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-I, the pre-discharge in termediate of the photochemical signal transduction cascade of rhodopsin, w hich is the 41 kDa integral membrane protein that triggers phototransductio n in vertebrate rod cells. The H-C10-C11-H torsional angles of the retinyli dene chromophore in bovine rhodopsin and metarhodopsin-I were determined si multaneously in the photo-activated membrane-bound state, using double-quan tum heteronuclear local field spectroscopy. The torsional angles were estim ated to be |phi| = 160 +/- 10 degrees for rhodopsin and phi= 180 +/- 25 deg rees for metarhodopsin-I. The result is consistent with current models of t he photo-induced conformational transitions in the chromophore, in which th e 11-Z retinal ground state is twisted, while the later photointermediates have a planar all-E conformation.