A method for detecting surface plasmon resonance with high resolution (ϳ10 Ϫ5 degrees or ϳ10 Ϫ8 refractive index units͒ and fast response time ͑1 s͒ is described. In the method, light is focused through a prism onto a metal film on which molecules to be detected are adsorbed. The total internal reflection of the incident light is collected with a bicell photodetector instead of a single cell or an array of photodetectors that are widely used in previous works. The ratio of the differential signal to the sum signal of the bicell photodetector provides an accurate measurement of shift in surface plasmon resonance angle caused by the adsorption of molecules onto the metal films or by conformational changes in the adsorbed molecules. Using the method, we have studied subtle conformational changes in redox protein, cytochrome c, due to an electron transfer reaction.

The structural and electron-transfer properties of cytochrome c (Cyt c) Langmuir−Blodgett (LB) films have been studied on graphite electrode with tapping mode atomic force microscopy and cyclic voltammetry (CV). Cyt c in the LB films forms an ordered monolayer in which the individual proteins pack into a quasi-hexagonal structure. The monolayer undergoes a reversible electron-transfer reaction in phosphate buffer. The interactions of Cyt c with cardiolipin (CL) and phosphatidylcholine (PC) LB films have been studied. The LB films of CL and PC are both ordered on graphite, but their interactions with Cyt c are quite different. On a CL monolayer, Cyt c adsorbs spontaneously and the adsorbed protein preserves the electron-transfer reaction. However, on a PC monolayer, Cyt c does not adsorb.

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