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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
Published on: April 19, 2019
Carotenoid radical formation: dependence on conjugation length
A Ligia Focsan1, Michael K Bowman, Péter Molnár
1Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0336, United States.
The Journal of Physical Chemistry. B
|June 30, 2011
Summary
Carotenoid radical cation deprotonation varies with conjugation length and structure. This study identifies specific proton loss sites, aiding in the detection of carotenoid radicals using Mims ENDOR.
Area of Science:
- Biochemistry
- Spectroscopy
- Organic Chemistry
Background:
- Carotenoids are vital pigments with diverse biological roles.
- Understanding carotenoid radical formation is crucial for biological studies.
- Proton loss from carotenoid radical cations generates neutral radicals.
Purpose of the Study:
- To investigate the relative energies of neutral carotenoid radicals formed by proton loss.
- To determine how conjugation length and structural modifications affect proton loss sites.
- To establish a method for identifying carotenoid radicals in biological systems.
Main Methods:
- Computational examination of relative energies of carotenoid neutral radicals.
- Analysis of proton loss from radical cations of linear carotenoids with varying conjugation lengths (n=9-15).
- Utilizing Mims Electron Nuclear Double Resonance (ENDOR) spectroscopy principles.
Main Results:
- For bisdehydrolycopene (n=15), proton loss is most favorable from methyl groups at C1/C1'.
- For shorter conjugated carotenoids (lycopene, n=11), proton loss occurs from exocyclic methylene groups.
- Specific structural saturations and methoxy group additions direct proton loss to distinct methylene groups.
- Neutral carotenoid radicals exhibit larger beta-methyl proton couplings (13-16 MHz) than radical cations (7-10 MHz).
Conclusions:
- The site of proton loss from carotenoid radical cations is dependent on conjugation length and specific structural features.
- The observed differences in electron spin distribution provide a spectroscopic signature for identifying carotenoid radicals.
- Mims ENDOR spectroscopy can be employed to detect and characterize these radicals in biological contexts.
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