David Johnathan Kitko

Oxidative Reactivity Difference among the Metal Oxo and Metal Hydroxo Moieties: pH Dependent Hydrogen Abstraction by a Manganese(IV) Complex Having Two Hydroxide Ligands

Clarifying the difference in redox reactivity between the metal oxo and metal hydroxo moieties for the same redox active metal ion in identical structures and oxidation states, that is, M(n+)O and M(n+)-OH, contributes to the understanding of natures choice between them (M(n+)O or M(n+)-OH) as key active intermediates in redox enzymes and electron transfer enzymes, and provides a basis for the design of synthetic oxidation catalysts. The newly synthesized manganese(IV) complex having two hydroxide ligands, [Mn(Me(2)EBC)(2)(OH)(2)](PF(6))(2), serves as the prototypic example to address this issue, by investigating the difference in the hydrogen abstracting abilities of the Mn(IV)O and Mn(IV)-OH functional groups. Independent thermodynamic evaluations of the O-H bond dissociation energies (BDE(OH)) for the corresponding reduction products, Mn(III)-OH and Mn(III)-OH(2), reveal very similar oxidizing power for Mn(IV)O and Mn(IV)-OH (83 vs 84.3 kcal/mol). Experimental tests showed that hydrogen abstraction proceeds at reasonable rates for substrates having BDE(CH) values less than 82 kcal/mol. That is, no detectable reaction occurred with diphenyl methane (BDE(CH) = 82 kcal/mol) for both manganese(IV) species. However, kinetic measurements for hydrogen abstraction showed that at pH 13.4, the dominant species Mn(Me(2)EBC)(2)(O)(2), having only Mn(IV)O groups, reacts more than 40 times faster than the Mn(IV)-OH unit in Mn(Me(2)EBC)(2)(OH)(2)(2+), the dominant reactant at pH 4.0. The activation parameters for hydrogen abstraction from 9,10-dihydroanthracene were determined for both manganese(IV) moieties: over the temperature range 288-318 K for Mn(IV)(OH)(2)(2+), DeltaH(double dagger) = 13.1 +/- 0.7 kcal/mol, and DeltaS(double dagger) = -35.0 +/- 2.2 cal K(-1) mol(-1); and the temperature range 288-308 K for for Mn(IV)(O)(2), DeltaH(double dagger) = 12.1 +/- 1.8 kcal/mol, and DeltaS(double dagger) = -30.3 +/- 5.9 cal K(-1) mol(-1).

Isolation of corneocyte envelopes from porcine epidermis

SummarySheets of porcine stratum corneum were dispersed into individual corneocytes after 4 h in a solution consisting of 8 mM N,N-dimethyldodecylamine oxide and 2 mM sodium dodecylsulfate in phosphate-buffered isotonic saline, at 45°C. With continued detergent treatment and moderate sonication, most of the cells lost their keratin contents and were then separated from the remaining intact cells by centrifugation in cesium chloride solution of density 1.280. Electron microscopy showed that the cell envelopes retained both the cross-linked protein envelope and its attached lipid envelope. The dry weight of envelopes was approximately 7% of the estimated dry weight of the original stratum corneum, while the corneocytes surviving intact also amounted to 7% of the starting weight. Mild alkaline hydrolysis of the corneocyte envelopes allowed the extraction of hydroxyceramides amounting to 10% of the dry weight of the envelopes. The procedure therefore provides isolated corneocyte envelopes suitable for studying both the protein and lipid components of this compound sheath.

Manganese complexes with a lengthy o -xylylene cross-bridged cyclam ligand: synthesis, characterization and catalytic hydrogen abstraction by dioxygen activation

Two ultra rigid, o-xylylene cross-bridged macrobicyclic ligands, 1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (H2XBC), and 13,19-dimethyl-1,10,13,19-tetraazatricyclo[8.6.6.03,8]docosa-3,5,7-triene (Me2XBC), have been synthesized and the manganese complexes have been synthesized and characterized, including an X-ray structure determination. Mn(Me2XBC)Cl2 displays a relatively high redox potential for the Mn2+/Mn3+ couple (+0.947V vs SHE, measured in CH3CN), suggesting that the manganese(III) complex may be capable of hydrogen abstraction from moderately active substrates. Direct reaction of the freshly synthesized manganese(III) complex, [Mn(Me2XBC)Cl2]PF6, with 1,4-cyclohexadiene confirmed its hydrogen abstracting ability. The manganese(II)/Me2XBC complex is activated by dioxygen in buffered basic aqueous solutions and catalyzes hydrogen abstraction from selected substrates. A possible mechanism for this manganese complex catalyzed dioxygen activation and hydrogen abstraction is proposed.