Effect of 13-cis violaxanthin on organization of light harvesting complex II in monomolecular layers

Lutein, neoxanthin and violaxanthin are the main xanthophyll pigment consti tuents of the largest light-harvesting pigment-protein complex of photosyst em II (LHCII). High performance liquid chromatography analysis revealed pho toisomerization of LHCII-bound violaxanthin from the conformation all-trans to the conformation 13-cis and 9-cis. Maximally, the conversion of 15% of all-trans violaxanthin to a cis form could be achieved owing to the light-d riven reactions. The reactions were dark-reversible. The all-trans to cis i somerization was found to be driven by blue light, absorbed by chlorophylls and carotenoids, as well as by red light, absorbed exclusively by chloroph yll pigments. This suggests that the photoisomerization is a carotenoid tri plet-sensitized reaction. The monomolecular layer technique was applied to study the effect of the 13-cis conformer of violaxanthin and its de-epoxidi zed form, zeaxanthin, on the organization of LHCII as compared to the all-t rans stereoisomers. The specific molecular areas of LHCII in the two-compon ent system composed of protein and exogenous 13-cis violaxanthin or 13-cis zeaxanthin show overadditivity, which is an indication of the xanthophyll-i nduced disassembly of the aggregated forms of the protein. Such an effect w as not observed in the monomolecular layers of LHCII containing all-trans c onformers of violaxanthin and zeaxanthin. 77 K chlorophyll a fluorescence e mission spectra recorded from the Langmuir-Blodgett (L-B) films deposited t o quartz from monomolecular layers formed with LHCII and LHCII in the two-c omponent systems with all-trans and 13-cis isomers of violaxanthin and zeax anthin revealed opposite effects of both conformers on the aggregation of t he protein. The cis isomers of both xanthophylls were found to decrease the aggregation level of LHCII and the all-trans isomers increased the aggrega tion level. The calculated efficiency of excitation energy transfer to chlo rophyll a from violaxanthin assumed to remain in two steric conformations w as analyzed on the basis of the chlorophyll a fluorescence excitation spect ra and the mean orientation of violaxanthin molecules in LHCII (71 degrees with respect to the normal to the membrane), determined recently in the lin ear dichroism experiments [Gruszecki et al., Biochim. Biophys. Acta 1412 (1 999) 173-183]. The calculated efficiency of excitation energy transfer from the violaxanthin pool assumed to remain in conformation all-trans was foun d to be almost independent on the orientation angle within a variability ra nge. In contrast the calculated efficiency of energy transfer from the form cis was found to be strongly dependent on the orientation and varied betwe en 1.0 (at 67.48 degrees) and 0 (at 70.89 degrees). This is consistent with two essentially different, possible functions of the cis forms of violaxan thin: as a highly efficient excitation donor (and possibly energy transmitt er between other chromophores) or purely as a LHCII structure modifier. (C) 2001 Elsevier Science B.V. All rights reserved.