Ab initio Hartree-Fock (HF) molecular orbital calculations were carried out for triglycine and its six singly-protonated species (at the three different nitrogen and carbonyl oxygen sites). Minimum-energy structures were first determined using the 3-21G basis set with full geometry optimizations; single-point calculations were next performed in the larger 6-31G* basis set to gain better electronic protonation energies. Based on the 6-31G*//3-21G electronic protonation energies, the relative basicity of different atomic sites is as follows : amino N > amide carbonyl O > carboxylic carbonyl O > amide N. To obtain proton affinity (PA) and gas-phase basicity (GB) values for triglycine, electronic energies at the 6-31G*//6-31G* level and zero-point and thermal energies at the 3-21G//3-21G level were calculated for the most stable structures of the neutral and amino N-protonated species. The resulting theoretical PA and GB are 227.9 and 219.6 kcal/mol; the latter is in close agreement with the experimental GB of 218.5 +/- 2.4 kcal/mol which was obtained by proton transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer. Among the calculated structures, several ring forms (C-5, C-7, C-10 and C-11) arising from intramolecular hydrogen bonding are identified. A novel intramolecular interaction between an amide carbonyl oxygen atom and a trivalent carbon atom is also uncovered. Effects of H-bonds and the geometry of the relevant protonated 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 theoretical estimates of GB with relative economy. Finally, comparisons are made between experimental and theoretical data on the thermodynamic and structural aspects of the protonation of triglycine.