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The relative Cu (I) ion affinities of amino acids (AA) are determined in the gas phase based on the unimolecular dissociations of their Cu+-bound heterodimers, AAi—Cu+—AA 2 (kinetic method). For the 20 common-amino acids, the Cu+ affinities increase in the order Gly< Ala< Ser< Val< Leu< lie< Thr< Pro< Asp< Asn< Glu< Phe< Tyr< Cys< Gin< Met< Trp< His< Lys< Arg and their values fall within< 20 kcal/mol. For comparison, the proton affinities of amino acids cover a range of> 33 kcal/mol. Correlation of the experimentally derived Cu+ affinity order to the reported proton affinity order of amino acids points out that the Cu+—AA bond is longer and less covalent in nature than the H+—AA bond. Increasing the alkyl side chain of the amino acid, and hence inductive effects, augments the proton affinity substantially more than the Cu (I) affinity. Further, soft donor groups, such as SH of cysteine, SCH3 of methionine, or …