Temperature-induced isomerization of violaxanthin in organic solvents and in light-harvesting complex II

Three main xanthophyll pigments are bound to the major photosynthetic pigment-protein complex of Photosystem II (LHCII): lutein, neoxanthin and violaxanthin. Chromatographic analysis of the xanthophyll fraction of LHCII reveals that lutein appears mainly in the all-trans conformation, neoxanthin in the 9′-cis conformation and major fraction of violaxanthin in the all-trans conformation. Nevertheless, a small fraction of violaxanthin appears always in a cis conformation: 9-cis and 13-cis (approximately 4% and 2% in the darkness, respectively). Illumination of the isolated complex (5 min, 445 nm, 250 μmol m−2 s−1) results in the substantial increase in the concentration of the cis steric conformers of violaxanthin: up to 6% of 9-cis and 4% of 13-cis. Similar effect can be obtained by dark incubation of the same preparation for 30 min at 60 °C. Heating-induced isomerization of the all-trans violaxanthin can also be obtained in the organic solvent system but the formation of the 9-cis stereoisomer has not been observed under such conditions. The fact that the appearance of the 9-cis form of violaxanthin is specific for the protein environment suggests that violaxanthin may replace neoxanthin in LHCII in the N1 xanthophyll binding pocket and that the protein stabilizes this particular conformation. The analysis of the electronic absorption spectra of LHCII and the FTIR spectra of the protein in the Amid I band spectral region indicates that violaxanthin isomerization is associated with the disaggregation of the complex. It is postulated that this reorganization of LHCII provides conditions for desorption of violaxanthin from the pigment protein complexes, its diffusion within the thylakoid membrane and therefore, availability to the enzymatic deepoxidation within the xanthophyll cycle. It is also possible that violaxanthin isomerization plays the role of a security valve, by consuming an energy of excessive excitations in the antenna pigment network (in particular, exchanged at the triplet state levels).