Formation and fragmentation of α-amino acids complexed by Cu+

Gas phase complexes of α-amino acids with Cu+ have been engendered in a plasma desorption ion source by bombarding a mixture of α-amino acids and cupric salts. The resulting adducts MCu+ and the fragments that include copper were studied using a time-of-flight analyser. The main fragments result from a loss of 46 u, this fragmentation being very similar to the one observed for protonated molecules. One noticeable exception, however, is arginine and lysine which give very little loss of 46 u as protonated molecules but do give a significant amount of it when cationized by Cu+. The study of valine-d8 has shown that the migration accompanying the fragmentations principally involves the hydrogens linked to heteroatoms. The deuteration of exchangeable hydrogens, done on several amino acids, confirmed these results. This fact, plus evidence of an interaction between Cu+ and the side chain of non-aliphatic α-amino acids, has led us to suggest several mechanisms.