Applying an Anti-Kasha Model Resolves Differences Between Photosynthetic and Artificial Pigments

Area of Science:

• Photosynthesis research
• Quantum biology
• Biophysics

Background:

• Kasha's rule typically governs excitation energy transfer (EET) in natural photosynthesis, prioritizing internal conversion over transfer between higher excited states.
• Artificial systems and dyes demonstrate exceptions to Kasha's rule, exhibiting anti-Kasha EET, especially in aggregated forms.

Purpose of the Study:

• To investigate the potential for anti-Kasha EET in natural photosynthetic pigments using a semiempirical Förster-type model.
• To analyze how pigment mixtures in light-harvesting complexes influence EET pathways and absorbance properties.

Main Methods:

• Application of a semiempirical Förster-type model to chlorophylls (Chl a, b, c1) and carotenoids.
• Calculations of Coulomb coupling elements and exciton delocalization for photosynthetic pigments.
• Modeling of pigment compositions in natural light-harvesting complexes (LHCII, CP24, CP26, CP29, FCP).

Main Results:

• All investigated natural photosynthetic pigments show strong potential for anti-Kasha EET due to high Coulomb coupling.
• Photosynthetic pigments form delocalized excitons, particularly at higher excited states relevant to anti-Kasha pathways.
• Accessory pigments in light-harvesting complexes suppress anti-Kasha EET in Chl a-only networks through exciton disruption, spectral competition, energy sinks, and rapid internal conversion.

Conclusions:

• Natural photosynthetic systems exhibit "special" behavior not due to inherent pigment properties but due to the complex interplay within pigment mixtures.
• Accessory pigments play a crucial role in regulating EET, preventing anti-Kasha pathways and ensuring efficient energy funneling in light-harvesting complexes.
• The findings challenge the universal applicability of Kasha's rule in natural photosynthesis and highlight the importance of pigment composition in determining EET dynamics.