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 ( λ 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.×(1/ λ 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/λ 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/ λ max remained highly significant ( r 2 =0.87), 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 λ max shift directly reflects the difference in photoactivation energies, (2) when the opsin is modified by amino acid substitutions, λ 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 λ 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 λ 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/ λ max )-relation is shallower than predicted.