Changes in Binding Motif of Protonated Heterodimers Containing Valine and Amines Investigated Using IRMPD Spectroscopy between 800 and 3700 cm(-1) and Theory

Proton-bound dimers consisting of valine and basic primary and secondary amines of varying gas-phase basicity (GB) were investigated using infrared multiple photon dissociation (IRMPD) spectroscopy between 800 and 3700 cm(-1), collisionally activated dissociation, and theory. The low-energy dissociation of these dimers results in a sharp transition from formation of primarily protonated valine to protonated base for dimers with ethylamine and propylamine, respectively, from which a GB of similar to 880 kJ/mol is deduced for valine, a value that is slightly higher than previously reported. The IRMPD spectra clearly indicate that, for bases with GB values within 20 kJ/mol of that of valine, the base coordinates to the N-terminus of a nonzwitterionic form of valine. In contrast, calculations indicate that valine is zwitterionic for complexes where the base is less basic. For bases with GB values at least 20 kJ/mol greater than that of valine, the spectra indicate a transition in structure, and for diethylamine (Delta GB = 40 kJ/mol), the dominant structure is one in which the base coordinates to the carbonyl oxygen of a nonzwitterionic form of valine and the carboxylic acid donates an intramolecular hydrogen bond to the N-terminus. These results are consistent with the destabilization of the N-terminally coordinated structure due to the increasing difference in proton affinities of the constituent molecules and the increasing importance of a stabililizing hydrogen bond formed in the C-terminally coordinated structure. Even when the GB of the base is 40 kJ/moI higher than that of valine, the form of the amino acid is nonzwitterionic, indicating that careful application of the kinetic method should provide reliable information about the basicity of valine and other aliphatic amino acids