Absorption characteristics of oriented photopigments in microvilli

Abstract

We have analysed the effects of photopigment orientation in microvilli membranes on the absorption properties of rhabdomeres. 1. Assuming a uniform distribution of rhodopsin around the microvillus membrane, theE vector for maximum absorption of polarised light must be either parallel or perpendicular to the microvillus axis regardless of the orientation of the chromophores in the membrane. Polarised light absorption follows an (A+B cos2Θ) dependence, as found experimentally. 2. The orientation of a chromophore in a microvillus membrane is completely defined in terms of two angles: the angle between the chromophore and the local membrane surface, and the angle between the chromophore and the microvillus axis. The dichroic ratio of a single microvillus is a function of chromophore orientation angle relative to the microvillus axis only. The maximum absorption of polarised light and the total absorption of unpolarised light are also functions of this angle. Thus measurements of the dichroism or polarisation sensitivity alone cannot yield the chromophore angle relative to the membrane surface. 3. Whereas large dichroic ratios, or high PS values, require a high degree of chromophore alignment relative to the microvillus axis, low values cannot be interpreted unambiguously. For example, chromophores perfectly oriented at 55° to the microvillus axis will have a dichroic ratio of ∼1, which is the same as for a completely random distribution of chromophore orientations. 4. Our analysis suggests that adaptation to both polarised and unpolarised light can produce a non-uniform distribution of rhodopsin around the microvillus if rhodopsin and metarhodopsin do not share the same chromophore orientation angle in the membrane. In particular, after adaptation to polarised light theE direction for maximum absorption may be other than parallel or perpendicular to the microvillus axis. In principle, a complete description of rhodopsin and metarhodopsin chromophore orientations may be obtained from polarisation sensitivity measurementsafter adaptation to polarised light. Assuming a uniform distribution of rhodopsin around the microvillus membrane, theE vector for maximum absorption of polarised light must be either parallel or perpendicular to the microvillus axis regardless of the orientation of the chromophores in the membrane. Polarised light absorption follows an (A+B cos2Θ) dependence, as found experimentally. The orientation of a chromophore in a microvillus membrane is completely defined in terms of two angles: the angle between the chromophore and the local membrane surface, and the angle between the chromophore and the microvillus axis. The dichroic ratio of a single microvillus is a function of chromophore orientation angle relative to the microvillus axis only. The maximum absorption of polarised light and the total absorption of unpolarised light are also functions of this angle. Thus measurements of the dichroism or polarisation sensitivity alone cannot yield the chromophore angle relative to the membrane surface. Whereas large dichroic ratios, or high PS values, require a high degree of chromophore alignment relative to the microvillus axis, low values cannot be interpreted unambiguously. For example, chromophores perfectly oriented at 55° to the microvillus axis will have a dichroic ratio of ∼1, which is the same as for a completely random distribution of chromophore orientations. Our analysis suggests that adaptation to both polarised and unpolarised light can produce a non-uniform distribution of rhodopsin around the microvillus if rhodopsin and metarhodopsin do not share the same chromophore orientation angle in the membrane. In particular, after adaptation to polarised light theE direction for maximum absorption may be other than parallel or perpendicular to the microvillus axis. In principle, a complete description of rhodopsin and metarhodopsin chromophore orientations may be obtained from polarisation sensitivity measurementsafter adaptation to polarised light.

DOI: 10.1007/BF00326667

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@article{Israelachvili1976AbsorptionCO, title={Absorption characteristics of oriented photopigments in microvilli}, author={Jacob N Israelachvili and M. Judy Wilson}, journal={Biological Cybernetics}, year={1976}, volume={21}, pages={9-15} }