Light absorption and excitation energy transfer calculations in primitive photosynthetic bacteria

Bridging Carotenoid-to-Bacteriochlorophyll Energy Transfer of Purple Bacteria LH2 With Temperature Variations: Insights From Conformational Changes

Photosynthesis is a key process for converting light energy into chemical energy and providing food for lives on Earth. Understanding the mechanism for the energy transfers could provide insights into regulating energy transfers in photosynthesis and designing artificial photosynthesis systems. Many efforts have been devoted to exploring the mechanism of temperature variations affecting the excitonic properties of LH2. In this study, we performed all-atom molecular dynamics (MD) simulations and quantum mechanics calculations for LH2 complex from purple bacteria along with its membrane environment under three typical temperatures: 270, 300, and 330 K. The structural analysis from validated MD simulations showed that the higher temperature impaired interactions at N-terminus of both α and β polypeptide helices and led to the dissociation of this hetero polypeptide dimer. Rhodopin-β-D-glucosides (RG1) moved centripetally with α polypeptide helices when temperature increased and enlarged their distances with bacteriochlorophylls molecules that have the absorption peak at 850 nm (B850), which resulted in reducing the coupling strengths between RG1 and B850 molecules. The present study reported a cascading mechanism for temperature regulating the energy transfers in LH2 of purple bacteria.

Proposal for verifying dipole properties of light-harvesting antennas

For light harvesters with a reaction center complex (LH1-RC complex) of three types, we propose an experiment to verify our analysis based upon antenna theories that automatically include the required structural information. Our analysis conforms to the current understanding of light-harvesting antennas in that we can explain known properties of these complexes. We provide an explanation for the functional roles of the notch at the light harvester, a functional role of the polypeptide called PufX or W at the opening, a functional role of the special pair, a reason that the cross section of the light harvester must not be circular, a reason that the light harvester must not be spherical, reasons for the use of dielectric bacteriochlorophylls instead of conductors to make the light harvester, a mechanism to prevent damage from excess sunlight, an advantage of the dimeric form, and reasons for the modular design of nature. Based upon our analysis we provide a mechanism for dimerization. We predict that the dimeric form of light-harvesting complexes is favored under intense sunlight. We further comment upon the classification of the dimeric or S-shape complexes. The S-shape complexes should not be considered as the third type of light harvester but simply as a composite form.