Search

The syntheses of 10 ring alkylated azulenedienal analogs of retinal are reported. They form bacteriorhodopsin pigment analogs at varying efficiency, in fact in a manner suggesting a possible t-butyl recognition site within the binding pocket. © 1997 Published by Elsevier Science Ltd. All rights reserved.

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 S1 state of the azulene chromophore. We further demonstrated that for slow-binding bR analogues, the samples can easily be contaminated with small amounts of native bR.

[structure: see text] Experimental and calculated data show that the relative energy levels of the two lowest excited states of azulene are sensitive to the nature and position of substituents on the nonalternant hydrocarbon. Extending such investigations led to a rational explanation for some of the baffling data on azulenic bacteriorhodopsin analogues in the literature.

Depending on the size and shape of their azulenic chromophores, azulenic bacteriorhodopsin (bR) pigment analogs can exist as either an initial pigment P1, a more red-shifted final pigment P2 or an equilibrium mixture of both. The absorption spectra of red-shifted bR analogs exhibit characteristic narrow-band shapes similar to charge fully delocalized cyanine-like dyes. Therefore, all such red-shifted pigments are believed to be highly delocalized, bond-equalized carbocations. We have determined structural requirements that facilitate their formation. To describe fully the red-shift potentials of these retinal analogs, we have introduced a new parameter—percent red-shift (PRS). A large PRS value not only reflects the extent of red-shift, but is also suggestive of extensive delocalization of the positive charge. Relevance of these findings in consideration of the possibility of forming stable O-intermediates is presented. The postulated resonance hybrid-like structures for different cations of the ...