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

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. (Int J Mass Spectrom 185/186/187 (1999) 107-116) (C) 1999 Elsevier Science B.V.