2ZZL STRUCTURE OF BACTERIORHODOPSIN'S M INTERMEDIATE AT PH 7 PROTON TRANSPORT, RETINAL, PROTEIN-LIPID COMPLEX, REACTION INTERMEDIATE, CELL MEMBRANE, CHROMOPHORE, HYDROGEN ION TRANSPORT, ION TRANSPORT, MEMBRANE, PHOTORECEPTOR PROTEIN, PYRROLIDONE CARBOXYLIC ACID, RECEPTOR, RETINAL PROTEIN, SENSORY TRANSDUCTION, TRANSMEMBRANE, TRANSPORT