Carotenoids (Car), P-Carotene (I), 8'-apo-beta-caroten-8'-al (II), and canthaxanthin (III), incorporated into activated Cu-MCM-41 were examined by UV/vis and EPR spectroscopies. A Cu2+-Car complex was formed for I and III but not for II. Formation of a complex results in distortion of the all-trans carotenoid geometry and a tetragonal geometry for Cu2+. The binding energies of Car and Cu2+, the changes in the maximum absorption, and the bond lengths of Car after the formation of the Cu2+-Car complex were examined by semiempirical ZINDO/1 and ZINDO/S calculations. Formation of a complex between Car and Cu2+ favors both forward and back electron transfer (ET) reactions due to the short distance (similar to2 Angstrom) between Car and Cu2+, and reversible ET appears upon temperature cycling.