Ring-to-chain conformation may be a determining factor in the ability of xanthophylls to bind to the bulk light-harvesting complex of plants

The binding of xanthophylls to the main light-harvesting complex (LHC) of higher plants has been studied using the technique of in vitro reconstitution. This demonstrated that the carotenoid diol lactucaxanthin (native to many LHC) would not support the assembly of LHC whilst other diols, notably zeaxanthin and lutein would. Analysis of the most stable forms of the carotenoid end-groups found in xanthophylls native to higher plant LHC (as determined by theoretical calculations) revealed profound differences in the adiabatic potential energy curves for the C5-C6-C7-C8-torsion angle for the e end-groups in lactucaxanthin (6-s-trans), in comparison to carotenoids possessing a 3-hydroxy beta end-group (zeaxanthin; 6-s-cis), 3-hydroxy-4-keto beta end-group (astaxanthin, 6-s-cis) or a 3-hydroxy-5,6-epoxy end-group (violaxanthin, distorted 6-s-cis). The epsilon end-groups of other carotenoids studied were 6-s-trans. We examine the possible relationship between carotenoid ring-to-chain conformation and binding to LHC. (C) 2002 Elsevier Science B.V. All rights reserved.