Model of the association of Chl with Lhcb1. The arrangement of the protein in thylakoid membranes is illustrated according to ref. 50. The "core" Chls (a1, a2, a4 and a5) are shown as Chl a according to ref. . The green color marks positions of Chl b as proposed in the text. Sites a3 and b3, although mixed in occupancy after reconstitution , were assigned as shown based on ligand strength. At least four of the five Chl b molecules are coordinated directly to the protein. The biological requirement of Chl b for accumulation of Lhcb1 (see text) suggests an alternate assignment for a4, as also proposed in ref. .