RESOLUTION OF SPECTRAL AND LIFETIME HETEROGENEITY OF TRYPTOPHAN FLUORESCENCE IN BACTERIORHODOPSIN

Abstract— The picosecond time-resolved fluorescence decay of bacteriorhodopsin (BR) was analyzed by the maximum entropy method. Results showed five distributions of lifetimes indicating at least five decay components. A wavelength-dependent study of emission decay of BR was carried out in the wavelength region from 310 to 390 nm. The decay at each wavelength was resolvable into four decay components by the discrete exponential analysis. The three short lifetime components (100 ± 20 ps, 400 ± 50 ps and 1.0 ± 0.1 ns) were independent of wavelength, whereas the longest lifetime component was wavelength dependent (varying from 4.1 ns at 310 nm to 5.7 ns at 390 nm). These results are inconsistent with the existing model of associating the fluorescence of bacteriorhodopsin with two or four lifetime components. An attempt is made to associate the five decay components with the emitting tryptophans of BR.     

Gas chromatographic quantitation of methoxyphenamine and three of its metabolites in plasma

Sensitive gas chromatographic procedures for the determination of methoxyphenamine and three of its metabolites in plasma have been developed. The metabolites were measured using an electron-capture detector. This simple procedure is based on the precipitation of protein from a 1-ml plasma sample with 10% trichloroacetic acid, followed by aqueous derivatization with pentafluorobenzoyl chloride at pH 9.2 and a single-step cyclohexane extraction. The lower limit of detection for the N-desmethyl, O-desmethyl and aromatic 5-hydroxy metabolites of methoxyphenamine were 1.6, 3.1 and 2.2 ng ml-1, respectively, with coefficients of variation less than 10%. The poor electron-capture response of fluorinated derivatives of methoxyphenamine necessitated the use of nitrogen-phosphorus detection. Extractive derivatization with pentafluorobenzoyl chloride, without the need for protein precipitation, enabled quantitation of methoxyphenamine down to 3.8 ng ml-1 from a 2-ml aliquot of plasma. In a pilot study involving healthy volunteers who received a single oral dose of methoxyphenamine hydrochloride plasma concentration could be followed in all three subjects for at least 24, 32, 12 and 4 h for methoxyphenamine and the O-desmethyl, 5-hydroxy and N-desmethyl metabolites, respectively.     

Oxidation – Reduction Reactions of Tetrachloroaurate(III) Anion with Triphenyl Derivatives of Group V Elements

The reduction of the tetrachloroaurate (III) anion by L (L = PPh₃, AsPh₃, SbPh₃) is quantitative in non-aqueous solution. The products are the gold(I)-complexes AuClL (L = AsPh₃, SbPh₃) and Au(PPh₃)⁺₂ together with the corresponding oxidation product LCl₂. Kinetic studies show that the reactions are first order in AuCl^?₁ and L. In addition a path independent of PPh₃ was found in dichloromethane. These data are interpreted in terms of mechanisms which involve reduction of AuCl^?₄ to AuCl^?₂ followed by equilibrium formation of AuClL for L = AsPh₃ and SbPh₃. For PPh₃, the data are consistent with a chloride replacement by PPh₃ to give AuCl₃ PPh₃, which is followed by a rapid reduction by a second mole of PPh₃. Equilibrium formation constants are reported for several Au(I) complexes.     

A water-mediated tautomerism mechanism in formamide and amidine. An ab initio study

Formamide, formamidic acid, and amidine water complexes were studied using 3-21G fully optimized structures and 6–31G energies. Hydrogen bonding and a water-mediated tautomerism mechanism were examined. The optimized complexes show that relaxation of the monomers has occurred. Hydrogen bond lengths and energies fall within the range of values found using other basis sets and other comparable systems.     

Metal complexes of sulfur-nitrogen chelating agents. Part 12. The chemistry of nickel(II), palladium(II), cobalt(II) and copper(II) complexes of N2S2 and N3S2 donor systems

Summary Nickel(II), palladium(II), cobalt(II) and copper(II) complexes of the ligandN,N-1,2-propane-bis(methyl 2-amino-cyclopent-1-ene-dithiocarboxylate) (H₂L¹),N,N-1,3-propane-bis(methyl 2-aminocyclopent-1-ene-dithiocarboxylate) (H₂L²) andN,N-[bis(methyl 2-aminocyclopent-1-ene-dithiocarboxylate)] diethylenetriamine (H₂L³) have been synthesised. Both H₂L¹ and H₂L² form complexes of the type ML, and all but the copper(II) complexes, are square planar. In the copper(II) complexes tetrahedral distortion is significantly more with CuL². From H₂L³ square planar complexes of the type [M(HL³)X] (M=Ni, X=Cl, Br, I or SCN; M=Pd, X=Cl or Br) have been obtained in which the donor unit involved is N₂SX. The composition of the cobalt(II) and copper(II) complexes is [M(H₂L³)X₂] (X=Cl or Br) which contain the chromophore [MN₃X₂].     

Differential thermal analysis studies on the crystallization of strontium tungstate from sodium tungstate melt. Nucleation mechanism and induction periods

The nucleation mechanism, kinetics and induction periods in strontium tungstate crystallization from sodium tungstate melt in platinum crucibles were investigated by differential thermal analysis. Heterogeneous nuclei first formed on the metal platinate particles within the solution during the induction periods (¯t); the main crystal growth started only after ¯t and few new nuclei were then formed. At any crystallization temperature, ¯t varied inversely with the cooling rate (R _T), and with the rate (R_C) of development of excess solute concentration according to the relation ¯t = 1/(k ₁ R _c ), wherek ₁ and are constants. The critical temperature (¯T), critical supersaturation(¯ S), k ₁ and were evaluated.

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Zusammenfassung Der Keimbildungsmechanismus, sowie die Kinetik und die Induktionsperioden wurden bei der Kristallisation von Strontiumwolframat aus Natriumwolframatschmelzen in Platintiegeln durch Differentialthermoanalyse untersucht. Die heterogenen Kristallkeime wurden zuerst während der Induktionsperiode () an den Metallplatinat-Teilchen in der Lösung gebildet: das Hauptkristallwachstum begann erst nach und nachfolgend wurden einige neue Keime gebildet. Bei jeder Kristallisationstemperatur änderte sich umgekehrt proportional zu den Abkühlungsgeschwindigkeiten (R _T) und den Geschwindigkeiten (R _c) der Bildung überschüssig gelösten Materials, entsprechend der Gleichung = 1/(k ₁ R _c , wobeik ₁ und Konstanten sind. Die kritische Temperatur (¯T), die kritische Übersättigung (¯S),k ₁ und wurden bewertet.

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Résumé On a étudié par analyse thermique différentielle le mécanisme de nucléation ainsi que la cinétique et les périodes d'induction de la cristallisation du tungstate de strontium, à partir des bains fondus de tungstate de sodium, dans des creusets de platine. Des noyaux hétérogènes se forment d'abord sur les particules de platinate métallique dans la solution, au cours des périodes d'induction (): la croissance principale des cristaux ne commence qu'après et il se forme alors quelques noyaux nouveaux. A une température quelconque de cristallisation, varie de façon inverse avec les vitesses de refroidissement (R _T) et les vitesses de développement (R _c ) de concentrations de sursaturation de la substance en solution, selon la relation = 1/(k ₁ R _c , oùk ₁ et sont des constantes. On a évalué la température critique (¯T), la sursaturation critiques (¯S),k ₁ et.

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, . - (¯ t). ¯t . - ,¯t (R _T), (R _c) ¯t=1/(K₁ R _c ), ₁ — . (¯), (_-S), K₁ .

     

Influence of methanol as a buffer additive on the mobilities of organic cations in capillary electrophoresis

The mobilities of a series of aromatic ammonium ions, ranging in charge from +1 to + 3, were investigated by capillary electrophoresis using buffers consisting of 0-75% v/v methanol. This is an extension of our previous studies involving anion mobility in methanol-water media [1]. Absolute mobilities were determined by extrapolation of the effective mobilities to zero ionic strength according to the Pitts' equation. For all of the buffer compositions studied, the ionic strength effect increased with increasing cation charge, and varied as a function of solvent 1/eta epsilon (1/2) as predicted by the electrophoretic term within the Pitts' equation. In the presence of methanol, the ionic strength effects became more dramatic. The absolute mobilities of the cations were altered by the addition of methanol to the electrophoretic media. For example, at 75% MeOH, a migration order reversal was observed between the + 2 and + 3 ammonium ions. These solvent-induced selectivity changes are attributed to dielectric friction. As predicted by the Hubbard-Onsager dielectric friction model, dielectric friction increased with increasing methanol content and with increasing analyte charge. Further, the changes in cation mobility correlated to the changes in solvent relaxation time (tau), epsilon and eta. Although not predicted by the Hubbard-Onsager theory, the + 3 ammonium ion experienced more dielectric friction than the - 3 sulfonate and - 3 carboxylate investigated previously [1]. This apparent failure of the Hubbard-Onsager model results from its continuum nature, whereby ion-solvent interactions are not taken into account.     

Synthesis and characterization of copper(I) amidinates as precursors for atomic layer deposition (ALD) of copper metal

A series of copper(I) amidinates of the general type [(R'NC(R)NR')Cu](2) (R' and R' = n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl; R = methyl, n-butyl) have been synthesized and characterized. These compounds are planar dimers, bridged by nearly linear N-Cu-N bonds. Their properties (volatility, low melting point, high thermal stability, and self-limited surface reactivity) are well-suited for atomic layer deposition (ALD) of copper metal films that are pure, highly conductive, conformal, and strongly adherent to substrates.     

Synthesis and structural studies of peptides containing a glucose-derived furanoid sugar amino acid

Conformational analysis of peptides containing a glucose-derived furanoid sugar amino acid (Gaa) by detailed NMR and constrained MD studies revealed that peptides with repeating Gaa-Leu-Val units had conformational signatures very similar to those of linear homooligomers of Gaa.     

Nonclassical 5-substituted tetrahydroquinazolines as potential inhibitors of thymidylate synthase

Classical inhibitors of thymidylate synthase such as N^l0-propargyl-5,8-dideazafolic acid (1), N-(5-[N-(3,4-dihydro-2-methyl-4-oxoquinazolin-6-ylmethyl)-N-methylamino]-2-thenoyl)-L-glutamic acid (ZD1694, 2) and N-[2-amino-4-oxo-3,4-dihydro(pyrrolo[2,3-d]pyrintidin-5-yl)ethylbenzoyl]-L-glutamic acid (LY231514, 3) while potent, suffer from a number of potential disadvantages, such as impaired uptake due to an alteration of the active transport system required for their cellular uptake, as well as formation of long acting, non-effluxing polyglutamates via the action of folylpolyglutamate synthetase, which are responsible for toxicity. To overcome some of the disadvantages of classical inhibitors, there has been considerable interest in the synthesis and evaluation of nonclassical thymidylate synthase inhibitors, which could enter cells via passive diffusion. In an attempt to elucidate the role of saturation of the B-ring of non-classical, quinazoline antifolate inhibitors of thymidylate synthase, analogues 7-17 were designed. Analogues 13-17 which contain a methyl group at the 7-position, were synthesized in an attempt to align the methyl group in an orientation which allows interaction with tryptophan-80 in the active site of thymidylate synthase. The synthesis of these analogues was achieved via the reaction of guanidine with the appropriately substituted cyclohexanone-ketoester. These ketoesters were in turn synthesized via a Michael addition of the appropriate thiophenol with 2-carbethoxycyclohexen-1-one or 5-methyl-2-carbethoxycyclo-hexen-1-one to afford a mixture of diastereomers. The most inhibitory compound was the 3,4-dichloro, 7-methyl derivative 17 which inhibited the Escherichia coli and Pneumocystis carinii thymidylate syntheses 50% at 5 × 10⁵ M. Our results confirm the importance of the 7-CH₃ group and electron withdrawing groups on the aromatic side chain for thymidylate synthase inhibition.