Unraveling the Electronic Structure of Individual Photosynthetic Pigment-Protein Complexes

TLDR

Delocalized excitation in pigment assemblies may underlie the high energy‑transfer efficiency of photosynthetic complexes. Low‑temperature single‑molecule spectroscopy was used to probe the LH2 pigment‑protein complex from purple bacteria. The B800 ring’s excited states are localized on individual BChl a molecules, whereas the B850 ring’s excitation is fully delocalized across the ring.

Abstract

Low-temperature single-molecule spectroscopic techniques were applied to a light-harvesting pigment-protein complex (LH2) from purple photosynthetic bacteria. The properties of the electronically excited states of the two circular assemblies (B800 and B850) of bacteriochlorophyll a (BChl a) pigment molecules in the individual complexes were revealed, without ensemble averaging. The results show that the excited states of the B800 ring of pigments are mainly localized on individual BChl a molecules. In contrast, the absorption of a photon by the B850 ring can be consistently described in terms of an excitation that is completely delocalized over the ring. This property may contribute to the high efficiency of energy transfer in these photosynthetic complexes.