Metarhodopsin II (MII) provides the active conformation of rhodopsin for interaction with the G-protein, Gt. Fourier transform infrared spectra from samples prepared by centrifugation reflect the pH dependent equilibrium between MII and inactive metarhodopsin I. C-terminal synthetic peptides (Gtα(340–350) and Gtγ(60–71)farnesyl) stabilize MII. We find that both peptides cause similar spectral changes not seen with control peptides (Gtα (K341R, L349A) and non-farnesylated Gtγ). The spectra reflect all the protonation dependent bands normally observed when MII is formed at acidic pH. Beside the protonation dependent bands, additional features, similar with both peptides, appear in the amide I and II regions.
