Nature of low-energy exciton levels in light-harvesting complexII of green plants as revealed by satellite hole structure.

Abstract

Persistent non-photochemical hole burning at 4.2K is an efficient experimental tool to unravel position and nature of low-energy excitonic states in pigment-protein complexes. This is demonstrated here for the case of the trimeric chlorophyll (Chl)a/b light-harvesting complexes of Photosystem II (LHCII) of green plants, where previous work (Pieperetal. J Phys Chem B 103:2412, 1999a) reported a highly localized lowest energy state at 680nm. At that time, this finding appeared to be consistent with the contemporary knowledge about the LHCII structure, which mainly suggested the presence of weakly coupled Chl heterodimers. Currently, however, it is widely accepted that the lowest state is associated with an excitonically coupled trimer of Chl molecules at physiological temperatures. This raises the question, why an excitonically coupled state has not been identified by spectral hole burning. A re-inspection of the hole burning data reveals a remarkable dependence of satellite hole structure on burn fluence, which is indicative of the excitonic coupling of the low-energy states of trimeric LHCII. At low fluence, the satellite hole structure of the lowest/fluorescing ~ 680nm state is weak with only one shallow satellite hole at 649nm in the Chlb spectral range. These findings suggestthat the lowest energy state at ~ 680nm is essentially localized on a Chla molecule, which may belong to a Chla/b heterodimer. At high fluence, however, the lowest energy hole shifts blue to ~ 677nm and is accompanied by two satellite holes at ~ 673 and 663nm, respectively, indicating that this state is excitonically coupled to other Chla molecules. In conclusion, LHCII seems to possess two different, but very closely spaced lowest energy states at cryogenic temperatures of 4.2K.