On the space of quadruples of quinary alternating forms

We discuss some aspects of the invariant theory and arithmetic of the prehomogeneous vector space of quadruples of quinary alternating forms. In particular, we complete the explicit construction of all prehomogeneous covariants of this space and give the rational classification of maximal singular orbits.     

Prehomogeneous vector spaces and field extensions

Summary Letk be an infinite field of characteristic not equal to 2, 3, 5. In this paper, we construct a natural map from the set of orbits of certain prehomogeneous vector spaces to the set of isomorphism classes of Galois extensions ofk which are splitting fields of equations of certain degrees, and prove that the inverse image of this map corresponds bijectively with conjugacy classes of Galois homomorphisms.Oblatum 24-I-1992 & 23-IV-1992Both authors are supported by NSF Grant DMS-8803085, DMS-9101091; The first author was partially supported by NSA grant MDA904-91-H-0041     

An arrhythmic-inducing glycoside from Albizzia julibrissin Durazz, IV.

Three pyridoxine derivatives have been isolated from the fresh stem bark of Albizzia julibrissin DURAZZ.. One of them, named julibrin II, was found to exhibit arrhythmic-inducing action. However, neither the others having the same aglycone nor some glycosides having the same sugar unit showed the action.     

A new simple preparation of platinum-nickel alloy nanoparticles and their characterization as an electrocatalyst for methanol oxidation

Pt-Ni alloy nanoparticles were produced by casting 2 or 10 mM H₂PtCl₆ solutions on a Ni column. The apparent particle size for the resultant Pt-Ni alloys increased with the concentration of the H₂PtCl₆ solution, while the content of Pt in the alloy decreased. The potential sweeps of 5 cycles in an H₂SO₄ aqueous solution for Pt-Ni (2 mM)/Ni and Pt-Ni (10 mM)/Ni electrodes led to electrochemical behavior similar to a polycrystalline Pt electrode, suggesting the formation of a few thin Pt layers on each Pt-Ni alloy surface. In electrochemical measurements, both Pt-Ni/Ni electrodes showed more negative onset potential of methanol oxidation and slower degradation of oxidation current of methanol than the polycrystalline Pt electrode. X-ray photoelectron spectroscopy of both Pt-Ni/Ni electrodes showed the shift of Pt4f peaks to a higher binding energy, suggesting that the increase in the d vacancy in the balance band 5d orbital of Pt contributed to the improved electrocatalytic activity and durability of the Pt-Ni/Ni electrodes.     

A plant type III polyketide synthase that produces pentaketide chromone

A novel plant-specific type III polyketide synthase (PKS) that catalyzes formation of a pentaketide chromone, 5,7-dihydroxy-2-methylchromone, from five molecules of malonyl-CoA, was cloned and sequenced from aloe (Aloe arborescens). Site-directed mutagenesis revealed that Met207 (corresponding to Thr197 in CHS) determines the polyketide chain length and the product specificity of the enzyme; remarkably, replacement of a single amino acid residue, Met207, with Gly yielded a mutant enzyme that efficiently produces aromatic octaketides, SEK4 and SEK4b, the products of the minimal PKS for actinorhodin (act from Streptomyces coelicolor), from eight molecules of malonyl-CoA. This provided new insights into the catalytic functions and specificities of the CHS-superfamily type III PKS enzymes.     

Synthesis of cyclotetrapeptides,am-toxin analogs,containing α-hydroxyalanine

Condensation of amide with α-keto acid to yield a α-hydroxyalanine (α-Hyala) or dehydroalanine residue was applied to syntheses of analogs of AM-toxins, cyclotetradepsipeptides. Cyclo(-α-Hyala-L-Ala-L-Val-L-Phe-), cyclo(-α-Hyala-L-Ala-L-Hmb-L-Phe-) and cyclo(-α-Hyala-L-Ala-L-Hmb-L-Tyr-) (Hmb, 2-hydroxy-3-methylbutanoic acid) were obtained from the corresponding pyruvyl-tripeptide amides in good yields by the treatment of anhydrous hydrogen fluoride.     

Field desorption mass spectrometry of natural products—II: Physiologically active pennogenin—and hederagenin-glycosides

Field desorption mass spectra of underivatized saponios, naturel oligoglycosides of physiological activity, showed intense ions formed by attachment of alkali cations to the neutral molecule and to structurally highly significant subunits. This cationization was generated by small amounts of alkali salts which were present in samples after isolation from biological material. When the mass spectre were produced by laser-supported desorption and were recorded at high resolution on vapor-deposited AgBr plates unambiguous determination of the molecular weghts (in the mass range from 700 to 1400) was performed. Moreover, ions formed by direct bond cleavages in the oligosaccharide moiety of the natural saponins clearly gave information about the sequence of the sugar units in the molecule and their individual chemical structures. The formation of these fragments in FD-MS is discussed in relation to the well established mechanism of glycosidic bond cleavage by acidic solvolysis. In principle this comparison showed the usefulness of this approach for the interpretation of FD mass spectra of substances containing heteroatom linkages e.g., glycosides, esters, amides, imines, mercaptans, etc.     

Observation of micelle solution of decyltrimethylammonium bromide by electrospray ionization mass spectrometry

A micelle solution of decyltrimethylammonium bromide (DTAB) was analyzed by electrospray ionization mass spectrometry. Finding an appropriate range of a capillary-skimmer potential was a prerequisite for obtaining a satisfactory spectrum. The mean molecular weight of DTAB aggregates, 10,500, was deduced from a series of mass spectra acquired at different capillary-skimmer potentials. The value was comparable with the micelle weight, previously determined by the light-scattering method.     

Two 3,4-seco-lupane triterpenes from leaves of Acanthopanax divaricatus var. albeofructus

Two 3,4-seco-lupane triterpenoids (1, 2) were isolated from the leaves of Acanthopanax divaricatus var. albeofructus (Araliaceae). Based on the spectroscopic data, the chemical structures of 1 and 2 were characterized as 24-hydroxychiisanogenin and 22alpha-hydroxychiisanogenin, respectively. 1 was a new compound and 2 was isolated for the first time from the natural source, although it had been obtained as an enzymatically hydrolyzed artifact from 22alpha-hydroxychiisanoside.