Application Of Potential Step Research Articles

Electrochemical analysis of a phospholipid phase transition

Abstract Phospholipid monolayers in the presence of electric fields show three phase transitions characterised by capacitance peaks at potentials ∼−0.94 V, ∼−1.0 V and ∼−1.25 V versus Ag/AgCl. This paper focuses on studying the phase transition characterised by the capacitance peak at potential ∼−1.0 V versus Ag/AgCl. After the application of potential steps in the negative potential direction, the current transients are characteristic of a nucleation and growth process. After the application of potential steps in the positive potential direction the transition is faster and not characteristic of a nucleation and growth process. When the change in charge associated with the phase transition was calculated from potential pulses in either direction it increases through the same potential window of ∼0.015 V. Impedance data plotted in the complex capacitance plane shows a significant extra capacitative element within the potential window characterising the phase transition. For applied ΔE of 0.002 V, the extra element fits a Debye relaxation process but for higher ΔE of 0.005 V the fit is not so good and the relaxation time constant is shorter. The data is consistent with a model whereby a structured electrolyte-phospholipid emulsion breaks up into two phases: electrolyte and phospholipid at more negative potentials. The two phases remain in equilibrium with each other throughout the potential window characterising the transition. An explanation for the Debye relaxation in the impedance data is that it represents the diffusion of phospholipids and increased adsorption of phospholipid head groups coincident with the increasing radius of phospholipid domains.

Protein conformational changes in tetraheme cytochromes detected by FTIR spectroelectrochemistry: Desulfovibrio desulfuricans Norway 4 and Desulfovibrio gigas cytochromes c3

The conformational change coupled to the redox processes of two tetraheme cytochromes c3 from bacteria of the genus Desulfovibrio have been studied by UV-vis and FTIR difference spectroscopy combined with protein electrochemistry. Two pairs of equivalent hemes were found in Desulfovibrio desulfuricans Norway 4 cytochrome c3 by UV-vis spectroelectrochemical redox titration in an optically transparent thin-layer electrochemical cell. In contrast to this, Desulfovibrio gigas cytochrome c3 showed a UV-vis difference spectrum for the highest potential heme different from that of the others. The redox titrations were monitored by FTIR difference spectroscopy using the same spectroelectrochemical cell. They show that in both cytochromes the overall redox process from the fully oxidized (III4) to the fully reduced oxidation state (II4), III4<==>II4, proceeds via an intermediate oxidation stage (III2II2) which is formed after the second electron uptake. The small amplitude of the difference signals in the reduced-minus-oxidized FTIR difference spectra obtained for the overall redox process in both Desulfovibrio cytochromes indicates a very small conformational change induced by the redox transition. Nevertheless, by application of potential steps from the fully oxidized or reduced form to the midwave potential (as obtained from the UV-vis redox titrations), the reduced-minus-oxidized IR difference spectra corresponding to the intermediate redox transitions (III4<==>III2II2 and III2II2<==>II4) were obtained, reflecting separately the contributions of the high- and low-potential heme pairs to the overall redox-induced conformational change. The overall redox process and both intermediate redox transitions were fully reversible. In the spectral region between 1500 and 1200 cm-1 the IR difference spectra of both cytochromes show several signals previously observed in the reduced-minus-oxidized IR difference spectra of spinach cytochrome b559 and iron-protoporphyrin IX-bis(imidazole) model compounds [Berthomieu, C., Boussac, A., Mäntele, W., Breton, J., & Nabedryk, E. (1992) Biochemistry 31, 11460-11471]. Moreover, Raman spectra of Desulfovibrio vulgaris cytochrome c3 and cytochrome b5 show signals attributed to Raman active heme skeletal modes at nearly the same positions [Kitagawa, T., Kyogoyu, Y., Izuka, T., Ikeda-Saito, M., & Yamanaka, T. (1975) J. Biochem. 78, 719-728], thus allowing their assignment to signals arising from heme vibrational modes. Comparatively strong IR difference signals at 1618 cm-1, which are tentatively assigned to phenylalanine residues, were found in D. desulfuricans cytochrome c3. In the spectra of D. gigas cytochrome c3, IR signals at 1614 cm-1 were detected only for the first redox transition (III4<==>III2II2).(ABSTRACT TRUNCATED AT 400 WORDS)

Nucleation of Silver (I) Oxide Investigated by Spectroscopic Ellipsometry

The early stages of the anodic oxidation of Ag(111) in during the application of potential steps, current steps, and potential sweeps have been investigated by spectroscopic ellipsometry. Film thicknesses and surface coverages have been derived from the measurements by use of a dual‐layer optical model. The results are supported by SEM observations on selected specimens. Oxide crystals are found to nucleate from a previously formed homogeneous oxide layer. A decrease in number density of crystals of several orders of magnitude during film growth has been found and is attributed to the growth of larger crystals at the expense of smaller ones.

Determination of Charge Passed Following Application of Potential Step in Study of Electrode Processes.

Determination of Charge Passed Following Application of Potential Step in Study of Electrode Processes.