Determination of the gas-phase basicities of proline and its di- and tripeptides with glycine: the enhanced basicity of prolylproline.

The gas-phase basicity (GB) of proline, which is the only imino acid and is reported to play a significant role in protein folding, was determined by deprotonation reactions in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Protonated peptide ions were generated by fast atom bombardment in an external ion source. The GB of proline was found to be 213.3 kcal/mol. Among the dipeptides studied, the GB of glycylproline (GlyPro) was determined to be 214.8 kcal/mol, while prolyglycine (ProGly) was 4.2 kcal/mol more basic (GB = 219.0 kcal/mol). The basicity of the tripeptide prolylglycylglycine (ProGlyGly, GB = 219.0 kcal/mol) is 2 kcal/mol higher than the basicities of glycylglycylproline (GlyGlyPro) and glycylprolylglycine (GlyProGly), both of which have GB = 217.0 kcal/mol. The enhanced basicity of ProGlyGly is consistent with the protonation site being the terminal amino nitrogen with enhanced stabilization of the charge by the nearby bulkier residue. Interestingly, prolyproline (ProPro), which has a GB of 223.3 kcal/mol, is more basic than the other di- and tripeptides studied. Consequently, semi-empirical AM1 calculations were employed to probe the structural characteristics and intramolecular hydrogen bonding interactions for ProPro, GlyPro, ProGly and glycylglycine (GlyGly). ProPro was found to have a unique structure with two potential hydrogen bonds between amino hydrogens and both carbonyl oxygens.