Absolute affinities of α-amino acids for Cu⁺ in the gas phase. A theoretical study

Ab initio calculations have been carried out on the [glycine-Cu]⁺, [serine-Cu]⁺, and [cysteine-Cu]⁺ complexes. Investigation of several types of structures for each complex shows that the preferred binding site of Cu⁺ involves chelation between the carbonyl oxygen and the amino nitrogen. With glycine, this leads to a complexation energy (best estimate of D₀) of 64.3 kcal/mol. Additional chelation with the alcohol group of serine or the thiol group of cysteine leads to larger binding energies, with cysteine binding more strongly than serine, in good agreement with a recent experimental scale of relative Cu⁺ affinities of all α-amino acids present in natural peptides. Combining this scale to the accurate determination of the Cu⁺ affinity of glycine from the present work leads to absolute values of Cu⁺ affinities of all amino acids. Calculations were also carried out on the complexes of Cu⁺ with water, ammonia, formaldehyde, and hydrogen sulfide. The geometrical and electronic structures of these complexes are used to analyze the binding of Cu⁺ to amino acids.