Electronic Absorption and Ground State Structure of Carotenoid Molecules

Abstract

Predicting the complete electronic structure of carotenoid molecules remains an extremely complex problem, particularly in anisotropic media such as proteins. In this paper, we address the electronic properties of nine relatively simple carotenoids by the combined use of electronic absorption and resonance Raman spectroscopies. Linear carotenoids exhibit an excellent correlation between (i) the inverse of their conjugation chain length N, (ii) the energy of their S0 → S2 electronic transition, and (iii) the position of their ν1 Raman band (corresponding to the stretching mode of their conjugated C═C bonds). For cyclic carotenoids such as β-carotene, this correlation is also observed between the latter two parameters (S0 → S2 energy and ν1 frequency), whereas their "nominal" conjugation length N does not follow the same relationship. We conclude that β-carotene and cyclic carotenoids in general exhibit a shorter effective conjugation length than that expected from their chemical structure. In addition, the effect of solvent polarizability on these molecular parameters was investigated for four of the carotenoids used in this study. We demonstrate that resonance Raman spectroscopy can discriminate between the different effects underlying shifts in the S0 → S2 transition of carotenoid molecules.

Mechanisms Underlying Carotenoid Absorption in Oxygenic Photosynthetic Proteins

“…This correlation is plotted in Fig. 2 for ␤-carotene (blue) and lutein (red), and displays a different slope to that for different Cars in the same solvent (black), as already observed and discussed for several Car molecules (26).…”

“…2 and Ref. 26). These cyclic Cars, when isolated in solvents, actually display effective conjugation lengths of 9.3 and 9.6, respectively.…”

“…These cyclic Cars, when isolated in solvents, actually display effective conjugation lengths of 9.3 and 9.6, respectively. This result was attributed to rotation of the conjugated end cycles out of the plane, such that the ring CϭCs are only partially conjugated (26,42). The position of the main absorption transition of Car molecules also tightly depends on the properties of the solvent in which they are dissolved, and particularly on its refractive index (17)(18)(19)(20)(21)(22).…”

“…In RR spectra, the frequency of the CϭC stretching mode of these molecules (the 1 band) gives direct access to the structure of the alternated system of their electronic ground state. As we recently showed (26) and as illustrated in Fig. 2, a different relationship is observed between the frequency of the 1 band and the position of the S 0 3 S 2 transition, depending on the molecular mechanism tuning this transition.…”

“…It is striking that in lutein, where only one rocking mode should be concerned, the intensity of the additional component is weaker than in ␤-carotene (where potentially two methyl groups may be concerned). In solvents, where it was concluded that the end cycles are at least partially out of the plane, the 3 is also observed to be broader in Raman spectra at room temperature (26). The structure of this band could thus be a direct indication of the conformation of the end cycles in both ␤-carotene and lutein, representing a probe for this mechanism of conjugation-length modulation.…”