The orange carotenoid protein, which contains 3'-hydroxyechinenone (hECN), changes color from orange to red when irradiated with blue-green light. In this study, the origins of the color change have been investigated. The conformation of hECN in the red form is more planar than that in the orange form; consequently, the absorption band is red-shifted on conversion from the orange form to the red form. Another source of the red shift is that the electrostatic field generated by the protein in the red form stabilizes the excited state better than that generated by the protein in the orange form. (C) 2016 Elsevier B.V. All rights reserved.