P-007

Gabrielė Rankelytė

gabriele.rankelyte@gmail.com

Jevgenij Chemliov, Andrius Gelzinis, Leonas Valkunas

Center for Physical Sciences and Technology (FTMC), Lithuania

Impact of Protein Geometry and Protonation Pattern on the Excited States of Pigments in Photosynthetic Complexes

In the thylakoid membrane of chloroplasts there are two systems that carry out photosynthesis – Photosystem I (PSI) and Photosystem II (PSII), both with their own light harvesting complexes – LHCI and LHCII. PSI is the most efficient light-to-energy conversion apparatus with quantum yield almost equal to 100%. One of the conditions needed for high efficiency is very fast energy transfer between the molecules in light harvesting complex (LHCI). Light-harvesting complex of PSI absorbs and emits light at the longest wavelengths compared to other pigment-protein complexes. In plants, light harvesting antenna of PSI is composed of four species of LHCI complexes. They all have very similar structure; however, their spectral properties are different.

The excitation dynamics in LHCI is highly affected by the charge-transfer (CT) states that occur between two or more pigments. Some sites in which CT states occur in LHCI are known; however, they do not completely explain the spectral properties of this antenna, such as the red-shifted peak in fluorescence spectrum. The energy of the excited states of pigments (including the CT states) is highly affected by the surrounding environment, consisting of other pigments and the protein chain. Therefore, it is necessary to account for the environment to model light-harvesting complexes properly. 

LHCI structure was obtained as the 1st–4th chains of PSI complex structure, freely available at Protein Data Bank (PDB ID: 5L8R). We performed quantum chemical calculations to obtain energies of chlorophyll dimer CT states in vacuo using “VU HPC” Saulėtekis supercomputer. For all four protein chains, we estimated the most probable protonation pattern in neutral solution. We then included the environment (chlorophylls, carotenoids and the protein chain) in our calculations by obtaining atomic partial charges of both environmental blocks and dimers of interest and evaluating the electrostatic interaction between these charges using CDC (charge-density coupling) method. Our recent findings demonstrate the sensitivity of pigment excited state properties to changes in the surrounding environmental geometry. Besides the geometry, amino acids can change their protonation state as well affecting the charge distribution within the protein chain and the contribution of each part to the shift of the pigment energies.