Tetsuya Yanagimoto

Photoinduced electron transfer between lipidporphyrinato-zinc(II) and -iron(III) complexes in a phospholipid vesicular membrane

Abstract A tetraphenylporphyrin derivative having four alkylphosphocholine groups and a covalently-bound axial imidazole (lipidporphyrin) is easily self-organized in water to form spherical micelles or bilayer membranes with phospholipid molecules. The photoinduced electron transfer reactions between lipidporphyrinato-zinc(II) (1) and -iron(III) (2) complexes in these molecular assemblies were studied by fluorescence spectroscopy and laser flash photolysis. A mixture of 1 and 2 (molar ratio: 1/1) produced non-fluorescent micelles. The red-shifted Soret band absorption, relative to that of the methanolic monomer solution, suggests the formation of the photodeactive complex made of 1 and 2 in the ground state. On the other hand, both chromophores were homogeneously dispersed into the bilayer membrane of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC), and the reductive electron transfer from the excited singlet state of 1 to 2 was observed. While this resulted in a significant decrease in the yield of intersystem crossing, the gradual quenching of the excited triplet state of 1 by 2 was also measured. In both cases, the corresponding Stern–Volmer plots showed a linear relationship and yielded quenching rate constants of 1.2×1011 and 6.2×104 M−1 s−1 via the excited singlet state and the triplet state, respectively. In the presence of excess triethanolamine as a sacrificial reagent, the intermolecular electron transfer became partly irreversible, giving an intramolecularly imidazole-coordinated Fe(II) complex which is capable of reversibly binding dioxygen like hemoglobin.

11,11,12,12-Tetracyano-2,6-anthraquinodimethane (TANT) as a novel extensive electron acceptor

The title compound is prepared as a higher homologue in an extensively conjugated series of TCNQ (7,7,8,8-tetracyanoquinodimethane) and TNAP (9,9,10,10-tetracyano-2,6-naphthoquinodimethane); its complexes with TTT (tetrathiatetracene) and HMTTeF (hexamethylenetetratellurafulvalene) are electrically highly conductive.

Molecular environment effect on O2 binding to lipidporphyrinatoiron(II) complexes in aqueous media

Lipidporphyrinatoiron(II) complexes are tetrakis(o-substituted)phenylporphinatoiron(II) derivatives which can be easily dispersed in water by molecular assembling. The most remarkable aspect of lipidporphyrinatoiron(II) assemblies is their reversible binding of dioxygen under physiological conditions (in aqueous media, pH 7.3, 37 °C) like hemoglobin and myoglobin. In these structures the O2-binding properties are largely influenced by the molecular environment around the coordination site. Tetrakis(o-pivalamido)phenylporphinatoiron(II) with a covalently linked axial imidazole (lipidporphyrinatoiron(II), 1) is incorporated into recombinant human serum albumin (rHSA), providing a totally synthetic O2-carrying hemoprotein (rHSA–1). Electrospray ionization mass spectrometry revealed the molecular mass of this non-covalent albumin–porphyrin hybrid. The O2 rebinding after laser flash photolysis represented a three-phase decay, suggesting that each porphyrin is embedded into different cavities in the albumin structure. On the other hand, amphiphilic lipidporphyrinatoiron(II) with four alkylphosphocholine chains (2) is self-organized in aqueous solution to produce bimolecular fibers with a uniform thickness of 10 nm. This fiber also gave a stable O2 adduct, and the O2 rebinding after laser flash irradiation showed monophasic kinetics. Up to 20 vol% of methanol, which is a critical concentration for fiber formation, the morphology was gradually dissociated into spherical micelles, and the stability of the dioxygenated species suddenly decreased to 10% of that of the fibers.