Influence of detergent concentration on aggregation and spectroscopic properties of light-harvesting complex II

Aggregation of photosynthetic light-harvesting complexes strongly influences their spectroscopic properties. Fluorescence yield and excited state lifetimes of the main light-harvesting complex (LHC II) of higher plants strongly depend on its aggregation state. Detergents are commonly used to solubilize membrane proteins and/or to circumvent their aggregation in aqueous environments. Nonlinear polarization spectroscopy in the frequency domain (NLPF) was performed with LHC II over a wide concentration range of the mild detergent n-dodecyl β-d-maltoside (β-DM). Additionally, conventional absorption-, fluorescence- and circular dichroism-spectra were measured. The results indicate that: (i) conventional spectroscopic techniques are not well suited to investigate aggregation effects. NLPF provides a novel approach to overcome this problem: NLPF spectra display dramatic alterations upon even minor β-DM concentration changes. (ii) Commonly used detergent concentrations (around or slightly above the critical micellar concentration) apparently do not lead to complete trimerization of LHC II. A long-wavelength species in the NLPF spectra (peaking at about 685 nm), indicative of residual aggregation, persists up to DM-concentrations of 0.06%. (iii) High-resolution NLPF spectra indicate the existence of a species with a considerably shortened excited state lifetime. (iv) No indication of denaturation was found even at the highest β-DM concentrations used. (v) A specific change in interaction between certain chlorophyll(s) b and a xanthophyll molecule, probably neoxanthin, was detected upon aggregation as well as at higher β-DM concentrations. The results are discussed with respect to the still elusive mechanism of nonradiative dissipation of excess excitation energy in the antenna system.