Chromophore reorientation relative to the membrane plane detected by time-resolved linear dichroism during the bacteriorhodopsin photocycle in oriented purple membrane

Time-resolved linear dichroism measurements on samples of oriented purple membranes enable reorientations in the plane of the membrane to be distinguished from reorientations with respect to the membrane normal. Our measurements reveal reorientation of the transition dipole of the retinylidene chromophore toward the membrane normal by 14 degrees in the M-state. A smaller reorientation (3 degrees) in the same direction is observed in the L-state and possibly as early as the K-state. The purple membrane was oriented in an 11 T magnetic field and immobilized in a polyacrylamide gel. Time-resolved linear dichroism measurements on purple membrane oriented so that pump and probe beams propagate parallel to the membrane plane are highly sensitive to reorientation with respect to the membrane normal due to the initial orientation of the transition dipole at about 67 degrees with respect to the membrane normal. Measurements where pump and probe beams propagate perpendicular to the membrane plane reveal no significant in-plane reorientation, although the sensitivity of these measurements is not as high. The observed reorientation in the M-state may be related to conformational changes that have been detected in helices C, F, and G.