Proton migration in protonated glycylglycylglycine (GGG) has been investigated by using density functional theory at the B3LYP/6-31++G(d,p) level of theory. On the protonated GGG energy hypersurface 19 critical points have been characterized, 11 as minima and 8 as first-order saddle points. Transition state structures for interconversion between eight of these minima are reported, starting from a structure in which there is protonation at the amino nitrogen of the N-terminal glycyl residue following the migration of the proton until there is fragmentation into protonated 2-aminomethyl-5-oxazolone (the b(2) ion) and glycine. Individual free energy barriers are small, ranging from 4.3 to 18.1 kcal mol(-)(1). The most favorable site of protonation on GGG is the carbonyl oxygen of the N-terminal residue. This isomer is stabilized by a hydrogen bond of the type O-H.N with the N-terminal nitrogen atom, resulting in a compact five-membered ring. Another oxygen-protonated isomer with hydrogen bonding of the type O-H.O, resulting in a seven-membered ring, is only 0.1 kcal mol(-)(1) higher in free energy. Protonation on the N-terminal nitrogen atom produces an isomer that is about 1 kcal mol(-)(1) higher in free energy than isomers resulting from protonation on the carbonyl oxygen of the N-terminal residue. The calculated energy barrier to generate the b(2) ion from protonated GGG is 32.5 kcal mol(-)(1) via TS(6-->7). The calculated basicity and proton affinity of GGG from our results are 216.3 and 223.8 kcal mol(-)(1), respectively. These values are 3-4 kcal mol(-)(1) lower than those from previous calculations and are in excellent agreement with recently revised experimental values.

Collision-induced dissociation experiments on the Ag ϩ -phenylalanine complex using several collision energies were shown to yield ten different fragment ions. Unambiguous assignment of these fragment ions were made by careful analysis of deuterium labeling experiments. The losses of H 2 O, CO, CO 2 , and AgH were commonly observed; also encountered were the losses of H 2 , Ag, and H. Deuterium labeling experiments and density functional calculations have been employed to probe fragmentation mechanisms that account for all experimental results. (J Am Soc Mass Spectrom 2002, 13, 408 -416) © 2002 American Society for Mass Spectrometry T he interactions of metal ions with amino acids and peptides in the gas phase is a topic of much current interest [1][2][3]. Attempts have been made to further our understanding of complicated biological processes by modeling them using carefully planned gas-phase studies. In the absence of solvent, one may study the intrinsic modes of binding governing metalamino acid complexes. Mass spectrometric experiments thus offer great potential for exploring the formation and reactivity of isolated cationic species.Amino acid complexes of transition metal ions were first observed and studied using fast atom bombardment (FAB) [4,5]. However, the limited solubility of metal salts and poor sensitivity prevented widespread systematic study of these complexes. Sensitivity is not typically an issue in matrix-assisted laser desorption/ ionization (MALDI) [6,7], but the fragmentation yields of its metastably decomposing ions are often poor. As the fragment ions potentially give information on atom connectivity and hence structure, MALDI is therefore non-ideal for examining fragmentation pathways. One technique that has proven effective for the study of metal-containing amino acids and peptides, and the one employed here, is electrospray tandem mass spectrometry. First, electrospray ionization [8] is a soft ionization method and one that is efficient at producing gas-phase metal-containing ions [9 -15], whose solubility in water or water/methanol is typically high. Second, fragmentation is efficient and is easily controlled via collision-induced dissociation, the energy of which is crucial in directing fragmentation products and their yield.Molecular orbital calculations are frequently combined with experiments in studies to obtain a better understanding of the structure and thermochemistry of the organometallic species of interest [1,2,16,17]. Recent density functional molecular orbital studies have shown that Ag ϩ can be mono-, di-, or tricoordinate in complexes with ␣-amino acids [18]; tetracoordinate Ag ϩ has been postulated for relatively small peptides [19,20]. The binding of silver(I) to glycine, diglycine, and triglycine, and to a number of other polypeptides, has been investigated [19,20]. In particular, the structures of argentinated glycine and its oligomers have been examined in detail by means of density functional theory [21].The focus of this paper is the Ag ϩ -phenylalanine complex. ...

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 Å, whereas protonation at the carbonyl oxygen leads to a decrease in the C−N bond distance from 1.336 to 1.321 Å. Solvent-catalyzed tautomerism may play an important role in the fragmentation of electrosprayed, protonated peptides in the gas phase.

Gas-phase methyl cation affinities (MCAs) for rare gases Ne, Kr, and Xe were measured with a pulsed electron-beam high-pressure mass spectrometer. The MCAs for Ne and Kr were determined to be 1.2 ± 0.3 and 19.8 ± 2.0 kcal/mol, respectively, by the observation of the clustering reaction, CH3 + + Rg = CH3 +(Rg) (Rg = Ne and Kr). The MCA of Xe was measured to be 2.0 ± 0.6 kcal/mol larger than that of N2 by the observation of the substitution reaction CH3 +(N2) + Xe = CH3 +(Xe) + N2. Based on the MCA of N2 of 44.1 kcal/mol proposed by McMahon et al., the MCA of Xe is determined to be 46.1 ± 0.6 kcal/mol. Molecular orbital calculations at six different levels consistently gave almost identical MCA values for each of the rare gases. At QCISD(T)(full)/6-311++G(2df,p)//B3LYP/6-311++G(d,p), the calculated values (all in kcal/mol) are as follows:  He, 0.6; Ne, 2.2; Ar, 15.9; Kr, 24.1; and N2, 43.2. For Xe at B3LYP/DZVP//B3LYP/DZVP, the calculated MCA is 39.0 kcal/mol. The ethyl cation affinities of Ar, Kr, and Xe were also measured. They are ∼1.7, 3.2 ± 0.3, and 6.8 ± 0.3, respectively. The stabilities of C2H5 +(Rg) and C2H5 +(N2) were discussed in terms of nonclassical (bridge) and classical (open) structures of C2H5 +.

The potential energy surface for C4H5 + as calculated by ab initio molecular orbital theory is reported at two levels of theory, HF/6-31G(d,p) and MP2(full)/6-311G(d,p). Fourteen minima have been located at HF/6-31G(d,p), but inclusion of electron correlation reduced this number to nine. The methylcyclopropenyl cation, 1, is the global minimum, and the 2-cyclobutenyl (2), α-vinylvinyl (3), γ-methylpropargyl (4), α-methylpropargyl (5), and 1-cyclobutenyl (6) cations are 9.1, 19.9, 25.3, 27.5, and 26.9 kcal/mol, respectively, above 1. Eleven transition structures, permitting interconversion between the nine minima, are reported. Enthalpies of formation (in kcal/mol) calculated at MP4SDTQ/6-311++G(2df,p) are 231.4 for 1, 241.7 for 2, 246.9 for 3, 255.7 for 5, 259.9 for 6, and 264.3 for 7.