To determine whether selenolates are viable alternatives to thiolates for self-assembled monolayers (SAMs), the formation and oxidative stability of monolayers made from diphenyl diselenide (DPDSe) solution were assessed by surface-enhanced Raman spectroscopy. Upon adsorption, the diselenide bond is cleaved to form benzeneselenolate, analogous to formation of benzenethiolate monolayers from diphenyl disulfide (DPDS). DPDSe displaces benzenethiolate from gold, but DPDS does not displace benzeneselenolate. Competitive adsorption experiments show that adsorption of DPDSe is more favorable by ∼0.7 kcal/mol. Unlike benzenethiolate, the benzeneselenolate monolayer is unstable both in air and to UV light. Long-term exposure to air results in oxidation to protonated and deprotonated benzeneseleninic acid. Exposure to UV results in C−Se bond cleavage (analogous to C−S bond cleavage in benzenethiolate) and formation of SeO2 and SeO3 2-. The higher adsorptivity of benzeneselenolate and its similar oxidative behavior to benzenethiolate suggests that selenolates are an attractive alternative to thiolates for building SAMs.

Ubiquinone functions in the mitochondrial electron transport chain. Recent evidence suggests that the reduced form of ubiquinone (ubiquinol) may also function as a lipid soluble antioxidant. The biosynthesis of ubiquinone requires two O-methylation steps. In eukaryotes, the first O-methylation step is carried out by the Coq3 polypeptide, which catalyzes the transfer of a methyl group from S-adenosylmethionine to 3,4-dihydroxy-5-polyprenylbenzoate. In Escherichia coli, 2-polyprenyl-6-hydroxyphenol is the predicted substrate; however, the corresponding O-methyltransferase has not been identified. The second O-methylation step in E. coli, the conversion of demethylubiquinone to ubiquinone, is carried out by the UbiG methyltransferase, which is 40% identical in amino acid sequence with the yeast Coq3 methyltransferase. On the basis of the chemical similarity of the first and last methyl-acceptor substrates and the high degree of amino acid sequence identity between Coq3p and UbiG, the ability of UbiG to catalyze both O-methylation steps was investigated. The current study shows that the ubiG gene is able to restore respiration in the yeast coq3 mutant, provided ubiG is modified to contain a mitochondrial leader sequence. The mitochondrial targeting of O-methyltransferase activity is an essential feature of the ability to restore respiration and hence ubiquinone biosynthesis in vivo. In vitro import assays show the mitochondrial leader sequence present on Coq3p functions to direct mitochondrial import of Coq3p in vitro and that processing to the mature form requires a membrane potential. In vitro methyltransferase assays with E. coli cell lysates and synthetically prepared farnesylated-substrate analogs indicate that UbiG methylates both the derivative of the eukaryotic intermediate, 3,4-dihydroxy-5-farnesylbenzoate, as well as that of the E. coli intermediate, 2-farnesyl-6-hydroxyphenol. The data presented indicate that the yeast Coq3 polypeptide is located in the mitochondria and that E. coli UbiG catalyzes both O-methylation steps in E. coli.

The formation and stability of self-assembled monolayers (SAMs) of aryl sulfmates and, for the first time, aryl sulfonates are described. The ways in which the molecules interact with the surface and the stability of the resulting SAMs were characterized by surface enhanced Raman (SER) spectroscopy. Aryl sulfinate monolayers can be reversibly oxidized to sulfonate monolayers, but the sulfonate is readily displaced by sulfinate in solution.The relative adsorptivities of aryl sulfur species were found to be ArSC>3_ < < ArSC>2-< ArS-. Through a novel application of perturbation theory, in which the adsorbate-surface Coulombic and charge transfer interactions and the change in the solvation free energy of the adsorbate are taken into account, we have been able to explain this trend. The differences in the adsorptivities of the anions studied here are primarily attributable to differences in the adsorbate-surface charge transfer interactions. These were evaluated by calculating the adsorbate HOMO energies at the ab initio HF/3-21G(d) level. Higher adsorbate HOMO energies are correlated with higher adsorptivities, consistent with established trends in the adsorptivities of soft, basic anions on metal electrodes. Statistical perturbation theory was used to calculate the relative free energies of hydration of the three anions. The sulfonate is most strongly solvated, followed by the sulfinate and thiolate. Thus, the trend in solvation energies is consistent with and probably reinforces the trend in the adsorbate-surface charge transfer interactions. The combination of computational methods used here may prove generally useful for predicting the relative adsorptivities of molecules and ions on metal surfaces.

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Ubiquinone (coenzyme Q or Q) is a lipid that functions in the electron transport chain in the inner mitochondrial membrane of eukaryotes and the plasma membrane of prokaryotes. Q-deficient mutants of Saccharomyces cerevisiae harbor defects in one of eight COQ genes (coq1-coq8) and are unable to grow on nonfermentable carbon sources. The biosynthesis of Q involves two separate O-methylation steps. In yeast, the first Omethylation utilizes 3,4-dihydroxy-5-hexaprenylbenzoic acid as a substrate and is thought to be catalyzed by Coq3p, a 32.7-kDa protein that is 40% identical to the Escherichia coli O-methyltransferase, UbiG. In this study, farnesylated analogs corresponding to the second O-methylation step, demethyl-Q 3 and Q 3 , have been chemically synthesized and used to study Q biosynthesis in yeast mitochondria in vitro. Both yeast and rat Coq3p recognize the demethyl-Q 3 precursor as a substrate. In addition, E. coli UbiGp was purified and found to catalyze both O-methylation steps. Futhermore, antibodies to yeast Coq3p were used to determine that the Coq3 polypeptide is peripherally associated with the matrixside of the inner membrane of yeast mitochondria. The results indicate that one O-methyltransferase catalyzes both steps in Q biosynthesis in eukaryotes and prokaryotes and that Q biosynthesis is carried out within the matrix compartment of yeast mitochondria.Ubiquinone is an essential lipid in the electron transport chain that is found in the inner mitochondrial membranes of eukaryotes and in the plasma membrane of prokaryotes (1). The structure of Q 1 consists of a quinone head group and a hydrophobic isoprenoid tail that can vary in length depending on the species in which it is found. The quinone group undergoes reversible single electron transfers, interchanging between the quinone, semiquinone, and hydroquinone, whereas the isoprenoid tail functions to anchor Q in the membrane. In eukaryotes, Q functions to shuttle electrons from either Complex I or Complex II to Complex III/bc 1 complex. The transfer of electrons from Q to the bc 1 complex is coupled to proton-translocation via the Q cycle mechanism that was first proposed by Mitchell (2). A number of studies support such a mechanism (for a review, see Ref. 1) including the recently determined complete structure of the bc 1 complex (3).The redox properties of Q also allow it to function as a lipid soluble antioxidant. Q functions by either directly scavenging lipid peroxyl radicals (4) or indirectly reducing ␣-tocopherol radicals to regenerate ␣-tocopherol (5, 6). Additionally, Q protects cells from oxidative damage generated by the autoxidation of polyunsaturated fatty acids (7). Q is found in many eukaryotic intracellular membranes, including the plasma membrane, where, in conjunction with a plasma membrane electron transport system, it functions to scavenge ascorbate free radicals (8, 9). In the plasma membrane of prokaryotes, Q participates in the maintenance of the enzymatic activity of DsbA/DsbB disulfide bond forming proteins (10), and Q-def...