Novel building blocks for biomimetic assemblies. Synthesis, characterization, and spectroscopic and electrochemical properties of new bidentate ligands derived from lysine and cysteine and their complexes with bis(2,2'-bipyridine)ruthenium(II)

Synthetic amino acids suitable for the assembly of small, redox-active metallopeptides are described. N-alpha-((1,1-Dimethylethoxy)carbonyl)-N-epsilon-(2-pyridylmethyl)-L-lysine (1), N-alpha-acetyl-S-(2-pyridylmethyl)-L-cysteine (2), and N-alpha-acetyl-S-(2-(2-pyridyl)ethyl)-L-cysteine (3) have been synthesized by alkylation of the N-alpha-protected amino acids. Their [Ru(bipy)(2)](2+) complexes [(bipy)(2)Ru(BocLysCH(2)py)](2+) (4), [(bipy)(2)Ru(AcCysCH(2)py)](2+) (5), and [(bipy)(2)Ru(AcCys(CH2)(2)py)](2+) (6) have been prepared by reactions of the ligands with [Ru(bipy)(2)Cl-2]. On the basis of H-1-NMR spectroscopy, 4-6 can be described best as trans-tetrapyridine complexes with the lysine amino N atom and the cysteine S atom occupying one of the apical positions. It was shown by luminescence spectroscopy that 4 can serve as a possible photoredox-active module for the construction of photochemically active peptides. The redox properties of the complexes are described with the aid of the Lever parameters. It was demonstrated that the amino acid ligands in 5 and 6 can be viewed as methionine units. particularly interesting is the unique redox chemistry of 4. Upon metal oxidation, a two-electron ligand oxidation occurred, followed by fast hydrolytic cleavage of the lysine-methylpyridine N-C bond. The physical and chemical properties of the compounds are discussed in terms of future applications in biomimetic chemistry such as the activation of small molecules.