AG codes from vector bundles

We discuss the properties of algebraic-geometric codes defined by a vector bundle on a curve X over a finite field. We investigate the correction capacities of the codes defined from vector bundles constructed by means of finite coverings of X.     

Fluorescent behavior of 2‐(3,4,5,6‐tetrafluoro‐2‐hydroxyphenyl)imidazo‐[1,2‐a]pyridine in the presence of metal perchlorate

2‐(3,4,5,6‐Tetrafluoro‐2‐hydroxyphenyl)imidazo[1,2‐a]pyridine ( 1 ) emits long wavelength light around 540 nm both in polar and in nonpolar solvents. Zn²⁺ perchlorate in acetonitrile causes the intermediate wavelength emission around 430 nm, which is ascribed to the species where the imidazole nitrogen atom and the phenolate oxygen atom bridge Zn²⁺. In the presence of Hg²⁺ and Al³⁺ perchlorates, short wavelength emission around 370 nm is strongly increased and this fluorescent enhancement is attributable not to the coordination of Hg²⁺ and Al³⁺ to 1 but to the formation of the salt of perchloric acid of 1 .     

Synthetic studies on chartelline C: stereoselective construction of the core skeleton

What a core-ker! The title synthesis was achieved using a route featuring an intramolecular Mitsunobu reaction of a nosyl amide, stereoselective construction of the β-lactam, and formation of an enamide moiety by selenoxide elimination. The stereochemistry of the alkylation for the formation of the β-lactam was controlled by a secondary hydroxy group on the ten-membered ring. SEM=2-(trimethylsilyl)ethoxymethyl; TBS=tert-butyldimethylsilyl.     

Amphotericin B covalent dimers forming sterol-dependent ion-permeable membrane channels

Polyenemacrolides such as amphotericin B (AmB) were thought to assemble together and form an ion channel across plasma membranes. Their antimicrobial activity has been accounted for by this assemblage, whose stability and activity are dependent on sterol constituents of lipid bilayer membranes. The structure of this channel-like assemblage formed in biomembranes has been a target of extensive investigations for a long time. For the first step to this goal, we prepared several AmB dimers with various linkers and tested for their channel-forming activity. Among these, AmB dimers that bore an aminoalkyl-dicarboxylate tether covalently linked between amino groups of AmB showed potent hemolytic activity. Furthermore, K+ influx actions monitored by measuring the pH of the liposome lumen by 31P NMR revealed that the dimers formed the molecular assemblage similar to that of AmB in phospholipid membrane. Judging from changes in 31P NMR spectra, the dimers appeared to induce "all-or-none"-type ion flux across the liposome membrane in the presence of ergosterol, which suggested that the ion channel formed by ergosterol/dimer is similar to that of AmB. With these data in hand, we are now trying to elucidate the structure of the ion-channel complex by making the labeled conjugates of AmB for NMR measurements.     

Mechanisms of water oxidation catalyzed by the cis,cis-[(bpy)2Ru(OH2)]2O4+ ion

The cis,cis-[(bpy)(2)Ru(III)(OH(2))](2)O(4+) micro-oxo dimeric coordination complex is an efficient catalyst for water oxidation by strong oxidants that proceeds via intermediary formation of cis,cis-[(bpy)(2)Ru(V)(O)](2)O(4+) (hereafter, [5,5]). Repetitive mass spectrometric measurement of the isotopic distribution of O(2) formed in reactions catalyzed by (18)O-labeled catalyst established the existence of two reaction pathways characterized by products containing either one atom each from a ruthenyl O and solvent H(2)O or both O atoms from solvent molecules. The apparent activation parameters for micro-oxo ion-catalyzed water oxidation by Ce(4+) and for [5,5] decay were nearly identical, with DeltaH(++) = 7.6 (+/-1.2) kcal/mol, DeltaS() = -43 (+/-4) cal/deg mol (23 degrees C) and DeltaH(++) = 7.9 (+/-1.1) kcal/mol, DeltaS(++) = -44 (+/-4) cal/deg mol, respectively, in 0.5 M CF(3)SO(3)H. An apparent solvent deuterium kinetic isotope effect (KIE) of 1.7 was measured for O(2) evolution at 23 degrees C; the corresponding KIE for [5,5] decay was 1.6. The (32)O(2)/(34)O(2) isotope distribution was also insensitive to solvent deuteration. On the basis of these results and previously established chemical properties of this class of compounds, mechanisms are proposed that feature as critical reaction steps H(2)O addition to the complex to form covalent hydrates. For the first pathway, the elements of H(2)O are added as OH and H to the adjacent terminal ruthenyl O atoms, and for the second pathway, OH is added to a bipyridine ring and H is added to one of the ruthenyl O atoms.     

Gravitational compression dynamics of charged colloidal crystals

We examine the compression of charged colloidal crystals under the influence of gravitational force by monitoring the spatiotemporal variations of Bragg diffraction from the crystal lattice. We use the dilute aqueous dispersions of colloidal silica particles (diameter=216 nm, charge number=733, a particle volume fraction φ=0.06) in the presence of 5-15 μM sodium chloride. The sedimentation profiles of the colloidal crystals along the crystal height are determined by in situ fiber optics reflection spectroscopy. The time evolutions of the sedimentation profiles are calculated by numerical simulations based on a phenomenological continuum model that explicitly incorporates the electrostatic interparticle interactions. The simulation results correctly describe the experiments at sufficiently high ionic strength.     

Star-shaped pyrrole-fused tetrathiafulvalene oligomers: synthesis and redox, self-assembling, and conductive properties

A series of star-shaped pyrrole-fused tetrathiafulvalene (TTF) oligomers 1-3 was synthesized via an S(N)Ar reaction of fluorinated benzenes with the pyrrolyl sodium salts. Electrochemical and chemical oxidations of 1-3 revealed that a radical cation moiety on each TTF unit was successfully accumulated in all oligomers. Self-assembled structures of neutral and oxidized species were characterized by SEM and XRD, and their conductive properties of the iodine-doped 1-3 as well as an intermolecular mixed-valence ion radical salt were investigated.     

Dioxygenases, key enzymes to determine the aglycon structures of fusicoccin and brassicicene, diterpene compounds produced by fungi

Fusicoccin A and cotylenin A are structurally related diterpene glucosides and show a phytohormone-like activity. However, only cotylenin A induces the differentiation of human myeloid leukemia cells. Since the cotylenin A producer lost its ability to proliferate during preservation, a study on the relationship between structure and activity was carried out and a modified fusicoccin A with hydroxyl group at the 3-position showed a similar biological activity with that of cotylenin A. We then searched for an enzyme source that catalyzes the introduction of a hydroxyl group into the 3-position and found that brassicicene C, which is structurally related to fusicoccin A with hydroxyl group at the 3-position, was produced by Alternaria brassicicola ATCC96836. We recently cloned a brassicicene C biosynthetic gene cluster including the genes encoding fusicocca-2,10(14)-diene synthase and two cytochrome P450s, which were responsible for the formation of fusicocca-2,10(14)-diene-8β,16-diol. In this study, we report that a α-ketoglutarate dependent dioxygenase, the gene coding for which was located in the cluster, catalyzed a hydroxylation at the 3-position of fusicocca-2,10(14)-diene-8β,16-diol. On the other hand, a α-ketoglutarate-dependent dioxygenase, which had been identified in a fusicoccin A biosynthetic gene cluster, catalyzed the 16-oxidation of fusicocca-2,10(14)-diene-8β,16-diol to yield an aldehyde (8β-hydroxyfusicocca-1,10(14)-dien-16-al), although both dioxygenases had 51% amino acid sequence identity. These findings suggested that the dioxygenases played critical roles for the formation of the fusicoccin A-type and cotylenin A-/brassicicene C-type aglycons. Moreover, we showed that short-chain dehydrogenase/reductase located in the fusicoccin A biosynthetic gene cluster catalyzed the reduction of the aldehyde to yield fusicocca-1,10(14)-diene-8β,16-diol.     

Total synthesis of citridone A

The first total synthesis of citridone A has been achieved through regioselective intramolecular iodocyclization and regio- and stereoselective Pd(0)-catalyzed coupling as key reactions.