The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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The Relation Between Spectral and Thermal Properties of Vertebrate Visual Pigments

Petri Ala-Laurila

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium F1 at Helsinki University of Technology (Espoo, Finland) on the 13th of December, 2003, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-6831-0)   [7158 KB]
Dissertation is also available in print (ISBN 951-22-6830-2)

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.

This thesis consists of an overview and of the following 5 publications:

  1. Koskelainen A., Ala-Laurila P., Fyhrquist N. and Donner K., 2000. Measurement of thermal contribution to photoreceptor sensitivity. Nature 403, No. 6766, pages 220-223. © 2000 Nature Publishing Group. By permission.
  2. Ala-Laurila P., Saarinen P., Albert R., Koskelainen A. and Donner K., 2002. Temperature effects on spectral properties of red and green rods in toad retina. Visual Neuroscience 19, No. 6, pages 781-792.
  3. Ala-Laurila P., Albert R.-J., Saarinen P., Koskelainen A. and Donner K., 2003. The thermal contribution to photoactivation in A2 visual pigments studied by temperature effects on spectral properties. Visual Neuroscience 20, No. 4, pages 411-419.
  4. Ala-Laurila P., Pahlberg J., Koskelainen A. and Donner K., 2003. On the relation between the photoactivation energy and the absorbance spectrum of visual pigments. Vision Research, accepted for publication.
  5. Ala-Laurila P., Donner K. and Koskelainen A., 2003. Thermal activation and photoactivation of visual pigments. Biophysical Journal, accepted for publication.

Keywords: rhodopsin, photoreception, activation energy, thermal noise, spectral sensitivity, absorbance, vision, retina

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© 2003 Helsinki University of Technology

Last update 2011-05-26