Heterogeneous electron transfer of cytochrome c on coated silver electrodes. Electric field effects on structure and redox potential

Citation
Dh. Murgida et P. Hildebrandt, Heterogeneous electron transfer of cytochrome c on coated silver electrodes. Electric field effects on structure and redox potential, J PHYS CH B, 105(8), 2001, pp. 1578-1586
Citations number
64
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF PHYSICAL CHEMISTRY B
ISSN journal
15206106 → ACNP
Volume
105
Issue
8
Year of publication
2001
Pages
1578 - 1586
Database
ISI
SICI code
1520-6106(20010301)105:8<1578:HETOCC>2.0.ZU;2-E
Abstract
Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolaye rs (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 wer e 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 ads orbed Cyt-c display the characteristic vibrational band pattern of the nati ve protein conformation denoted as state B1. The enhancement of the SERR si gnals decreases with increasing chain length, but even at distances as larg e 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 ele ctrostatic interactions. On the basis of potential-dependent SERR experimen ts, the redox equilibria of the adsorbed Cyt-c (B1) were analyzed, revealin g ideal Nernstian behavior (n congruent to 1). However, the redox potential s exhibit negative shifts compared to that of Cyt-c in solution, which incr ease with the chain length of the SAMs. In the absence of excess Cyt-c in s olution (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 coor dination 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 electr ic field at the binding site, generated by the potential drop across the el ectrode/SAM interface. When an electrostatic model for the interfacial pote ntial distribution for the electrode/ SAM/protein device is employed, both the redox potential shifts and the electric-field-induced structural change s can be consistently explained. The impact of these findings for the proce sses of Cyt-c at biological interfaces is discussed.