The novel nonlinear optical method of second harmonic generation-circular dichroism (SHG-CD) has
been used to follow the adsorption and redox properties of a peripheral membrane protein, horse heart
cytochrome c, adsorbed at several model membrane surfaces. The SHG-CD response is shown to be affected
by the oxidation state of the heme within surface-adsorbed cytochrome c, as is consistent with the sensitivity
of SHG to the chirality of the heme. SHG-CD measurements show that adsorbed cytochrome c is reducible
by ascorbate at alkanethiol surfaces, but not at phospholipid/alkanethiol hybrid bilayer membranes (HBMs).
The adsorption of cytochrome c at the model membrane surfaces was verified by surface plasmon resonance.
Surface-enhanced resonance Raman measurements show that cytochrome c adsorbed on a hybrid bilayer
membrane retains the Fe-heme conformation associated with solution-phase cytochrome c and is reducible
by applying potential to the supporting electrode. The inability of ascorbic acid to reduce cytochrome c
associated with the HBM is attributed not to a change in its redox potential, but rather to the nature of
the interaction of cytochrome c with the HBM.
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