EXPERIMENTAL AND AB-INITIO STUDIES OF THE GAS-PHASE BASICITIES OF POLYGLYCINES

The gas-phase basicities of polyglycines, Gly(n) (n = 1-6), were deter mined by proton transfer reactions in a Fourier transform ion cyclotro n resonance mass spectrometer. The basicities were found to increase a s the peptide chain length increased: 206.2, 215.3, 218.9, 225.0, 225. 3, and 227.4 kcal/mol for n = 1-6, respectively. Comparable results, b ut with some deviations, were obtained using the kinetic method of col lision-induced dissociation on a proton-bound dimer. The large change in basicity between glycine and diglycine suggests significant differe nces in the strength of intramolecular hydrogen bonding in their proto nated forms. This is consistent with ab initio Hartree-Fock calculatio ns, which were carried out using the 6-31G basis set for glycine, dig lycine, and their protonated species. Full optimization of geometry yi elded eight minimum-energy structures: glycine (1), protonated glycine s at the amino nitrogen (2) and the carbonyl oxygen (3), diglycine (4) , and protonated diglycines at the amino nitrogen (5), the amide carbo nyl oxygen (6), the amide nitrogen (7), and the carboxyl carbonyl oxyg en (8). The ideal-gas basicities for the relevant protonations at 298. 15 K and 1 atm, calculated after inclusion of zero-point energy and te mperature changes in enthalpy and entropy, were found to be 207.5 (1 - -> 2), 194.3 (1 --> 3), 214.0 (4 --> 5), 212.9 (4 --> 6), 196.9 (4 --> 7), and 188.3 (4 --> 8) kcal/mol. For the glycines, more accurate ele ctronic energies were obtained with geometry optimization in the large r 6-31+G(d,p) basis, including electron correlation corrected to the f ourth order in the Moller-Plesset perturbation treatment. Scaling the zero-point energy with a factor of 0.91 was also found to improve the calculated basicities. The best theoretical estimates of basicities co rresponding to protonation at the terminal amine were 206.3 and 215.2 kcal/mol for the respective glycine and diglycine; these numerical val ues are in excellent agreement with the experimental gas-phase basicit ies. The corresponding theoretical and experimental proton affinities are also presented. Detailed analysis is made on the structural featur es that affect the relative stabilities of the various molecular speci es. In addition, several topics relevant to the theoretical and experi mental determinations of basicity are discussed.