On the molecular mechanism of the blue to purple transition of bacteriorhodopsin. UV-difference spectroscopy and electron spin resonance studies.

Conformational changes in the bacteriorhodopsin molecule related to the blue to purple transition have been monitored using UV-difference spectrophotometry. Mn2+ binding to the deionized blue membrane, which restores the purple form, promotes the appearance of a difference spectrum that can be interpreted as arising from tryptophan perturbation. Similar difference spectra were found upon pH increase of the blue membrane suspensions. Such pH increase yields the deionized purple membrane and shows an apparent pK of 5.4. Binding of Hg2+ to the blue membrane does not induce any UV-difference spectrum or change the apparent pK of the transition. ESR studies of Mn2+ binding show that in the pink membrane several high and medium affinity binding sites have been converted to low affinity ones. In the NaBH4-reduced membrane, a medium affinity site has been converted to a low affinity site. Upon Mn2+ binding to the reduced membrane or pH increase, absorption changes were found in the visible region which showed a dependence upon bound Mn2+ as well as an apparent pK similar to those of the nonreduced membrane. It is proposed that the functional form of the membrane depends primarily on the deprotonated state of a control group and that cation binding only affects the pK of this deprotonation through changes in the membrane surface potential.