Figure 1. Chemical structures of lutein, zeaxanthin and meso-zeaxanthin.

Lutein is isomeric with zeaxanthin, differing only in the placement of one double bond in one of the end rings. The principal natural stereoisomer of lutein is (3R,3′R,6′R)-beta,epsilon-carotene-3,3′-diol. Zeaxanthin exists in three stereoisomeric forms that result from the configurations at its two chiral centers: the (3R,3′R), (3R,3′S) and (3S,3′S). The principal natural form of zeaxanthin is the (3R, 3′R). Lutein and zeaxanthin can be interconverted in the body through an intermediate called meso-zeaxanthin, which is the (3R,3′S) stereoisomer.