Katsumi Niki

Reductive desorption of carboxylic-acid-terminated alkanethiol monolayers from Au(111) surfaces

Abstract The peak potential E p of the reductive desorption for COOH-terminated alkanethiols at the gold(111)|0.1 M KOH solution interface has been studied at several different alkyl chain lengths and compared with those for n -alkanethiols. The desorption of COOH-terminated alkanethiols is also confirmed by the increase in the capacitance of gold|solution interface and the reflectivity of gold electrode surface. The E p for both CH 3 - and COOH-terminated alkanethiols shift by 15 mV to the negative direction per methylene group as the alkyl chain length n increases. The E p vs. n plots for the two series of alkanethiols are parallel with each other, suggesting that the chemical interaction of thiol head group with gold, the chain-chain attractive interaction, and the electrostatic repulsion between the carboxylate groups make an additive contribution to the determination of E p . The E p values for COOH-terminated alkanethiols are 110 mV more positive than those for n -alkanethiols having the same alkyl chains, owing in part to the repulsive interaction between the negatively-charged carboxylate groups in a COOH-terminated alkanethiol monolayer. The positive shift of E p for mercaptopropionic acid with decreasing concentration of KOH solution also suggests the primary role of the electrostatic repulsive interaction between carboxylate groups in the shift of E p .

Phase separation of binary self-assembled thiol monolayers composed of 1-hexadecanethiol and 3-mercaptopropionic acid on Au(111) studied by scanning tunneling microscopy and cyclic voltammetry

Abstract The mixing characteristics of binary self-assembled monolayers composed of 1-hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) on Au(111) have been studied by cyclic voltammetry and scanning tunneling microscopy. Two distinctive peaks, ∼0.45V apart, are observed over the entire range of surface composition on cyclic voltammograms for the reductive desorption of the adsorbed thiol molecules, which reflects the presence of two different types of phase-separated domains greater than several tens of nm 2 which can be imaged by scanning tunneling microscopy. The peak potential of 1-hexadecanethiol is nearly independent of the surface composition whereas a slight shift of the peak potential is observed in the case of MPA, suggesting that the HDT is slightly soluble in the MPA domains, while MPA is insoluble in the HDT domains. The minimum number of the adsorbed thiol molecules required for exhibiting two distinctive peaks (i.e. two-dimensional bulk properties) is estimated to be ca. 50 by comparing the cyclic voltammograms with the distribution of the domain size observed by scanning tunneling microscopy.

Bioelectrocatalysis at electrodes coated with alcohol dehydrogenase, a quinohemoprotein with heme c serving as a built-in mediator

Alcohol dehydrogenase (ADH), a bacterial membrane-bound protein containing pyrroloquinoline quinone (PQQ) and heme c was held by adsorption on electrodes of gold, silver, glassy carbon, or pyrolytic graphite. All the electrodes with adsorbed ADH produced anodic currents which oxidized ethanol, in which the adsorbed ADH catalysed the electrolysis of ethanol. The electrocatalysis behavior could be described by a theoretical equation for bioelectrocatalysis at an enzyme-coated electrode, and was characterized by two quantities, the Michaelis constant Km, and maximum current density Imax/A. Using electroreflectance measurements with an ADH-coated gold electrode it was revealed that electron transfer occurred between heme c of the adsorbed ADH and the electrode. On the basis of these results, the reaction mechanism of the bioelectrocatalysis is discussed and oriented adsorption of ADH is proposed with the heme c moiety being in close contact with the electrode and with the PQQ moiety, the site reacting with the substrate, facing toward the solution.

Long-range electron-transfer reaction rates to cytochromec across long- and short-chain alkanethiol self-assembledmonolayers: Electroreflectancestudies

The kinetics of electron transfer (ET) between cytochrome nc and a gold (111) electrode through self-assembled nmonolayers of alkanethiols with terminal carboxylic acid groups, nCOOH(CH n 2 n) n n nSH, have been studied for nn=2–11 using an ac potential-modulated UV–VIS nreflectance spectroscopic technique (electroreflectance spectroscopy, nER). For 9⩽n⩽11, the standard ET rate constant, nk n app n, depends exponentially on the chain lengths nand the exponential decay factor is 1.09 per methylene group; for nn<9, however, k n app n deviates from the nexponential plot. The ET reaction through short-chain alkanethiol nmonolayers is controlled by the preceding chemical reaction. The nrate-controlling step is very likely to be the reorganization of ncytochrome c to the favourable conformation for the ET nreaction. The ET reaction rate constant from cytochrome c in nthe favourable conformation to the electrode surface obeys Marcus ntheory for long-range ET. The ET reaction through long-chain nalkanethiol monolayers is controlled by the ET rate through nalkanethiols.

Investigation of the electrode reaction of cytochrome c through mixed self-assembled monolayers of alkanethiols on gold(111) surfaces☆

Horse heart cytochrome c was immobilized on mixed self-assembled monolayers (SAMs) of carboxyl- and methyl-terminated alkanethiolates (HOOC(CH2)10S + CH3(CH2)9S) on Au(111) electrodes. The apparent standard rate constant kobs of electron transfer for the electrode system of cytochrome c⧹mixed SAM⧹gold electrode was measured by a.c. impedance and a.c. modulated UV-visible electro reflectance techniques. A strong dependence of kobs on the composition of the mixed alkanethiol solution, from which the monolayers had been assembled, was observed. A maximum of kobs ≈ 200s−1 was found for SAMs formed from solutions containing a mole fraction χ ≈ 0.8 of carboxyl-terminated alkanethiols. This rate constant is about six times faster than that of a system with a single-component monolayer of HOOC(CH2)10S (ξ = 1). Reductive desorption suggested that the two alkane thiolates mix homogeneously in the monolayer and there is no sizable phase separation on the Au(111) electrode. Double-layer capacitance measurements determined the degree of protonation of the mixed SAMs in the pH range 6–9. The enhancement of electron-transfer rate is attributed to a favorable distribution of protein binding sites in the mixed monolayers.

Electroreflectance spectroscopic study of the electron transfer rate of cytochrome c electrostatically immobilized on the ω-carboxyl alkanethiol monolayer modified gold electrode

Abstract Horse heart cytochrome c was immobilized electrostatically on self-assembled monolayers (SAMs) of carboxylic-acid-terminated alkanethiols, HS(CH 2 ) n COOH ( n = 2, 10) on gold electrodes, The rate constants of the electrode reaction of cytochrome c at the SAM-modified gold electrodes were determined by the frequency dependence of a.c. modulated UV-visible electroreflectance signals. The apparent standard electrode reaction rate constants of cytochrome c were 880 s −1 and 72 s −1 at the HS(CH 2 ) 2 COOH and HS(CH 2 ) 10 COOH SAM modified gold electrodes respectively. The rate constant at HS(CH 2 ) 2 COOH was much smaller than expected from Marcus theory. However, the rate constant at HS(CH 2 ) 10 COOH could be explained in terms of the through-bond tunnelling mechanism with tunnelling parameter β = 8.2 nm −1 .

Reversible voltammetric response for a molecule containing four non-equivalent redox sites with application to cytochrome c3 of Desulfovibrio vulgaris, strain Miyazaki

Abstract The cyclic voltammetric and differential pulse polarographic behavior of cytochrome c 3 of Desulfovibrio vulgaris , strain Miyazaki, has been evaluated in terms of a model employing four reversible redox centers. Both types of experiments can be fit by digital simulations using the four standard potentials: E 1 0 =−0.467, E 2 0 =−0.519, E 3 0 =−0.539 and E 4 0 =−0.580 V vs. SCE. The results are interpreted to mean that the four redox centers are chemically different and only weakly interacting. The relationships between the observed macroscopic standard potentials and the microscopic standard potentials for reduction of individual sites are discussed.

Polarographic Determination of Subnanogram Quantities of Free Platinum in Reaction Mixture with Dna

Abstract A simple and rapid differential pulse polarographic assay for measurement of platinum drug binding to DNA is described. The method makes it possible to determine the free (unbound) platinum compound in the presence of DNA or platinum - DNA complex. The method is based on the polarographic catalytic hydrogen current yielded by platinum (II) or (IV) complexes in formaldehyde - hydrazine - sulphuric acid background electrolyte, in which DNA or platinum - DNA complex precipitate. The lower level of analytical utility of this method is c. 1 × 10−9 M.

Raman scattering from nucleic acids adsorbed at a silver electrode

Adsorption of nucleic acids at a silver electrode polarized to -0.6 to -0.1 V (vs. Ag/AgCl) was investigated by means of surface enhanced Raman scattering (SERS) spectroscopy. Single-stranded polyriboadenylic acid and thermally denaturated DNA adsorbed at the silver electrode yield two intense bands at 734 and 1335 cm-1 on the SERS spectra. These bands, assigned to the vibrations of adenine residue rings, were much less intense if the SERS spectra were recorded for double-helical complex polyadenylic X polyuridylic acid and native DNA. Moreover, the courses of alkaline denaturation of DNA and its digestion by deoxyribonuclease I were observed by SERS spectroscopy. The results were interpreted as support for the view that intact double-helical segments of nucleic acids are not denatured or destabilized due to their adsorption at the positively charged and roughened surface.

Regulation of the redox order of four hemes by pH in cytochrome C3 from D. vulgaris Miyazaki F

The assignment of 1H-NMR signals of the heme methyl and propionate groups of cytochrome c3 of D. vulgaris Miyazaki F was performed. The heme assignment was revised for hemes 2 and 3 (sequential heme numbering). Namely, heme 4 is mainly reduced at first with hemes 1, 2 and 3 following it in this order. The p2H titration of heme methyl signals in four macroscopic oxidation states was performed in the p2H range of 5.2 to 9.0. While the heme methyl resonances in the fully oxidized state showed just small changes with p2H, most resonances in the intermediate oxidation states revealed clear p2H dependence. In particular, the methyl resonances of heme 1 shifted significantly in the acidic region. Then, the chemical shifts of beta-CH2 (next to the carboxyl group) of all propionate groups in the fully oxidized state were observed at various p2H in the range of 4.5 to 9.0. Only the propionate group at C-13 (IUPAC-IUB nomenclature) of heme 1 showed a clear change in this p2H range, its titration curve being similar to those of the methyl resonances of heme 1 in the intermediate oxidation states. pKa of the propionate group was 5.95 +/- 0.05. Analysis of the microscopic formal redox potentials was carried out for the observations at p2H 5.2, 7.1 and 9.0. The redox potentials of heme 1 showed the most remarkable p2H dependence, resulting in the change of the order of the redox potentials of four hemes. A significant change was also found in the interacting potential between hemes 1 and 2. In the light of the p2H-titration experiments, the propionate at C-13 of heme 1 was identified as the most plausible ionizable group responsible for the p2H dependence of microscopic redox potentials of heme 1 in the acidic region.