An M-type intermediate is formed in the 13-cis-bR photocycle in purple membranes at high pH. This is presumably due to deprotonation of the same group whose deprotonation causes a large increase in rate of M formation in the trans-bR photocycle (the ‘alkaline transition’). For Triton X-100-solubilized bR, the alkaline transition is shifted to a lower pH value by more than 2 pH units. The alkaline transition in Triton-solubilized preparations changes the efficiency of the M intermediate formation in the 13-cis-sbR photocycle. The M intermediate formation in 13-cis-sbR, as in the case of trans-sbR, is completely inhibited when the blue ‘acidic’ bR is formed at low pH. The protonation state of the group affecting formation of the M intermediate in 13-cis-bR at high pH and the group which is responsible for the transition to the blue acidic form influence in a similar way the equilibrium between bR isomers in the dark-adapted form as well as the rate of dark adaptation.
