Kinetics of reaction of tris[5,5′ {3-(2-pyridyl)-1,2,4, triazine-5,6 diyl} bis-2-furansulphonic acid]iron(II) with hydroxide ion

Summary The kinetics of dissociation of the iron(II) tris complex of ferene, [5,5{3-(2-pyridyl)-1,2,4-triazine-5,6-diyl}] bis-2-furansulphonic acid in aqueous basic and acidic media have been studied spectrophotometrically at 298.2K. Rate constants at several hydroxide ion concentrations are reported for both steps observed in basic solution. A possible dissociation mechanism is presented and the comparative reactivity of this complex with other iron(II)-diimine complexes is assessed.Presented in part at the 191st Am. Chem. Soc. Nat. Meeting, New York, NY, April 1986. Abs. Inorg. 0130     

Molecular dynamics study of the surface tension of a binary immiscible fluid

The planar interface between two liquids having two degrees of affinity to mix has been studied by molecular dynamics simulations. The surface tension is calculated from the normal, PN, and transverse, PT, components of the pressure tensor P for a wide range of temperatures. An unusual increase in surface tension with increasing temperature is attributed to a pressure induced void transfer mechanism that is justified by basic thermodynamic arguments. This effect is diminished on the addition of a modest attractive potential between the two species, and there is a turnover point at higher temperatures beyond which the surface tension decreases with increasing temperature. An order parameter is identified as the gradient of the mole fraction distribution through the interfacial region. An additional effect is the dramatic inversion of the kinetic and potential contributions to the PN profile as the temperature is varied. It is found that a commonly used approximation for P, the Irving-Kirkwood 1 or IK1 method, results in a relatively modest unphysical variability in PN that weakly violates the condition of local mechanical stability. However, this artifact does not prevent the IK1 method from producing an interfacial tension which is nearly identical to that derived from the complete IK formula with no additional approximations.     

Structure-activity relationships in purine-based inhibitor binding to HSP90 isoforms.

Inhibition of the ATPase activity of the chaperone protein HSP90 is a potential strategy for treatment of cancers. We have determined structures of the HSP90alpha N-terminal domain complexed with the purine-based inhibitor, PU3, and analogs with enhanced potency both in enzyme and cell-based assays. The compounds induce upregulation of HSP70 and downregulation of the known HSP90 client proteins Raf-1, CDK4, and ErbB2, confirming that the molecules inhibit cell growth by a mechanism dependent on HSP90 inhibition. We have also determined the first structure of the N-terminal domain of HSP90beta, complexed with PU3. The structures allow a detailed rationale to be developed for the observed affinity of the PU3 class of compounds for HSP90 and also provide a structural framework for design of compounds with improved binding affinity and drug-like properties.     

Solvation and reactivity of a helical binuclear iron(II) complex with a tetradentate Schiff-base ligand

Reaction of 2-acetylpyridine, hydrazine, and an iron(II) salt gives a low-spin binuclear iron(II) complex, [Fe₂(pmk)₃]⁴⁺, in which three tetradentate Schiff base ligands span the two metal centres to give a complex of helical geometry. We report solubilities of the iodide and perchlorate salts, in H₂O, in aqueous MeOH, and in aqueous DMSO (perchlorate only), up to 60% by vol of organic cosolvent. Transfer chemical potentials for the complex cation from H₂O into the respective mixed solvents have been derived from these solubilities, to give a picture of solvation of this complex in the media studied. Kinetics of base hydrolysis of [Fe₂(pmk)₃]⁴⁺ have been established, in H₂O and in 40% MeOH, 20 and 40% i -PrOH, and 20 and 40% DMSO, at 298.2 K. The reactivity of [Fe₂(pmk)₃]⁴⁺ is discussed in relation to reactivities of a selection of iron(II)-diimine complexes, ranging from [Fe(bipy)₃]²⁺ and [Fe(phen)₃]²⁺ to ligand-encapsulated cage complexes.     

Formation of beta-hydroxy histidine in the biosynthesis of nikkomycin antibiotics

Nikkomycins, a group of peptidyl nucleoside antibiotics produced Streptomyces tendae Tü901, are potent competitive inhibitors of chitin synthase. In this study, three nikkomycin biosynthetic enzymes, NikP1, NikQ, and NikP2, were overexpressed, purified, and characterized. The NikP1 activated L-His and transferred it to the carrier protein domain to form L-His-S-NikP1, which served as the beta-hydroxylation substrate of NikQ. The beta-OH-His was then hydrolytically released from NikP1 by NikP2. The results reported here substantiate our earlier proposal that the covalent tethering of an amino acid onto a carrier protein domain prior to downstream modification is a general strategy for diverting a fraction of the amino acid into secondary metabolism.     

Enantioselective gas chromatographic assays with electron-capture detection for methoxyphenamine and its three primary metabolites in human urine

Sensitive and enantioselective gas chromatographic assays have been developed and applied to the quantitation in human urine of the enantiomers of methoxyphenamine and its three primary oxidative metabolites, namely, N-desmethylmethoxyphenamine, O-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine. The separation of the various analytes was achieved through the combined use of high-resolution gas chromatography coupled with electron-capture detection and employing a capillary OV-225 column. The formation of diastereometric derivatives involved the chiral acylating reagent N-heptafluorobutyryl-L-prolyl chloride. The assays for methoxyphenamine and O-desmethylmethoxyphenamine were linear over the range 0.25-2.0 micrograms/ml for each analytes' enantiomers, while in the case of the enantiomers for N-desmethylmethoxyphenamine and 5-hydroxymethoxyphenamine linearity was shown over the ranges 0.094-0.75 and 0.188-1.5 micrograms/ml, respectively. The mean coefficients of variation in all cases were less than 4%.     

Palladium-catalyzed N-heteroannulation of N-allyl- or N-benzyl-2-nitrobenzenamines: synthesis of 2-substituted benzimidazoles

A palladium-catalyzed reductive N-heteroannulation of N-allyl- or N-benzyl-2-nitrobenzenamines, using carbon monoxide as the ultimate reducing agent, affording 2-substituted benzimidazoles has been developed.     

Formal hydride transfer mechanism for photoreduction of 3-phenylquinoxalin-2-ones by amines. Association Of 3-phenylquinoxalin-2-one with aliphatic amines

The photophysical and photochemical behavior of 1-methyl-3-phenylquinoxalin-2-one (MeNQ) and 3-phenylquinoxalin-2-one (HNQ) in the presence of amines is reported. While HNQ fluorescence shows an auxochromic effect and a bathochromic shift with added amines, explained by association of HNQ with amine in the ground state and emission from both excited species HNQ and [HNQ-amine], both MeNQ and HNQ are photoreduced efficiently on irradiation in the presence of amines, leading to the semireduced quinoxalin-2-ones, MeNQH(-) and HNQH(-), respectively, via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. The semireduced quinoxalin-2-ones XNQH(-) (X = H, Me) revert almost quantitatively to the parent XNQ in a dark thermal reaction with an activation free energy for MeNQH(-) of 17.4 and 25.9 kcal/mol in acetonitrile and benzene, respectively. Kinetic and spectroscopic (UV and NMR) evidence supports the proposed reaction mechanism for the reversible photoreduction.