Photoactivities of the red-shifted azulenic bacteriorhodopsin analogues

Incorporating red-shifted retinal analogues, such as azulenic retinals, into bacterioopsin provides a method to evaluate the low-energy limit for bacteriorhodopsin (bR) proton-pumping activity. We report results of nanosecond time-resolved studies of three azulenic bR analogues. In contrast to a recent report on an azulenic bR analogue (Druzhko et al., 1996),(4) we conclude that such pigments do not exhibit photocycles (no detectable intermediates) and suspect that the preferred direction of deactivation is internal conversion to the low-lying forbidden S-1 state of the azulene chromophore. We further demonstrated that fur slow-binding bR analogues, the samples can easily be contaminated with small amounts of native bR.