Determination of a molecular torsional angle in the metarhodopsin-I photointermediate of rhodopsin by double-quantum solid-state NMR.

We present a solid-state NMR study of metarhodopsin-1, the pre-discharge intermediate of the photochemical signal transduction cascade of rhodopsin, which is the 41 kDa integral membrane protein that triggers phototransduction in vertebrate rod cells. The H-C10-C11-H torsional angles of the retinylidene chromophore in bovine rhodopsin and metarhodopsin-I were determined simultaneously in the photo-activated membrane-bound state, using double-quantum heteronuclear local field spectroscopy. The torsional angles were estimated to be [phi] = 160+/-10 degrees for rhodopsin and phi = 180+/-25 degrees for metarhodopsin-I. The result is consistent with current models of the photo-induced conformational transitions in the chromophore, in which the 11-Z retinal ground state is twisted, while the later photointermediates have a planar all-E conformation.