Structural changes in the lumirhodopsin-to-metarhodopsin I conversion of air-dried bovine rhodopsin

Structural changes during the photochemical reactions of unhydrated air-dried films of bovine rhodopsin in rod outer segments were examined by visible and Fourier transform infrared (FTIR) spectroscopy at 200, 240, and 280 K. These films exhibited conversion from a lumirhodopsin state to a metarhodopsin I state with a time constant of 13.5 min at 280 K, but did not form metarhodopsin II at all, as observed earlier for digitonin-extracted rhodopsin in dry gelatin films [Wald, Durell, and St. George (1950) Science 111, 179-181]. Lumirhodopsin which was stable in the dry film was very similar to normal lumirhodopsin. The metarhodopsin I-like state retained properties characteristic of lumirhodopsin in regard to a twisted structure between the C14-H and the Schiff base of the chromophore, and perturbation around Glu122, although the C-C stretch frequencies of the chromophore were identical with those of metarhodopsin I. Thus, under dry conditions some of the structural changes that lead to metarhodopsin I are partially inhibited. These defects could result in stable lumirhodopsin and the failure to form metarhodopsin II, which is in equilibrium with metarhodopsin I.