AB-INITIO STUDIES OF NEUTRAL AND PROTONATED TRIGLYCINES - COMPARISON OF CALCULATED AND EXPERIMENTAL GAS-PHASE BASICITY

Ab initio Hartree-Fock (HF) molecular orbital calculations were carrie d out for triglycine and its six singly-protonated species (at the thr ee different nitrogen and carbonyl oxygen sites). Minimum-energy struc tures were first determined using the 3-21G basis set with full geomet ry optimizations; single-point calculations were next performed in the larger 6-31G basis set to gain better electronic protonation energie s. Based on the 6-31G//3-21G electronic protonation energies, the rel ative basicity of different atomic sites is as follows: amino N > amid e carbonyl O > carboxylic carbonyl O > amide N. To obtain proton affin ity (PA) and gas-phase basicity (GB) values for triglycine, electronic energies at the 6-31G//6-31G* level and zero-point and thermal energ ies at the 3-21G//3-21G level were calculated for the most stable stru ctures of the neutral and amino N-protonated species. The resulting th eoretical PA and GB are 227.9 and 219.6 kcal/mol; the latter is in clo se agreement with the experimental GB of 218.5 +/- 2.4 kcal/mol which was obtained by proton transfer reactions in a Fourier transform ion c yclotron resonance mass spectrometer. Among the calculated structures, several ring forms (C-5, C-7, C-10 and C-11) arising from intramolecu lar hydrogen bonding are identified. A novel intramolecular interactio n between an amide carbonyl oxygen atom and a trivalent carbon atom is also uncovered. Effects of H-bonds and the geometry of the relevant p rotonated peptide bond on the relative stability of protonated species are discussed in detail. The 6-31G//3-21G approach is shown to be a practical alternative to 6/31G//6-31G* for obtaining reasonable theor etical estimates of GB with relative economy. Finally, comparisons are made between experimental and theoretical data on the thermodynamic a nd structural aspects of the protonation of triglycine.