Molecular Design of Artificial Proteins Bearing Novel Structural Motif by Utilizing an Unnatural Amino Acid

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The purpose of this work is to design and synthesize artificial proteins, which consist of peptides containing an unnatural and coordinating amino acid, 5'-amino-2,2'-bipyridine-5-carboxylic acid. The peptides containing a couple of residues of the unnatural amino acid are expected to coordinate with a metal ion, affording the artificial proteins with unique structural motifs. In the synthesis of peptides, the dipeptide units, which were separately prepared, were introduced into the common solid phase synthetic methods ; otherwise the synthesis was difficult due to its less reactive amino group. The 21-mer peptides with three residues of the unnatural amino acids were successfully synthesized by the methodology. Furthermore, formation of the iron(II) complexes by reacting the peptides with iron(II) ion was observed in absorption spectra, showing Metal-to-Ligand Charge Transfer (MLCT) band at 541 nm. On the other hand, if ruthenium(II) ion was used, the artificial proteins had ruthenium tris(bipyridine) type complexes as the core, which were expected to appear photo-functions, such as emission, photo-catalytic, and photo-induced electron transfer abilities. Since there are, however, few works on ruthenium tris(5,5'-disubstitueted-2,2'-bipyridine) complexes, we synthesized various ruthenium complexes of the unnatural amino acid derivatives and 5,5'-diamide-2,2'-bipyridines and investigated their photochemical properties. In particular, these complexes were found to act as sensors for a variety of anions by strongly binding with them via cooperative hydrogen bonding between the amide hydrogen (NH) in three the ligands and anions.