Amino acid- and pterin-metal chemistry as an approach to biological functions
The aromatic rings of phenylalanine, tyrosine, and tryptophan (Trp) have been shown to be involved in aromatic ring stacking with other aromatic rings such as 2,2'-bipyridine (bpy) in ternary metal complexes as prototypes of the enzyme-metal- substrate intermediate in metalloenzyme catalysis. Stability enhancement primarily due to the intramolecular stacking in ternary Cu(I1) complexes was concluded from the equilibrium constants for hypothetical equilibria involving formation of ternary complexes with stacked rings. X-ray crystal structure analysis of the ternary Cu(I1) complexes revealed the stacking interaction in the solid state, supporting the conclusion from the solution studies. Theoretical calculation on (Cu(Trp)(bpy)(H20))+ afforded the fully optimized structure that is very similar to the one revealed by X-ray crystal structure analysis. The structures of the metal complexes with pterin derivatives and the redox reactions with tetrahydropterins have been revealed as a step toward understanding the mechanism of the hydroxylation of phenylalanine by phenylalanine hydroxylase in the presence of a pterin cofactor which is regarded as a functionaliza- tion of aromatic amino acids.