reductive-elimination reaction in which H2 is lost as a neutral. Selective and nonselective deuterium labeling experiments and hybrid density functional calculations have been employed to probe fragmentation mechanisms that account for all experimental results. The mechanisms for the competitive losses of AgH and H2 from the Ag+-proline complex have been calculated at B3LYP/DZVP.

We have studied the silver ion affinities of several RCN ligands using density functional theory. It was found that they correlate linearly with experimental proton affinities, a contradiction to the HSAB principle as H + is a ‘hard’ acid, and Ag + is a ‘soft’ acid. This linear correlation between the Ag + and proton affinities diminishes as the number of RCN ligands attached to the Ag + ion increases from one to three. In the third addition step, the silver ion affinities nearly level off in agreement with the expectation that the electron donating or withdrawing properties of the R group become much less important than in the previous two steps, where the positive charge on the metal ion is not well delocalized. Delocalization of the charge of a metal ion occurs only when a sufficiently large number of ligands are attached to the metal ion. When this condition is satisfied, our data suggests that the HSAB principle may be applicable.

Density functional calculations at B3LYP/DZVP were used to obtain structural information, relative free energies of different isomers and binding energies for the following reaction in the gas phase: M+ + (glycyl)nglycine → M–(glycyl)nglycine+, where M = Ag or Cu and n = 0–2. For the complexes with Cu+, optimizations were also performed at B3LYP/6–31++G(d,p) and single-point calculations at MP2(fc)/6–311++G(2df,2p)//B3LYP/DZVP. The calculated binding energies for the Cu+ complexes are all higher than those of the structurally similar Ag+ ions. These calculated binding energy differences become larger as the size of the ligand increases. For all the Cu+ complexes examined, the coordination number of the copper ion does not exceed two, whereas for the silver complexes tri- and tetracoordinate Ag+ structures are calculated to be at low energy minima. Significant structural and relative free energy differences occur between the lowest energy ‘zwitterionic ’ forms of the M–(glycyl)nglycine+ complexes.

Density functional calculations at B3LYP/6-31+G(d) and B3LYP/DZVP are reported for Mg(NH 3 ) n +· , where n = 1–6 and for some solvated ions Mg(NH 3 ) n +· … NH 3 ( n = 1–3, 6). After correction for basis set superposition errors, the enthalpies for sequential addition of NH 3 to Mg +· resulting from direct coordination to the metal are 38.1, 26.6, 21.2, 13.7, 12.1, and 11.3 kcal mol −1 . The free energies for these same addition reactions are all negative, although for complexes with n ≥ 4 the values are very small. Attempts at optimising structures with higher coordination numbers all resulted in the formation of solvated octahedral complexes. Enthalpies for solvation through hydrogen bonding to one of the ligated NH 3 molecules are all less than 16 kcal mol −1 and decrease rapidly as the number of ligated NH 3 molecules increases. Molecular orbital calculations at B3LYP/DZVP have been used to optimise structures for ions Ag(NH 3 ) n + , where n = 1–6. The five-coordinate and six-coordinate structures have very small binding enthalpies (4.3 and 2.6 kcal mol −1 ) and the free energies for formation of these ions are positive. The binding energies for the addition of the first and second NH 3 molecules added to Ag + are 40.1 and 36.1 kcal mol −1 , while those for the third and fourth additions are much smaller (15.1 and 11.0 kcal mol −1 ). Adducts up to n = 3 have been detected in electrospray experiments. The first three adducts of Ag + with NH 3 have been formed in the selected ion flow tube apparatus and multicollision induced dissociation experiments show Ag(NH 3 ) 3 + to have a lower binding enthalpy than both Ag(NH 3 ) 2 + and Ag(NH 3 ) + .

Molecular orbital calculations at B3LYP/6-31++G(d,p) are reported for bases 2-(aminomethyl)-5-oxazolone, 2-(aminomethyl)-4-methyl-5-oxazolone, 2-phenyl-5-oxazolone, and 2-phenyl-4 methyl-5-oxazolone and for the cations formed by protonation of these bases on their imino nitrogens. Structures and relative energies for isomers generated by protonation at each of the four heteroatoms of 2-(aminomethyl)-5-oxazolone are reported. Lithium and silver cations both add to 2-(aminomethyl)-5-oxazolone, but unlike the proton, they bind with two heteroatoms simultaneously. For both the lithiated and argentinated 2-(aminomethyl)-5-oxazolone cations the lowest energy isomers have the metal coordinated with the two nitrogen atoms. Proton affinities of these bases are in the range 217.0-221 kcal mol -1 , with the methyl group at C4 increasing the proton affinity by 3 kcal mol -1 . Single-point calculations were performed at MP4(fc)/6-311++G(2df,p)//B3LYP/6-31++G(d,p) for 2-(aminomethyl)-5-oxazolone, diketopiperazine, glycine, and alanine and their conjugate acids. The proton affinities from this level of theory are lower by as much as 2.7 kcal mol -1 than those calculated at B3LYP/6-31++G(d,p). Enthalpies of formation calculated at B3LYP/6-31++G(d,p) from isodesmic reactions for glycine, alanine, and their conjugate acids are all within 1 kcal mol -1 of the experimental values, but those calculated at MP4 deviate by as much as 4.8 kcal mol -1 . Enthalpies of formation from atomization reactions at the MP4 level are in larger disagreement with experimental values.

The presence of an interacting water or methanol molecule has been shown to catalyze the 1,3-proton shift in a peptide linkage between the tautomers of protonated formamide and glycylglycylglycine. Density functional theory calculations at the B3LYP/6-31++G(d,p) level of theory show that, for glycylglycylglycine, the forward barrier of this shift decreases from a free energy at 298 K of 39.6 kcal/mol in the absence of solvent to 26.7 kcal/mol in the presence of water and to 22.0 kcal/mol in the presence of methanol. Protonation at the amide nitrogen of the second residue results in a large increase in the C -N bond distance from 1.336 to 1.519 A, whereas protonation at the carbonyl oxygen leads to a decrease in the C-N bond distance from 1.336 to 1.321 A. Solvent-catalyzed tautomerism may play an important role in the fragmentation of electrosprayed, protonated peptides in the gas phase.