Shigero Oishi

Direct comparison of electron transfer properties of two distinct semisynthetic triads with non-protein based triad: unambiguous experimental evidences on protein matrix effects.

In order to understand the roles of protein matrix in electron transfer processes (ET) within biological systems, a heme-based donor (Zn-heme: ZnPP)-sensitizer (Ru2+(bpy)3)-acceptor (cyclic viologen: BXV4+) triad 1 was used as a probe molecule. Two semi-synthetic systems, Cyt-b562(1) and Mb(1), in which the triad is incorporated into cytochrome b562 (Cyt-b562) or into myoglobin (Mb), were constructed by cofactor reconstitution. These two semi-synthetic proteins were compared with the triad itself (i.e., without the protein matrix) using absorption spectroscopy, steady state emission and excitation studies, laser flash photolysis experiments, and molecular modeling. Photoexcitation of the ZnPP moiety of Cyt-b562(1) or Mb(1) leads to a direct ET from the triplet state of ZnPP state (3ZnPP) to BXV4+ to generate a final charge-separated (CS) state, Cyt-b562(Zn+)-Ru2+-BXV3+* or Mb(Zn+)-Ru2+-BXV3+*. On the other hand, direct ET from the excited ZnPP moiety to the BXV4+ moiety is also involved in 1 in the absence of the protein matrix, but the excited state of ZnPP involved is not 3ZnPP, but the singlet excited state (1ZnPP) in this pathway. When the Ru2+(bpy)3 moiety of Cyt-b562(1) or Mb(1) is excited, a stepwise ET relay occurs with the ion-pair, Cyt-b562(Zn)-Ru3+-BXV3+* or Mb(Zn)-Ru3*-BXV3+*, as an intermediate, leading to the same final CS state as that generated in the direct ET pathway. The lifetimes of the corresponding final CS states were determined to be 300 ns for 1 in the absence of the protein matrix, 600-900 ns for Cyt-b562(1) and 1.1-18 micros for Mb(1), the values of which are greatly affected by the protein matrix. Molecular modeling study of the three systems consistently explained the differences of their photophysical behavior.

Artificial metalloproteins with a ruthenium tris(bipyridyl) complex as the core

Creating artificial proteins such as artificial enzymes, artificial receptors, and artificial antibodies is one of the dreams of modern chemists. Until now, biomimetic chemistry, in which the amino acid residues around active sites in target proteins (enzymes) are extracted, has helped in understanding enzymatic reactions and/or designing new functional molecules. However, this approach excludes the protein backbones around the active sites, and therefore, it effectively reduces protein science to organic chemistry. On the other hand, research towards creating functional peptides, which possess similar backbones to proteins, has actively been carried out. As peptides, being smaller (MW ~10) than proteins, are generally flexible, it is important for designing functional peptides to precisely control their three-dimensional structures. As examples, the template-assisted synthetic proteins (TASP) have actively been researched; the template molecules associate with secondary structures such as a-helices and b-sheets to successfully produce functional peptides. The introduction of rigid, unnatural amino acids to control the peptides’ conformation is also effective. Ishida and co-workers have reported the molecular design of cyclic peptides with alternative natural amino acid and 3-aminobenzoic acid sequences that act as efficient phosphoester binders and as artificial ion channels. Bipyridyl groups, which can coordinate with a variety of metal ions, have sometimes been utilized to control peptide conformations and/or design functional peptides. a-Helical or collagenous peptides have been assembled by using bipyridyl moieties through coordination with a metal ion. A peptide with a bipyridyl group has also been found to produce the b-sheet conformation upon treatment with a metal ion. Many amino acids with a bipyridyl group as a side chain or a backbone have been synthesized. We propose the molecular design of artificial proteins utilizing 5’-amino-2,2’-bipyridine-5-carboxylic acid (5Bpy) as an unnatural amino acid (Scheme 1). In the design, the peptides containing three 5Bpy residues fold by coordination with a metal ion. The designed proteins are considered to have an artificial motif, not naturally occurring protein motifs. Furthermore, by using the ruthenium(II) ion, the central metal com-

Cyclodextrin-appended myoglobin as a tool for construction of a donor–sensitizer–acceptor triad on a protein surface

A protein-based and noncovalently-linked donor–sensitizer–acceptor triad has been prepared by self-assembly via mechanical linkages and hydrophobic interactions, and its photoinduced electron transfer properties have been studied.

Cr(CO)5(phen) as an intermediate of photochemical CO substitution in Cr(CO)6 with 1,10-phenanthroline

Abstract Cr(CO) 5 (phen) generated as an intermediate of photochemical carbonyl substitution in Cr(CO) 6 with 1,10-phenanthroline (phen) has been investigated by laser flash photolysis with infrared detection. The Cr(CO) 5 moiety was found to have a square-pyramidal structure by analyzing with the Cotton-Kraihanzel force field. Simulation, based on the correlation of CO-stretching force constants with the bond indices calculated by ZINDO-MO, revealed that phen coordinated unusually in a monodentate fashion and the interaction between Cr and another nitrogen was antibonding.

Absolute rate constants for the coordination of olefins to a transient coordinatively unsaturated cobalt complex

Abstract Absolute second-order rate constants for the coordination of diethyl phenylphosphonite and various olefins to a transient coordinatively unsaturated cobalt complex, i.e. hydridotris(diethyl phenylphosphonite)cobalt(I), have been measured in cyclohexane at 23° C using laser flash photolysis techniques. The rate constants have been found to depend markedly on the structures of the olefins, e.g. 1.2 × 108 and 6.5 × 104 M−1 s−1 for 1-hexene and tetramethylethylene, respectively. The mechanism of photochemical double-bond migration catalyzed by the transient is discussed on the basis of these rate constants.

Notes. Co-ordination kinetics of some aliphatic amines towards the photogenerated transient ‘CoH{PPh(OEt)2}3’

Time-resolved absorption spectra were observed after nitrogen-laser flash photolysis of toluene solutions of [CoHL4][L = PPh(OEt)2] in the presence of some aliphatic amines, and second-order rate constants were evaluated for co-ordination of the amines to the co-ordinatively unsaturated transient ‘CoHL3’ photogenerated. The steric bulk of substituents on the amine nitrogens is a dominant factor in the magnitude of the constants. The CoHL3(amine) adducts were also formed upon continuous photolysis with a high-pressure mercury lamp, and found to be stable at temperatures lower than –40 °C, on the basis of their 31P n.m.r. and visible absorption spectra.

Laser flash photolysis of hydridotetrakis(diethyl phenylphosphonite)cobalt(I). Direct evidence for the existence of a co-ordinatively unsaturated species

A transient species (λmax: 580 nm, Iµ: 1.7 × 103 dm3 mol–1 cm–1) observed 0.43 µs after nitrogen laser pulse excitation of hydridotetrakis(diethyl phenylphosphonite)cobalt(I) in cyclohexane has been identified as a co-ordinatively unsaturated species, to which diethyl phenylphosphonite and hex-1-ene co-ordinate with second-order rate constants of 1.4 × 108 and 1.1 × 108 dm3 mol–1s–1 respectively.