The role of the local environment on the structural heterogeneity of carotenoid [beta]-ionone rings

Our analysis of the X-ray crystal structure of canthaxanthin (CAN) showed that its ketolated [beta]-ionone rings can adopt two energetically equal, but structurally distinct puckers. Quantum chemistry calculations revealed that the potential energy surface of the [beta]-ionone ring rotation over the plane of the conjugated [pi]-system in carotenoids depends on the pucker state of the [beta]-ring. Considering different pucker states and [beta]-ionone ring rotation, we found six separate local minima on the potential energy surface defining the geometry of the keto-[beta]-ionone ring-two cis and one trans orientation for each of two pucker states. We observed a small difference in energy and no difference in relative orientation for the cis-minima, but a pronounced difference for the position of trans-minimum in alternative pucker configurations. An energetic advantage of [beta]-ionone ring rotation from a specific pucker type can reach up to 8 kJ/mol ( [Formula omitted]). In addition, we performed the simulation of linear absorption of CAN in hexane and in a unit cell of the CAN crystal. The electronic energies of [Formula omitted] transition were estimated both for the CAN monomer and in the CAN crystal. The difference between them reached [Formula omitted], which roughly corresponds to the energy gap between A and B pucker states predicted by theoretical estimations. Finally, we have discussed the importance of such effects for biological systems whose local environment determines conformational mobility, and optical/functional characteristics of carotenoid.