ELECTRONIC EXCITED STATES OF CHLOROPHYLLS IN PHOTOSYNTHETIC COMPLEX CP29

Light harvesting – the first step of photosynthesis – is carried out by photosystems I and II (PSI and PSII) that are located in the thylakoid membrane of chloroplasts. PSII oxidizes water and produces oxygen molecules that can react with the excited triplet state of chlorophyll and generate singlet oxygen which can cause photo-oxidative damage to PSII. For this reason, plants have developed protective mechanisms such as non-photochemical quenching (NPQ) [1]. Quenching sites have been identified in LHCII and CP29 antenna complexes of PSII. NPQ in LHCII is thought to be correlated with Chl–Chl charge-transfer (CT) states [2]; this could also be the case for CP29 because it is positioned between the outer antenna complexes and the reaction center.

In this work, the high resolution crystal structure of spinach photosynthetic complex CP29 [3] was chosen for the identification of Chl–Chl CT states in CP29. CT states can form between strongly interacting chlorophylls, so only those chlorophyll dimers with distances less than 10 Å between chlorophylls were used in calculations. The geometries of chlorophylls forming the dimers were optimized in vacuum using density functional theory (DFT). In all calculations performed in this work, the phytyl group in chlorophyll structure was changed to methyl group. The excited state energy spectra and related parameter values for each dimer were calculated using time-dependent density functional theory (TD-DFT). The values of static and transition dipole moments and the sums of Mulliken charges were taken into consideration to identify CT states in dimers.

Nine CT states were identified in five of the six analyzed chlorophyll dimers and are presented in Table 1 together with the energy values. CT states with the lowest energies could possibly be related to NPQ and other processes in PSII; the lowest energy CT state was identified in dimer b614–a613. The results were compared to the experimental data in Ref. [4], where time-resolved fluorescence was measured in CP29 and its knock-out mutants, i.e. CP29 lacking either chlorophyll a612 or a603; these chlorophylls form strongly coupled dimers a611–a612 and a603–a609 that were analyzed in this work.