CHROMOPHORE REORIENTATION RELATIVE TO THE MEMBRANE PLANE DETECTED BY TIME-RESOLVED LINEAR DICHROISM DURING THE BACTERIORHODOPSIN PHOTOCYCLEIN ORIENTED PURPLE MEMBRANE

Time-resolved linear dichroism measurements on samples of oriented pur ple 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 th e retinylidene chromophore toward the membrane normal by 14 degrees in the M-state. A smaller reorientation (3 degrees) in the same directio n 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 immobilize d in a polyacrylamide gel. Time-resolved linear dichroism measurements on purple membrane oriented so that pump and probe beams propagate pa rallel to the membrane plane are highly sensitive to reorientation wit h respect to the membrane normal due to the initial orientation of the transition dipole at about 67 degrees with respect to the membrane no rmal. Measurements where pump and probe beams propagate perpendicular to the membrane plane reveal no significant in-plane reorientation, al though the sensitivity of these measurements is not as high. The obser ved reorientation in the M-state may be related to conformational chan ges that have been detected in helices C, F, and G.