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Heterogeneous electron transfer of cytochrome c on coated silver electrodes. Electric field effects on structure and redox potential
Citation key ISI:000167267600012
Author Murgida, D H and Hildebrandt, P
Pages 1578-1586
Year 2001
ISSN 1089-5647
DOI 10.1021/jp003742n
Address 1155 16TH ST, NW, WASHINGTON, DC 20036 USA
Journal J. Phys. Chem. B
Volume 105
Number 8
Month MAR 1
Abstract Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolayers (SAM) of omega -carboxylalkanethiols that were covalently attached to Ag electrodes. Employing surface-enhanced resonance Raman (SERR) spectroscopy, the redox equilibria and the structural changes of the adsorbed Cyt-e were analyzed quantitatively for SAMs of different chain lengths ranging from 2-mercaptoacetic acid (C-2-SAM) to 16-mercaptohexadecanoic acid (C-16-SAM). In the presence of Cyt-c in the bulk solution, the SERR spectra of the adsorbed Cyt-c display the characteristic vibrational band pattern of the native protein conformation denoted as state B1. The enhancement of the SERR signals decreases with increasing chain length, but even at distances as large as 24 Angstrom (C-16-SAM), SERR spectra of high quality could be obtained. Conversely, no SERR signals could be detected for SAMs including hydroxyl instead of carboxylate headgroups, implying that Cyt-c is adsorbed via electrostatic interactions. On the basis of potential-dependent SERR experiments, the redox equilibria of the adsorbed Cyt-c (B1) were analyzed, revealing ideal Nernstian behavior (n congruent to 1). However, the redox potentials exhibit negative shifts compared to that of Cyt-c in solution, which increase with the chain length of the SAMs. In the absence of excess Cyt-c in solution (i.e., 0.2 muM), a new conformational state B2 of the adsorbed Cyt-c is observed. This state B2, which differs from the native state B1 by the heme pocket structure, includes three substates of different spin and coordination configurations. The distribution among these substates as well as the total contribution of state B2 varies with the chain length of the SAM such that the latter decreases from 73\% at C-2-SAM to 0\% at C-11- and C-16-SAMs. These results imply that the formation of B2 is induced by the electric field at the binding site, generated by the potential drop across the electrode/SAM interface. When an electrostatic model for the interfacial potential distribution for the electrode/ SAM/protein device is employed, both the redox potential shifts and the electric-field-induced structural changes can be consistently explained. The impact of these findings for the processes of Cyt-c at biological interfaces is discussed.
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