3'-Hydroxyechinenone (3'-hECN) is a chromophore of the orange carotenoid protein (OCP). Excitation of 3'-hECN bound to OCP to the S-2(1(1)B(u)(+)) state yields an emission characteristic of an intramolecular charge transfer (ICT) state. The present computational results indicate that the Si state of 3'-hECN is of the 2(1)A(g)(-) character and slightly gains an ICT character through the mixing with the S-2(1(1)B(u)(+)) state when the polyene chain is twisted. Formation of hydrogen bonds with OCP enhances the 2(1)A(g)(-)/1(1)B(u)(+) mixing and hence the ICT character in the Si state. Such carotenoid-protein interaction may explain the observation of the ICT emission. (C) 2016 Elsevier B.V. All rights reserved.