Stark spectroscopy of the light-harvesting complex II in different oligomerisation states

Abstract

The electric field-induced absorption changes (Stark effect) of light-harvesting complex II (LHCII) in different oligomerisation states—monomeric, trimeric and aggregated—have been probed at 77 K. All the chlorophyll (Chl) a molecules exhibit electro-optic properties in the Q y absorption region characterized by a change in dipole moment | Δ μ → |=0.6±0.06 D/f and polarizability, Tr(Δ α̃)∼55±5 Å 3/ f 2 upon electronic excitation, which are similar to those of unbound monomeric Chl a, indicating the absence of strong delocalization of the excitations which would be expected in the presence of strong excitonic interactions. The Stark effect in the Chl b absorption region is significantly bigger with | Δ μ → | values of the order of 2.0±0.2 D/ f and it is attributed to strong interactions with neoxanthin molecules. Clear oligomerisation-dependent differences are observed in the carotenoid region, mainly due to the appearance of a new xanthophyll absorption band at 509 in the spectra of trimers and oligomers. It is ascribed to some lutein molecules, in agreement with previous experimental observations. The electro-optic properties of these lutein molecules are significantly different from those of the other xanthophylls in LHCII, which do not exhibit such a big change in dipole moment upon electronic excitation ( | Δ μ → | =14.6±2.0 D/ f). Upon aggregation of LHCII some extra absorption appears on the red side of the main Chl a Q y absorption band. In contrast to an earlier suggestion [J. Phys. Chem., A 103 (1999) 2422], no indications are found for the charge-transfer character of the corresponding band. The assignments of the S 2 electronic transitions of neoxanthin and lutein in LHCII and possible origins of the Stark effect are discussed.