Background: Tuning of carotenoid function, through modulation of their electronic properties, is seen throughout Nature. Results: Two photosynthetic proteins are able to modulate the effective conjugation length of bound carotenoid cofactors. Conclusion: Altering the conformation of conjugated end cycles via steric hindrance provides a means of tuning the electronic properties of carotenoids. Significance: A novel mechanism tuning the functional properties of carotenoids is revealed.

We report here the resonance Raman spectra and the quantum chemical calculations of the Raman spectra for β-carotene and 13,13'-diphenyl-β-carotene. The first aim of this approach was to test the robustness of the method used for modeling β-carotene, and assess whether it could accurately predict the vibrational properties of derivatives in which conjugated substituents had been introduced. DFT calculations, using the B3LYP functional in combination with the 6-311G(d,p) basis set, were able to accurately predict the influence of two phenyl substituents connected to the β-carotene molecule, although these deeply perturb the vibrational modes. This experimentally validated modeling technique leads to a fine understanding of the origin of the carotenoid resonance Raman bands, which are widely used for assessing the properties of these molecules, and in particular in complex media, such as binding sites provided by biological macromolecules.