期刊
BIOELECTROCHEMISTRY
卷 55, 期 1-2, 页码 139-143出版社
ELSEVIER SCIENCE SA
DOI: 10.1016/S1567-5394(01)00154-2
关键词
cytochrome c; electron transfer; electrode; self-assembled monolayer; electric field; surface enhanced resonance Raman spectroscopy
Cytochrome c (Cyt-c) was electrostatically immobilised on Ag electrodes coated with self-assembled monolayers (SAM) that are formed by omega-carboxyl alkanethiols with different alkyl chain lengths (C-x). Surface enhanced resonance Raman (SERR) spectroscopy demonstrated that electrostatic binding does not lead to conformational changes of the heme protein under the conditions of the present experiments. Employing time-resolved SERR spectroscopy, the rate constants of the heterogeneous electron transfer (ET) between the adsorbed Cyt-c and the Ag electrode were determined for a driving force of zero electronvolts. For SAMs with long alkyl chains (C-16, C-11), the rate constants display a normal exponential distance dependence, whereas for shorter chain lengths (C-6, C-3, C-3) the ET rate constant approaches a constant value (ca. 130 s(-1)). The onset of the non-exponential distance-dependence is paralleled by an increasing kinetic H/D effect, indicating a coupling of the redox reaction with proton transfer (PT) steps. This unusual kinetic behaviour is attributed to the effect of the electric field at the Ag/SAM interface that increasingly raises the energy barrier for the PT processes with decreasing distance of the adsorbed Cyt-c from the electrode. The distance-dependence of the electric field strength is estimated on the basis of a simple electrostatic model that can consistently describe the redox potential shifts of Cyt-c as determined by stationary SERR spectroscopy for the various SAMs. At low electric fields, PT is sufficiently fast so that rate constants, determined as a function of the driving force, yield the reorganisation energy (0.217 electronvolts) of the heterogeneous ET. (C) 2002 Elsevier Science B.V. All rights reserved.
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