The role of the local environment on the structural heterogeneity of carotenoid β-ionone rings

Our analysis of the X-ray crystal structure of canthaxanthin (CAN) showed that its ketolated β-ionone rings can adopt two energetically equal, but structurally distinct puckers. Quantum chemistry calculations revealed that the potential energy surface of the β-ionone ring rotation over the plane of the conjugated π-system in carotenoids depends on the pucker state of the β-ring. Considering different pucker states and β-ionone ring rotation, we found six separate local minima on the potential energy surface defining the geometry of the keto-β-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 β-ionone ring rotation from a specific pucker type can reach up to 8 kJ/mol ( 669 cm - 1 ). 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 S 0 → S 2 transition were estimated both for the CAN monomer and in the CAN crystal. The difference between them reached 690 cm - 1 , 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.