On the relation between the photoactivation energy and the absorbance spectrum of visual pigments

Abstract

We relate the collected experimental data on the minimum energy for photoactivation (E(a)) to the wavelengths of peak absorbance (lambda(max)) of 12 visual pigments. The E(a) values have been determined from the temperature-dependence of spectral sensitivity in the long-wavelength range. As shown previously, the simple physical idea E(a) =const. x (1/lambda(max)) (here termed the Stiles-Lewis-Barlow or SLB relation) does not hold strictly. Yet there is a significant correlation between E(a) and 1/lambda(max) (r(2)=0.73) and the regression slope obtained by an unbiased fit is 84% of the predicted value of the best SLB fit. The correlation can be decomposed into effects of A1 --> A2 chromophore change and effects of opsin differences. For a chromophore change in the same opsin, studied in two A1/A2 pigment pairs, the SLB relation holds nearly perfectly. In seven pigments having different opsins but the same (A2) chromophore, the correlation of E(a) and 1/lambda(max) remained highly significant (r(2)=0.78), but the regression coefficient is only 72% of the best SLB fit. We conclude that (1) when the chromophore is exchanged in the same opsin, the lambda(max) shift directly reflects the difference in photoactivation energies, (2) when the opsin is modified by amino acid substitutions, lambda(max) and E(a) can be tuned partly independently, although there is a dominant tendency for inverse proportionality. In four (A1) rhodopsins with virtually the same lambda(max), E(a) varied over a 4.5 kcal/mol range, which may be taken as a measure of the freedom for independent tuning. Assuming that low E(a) correlates with high thermal noise, we suggest that the leeway in lambda(max) - E(a) coupling is used by natural selection to keep E(a) as high as possible in long-wavelength-sensitive pigments, and that this is why the opsin-dependent E(a) (1/lambda(max))-relation is shallower than predicted.