The relation between spectral and thermal properties of vertebrate visual pigments

Abstract

This thesis investigates the relation between three important functional properties of visual pigments: 1) the absorbance spectrum, 2) the energy needed for activation and 3) the thermal stability.

The study is based on measurement of spectral absorbance and spectral sensitivity in 10 visual pigments in rod and cone photoreceptor cells from the retinas of 6 vertebrate species and on data from the literature. Absorbance spectra were recorded by single-cell mictrospectrophotometry (MSP) and spectral sensitivities by electroretinogram recording (ERG) across the isolated retina. For each pigment, measurements were conducted at two or more temperatures in the range 0-40 °C. The photoactivation energies of the visual pigments were determined from the temperature-dependence of spectral sensitivity in the long-wavelength range. Thermal activation rates of rod and cone pigments were collected from the literature.

One objective was to test the hypothesis that there is a strict coupling between the energy needed for photoactivation (Ea) and the wavelength of maximum absorbance (λmax) of visual pigments. The greater goal was to clarify the relation between the energies required for thermal and photic activation and thus explain the experimentally observed correlation between λmax and the rate of spontaneous, thermal activation of pigments.

The measurements showed that there is no necessary physical coupling between Ea and λmax. A strict inverse proportionality (Ea ∝ 1/λmax) holds only in the simple case where spectral tuning is achieved by a change of chromophore, with no change in the protein (opsin) part of the pigment. On the other hand, a significant correlation between Ea and 1/λmax was found in the full set of 12 visual pigments considered (including two invertebrate pigments).

A new model for thermal activation is proposed, with a consequent dependence of the activation rate on λmax. The crucial point is that the statistics of thermal activation is determined by the presence of internal energy in a large number of vibrational modes of the visual pigment molecule. The great discrepancy between photoactivation energies and thermal activation energies as estimated in earlier work then disappears as an analytical artifact. The main conclusion is that thermal and photic activation of visual pigments may follow the same molecular route from a very early stage (isomerization of the chromophore in the native conformation of rhodopsin). Furthermore, the model accurately predicts the correlation between the wavelength of maximum absorbance and the rate of thermal activation observed in the whole set of visual pigments studied.