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Updated: Aug 9, 2026
10:03
Proton Transfer and Protein Conformation Dynamics in Photosensitive Proteins by Time-resolved Step-scan Fourier-transform Infrared Spectroscopy
Published on: June 27, 2014
Is there an excitonic interaction or antenna system in bacteriorhodopsin?
M A El-Sayed1, C T Lin, W R Mason
1Department of Chemistry and Biochemistry, University of California at Los Angeles, Los Angeles, CA 90024.
Summary
This study challenges the long-held belief of excitonic interactions in bacteriorhodopsin (bR) trimers. New data suggest the biphasic circular dichroism (CD) arises from conformational variations, not exciton coupling.
Area of Science:
- Biophysics
- Spectroscopy
- Structural Biology
Background:
- Bacteriorhodopsin (bR) exhibits biphasic circular dichroism (CD) in its visible absorption spectrum.
- This phenomenon has been attributed to excitonic interactions within the trimeric structure of retinal chromophores.
- The functional implications of this proposed interaction, including antenna systems and reaction centers, remain debated.
Purpose of the Study:
- To investigate the nature of the excitonic interaction in bacteriorhodopsin trimers.
- To re-evaluate the cause of the biphasic CD spectrum observed in bR.
- To determine if bR possesses an antenna system or a specialized reaction center.
Main Methods:
- Magnetic circular dichroism (MCD) spectroscopy of bR trimers and monomers.
- Analysis of previously published CD spectra of bR photocycle intermediates.
- Review of linear polarization studies on fluorescence and daughter absorption.
Main Results:
- MCD spectra of bR trimers were found to be similar to monomer spectra.
- No evidence for absorption to a doubly degenerate state, as predicted by exciton theory, was observed.
- Previous CD and polarization data also cast doubt on excitonic interactions.
Conclusions:
- The study provides strong evidence against excitonic interactions in bR trimers.
- The observed biphasic CD spectrum is likely due to multiple bR types with varying protein conformations around retinal.
- The findings question the existence of a functional antenna system or a specialized reaction center in bR.
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