Xanthophyll pigments in light-harvesting complex II in monomolecular layers: localisation, energy transfer and orientation

Monomolecular layers of the largest light-harvesting pigment-protein comple x of Photosystem II (LHCII) were formed at the argon-water interface. The m olecular area of the LHCII monomer in monomolecular layers determined from the isotherms of compression is found to be close to 14 nm(2), which corres ponds well to the molecular dimensions of the protein evaluated on the basi s of crystallographic studies. Monolayers of LHCII were deposited on a glas s support by means of the Langmuir-Blodgett technique and subjected to spec troscopic studies: electronic absorption spectrophotometry and spectrofluor ometry. The fluorescence excitation spectra of chlorophyll a in monolayers of LHCII were analysed using gaussian deconvolution, Comparison of the abso rption and fluorescence excitation spectra enabled calculation of the rate of excitation energy transfer in the system. Excitation energy was found to be transferred to chlorophyll a from chlorophyll b with 97% efficiency, fr om neoxanthin with 85%, from lutein with 62% and from violaxanthin with at least 54% efficiency. The analysis of the position of the 0-0 absorption ba nd of the xanthophylls revealed that neoxanthin is located in the same prot ein environment as lutein but in a different environment than violaxanthin. The analysis of fluorescence excitation spectra of chlorophyll a in LHCII, recorded with the excitation light beam polarised in two orthogonal direct ions, enabled the determination of the mean orientation angle of the access ory xanthophyll pigments with respect to the plane of the sample. The mean orientation of lutein found in this study (approx. 51 degrees) corresponds well to the crystallographic data. Neoxanthin was found to adopt a similar orientation to lutein. The transition dipole moment of violaxanthin was fou nd to form a mean angle of 71 degrees with the axis spanning two polar regi ons of the protein, perpendicular to the plane of the monolayer, suggesting planar orientation of this pigment with respect to the plane of the thylak oid membrane. These experimentally determined xanthophyll orientations are discussed in terms of importance of peripheral xanthophyll pigments in supr amolecular organisation of LHCII and the operation of the xanthophyll cycle within the thylakoid membrane. (C) 1999 Elsevier Science B.V. All rights r eserved.