High-pressure control of photosynthetic excitons

In photosynthesis, collective pigment excitations – excitons – facilitate chemical reactions for sustainable biological function. Here, the effect of hydrostatic pressure – an important thermodynamic stress factor – on optical spectral properties of excitons in the cyclic light-harvesting 2 pigment-protein complex from photosynthetic bacterium Rhodoblastus acidophilus was first studied. The high pressure-induced modifications of absorption, fluorescence and polarized fluorescence excitation spectra were theoretically analyzed in terms of the disordered molecular exciton model. We uniquely show that the observed shift of the spectra under pressure is largely governed by the pressure-induced rise of two factors: the exciton displacement energy and the exciton coupling energy, which increases the spread of the exciton state manifold. A significant increase of static energy disorder revealed by model calculations suggests that high-pressure compressing of the complex is actually accompanied by altered spatial orientations and conformations of the protein-embedded pigment molecules.