Water molecules in the schiff base region of bacteriorhodopsin

The Schiff base region of bacteriorhodopsin (BR), a light-driven proton pump, contains a pentagonal cluster, being composed of three water molecules and one oxygen each of Asp85 and Asp212. Asp85 and Asp212 are located at similar distances from the retinal Schiff base, whereas the Schiff base proton is transferred only to Asp85 during the pump function. The present FTIR study experimentally established the stretching vibration of water402 hydrating with Asp85 by use of various BR mutants, whose frequency (2171 cm-1 as the O-D stretch) indicates very strong hydrogen bond.     

Accelerating effect of a proton on the reduction of CO2 dissolved in water under acidic conditions. Isolation, crystal structure, and reducing ability of a water-soluble ruthenium hydride complex

A water-soluble hydride complex [(eta6-C6Me6)RuII(bpy)H]+ {1, bpy = 2,2'-bipyridine} serves as a robust reducing agent for the reduction of CO2 in water in a pH range of about 3-5 at ambient temperature under stoichiometric conditions. The structure of 1 was unequivocally determined by X-ray analysis. The mechanism of acid-catalyzed reduction of CO2 promoted by 1 in water under acidic conditions is disclosed.     

A new series of molybdenum-(IV), -(V), and -(VI) dithiolate compounds as active site models of molybdoenzymes: preparation, crystal structures, spectroscopic/electrochemical properties and reactivity in oxygen atom transfer

A new set of molybdenum-(IV), -(V), and -(VI) compounds containing 3,6-dichloro-1,2-benzenedithiolate (bdtCl2) were isolated and characterised by crystallographic and other spectroscopic techniques as active site models of arsenite oxidase, one of the molybdoenzymes. MoO2 compounds were prepared in high yields by reaction of MoO2Cl2 with bdtCl2, related dithiolene and thiocatecholate in methanol at low temperature. The bdtCl2 ligand particularly stabilised the MoO compounds with oxidation numbers of +4 and +5 as well as the MoO2 compound with an oxidation number of +6. The compounds (Et4N)2[MoVIO2(bdtCl2)2], (Et4N)2[MoIVO(bdtCl2)2] and (Et4N)[MoVO(bdtCl2)2] were successfully isolated, whereas (Et3NH)2[MoO2(thiocatecholate)2] gradually decomposed in acetonitrile. A distorted octahedral structure similar to that of was suggested for the structure of the active site of the oxidised form of arsenite oxidase on the basis of a comparison of their bond distances and angles. The bond distances and angles around the molybdenum(IV) atom in were similar to those around the molybdenum(IV) centre in the reduced form of arsenite oxidase. The reversible / couple exhibited a more positive redox potential than common MoO dithiolene compounds. Underwent an irreversible proton-coupled reduction process to yield. An oxygen atom transfer reaction of with triphenylphosphine afforded and triphenylphosphine oxide, and proceeded in second order as v=-d/dt[MoO2]=k[MoO2][PPh3]. The structures and properties of the oxo-bridged dinuclear compound (Et4N)2[MoVIO2(bdtCl2)]2(micro-O), a dimer of bdtCl2 and were also characterised.     

Dual-action molecular superconductors with magnetic anions

Dual-action organic superconductors, whose conducting properties can be sharply controlled by an external magnetic field, have been discovered in systems consisting of organic conduction layers based on bis(ethylenedithio)tetraselenafulvalene (BETS) molecules and magnetic anions. Owing to the metamagnetic nature of the anion layers, the superconducting state of kappa-BETS2FeBr4 can be switched on or off by applying the external field. In lambda-BETS2Fe0.4Ga0.6Cl4, exhibiting a field-induced superconducting transition for the field parallel to the conduction plane, the insulating, metallic, and superconducting states can be realized in a stepwise manner by slightly tuning the external magnetic field.     

Highly regioselective diels-alder reactions of 2-trimethylsilylmethyl- 1,3-butadiene catalyzed by a lewis acid and applications to syntheses of terpenes

2-Trimethylsilylmethyl-1,3-butadiene undergoes highly regioselective Diels-Alder reactions with dienophiles such as acrolein and methyl vinyl ketone catalyzed by aluminum chloride in which the “para” isomers are obtained almost exclusively. The adducts are converted readily to a variety of naturally occurring mono and sesquiterpenes.     

Intermolecular Ferromagnetic and Antiferromagnetic Interactions in Halogen-Bridged Copper(I) Imino Nitroxides: Crystal Structures and Magnetic Properties of [Cu(I)(&mgr;-X)(imino nitroxide)](2) (X = I or Br)

Reaction of CuI or CuBr with some imino nitroxides in methanol gave the halogen bridged dinuclear Cu(I) complexes [Cu(&mgr;-I)(impy)](2) (1), [Cu(&mgr;-I)(immepy)](2) (2), [Cu(&mgr;-Br)(immepy)](2) (3), and [Cu(&mgr;-Br)(imph-NO(2))](2) (4), respectively (impy = 2-(2'-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl, immepy = 2-(6'-methyl-2'-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl, imph-NO(2) = 2-(4'-nitrophenyl)-4,4,5,5-tetramethyl-4,6-dihydro-1H-imidazolyl-1-oxyl). Crystal structures and magnetic properties have been studied. Complexes 1-4 have dimeric structures where two copper ions are doubly bridged by halide ions in a &mgr;(2) fashion. In 1-3, each copper ion is tetrahedral with a bidentate imino nitroxide and two halide ions, and the two copper ions are separated by 2.592(2), 2.6869(8), and 2.7357(6) ?, respectively. In 4, triangular coordination sites of the copper ions are completed with a nitrogen atom from the imino nitroxide and two bromide ions bridging the two copper ions with a separation of 3.074(2) ?. Ligand imino nitroxides in 1-4 form one-dimensional radical chains, and the chains are linked with halocuprate dimer units. Structural and magnetic susceptibility data support that radicals in 1 and 4 are ferromagnetically stacked, while radicals in 2 and 3 form an antiferromagnetic chain. The magnetic behaviors are discussed in connection with the stacking modes of the radicals and bridging conformations. Crystal data (Mo Kalpha, lambda = 0.71069 ?): 1, orthorhombic, space group P2(1)2(1)2(1), a = 17.807(2) ?, b = 8.595(2) ?, c = 19.336(6) ?, and Z = 4; 2, monoclinic, space group P2(1)/c, a = 9.941(2) ?, b = 18.482(2) ?, c = 8.337(2) ?, beta = 96.41(2) degrees, and Z = 2; 3, monoclinic, space group P2(1)/c, a = 9.964(6) ?, b = 18.167(4) ?, c = 8.009(7) ?, beta = 95.81(6) degrees, and Z = 2; 4, monoclinic, space group P2(1)/c, a = 11.991(7) ?, b = 17.998(8) ?, c = 7.215(6) ?, beta = 104.07(6) degrees, and Z = 2.     

Total synthesis of an antitumor antibiotic,Fostriecin (CI-920)

The total synthesis of an antitumor antibiotic, fostriecin (CI-920), via a highly convergent route is described. A characteristic feature of the present total synthesis is that the synthesis was achieved via a coupling procedure of three segments A, B, and C. The unsaturated lactone moiety of fostriecin, corresponding to segment A, was constructed from a known Horner-Emmons reagent, and the stereochemistry of the C-5 position was introduced by asymmetric reduction with (R)-BINAl-H. Segment B having a series of stereogenic centers was synthesized from (R)-malic acid and the stereogenic centers at the C-8 and C-9 positions were prepared by a combination of Wittig reaction and Sharpless asymmetric dihydroxylation reaction. The conjugated Z,Z,E-triene moiety of fostriecin, corresponding to segment C, was eventually constructed by Wittig reaction and Stille coupling reaction. The phosphate moiety, which is known to be essentially important for the antitumor activity, was introduced via two routes: (i) direct phosphorylation of the monohydroxyl derivative in which other hydroxyl groups are protected with silyl groups; (ii) cyclic phosphorylation and selective cleavage of the cyclic phosphate derivative. Although the former route is basically the same as those reported by other groups, the latter route is novel and more effective than the former one. The present total synthesis would serve as a versatile synthetic route to not only fostriecin, but also its various analogues including stereoisomers.     

Topochemical exploration of potent compounds using retro-enantiomer libraries of cyclic pentapeptides

Cyclic pentapeptides have been adopted as conformationally restricted peptide templates to dispose pharmacophores of bioactive peptides. In our recent study, use of two orthogonal cyclic pentapeptide libraries involving conformation-based and sequence-based libraries containing critical residues of a bioactive peptide led to the discovery of potent downsized peptides that possess activity comparable to that of the parent peptide. The present study demonstrates that a third library consisting of retro-enantiomers (retro-inverso peptides) that possess not only all residues with the opposite configuration to those in the corresponding original peptide but also amino acid sequences with reversed arrangement, is important as an alternative library for rationally finding active compounds.     

Synthetic studies on chemotherapeutics. II. Synthesis of phenyl-substituted 1,4-dihydro-4-oxonicotinic acid derivatives. [Studies on the syntheses of heterocyclic compounds. Part 704]

The thermal cyclization of the arninomethylenemalonates (8) gave the 4-hydroxynicotinates ( 9 ), ethylation of which yieldedN-ethylated ( 11 ) and O-ethylated products ( 12 ). Hydrolysis of 9, 11 , and 12 led to the desired nicotinic acids (10, 4, and 13), respectively.     

Hybridization-promoted and cytidine-selective activation for cross-linking with the use of 2-amino-6-vinylpurine derivatives

Recently, we have proposed a new concept for cross-linking agents with inducible reactivity, in which the highly reactive cross-linking agent, the 2-amino-6-vinylpurine nucleoside analogue (1), can be regenerated in situ from its stable precursors, the phenylsulfide (4) and the phenylsulfoxide (3) derivatives, by a hybridization-promoted activation process with selectivity to cytidine. The phenylsulfide precursor (4) exhibited cross-linking ability despite its high stability toward strong nucleophiles such as amines and thiols. In this study, we investigated the substituent effects of the phenylsulfide group on the cross-linking reaction, and determined the 2-carboxy substituent of the phenylsulfide derivative (11k) as an efficient cross-linking agent with inducible reactivity. Detailed investigations have shown that the phenylsulfoxide (3) and phenylsulfide (4) precursors produce the 2-amino-6-vinylpurine nucleoside (1) as the common reactive species. It has been concluded that the nature of the inducible reactivity of the precursors (3 and 4) is acceleration of their elimination to the 2-amino-6-vinylpurine nucleoside (1) through the selective process in the duplex with the ODN having cytidine at the target site.