TIME-RESOLVED DETECTION OF STRUCTURAL-CHANGES DURING THE PHOTOCYCLE OF SPIN-LABELED BACTERIORHODOPSIN

Bacteriorhodopsin was selectively spin labeled at residues 72, 101, or 105 after replacement of the native amino acids by cysteine. Only the electron paramagnetic resonance spectrum of the label at 101 was time -dependent during the photocycle. The spectral change rose with the de cay of the M intermediate and fell with recovery of the ground state. The transient signal is interpreted as the result of movement in the C -D or E-F interhelical loop, or in both, coincident with protonation c hanges at the key aspartate 96 residue. These results link the optical ly characterized intermediates with localized conformational changes i n bacteriorhodopsin during the photocycle.