The angles between the C1-, C5-, and C9-methyl bonds of the retinylidene chromophore and the membrane normal increase in the M intermediate of bacteriorhodopsin:: Direct determination with solid-state 2H NMR

The orientations of three methyl bonds of the retinylidene chromophore of bacteriorhodopsin were investigated in the M photointermediate using deuterium solid-state NMR (H-2 NMR), In this key intermediate, the chromophore has a 13-cis, 15-anti conformation and a deprotonated Schiff base. Purple membranes containing wild-type or mutant D96A bacteriorhodopsin were regenerated with retinals specifically deuterated in the methyl groups of either carbon C1 or Cs of the beta-ionone ring or carbon Cs of the polyene chain. Oriented hydrated films were formed by drying concentrated suspensions on glass plates at 86% relative humidity. The lifetime of the M state was increased in the wild-type samples by applying a guanidine hydrochloride solution at pH 9.5 and in the D96A sample by raising the pH. 2H NMR experiments were performed on the dark-adapted ground state (a 2:1 mixture of 13-cis, 15-syn and all-trans, 15-anti chromophores), the cryotrapped light-adapted state (all-trans, 15-anti), and the cryotrapped M intermediate (13-cis, 15-anti) at -50 degrees C. Bacteriorhodopsin was first completely converted to M under steady illumination of the hydrated films at +5 OC and then rapidly cooled to -50 OC in the dark. From a tilt series of the oriented sample in the magnetic field and an analysis of the 2H NMR line shapes, the angles between the individual C-CD3 bonds and the membrane normal could be determined even in the presence of a substantial degree of orientational disorder. While only minor differences were detected between dark- and light-adapted states, all three angles increase in the M state. This is consistent with an upward movement of the C-5-C-13 part of the polyene chain toward the cytoplasmic surface or with increased torsional strain. The C-9-CD3 bond shows the largest orientational change of 7 degrees in M. This reorientation of the chromophore in the binding pocket provides direct structural support for previous suggestions (based on spectroscopic evidence) for a steric interaction in M between the Cs-methyl group and Trp 182 in helix F.