On the relation between thermal and photic activation of visual pigments

Abstract

Abstract: : Purpose:To explain 1) the experimentally observed correlation between the rate of thermal isomerizations (k) and the wavelength of maximum absorbance (λ_max) of vertebrate visual pigments; 2) the large difference between photoactivation energies (40–50 kcal/mol) and the apparent thermal activation energies (20–25 kcal/mol) deduced by simple Arrhenius analysis from the temperature dependence of k. Methods:Hinshelwood’s theory of thermal unimolecular reactions with many internal degrees of freedom associated with the many vibrational modes of complex molecules¹ was used. We tested whether a model based on this could fit the experimentally observed temperature–dependence of k under the assumption that thermal and photic activation energies differ little or not at all but different numbers of vibrational modes are coupled to the two activation mechanisms. Results:A good fit to the temperature–dependence of k was achieved even under the simplest assumption that the activation energies for thermal and photic activation processes have the same value. The central parameter, the internal degrees of freedom of the molecule giving the best fit, was 79 (comprising ca. 39 vibrational modes) for thermal activation. With the same number of degrees of freedom, the model correctly predicts the experimentally found correlations between k and λ_max for both rod and cone visual pigments. Conclusions:The large difference between previous estimates of thermal activation energies and photoactivation energies can be explained as an analytical artifact arising from the failure of simple Arrhenius analysis to take into account the internal energy present in the vibrational modes of rhodopsin. The conclusion that the two may be similar (40–50 kcal/mol) implies that thermal and photic activation may follow the same molecular route from a very early stage. It also explains the correlation between high sensitivity to long–wavelength light and high rates of thermal isomerizations. ¹Hinshelwood, C.N. (1933). The Kinetics of Chemical Change in Gaseous Systems. Oxford: Clarendon Press. Grant identification: Emil Aaltonen Foundation & Academy of Finland Commercial Relationship: P. Ala–Laurila, None; K. Donner, None; A. Koskelainen, None.

Keywords: photoreceptors • protein structure/function • electrophysiology: non–clinical