The redox behavior of cytochrome c (cyt c ) adsorbed to gold electrodes modified with self-assembled monolayers (SAMs) depends on the SAM. This paper examines SAMs generated from alkanethiols terminating in trimethylammonium(1), sulfonate (2),methyl (3), amine (4), and carboxylic acid (5) groups and from an aromatic thiol (6). The redox potentials of cyt c adsorbed on SAMs of 1 and 5 are relatively close to the formal potential of native cyt measured in solution. The redox potentials of cyt c c adsorbed on SAMs of 3, 4, and 6 are significantly shifted from the formal potential, and a reduction peak at about 0.5 Vmore negative than the formal potential (that is, a value corresponding to amore difficult reduction) was observedinall three cases.These observations suggest that cyt c changes its conformationsignificantly on adsorption on these surfaces. No redox peaks were observed for cyt c adsorbed on SAMs of 2, although surface plasmon resonance (SPR) studies indicate that the SAMs of 2 irreversibly adsorbed approximately a double layer of cyt c .Mixed SAMs were also studied.Most interestingly, cyt c adsorbed onmixed SAMs formed fromthe combinations of 1 and 2 exhibited significantly slower electron transfer (0.3-1.2 s-1) than cyt c adsorbedonahomogeneousSAMof1(45s-1).Theseobservations suggest changes inproteinorientation due to the presence of the sulfonate groups at the interface. This study suggests that electrochemical measurement can be a useful probe for the conformation and orientation of protein adsorbed on surfaces.
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