INTERACTIONS BETWEEN THE PROTONATED SCHIFF-BASE AND ITS COUNTERION INTHE PHOTOINTERMEDIATES OF BACTERIORHODOPSIN

In a previous study, we have shown that halide salts of the protonated Schiff base (pSB) of all-transretinal with aniline crystallize in the 6-s-trans conformation. These compounds are therefore better solid st ate models for the chromophores of the all-trans states of bacteriorho dopsin (bR) than the conventional butyl imine compounds. We have now e xtended these studies to the halide salts of the pSB of 13-cis-retinal with aniline. The 5-C-13 chemical shifts again indicate 6-s-trans con formations. These compounds are thus good models for the chromophores of the 13-cis photocycle intermediates of bR. As for the all-trans com pounds, the frequencies of maximum visible absorbance and the N-15 che mical shifts of the 13-cis compounds are Linearly related to the stren gth of the pSB-counterion (CI) interaction as measured by (1/d(2)), wh ere d is the center-to-center distance between the pSB charge and the CI charge. However, the relationship is steeper for the 13-cis compoun ds. With these calibrations, we estimate that d = 4.0, 3.9, 3.7, 3.6, and 3.8 Angstrom (+/-0.3 Angstrom) for the J(625), K-590, L-550, N-520 , and bR(555) states of bR, respectively. These distances compare with similarly determined values of about 4.16 +/- 0.03 and 4.66 +/- 0.04 Angstrom for the all-trans bR(568) and O-640 states, respectively. The results suggest that the pSB-CI interaction is stronger in all the 13 -cis photocycle intermediates, including the red-shifted J and K inter mediates, than in bR(568) The apparent tightening of the pSB-CI intera ction in the J-->N sequence is consistent with models of the early pho tocycle in which electrostatic attractions between the pSB and counter ions constitute an important constraint.