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Redox and conformational equilibria and dynamics of cytochrome c at high electric fields
Citation key ISI:000184243600007
Author Wackerbarth, H and Hildebrandt, P
Pages 714-724
Year 2003
ISSN 1439-4235
DOI 10.1002/cphc.200200618
Address PO BOX 10 11 61, D-69451 WEINHEIM, GERMANY
Journal Chem. phys. chem.
Volume 4
Number 7
Month JUL 14
Abstract Cytochrome c (Cyt-c) adsorbed in the electrical double layer of the Ag electrode/electrolyte interface has been studied by stationary and time-resolved surface enhanced resonance Raman spectroscopy to analyse the effect or strong electric fields on structure and reaction equilibria and dynamics of the protein. In the potential range between +0.1 and -0.55V (versus saturated calomel electrode), the adsorbed Cyt-c forms a potential-dependent reversible equilibrium between the native state B1 and a conformational state B2. The redox potentials of the bis histidine coordinated six-coordinated low-spin and five-coordinated high-spin substrates of B2 were determined to be -0.425 and -0.385 V, respectively, whereas the additional six coordinated aquo-histidine-coordinated high-spin substrate was found to be redox inactive. The redox potential for the conformational state B1 was found to be the same as in solution in agreement with the structural identity of the adsorbed B1 and the native Cyt-c. For all three redox-active species, the formal heterogeneous electron transfer rate constants are small and of the same order of magnitude (3-13 s(-1)), which implies that the rate-limiting step is largely independent of the redox-site structure. These findings, as well as the slow and potential-dependent transitions between the various conformational (sub-)states, can be rationalized in terms of an electric field-induced increase of the activation energy for proton-transfer steps linked to protein structural reorganisation. Further increasing the electric field strength by shifting the electrode potential above +0.1 V leads to irreversible structural changes that are attributed to an unfolding of the polypeptide chain.
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