Dynamic Stark effect in β and γ carotenes induced by photoexcitation of bacteriochlorophyll c in chlorosomes from Chloroflexus aurantiacus.

Abstract

Chlorosomes of green bacteria can be considered as a prototype of future artificial light-harvesting devices due to their unique property of self-assembly of a large number of bacteriochlorophyll (BChl) c/d/e molecules into compact aggregates. The presence of carotenoids (Cars) in chlorosomes is very important for photoprotection, light harvesting and structure stabilization. In this work, we studied for the first time the electrochromic band shift (Stark effect) in Cars of the phototrophic filamentous green bacterium Chloroflexus (Cfx.) aurantiacus induced by fs light excitation of the main pigment, BChl c. The high accuracy of the spectral measurements permitted us to extract a small wavy spectral feature, which, obviously, can be associated with the dynamic shift of the Car absorption band. A global analysis of spectroscopy data and theoretical modeling of absorption spectra showed that near 60% of Cars exhibited a red Stark shift of ~ 25cm-1 and the remaining 40% exhibited a blue shift. We interpreted this finding as evidence of various orientations of Car in chlorosomes. We estimated the average value of the light-induced electric field strength in the place of Car molecules as ~ 106V/cm and the average distance between Car and the neighboring BChl c as ~ 10Å. We concluded that the dynamics of the Car electrochromic band shift mainly reflected the dynamics of exciton migration through the chlorosome toward the baseplate within ~ 1ps. Our work has unambiguously shown that Cars are sensitive indicators of light-induced internal electric fields in chlorosomes.