Light-induced Change of Configuration of the LHCII-Bound Xanthophyll (Tentatively Assigned to Violaxanthin): A Resonance Raman Study

Raman scattering spectra of light-harvesting complex LHCII isolated from spinach were recorded with an argon laser, tuned to excite the most red-absorbing LHCII-bound xanthophylls (514.5 nm). The intensity of the v(4) band (at ca. 950 cm(-1)) corresponding to the out-of-plane wagging modes of the C-H groups in the resonance Raman spectra of carotenoids appears to be inversely dependent on the probing laser power density. This observation can be interpreted in terms of excitation-induced change of configuration of the protein-bound xanthophyll owing to the fact that the intensity of this particular band is diagnostic of a chromophore twisting resulting from its binding to the protein environment. The comparison of the shape of the v(4) band of a xanthophyll involved in the light-induced spectral changes with the shape of the v(4) band of the xanthophylls present in LHCII, reported in the literature, lets us conclude that, most probably, violaxanthin is a pigment that undergoes light-driven changes of molecular configuration but also the involvement of lutein may not be excluded. Possible physical mechanisms responsible for the configuration changes and physiological importance of the effect observed are discussed.