Arrangement and interaction of pigment molecules in reaction centers of Rhodopseudomonas viridis Photodichroism and circular dichroism of reaction centers at 100 K

Abstract

The linear dichroism of light-induced absorbance changes (photodichroism, p) accompanying photooxidation of the primary electron donor ( P), a bacteriochlorophyll dimer (BChl) 2-960, or photoreduction of the intermediary electron acceptor, bacteriopheophytin (BPh), as well as circular dichroism (CD) of the reaction centers of Rhodopseudomonas uiridis at 100 K in their various redox states: P · BPh, P ⨥ · BPh ⨪ and P · BPh ⨪ , have been measured. Photooxidation of P is accompanied by bleaching of the absorption and CD bands at 980 nm and a considerable change of the frequency, a decrease of the dipole and rotational strengths of the transition at 850 nm with the preservation of dipole and rotational strengths of the BPh transition at 790 nm. On the other hand, the photoreduction of BPh is accompanied by bleaching the absorption and CD bands of BPh at 790 nm and a considerable change of the frequency, a decrease of dipole and rotational strengths of the transition at 830 nm with the preservation of the dipole and rotational strengths of the transitions at 850 and 980 nm. Consideration of these data in terms of the static quantum treatment of reaction centers as molecular aggregates shows: (i) a direct interaction between BPh and (BChl) 2-960 is not observed; (ii) BPh mainly interacts with BChl monomer absorbing at 830 nm (BChl-830); (iii) (BChl) 2-960 mainly interacts with BChl monomer absorbing at 850 nm (BChl-850); and (iv) there is also an interaction between BChl-830 and BChl-850 since the dipole strengths of these transitions differ from each other but their sum corresponds to the sum of the dipole strengths of two BChl monomers. The p values of the transitions have been used for the estimations of the angles between the dipole-vectors of the transitions in reaction centers. Estimations of the distances and interaction energy between the transitions of the pigment molecules in the reaction center are given.