Gas phase copper(I) ion affinities of valine, lysine, and arginine based on the dissociation of Cu +-bound heterodimers at varying internal energies

Abstract

The Cu + affinities of the amino acids valine (Val), lysine (Lys), and arginine (Arg) are determined in the gas phase based on the dissociations of Cu +-bound dimers [A + B i ]Cu +, in which A represents one of the three amino acids studied and B i a set of different amino acids of known Cu + affinity (kinetic method). In order to deconvolute entropic contributions from experimentally measured free energies, the decompositions of [A + B i ]Cu + are assessed as a function of internal energy using angle-resolved mass spectrometry. The Cu + affinities deduced for Val, Lys, and Arg are 283, 355, and 364 kJ mol −1, respectively. Lysine and arginine are found to have substantially larger entropies of Cu + attachment when compared to valine. The combined affinity and entropy data are consistent with participation of the flexible side chain substituents of lysine and arginine in the coordination of Cu +, yielding multidentate complexes of markedly higher stability than the aliphatic amino acid valine.