Yukito Murakami

Redox chemistry of niobium and molybdenum porphyrins

A. Introduction . . . . . . . 158 B. Preparation and chemical reduction and oxidation of pentavalent niobium and molybdenum complexes . . 166 (i) Niobium complexes . . . . 166 (ii) Molybdenum complexes 172 C. Electrochemistry 178 (i) Niobium porphyrins 178 (ii) Molybdenum porphyrins. . . . . 183 D. Photochemistry . . . . 186 (i) Niobium porphyrins . . . . . . 186 (ii) Molybdenum porphyrins. . . . . . . . . . 187 E. Catalytic functions . . . 189 References _..____.___.____._____....___.___.____.___.___.____.__ 191

Transition-metal complexes of pyrrole pigments. I. Electronic and vibrational spectra of cobalt(II), nickel(II) and copper(II) complexes of some dipyrromethenes

The cobalt, nickel(II), and copper(II) complexes of 3,4,5-trimethyl-, 3,3′,5,5′-tetramethyl- and 3,3′,4,4′,5,5′-hexamethyldipyrromethene have been synthesized in this work and investigated by visible, near-infrared, infrared and far-infrared spectroscopy. The ligand-field bands for these metal complexes have provided valuable information on their geometry around the central metal atoms. Namely, the copper complexes assume the tetragonally distorted tetrahedral configuration while the nickel and cobalt complexes are approximately regular tetrahedral. Moreover, the removal of the 5′-methyl group of a dipyrromethene moiety in the copper complexes results in a significant structural alteration to the further flattened tetrahedron. In the far-infrared spectra, the prominent absorption peaks which can be associated with the metal-ligand stretching modes have been observed in the 400–350 cm−1 range. In addition, strong bands appearing at ∼1600 cm−1 have been assigned to the pyrroles-skeletal stretching mode. After examination of these infrared spectral data, the strength of the coordinate bonds has been noticed to increase in the order: Cu<Ni≳Co. The lowered strength of coordinate bonds for the copper complexes and the nature of coordinate bonds involved in the metal complexes of this work were discussed in connection with their stereochemistry.

Molecular recognition by novel cage-type azaparacyclophanes bearing chiral binding sites in aqueous media

Novel cage-type cyclophanes which are constructed with two rigid macrocyclic skeletons, tetraaza(6.1.6.1 Iparacyclophane and tetraaza(3.3.3.3)- paracyclophane, and four chiral bridging components were prepared. An asymmetric character of the internal cavity was clarified by means of circular dichroism (CD) spectroscopy and a computer-aided molecular modeling study based on molecular mechanics and dynamics (BIOGRAF, Dreiding-l and Dreiding-ll) conformational search. In aqueous media, the present hosts strongly bound anionic and nonionic hydrophobic guests to form inclusion complexes in 1 :I stoichiometry and the CD phenomena were induced in an incorporated achiral guest molecule through its stereochemical interaction with the chiral host cavity. In addition, the present hosts exhibited discriminative recognition toward steroid hormones in D20/CD30D (3:l v/v) as effected by hydrophobic and x--x interactions. The chirality-based discrimination of estrogens was attributed to their different modes of hydrogen bonding with the hosts.

Chiral recognition by cyclic oligosaccharides. Enantioselective complexation of binaphthyl derivatives with cyclodextrins

Abstract Chiral recognition of binaphthyl derivatives, such as 1,1′-bi-2-naphthol (1), 1,1′-binaphthyl-2,2′-diyl hydrogen phosphate (2), and 2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid (3), by cyclodextrins (CDxs) has been studied. The S enantiomers of 1 and 2 are bound to heptakis(2,3,6-tri-O-methyl)-β-CDx (TMe-β-CDx) as well as β-CDx more strongly than the R enantiomers. The molecular mechanics and molecular dynamics calculations for the 1:1 complex of 1 and β-CDx suggest that more effective van der Waals contacts and intermolecular hydrogen bonding stabilize the complex of S-1 compared with that of R-1. Meanwhile the R enantiomer of 3 is the preferable guest for β- and TMe-β-CDxs. Circular dichroism spectroscopy suggests that the complex of S-3 is more unstable than that of R-3 because the dihedral angle of the naphthalene planes of S-3 needs to be reduced for forming the inclusion complex. The enantiomers of the guest binaphthyls are completely separated by means of capillary zone electropho...

Specific molecular recognition by chiral cage-type cyclophanes having leucine, valine, and alanine residue

Chiral cage-type cyclophanes were constructed with two rigid macrocyclic skeletons and four bridging components bearing chiral leucine, valine, and alanine residues, individually. These host molecules strongly bind anionic and hydrophobic guests, such as 8-anilinonaphthalene-1-sulfonate and 6-p-toluidinonaphthalene-2-sulfonate. Thermodynamic parameters were evaluated from temperature-dependent complexation constants determined by fluorescence spectroscopy, and gave negative ΔH and positive ΔS values; especially large values for the cage-type cyclophanes having leucine residues. The positive ΔS values come primarily from effective desolvation of the guest molecules when incorporated into the hydrophobic host cavities, as evidenced by fluorescence parameters. The four bridging segments of the cage-type hosts having chiral amino acid residues seem to undergo chiral twist in the same directions in the light of circular dichroism (CD) spectroscopy. Such helical conformations of the cyclophanes must be caused by chiral nature of the amino acid residues, and the extent of twist in helical conformations is as follows; leucine > valine > alanine. In addition, the twisted direction of bridging segments in the cage-type hosts having L-amino acid residues is opposite to that evaluated for those having D-amino acid residues, so that the former and latter cyclophanes furnish M- and P-helical cavities, respectively. The chirality-based molecular recognition of the cage-type hosts toward an enantiomeric guest, bilirubin-IXα, was investigated by CD spectroscopy in aqueous media.

Supramolecular Assemblies Formed with Synthetic Peptide Lipids. Functional Models of Biomembranes and Enzymes

Membrane-forming lipids have been prepared. These have a-amino acid residue(s) interposed between a polar head moiety and a hydrophobic double-chain segment, peptide lipids. Cationic, anionic, zwitterionic, and nonionic peptide lipids, except for some nonionic ones, undergo aggregation to form multiwalled bilayer vesicles and/or lamellae when these are dispersed in aqueous media. The number of carbon atoms in each hydrocarbon chain of the double-chain portion needs to be equal to or greater than 12. The multiwalled vesicles are transformed into single-walled bilayervesicles upon sonication and the latter are morphologically stable over a sufficiently prolonged period of time. The phase transition parameters, T m and ΔH, of the multiwalled bilayer aggregates are linearly correlated with the alkyl-chain length of the double-chain segment for a series of N+ C5Ala2Cn: ca. 20°C and ca. 10 kJmol-1 changes per two methylene groups, respectively. On the other hand, nonlamellar aggregates, inverted cubic and inverted hexagonal, are formed by proper adjustment of the critical packing parameter for the peptide lipids. Single-walled bilayer vesicles undergo fusion via the formation of the intermediate nonlamellar phase. A bilayer-type artificial enzyme, which can simulate the catalytic functions of vitamin B6-dependent enzymes, is constituted with a hydrophobic vitamin B6, single-walled vesicles of a peptide lipid having a histidyl residue, and copper(II) ions. The artificial enzyme catalyzes transamination between a relatively hydrophobic α-amino acid and a hydrophilic α-keto acid along with the turnover of the catalyst system under mild conditions. The reaction mechanism and the microenvironmental properties of the catalytic site have been clarified. The identical artificial enzyme also simulates the catalytic function of tryptophan synthase that converts serine into tryptophan by β-replacement of the former amino acid with indole.

Hydrophobic vitamin B12. Part 14. Ring-expansion reactions catalyzed by hydrophobic vitamin B12 under electrochemical conditions in nonaqueous medium☆

Abstract The electrolysis of alicyclic ketones (5, 6, 7, and 8-membered rings) with a carboxylic ester and a bromomethyl group was carried out in N , N -dimethylformamide in the presence of a catalytic amount of heptamethyl cobyrinate perchlorate under various reaction conditions. The ring-expansion reaction largely proceeded at −2.0 V vs sce for all the substrates. The electrolysis plausibly proceeds as follows: the Co(II) complex as a catalyst is electrochemically reduced to the Co(I) species; the corresponding alkylated complex is generated by reaction of the supernucleophilic Co(I) species with an alicyclic bromide; the alkylated complex is subsequently decomposed by electrolysis to afford the final products; and the cobalt complex acts as a mediator repeatedly.

Hydrophobic vitamin B12: 8. Carbon-skeleton rearrangement reactions catalyzed by hydrophobic vitamin B12 in octopus azaparacyclophane

Abstract Heptapropyl cobyrinate perchlorate catalyzed the carbon-skeleton rearrangements, which convert 2-acetyl-2-ethoxycarbonylpropane, 2-cyano-2-ethoxycarbonylpropane, 1-acetyl-1-ethoxycarbonylethane, and diethyl β-methyl- dl -aspartate into 1-acetyl-2-ethoxycarbonylpropane, 2-cyano-1-ethoxycarbonylpropane and 1-cyano-2-ethoxycarbonylpropane, 1-acetyl-2-ethoxycarbonylethane, and diethyl glutamate, respectively, in an octopus cyclophane placed in aqueous carbonate buffer (pH 7.0) at 20.0°C by utilizing vanadium trichloride as a cocatalyst under aerobic photolysis conditions. The migratory aptitude of the electron-withdrawing groups was found to follow the sequence: CN ≈ CO 2 C 2 H 5 3 . Yields of the rearrangement products were very low in the absence of heptapropyl cobyrinate perchlorate.

Preparation and host-guest interactions of novel cage-type cyclophanes bearing chiral binding sites provided by dipeptide residues

Abstract Novel cage-type cyclophanes, which are constructed with two rigid macrocyclic skeletons, tetraaza[3.3.3.3]paracyclophanes, and four bridging segments composed of either β- l -aspartyl- l -aspartyl or β- d -aspartyl- d -aspartyl residues were prepared [(−)-1 and (+)-1, respectively]. Structural and asymmetric properties of these cage-type cyclophanes were characterized by 1H NMR and circular dichroism (CD) spectroscopy. The guest-binding behavior of the cage-type hosts toward fluorescent guests, such as 8-anilinonaphthalene-1-sulfonate and 6-p-toluidinonaphthalene-2-sulfonate, was examined in comparison with those demonstrated by the corresponding non-cage hosts in aqueous media. The microenvironmental polarity around the fluorescent guests and their fluorescence polarization values in the presence of the cage-type hosts revealed that these guest molecules were incorporated into apolar host cavities, and their rotational motion was largely restricted. Furthermore, chiral host-guest interactions between the hosts and a hydrophobic guest, pamoic acid (PA), were examined by CD spectroscopy. PA was subjected to stereochemical changes to assume P- and M-helix configurations in the presence of (−)-1 and (+)-1, respectively.

Steroid Cyclophanes as Artificial Cell-surface Receptors. Molecular Recognition and its Consequence in Signal Transduction Behavior

Steroid cyclophanes, bearing four bile acid moieties covalently placed on a tetraazaparacyclophane skeleton, were designed and synthesized as artificial cell-surface receptors. Guest-binding behavior of the steroid cyclophanes embedded in a bilayer membrane formed with a synthetic peptide lipid was clarified by means of fluorescence and circular dichroism spectroscopy. We found that the steroid cyclophane effectively bound aromatic guests in both bilayer membranes and aqueous solution. In addition, copper(II) ions acted as a guest species for the steroid cyclophane and a competitive inhibitor toward a NADH-dependent lactate dehydrogenase (LDH). On these grounds, we constituted a supramolecular assembly as an artificial signaling system in combination with the steroid cyclophane, a cationic peptide lipid, and LDH. As a consequence, the steroid cyclophane acted as an effective artificial cell-surface receptor being capable of transmitting an external signal to the enzyme in collaboration with copper(II) ions as a signal transmitter.