Intrinsic basicity of oligomeric peptides that contain glycine, alanine, and valine—The effects of the alkyl side chain on proton transfer reactions

The gas-phase basicities of oligomers of alanine and valine have been determined by bracketing measurements in an external source Fourier transform mass spectrometer. The results are compared to the oligomers of glycine, which were reported in an earlier publication, to observe the effect of the alkyl group and the increasing gas-phase basicity of the monomer units on the rates of proton transfer reactions. Molecular orbital calculations were performed on protonated triglycine and trialanine to determine how the alkyl groups affect intramolecular interactions. The results show that a high degree of ordering of the carbonyl groups is present in the protonated species. The carbonyl groups in turn order the side chain alkyl groups and decrease the rates of proton transfer reactions in, for example, the oligomers of valine.