Heterogeneity of excitonic properties in ensemble of disordered bacterial reaction center complexes

Olga Rancovaa, Ryszard Jankowiakb, Darius Abramaviciusa
aDepartment of Theoretical Physics, Vilnius University, Sauletekio al 9-III, 10222 Vilnius, Lithuania; bDepartment of Chemistry and Department of Physics, Kansas State University, Manhattan, KS 66506, US

Pigment - protein complexes are inherently disordered. Usually both experimental and theoretical researches performed on bulk samples present and discuss ensemble-averaged properties of the complexes and make conclusions based on averaged values. However for the same averaged value the distribution of properties within the ensemble can be quite different thus revealing different features of pigment-protein complexes.

We simulate spectra of the photosynthetic Rhodobacter sphaeroides bacterial reaction center at low (5 K) temperature in different states and explore the excitonic properties [1]. At such low temperature more features appear in the spectra and it implies stronger restrictions on the simulations parameters. The obtained ensemble averaged theoretical values are in good agreement with the experimental absorption and spectral hole burning data and extracted lifetimes and band structure. Whereas the investigation of properties of the individual realizations of the statistical ensemble provides more insight on the undergoing excitation, energy transfer and relaxation processes and enables to interpret some experimental data from different viewpoint (Figure 1). Taking into account the heterogeinity of pigment-protein complexes allows to simulate and explain absorption and hole burned spectra of the (M)L214G mutant of bacterial reaction center assuming two subpopulations within the statistical ensemble. We find that excitation lifetimes become widely distributed. Some reaction centers get very long intermediate state lifetimes.

Figure 1: Distribution of lifetimes of 3rd exciton in ensemble calculations of spectra of oxidized bRC.

Reference

[1] O. Rancova, R. Jankowiak, A. Kell, M. Jassas, and D. Abramavicius, J. Phys. Chem. B 2016.