GAS-PHASE BASICITIES OF HISTIDINE AND LYSINE AND THEIR SELECTED DIPEPTIDE AND TRIPEPTIDE

The gas-phase basicities (GB) of histidine, lysine, and di- and trigly cyl peptides containing either one histidine or one lysine residue hav e been determined. In all, 12 compounds were examined in a Fourier tra nsform ion cyclotron resonance mass spectrometer. The GBs of the biomo lecules were evaluated by proton transfer reactions employing a range of reference compounds with varying gas-phase basicities. In addition, the GBs were determined by using the kinetic method of collision-indu ced dissociation on a proton-bound dimer containing the peptide and a reference compound. The GBs of histidine and lysine were both found to be 220.8 kcal/mol via proton transfer reactions. The kinetic method e xperiments, including dissociation of a proton-bound dimer containing both histidine and lysine, also suggest equivalent GBs for these amino acids. However, the small peptides containing lysine are generally mo re basic than the corresponding histidine-containing peptides. For the peptides, the data suggest that the protonation site is on the basic side chain functional group of the histidine or lysine residues. The G Bs of the di- and tripeptides are dependent upon the location of the b asic residue. For example, the GBs of the tripeptides glycylglycyl-L-l ysine (GlyGlyLys) and L-lysylglycylglycine (LysGlyGly) were both deter mined to be 230.7 kcal/mol while a GB of kcal/mol was obtained for gly cyl-L-lysylglycine (GlyLysGly). A similar GB trend is seen with the hi stidine-containing tripeptides. Generally, the GBs obtained by using t he kinetic method are slightly higher than those obtained by deprotona tion reactions; however, the trends in relative GB values are essentia lly the same with the two techniques. (C) 1996 American Society for Ma ss Spectrometry.