Cyclic and square-wave voltammetry have been used to measure the first oxidation potentials of beta-carotene, zeaxanthin, and violaxanthin. Zeaxanthin and violaxanthin are enzymatically interconverted in the xanthophyll cycle, one component of the process of non-photochemical quenching (NPQ), a molecular adaptation mechanism guarding plants against damage from high light stress. The voltammetric potentials were found to correlate well with computed highest occupied molecular orbital (HOMO) energies. The data are important for rationalizing the natural selection of zeaxanthin as an electron donor directly participating in the process of NPQ. (c) 2005 Elsevier B.V. All rights reserved.