Raman spectroscopy of polyconjugated molecules with electronic and mechanical confinement: the spectrum of Corallium rubrum

Abstract

The chemical nature of the red pigment of Corallium rubrum (CR) has not yet been clearly identified. We have recorded the Raman spectra of CR, canthaxantin, and parrot feather, and we propose an interpretation with the help of quantum chemistry and of the effective conjugation coordinate theory, which accounts for the main Raman lines (n 1 , n 2 , n 3 ) as originating from the polyene backbone. In this study, two additional lines in the 1000 cm À1 range (n 4 and n 5 ) are considered as well as a series of overtones and combinations in the second order Raman spectrum (2000-3000 cm À1 ). Density Functional Theory calculations predict that, moving with successive methylation from a simple unsubstituted polyene chain with 9 C¼C bonds (as psyttacofulvins) to a 9, 9′, 13, 13′ tetramethylated chain (as carotenoids), the geometric structures of the molecules and their Raman spectra show changes that can be rationalized if the effective conjugation coordinate theory is extended to account for mechanical confinement. This turns out to be a new concept that helps the interpretation of the Raman spectra of partially methylated conjugated oligoenes. Evidence is found that the main component of the pigment of CR does not posess a fully demethylated polyene chain (psyttacofulfine) nor a tetramethylated chain (carotenoid), thus indicating that the polyene chain is partially methylated. Because we consider resonance Raman spectra, the target of this study is the structure of the polyene backbone; no direct spectroscopic information can be obtained in this way on the chemical nature of the groups attached to both chain ends. Figure 2. (a) Simplified molecular model of a linear polyene chain with the numbering of the unit cells and the definition of the internal vibrational displacement coordinates R (C¼C stretch) and r (CÀC stretch) according to Wilson; [28] (b) sketch of the collective Я coordinate defined by Eqn. (3).Raman spectroscopy of polyconjugated molecules with electronic and mechanical confinement

Conjugated polyenes as chemical probes of life signature: use of Raman spectroscopy to differentiate polyenic pigments

“…On the other hand, the Raman spectra of the conjugated polyenes known as polyenals do not present such enhancement for the band at 1020 cm −1 [40] for the same vibrational mode. This can be explained by the presence of the methyl groups in the middle of the conjugated chain of the carotenoids and the complete absence of such groups at the same positions in the polyenal structure.…”

“…The wavenumber position of δ(C−CH 3 ) at ca 1020 cm −1 presents different behaviour from that of methyl groups arranged as in terpene units; in carotenoids, there is a significant contribution of the δ(C−CH 3 ) to the ν(C−C) mode [41]. The shift of the wavenumber positions of the ν(C−C) stretching mode by ca 30 cm −1 may be an indication that polyenals do not contain the same methylation pattern as occurs in carotenoids; the ν(C−C) mode downshifts as the number of methyl groups decreases in the polyenic chain [40]. Subtle differences in the wavenumber position of δ(C−CH 3 ) account for the identification of totally or partially unmethylated polyenes [40].…”

“…The shift of the wavenumber positions of the ν(C−C) stretching mode by ca 30 cm −1 may be an indication that polyenals do not contain the same methylation pattern as occurs in carotenoids; the ν(C−C) mode downshifts as the number of methyl groups decreases in the polyenic chain [40]. Subtle differences in the wavenumber position of δ(C−CH 3 ) account for the identification of totally or partially unmethylated polyenes [40]. Brambilla et al [40] proposed that pigments from the octocoral Corallium rubrum are not completely demethylated, as are psittacofulvins.…”

“…Subtle differences in the wavenumber position of δ(C−CH 3 ) account for the identification of totally or partially unmethylated polyenes [40]. Brambilla et al [40] proposed that pigments from the octocoral Corallium rubrum are not completely demethylated, as are psittacofulvins. Analysis of the overtones and combination bands indicated that the polyconjugated chain may contain -CH 3 groups; however, no combination band involving C−CH 3 was observed in the psittacofulvin spectra.…”

“…Raman spectroscopic analysis of polyenals or unsubstituted polyacetylenes has revealed that the position of the ν(C=C) and ν(C−C) bands are chain-size and matrix-effect dependent (figures 2 and 3) [27,29,40]. The wavenumber position of δ(C−CH 3 ) at ca 1020 cm −1 presents different behaviour from that of methyl groups arranged as in terpene units; in carotenoids, there is a significant contribution of the δ(C−CH 3 ) to the ν(C−C) mode [41].…”