Structural requirements in chiral diphosphine-rhodium complexes. X asymmetric homogeneous hydrogenation of z-N-acetyldehydroamino acids and esters with (1R,2R)-trans-1,2-bis(diphenylphosphinomethyl)cyclobutane/rhodium(I) complexes.

Z--acetamidocinnamate esters were hydrogenated with neutral rhodium(I) complexes containing (1R,2R)-trans-1,2-bis(diphenylphosphinomethyl)cyclobutane. Increasing the steric bulk of the alcohol moiety in the unsaturated esters had little influence upon the optical purity of the N-acetylphanylalanine ester products. In the series Me, Et, i-Pr, and t-Bu the optical purity decreased from 44 % ee-(R) [Me] to 40 % ee-(R) [t-Bu]. The chiral cyclobutane diphosphine appears to be only slightly more effective than the heterocyclic DIOP when ring-substituted Z--acetamidocinnamic acids are hydrogenated with neutral rhodium(I) complexes without the addition of triethylamine. Addition of triethylamine to the solvent blend seems to be more beneficial to the cyclobutane analogue than to DIOP.     

Evaluation of 3-hydroxy-3-methylglutaryl-COA-reductase, cholesterol-7alpha-hydroxylase and acyl-COA:cholesterol acyltransferase activities: alternative chromatographic methods to separate metabolites

Alternative HPLC and solid-phase extraction column methods were developed to separate metabolites of enzymes involved in cholesterol metabolism in rabbit liver microsomes: hydroxyl-methylglutaryl-CoA reductase, cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase. A comparison method of thin-layer chromatography and solid-phase extraction column were assayed to separate substrate and metabolite of hydroxy-methylglutaryl-CoA reductase, whereas for cholesterol-7alpha-hydroxylase and acyl-CoA:cholesterol acyltransferase, this comparison was done between thin layer chromatography and HPLC. The results obtained by the new analytical chromatographic methods are not significantly different than those observed in literature. Moreover a larger percentage recovery was obtained for analysed metabolites. Our results demonstrate the reliability of these alternative chromatographic techniques and showed that they are valuable tools to precisely and rapidly measure the activity of those enzymes.     

Spectroscopic and electrochemical study of the adsorption of [Co(en)2Cl2]Cl on gamma-alumina: influence of the alumina ligand on Co(III)/(II) redox potential

UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co(III)(en)2Cl2]Cl (en=ethylenediamine) complexes and the gamma-alumina-supported cis-Co((III)) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on gamma-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the gamma-alumina-supported cis-Co(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively.     

Infrared spectra of protonated uracil, thymine and cytosine.

The gas-phase structures of protonated uracil, thymine, and cytosine are probed by using mid-infrared multiple-photon dissociation (IRMPD) spectroscopy performed at the Free Electron Laser facility of the Centre Laser Infrarouge d'Orsay (CLIO), France. Experimental infrared (IR) spectra are recorded for ions that were generated by electrospray ionization, isolated, and then irradiated in a quadrupole ion trap; the results are compared to the calculated infrared absorption spectra of the different low-lying isomers (computed at the B3LYP/6-31++G(d,p) level). For each protonated base, the global energy minimum corresponds to an enolic tautomer, whose infrared absorption spectrum matched very well with the experimental IRMPD spectrum, with the exception of a very weak IRMPD signal observed at about 1800 cm(-1) in the case of the three protonated bases. This signal is likely to be the signature of the second-energy-lying oxo tautomer. We thus conclude that within our experimental conditions, two tautomeric ions are formed which coexist in the quadrupole ion trap.     

The gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene--energetics,structure and interconversion of dihydrotropylium ions

The hitherto unknown gas-phase basicity and proton affinity of 1,3,5-cycloheptatriene (CHT) have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Several independent techniques were used in order to exclude ambiguities due to proton-induced isomerisation of the conjugate cyclic C(7)H(9)(+) ions, [CHT + H](+). The gas-phase basicity obtained by the thermokinetic method, GB(CHT) = 799 +/- 4 kJ mol(-1), was found to be identical, within the limits of experimental error, with the values measured by the equilibrium method starting with protonated reference bases, and with the values resulting from the measurements of the individual forward and reverse rate constants, when corrections were made for the isomerised fraction of the C(7)H(9)(+) population. The experimentally determined gas-phase basicity leads to the proton affinity of cycloheptatriene, PA(CHT) = 833 +/- 4 kJ mol(-1), and the heat of formation of the cyclo-C(7)H(9)(+) ion, deltaH(f)(0)([CHT + H](+)) = 884 +/- 4 kJ mol(-1). Ab initio calculations are in agreement with these experimental values if the 1,2-dihydrotropylium tautomer, [CHT + H((1))](+), generated by protonation of CHT at C-1, is assumed to be the conjugate acid, resulting in PA(CHT) = 825 +/- 2 kJ mol(-1) and deltaH(f)(0)(300)([CHT + H((1))](+)) = 892 +/- 2 kJ mol(-1). However, the calculations indicate that protonation of cycloheptatriene at C-2 gives rise to transannular C-C bond formation, generating protonated norcaradiene [NCD + H](+), a valence tautomer being 19 kJ mol(-1) more stable than [CHT + H((1))](+). The 1,4-dihydrotropylium ion, [CHT + H((3))](+), generated by protonation of CHT at C-3, is 17 kJ mol(-1) less stable than [CHT + H((1))](+). The bicyclic isomer [NCD + H](+) is separated by relatively high barriers, 70 and 66 kJ mol(-1) from the monocyclic isomers, [CHT + H((1))](+) and [CHT + H((3))](+), respectively. Therefore, the initially formed 1,2-dihydrotropylium ion [CHT + H((1))](+) does not rearrange to the bicyclic isomer [NCD + H](+) under mild protonation conditions.     

Ultraviolet Radiation Effects on a Microscopic Green Alga and the Protective Effects of Natural Dissolved Organic Matter

Abstract— The population and photosynthetic responses of a microscopic green alga ( Selenastrum capricornutum ) to realistic levels of UV radiation (UVA and UVB) were assessed in natural lake waters of different dissolved organic carbon (DOC) concentration. Specific growth rates and photosynthetic competence (as reflected by F_v/F_m [measure of maximal quantum efficiency of photosystem II] and t_1/2 [estimate of electrons transported to the plastoquinone pool] measured by in vivo variable chlorophyll a fluorescence) were compared between two exposure levels of UVR and two concentrations of DOC (2.5 mg C L⁻¹ 7.7 mg C L⁻¹). Exposure periods of 6–9 days (five to nine generations) were used. Exposure to UVA primarily affected the efficiency of photosystem II, as evidenced by significant decreases of F_v/F_m but not growth rates or t_1/2 Exposure to UVB, in the presence of UVA, did not cause significant additional decreases of F_v/F_m but did diminish growth rates. In the low DOC water, t_1/2 was also diminished, suggesting different proximate sites of action from those for UVA. The high DOC water decreased the effective exposure to both UVA and UVB and diminished the negative impact of UV radiation on the cells, but the apparent protection was not explicable solely by the shading action of the DOC. Control values for F_v/F_m, growth rates and t_1/2 were all lower in the high DOC water, suggesting a negative side effect to the apparent protective action of the DOC against UVB.     

Reaction of functionalised thiols with oligoisobutenes via free-radical addition.: Some new routes to thermoplastic crosslinkable polymers

The functionalisation of vinylidene (isopropenyl) terminated oligoisobutenes (polyisobutenes; PIB) with several thiols has been realised in the presence of free-radical generators. Oxygen (air) is a significant accelerator for this reaction and in some cases, alone, is sufficient to cause the reaction to occur. Free-radicals were generated from peroxydicarbonates, AIBN or UV irradiation. The reaction is of an anti-Markovnikov type with the RS-function adding to the vinylidene CH₂ group selectively in the presence of more substituted olefin groups in commercial PIB. A variety of thiols has been investigated and the synthesis of elastomeric polymers resulting from the hydrolysis and condensation of a trialkoxysilane coupled to PIB via a sulphide link has been demonstrated. The synthesis of comb-like polymers made by the addition of PIB to poly(mercaptopropyl methylsiloxane) has been achieved in a way to leave some non-reacted thiol functions available for crosslinking. A perfluoroalkyl thiol modified PIB shows a significant reduction in surface tension compared to the starting PIB polymer.     

Cold storage and cryopreservation of hops (Humulus l.) shoot cultures through application of standard protocols

The USDA-ARS National Clonal Germplasm Repository (NCGR) stores the global diversity of Humulus for the US Plant Germplasm System as trellised plants in a field genebank. In vitro storage and cryopreservation are now considered excellent ways to provide medium and long-term storage for plant collections. Developing a new cryopreservation or cold storage protocol for every accession or genus of large multi-crop collections can be a very time consuming and long-term activity. We propose that standard cold storage and cryopreservation techniques used for other temperate crop genera would be successful for additional crops with few modifications. This study was initiated to determine if a large collection of hops germplasm could be successfully stored with techniques developed for unrelated genera. In this study we characterized the response of diverse Humulus genotypes to in vitro storage under low light at 4 degree C following techniques used for strawberry and mint plants, and cryopreservation in liquid nitrogen by slow cooling with a pear protocol. The average storage time without transfer for the 70 genotypes evaluated was 14 +/- 3.5 months with a range of 6 to 26 months. Mean recovery of cryopreserved shoot tips of accessions with 1-wk cold acclimation was 41 +/- 18 percent and increased to 54 +/- 13 percent with 2-wk cold acclimation. This demonstrates that application of a well-tested standard technique can provide a quick start for storing additional germplasm collections.