Figure 4.

Chemical structures of lutein (a), zeaxanthin (b) and meso-zeaxanthin (c). The ionone rings of lutein have different shapes. The β-ring contains a single stereocenter (3R); the ∊-ring contains two stereocenters (3′R, 6′R). Asymmetry in the ionone shape distinguishes lutein from the other ocular xanthophylls and may contribute to selectivity for lutein and exclusion of zeaxanthin and meso-zeaxanthin, which each have two β-ionone rings (3R and 3′R in zeaxanthin; 3R and 3′S in meso-zeaxanthin).