THE GROUND STATE Cu2+ION AFFINITIES OF GLYCINE, ALANINE AND CYSTEINE IN GAS AND AQUEOUS PHASE: A DFT BASED COMPUTATIONAL STUDY

A detailed study of Cu2+ion affinities of the amino acids namely Glycine (Gly), Alanine (Ala) and Cysteine (Cys) and their Cu2+complexes have been investigated using density functional theory. Interactions of a Cu2+ ion with oxygen, nitrogen, and sulfur (for cysteine) of the selected amino acids have been optimized. The results show that complex formation reactions are exothermic in both gas and aqueous phase and the neighboring stereochemical nature of Cu2+ion is more or less same in all amino acids. The computed Cu2+ affinity for both O-Cu2+ and N-Cu2+ interaction in the gas phase is in this order ∆ECys>∆EAla>∆EGly. In aqueous phase, Cu2+ion affinity for O-Cu2+ interaction follows the same order as above, whereas in N-Cu2+ interaction it differs as ∆EAla≥∆ECys>∆EGly. In NCu2+ interaction Zwitterterionic complexes (Cu2+ bind with both nitrogen and carbonyl oxygen atom) have been formed. The optimization energies are estimated to be lower relative to the other interactions and the Cu2+ ion affinities have been predicted more. The results have been well supported by the natural population analysis (NPA) of the atoms and hardness parameters. The charge, energetics, geometrical and electronic properties of the complexes signify that the interaction between the Cu2+ with the carbonyl oxygen and the amino nitrogen of free amino acids is predominantly a covalent interaction in the gas phase and which becomes more ionic in the aqueous phase.