Gated Electron Transfer of Yeast Iso-1 Cytochrome c on Self-Assembled Monolayer-Coated Electrodes

Iso-1 yeast cytochrome c (YCC) was adsorbed on Ag electrodes coated with self-assembled monolayers (SAMs) consisting either of 11-mercaptoundecanoic acid (MUA) or of 1:1 mixtures of MUA and either 11-mercaptoundecanol (MU) or 7-mercaptoheptanol (MH). The redox potentials and the apparent rate constants for the interfacial redox process as well as for the protein reorientation were determined by stationary surface-enhanced resonance Raman (SERR) and time-resolved SERR spectroscopy, respectively. For YCC immobilized on MUA and MUA/MU at pH 7.0 and 6.0, the negative shifts of the redox potentials with respect to that for the protein in solution can be rationalized in terms of the potential of the zero-charge determined by impedance measurements. The apparent electron transfer rate constants of YCC on MUA/MU and MU/MH at pH 6.0 were determined to be 8 and 18 s(-1), respectively. A decrease of the relaxations constants by a factor of ca. 2 was found for pH 7.0, and a comparable low value was determined for a pure MUA even at pH 6.0. In each system, the rate constant for protein reorientation was found to be the same as that for the electron transfer, implying that protein reorientation is the rate limiting step for the interfacial redox process. This gating step is distinctly slower than that for horse heart cytochrome c (HHCC) observed previously under similar conditions (Murgida, D. H.; Hildebrandt, P. J. Am. Chem. Soc. 2001, 123, 4062-4068). The different rate constants of protein reorientation for both proteins and the variations of the rate constants for the different SAMs and pH are attributed to the electric field dependence of the free energy of activation which is assumed to be proportional to the product of the electric field strength and the molecular dipole moment of the protein. The latter quantity is determined by molecular dynamics simulations and electrostatic calculations to be more than 2 times larger for YCC than for HHCC. Moreover, the dipole moment vector and the heme plane constitute an angle of ca. 10 and 45 degrees in YCC and HHCC, respectively. The different magnitudes and directions of the dipole moments as well as the different electric field strengths at the various SAM/protein interfaces allow for a qualitative description of the protein-, SAM-, and electrode- specific kinetics of the interfacial redox processes studied in this and previous works.