GAS-PHASE BASICITY MEASUREMENTS OF DIPEPTIDES THAT CONTAIN VALINE

Gas-phase basicities of 22 dipeptides that contain valine were measured by a double bracketing method in a Fourier transform ion cyclotron resonance spectrometer. Matrix-assisted laser desorption was used to generate protonated peptide ions which were reacted with reference compounds to bracket the gas-phase basicity. In addition, neutral peptide molecules were formed by substrate-assisted laser desorption and reacted with protonated reference ions to confirm the assignment of the gas-phase basicity. The rate of proton transfer between the protonated molecule of alanylvaline and six reference compounds was measured to examine the behavior of both exoergic and endoergic reactions. Gas-phase basicities of most of the dipeptides were found to be nearly equal to that of their most basic amino acid residue. The results are consistent with an intramolecular hydrogen bond between the N-terminus nitrogen and the amide carbonyl oxygen of a dipeptide. Furthermore, the results suggest that inductive effects cause an increase in the strength of the intramolecular hydrogen bond that parallels the increase in the basicity of the C-terminus amino acid residue. Dipeptides VF and VY are more basic than their constituent amino acids. These data and molecular mechanics calculations suggest that these two peptides are stabilized by an electrostatic interaction between the N-terminal ammonium ion and the polarizable electrons of the aromatic side chain of the C-terminus.