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Why are blue visual pigments blue? A resonance Raman microprobe study.
G R Loppnow1, B A Barry, R A Mathies
1Department of Chemistry, University of California, Berkeley 94720.
Summary
Blue visual pigments absorb light differently due to a protonated Schiff base in their retinal prosthetic group. This finding explains the pigment
Area of Science:
- Biophysics
- Spectroscopy
- Molecular Biology
Background:
- Visual pigments are crucial for light detection in photoreceptor cells.
- The chromophore's structure, specifically the retinal prosthetic group, determines light absorption wavelength.
- Understanding wavelength regulation is key to deciphering visual pigment function.
Purpose of the Study:
- To investigate the structural basis of wavelength regulation in blue visual pigments.
- To determine the role of the Schiff base linkage in the chromophore of toad green rod pigment.
Main Methods:
- Development of a resonance Raman microscope for studying individual photoreceptor cells.
- Acquisition of Raman vibrational spectra from intact photoreceptors at 77 K.
- Analysis of Schiff base and ethylenic band frequencies in normal and deuterium-substituted conditions.
Main Results:
- The 440-nm absorbing pigment in toad green rods contains a protonated Schiff base linkage (1662 cm-1).
- Protonation of the Schiff base is sufficient to explain the blue absorption maximum without additional protein interactions.
- Spectroscopic data indicate an unperturbed chromophore, similar to solution-state protonated retinal Schiff base.
Conclusions:
- Blue visual pigments can arise from an unperturbed protonated Schiff base.
- This challenges the necessity of protein charge perturbations for blue-shifted absorption.
- Experimental evidence supports the removal of protein charge perturbations in blue pigment opsins.