Electric-Field Induced Redox Potential Shifts of Tetraheme Cytochromes c3 Immobilised on Self-Assembled Monolayers. Surface Enhanced Resonance Raman Spectroscopy and Simulation Studies
Rivas Laura 1, Claudio M. Soares 2, Antonio M. Baptista 2, Jalila Simaan 2, Roberto E Di Paolo 2, Daniel H Murgida 1 and Peter Hildebrandt 1*
1 ITQB & Technische Universität Berlin
2 Universidade Nova de Lisboa
* To whom correspondence should be addressed. E-mail: hildebrandt{at}chem.tu-berlin.de.
Submitted on November 29, 2004
Revised on February 4, 2005
Accepted on 23 February 2005
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Abstract |
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The tetraheme protein cytochrome c3 (Cyt-c3) from Desulfovibrio gigas, immobilised on a self-assembled monolayer (SAM) of 11-mercaptoundecanoic acid, is studied by theoretical and spectroscopic methods. Molecular dynamics simulations indicate that the protein docks to the negatively charged SAM via its lysine-rich domain around the exposed heme IV. Complex formation is associated with only little protein structural perturbations. This finding is in line with the resonance Raman (RR) and surface enhanced RR (SERR) spectroscopic results that indicate essentially the same heme pocket structures for the protein in solution and adsorbed on SAM-coated Ag electrodes. Electron and proton binding equilibrium calculations reveal substantial negative shifts of the redox potentials compared to the protein in solution. The magnitude of these shifts decreases in the order heme IV (-161 mV) > heme III (-73 mV) > heme II (-57 mV) > heme I (-26 mV), resulting in a change of the order of reduction. These shifts originate from the distance dependent electrostatic interactions between the SAM head groups and the individual hemes, leading to a stabilisation of the oxidised forms. The results of the potential-dependent SERR spectroscopic analyses are consistent with the theoretical predictions and afford redox potential shifts of -160 mV (heme IV), -90 mV (heme III), -70 mV (heme II), and +20 mV (heme I) relative to the experimental redox potentials for Cyt-c3 in solution. SERR spectroscopic experiments reveal electric-field induced changes of the redox potentials also for the structurally very similar Cyt c3 from D. vulgaris, although the shifts are somewhat smaller compared to Cyt-c3 from D. gigas. The present study suggests that electric-field induced redox potential shifts may also occur upon binding to biomembranes or partner proteins and thus may affect biological electron transfer processes.
Key Words:
SERR spectroscopy, electrostatic calculations, heme protein, interface, molecular dynamics simulation
