Cooperativity of Photocycle in Purple Membrane

This chapter discusses the kinetic coupling among bacteriorhodopsin molecules that constitutes the various clusters in the hexagonal or the orthorhombic purple membrane. To detect kinetic coupling among bacteriorhodopsin (BR) molecules both in the native hexagonal and in the orthorhombic purple membrane, it is necessary to correlate the kinetics of the thermal transitions of the photocycle with the population of a certain metastable state within a certain cluster. Because the M₄₁₂ state is the longest-lived intermediate, it is possible by choosing the appropriate environmental conditions (temperature, pH, ionic strength) to obtain a high conversion yield of BRsro into M₄₁₂ under photosteady light illumination, and thus by changing the light intensity to induce a variable number of BR molecules in the M₄₁₂ state within a certain cluster. The dependence of the thermal decay of M₄₁₂ on the occupancy of M₄₁₂ is measured by turning the light off after reaching the various photostationary states of M₄₁₂.