Density Functional Theoretical Analysis of the Molecular Structural Effects on Raman Spectra of β‐Carotene and Lycopene

Abstract

The molecular structural and Raman spectroscopic characteristics of β-carotene and lycopene are investigated by density functional calculations. The effects of molecular structure and solvent environment on the Raman spectra are analyzed by comparing the calculated and measured results. It is found that the B3LYP/6-31G(d) method can predict the reasonable result for β-carotene, but the ν1 Raman activities of lycopene overflow at all the used theoretical methods because of the longer conjugation length. The calculated results indicate that the rotation of β-rings in β-carotene impedes the delocalization of π-electrons, shortens the effective conjugation length, and results in higher frequency and lower activity of the ν1 mode in β-carotene than lycopene. The measured ν1 bands of β-carotene and lycopene shift respectively to higher and lower frequencies in solution compared with that in crystals since the crystal packing forces can lead to different conformational variations in the carotenoids molecules. The polarized continuum model theoretical analysis suggests that solvent has slight (significant) effects on the Raman frequencies (intensities) of both carotenoids. Figure 1Constitution formulas of (a) β-carotene and (b) lycopene.

Mechanisms Underlying Carotenoid Absorption in Oxygenic Photosynthetic Proteins

“…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).…”

“…This is illustrated in Fig. 9, A-D, where the four end rings are compared with that of the DFT-calculated in vitro structure (42). Both cycles of the (blue-absorbing) ␤-carotene perpendicular to the membrane plane (Bcr645 in Protein Data Bank structure 3ARC) make a large angle with the conjugated CϭC chain (dihedral angles 59º and 68°; Fig.…”

“…Although no solution structure currently exists for either carotenoid, density functional theory (DFT) simulations performed on ␤-carotene predict dihedral angles for the end cycles of ϳ47° (42, 50, 51; see Fig. 9), and this has been calculated to shorten the conjugation length by the exact amount predicted from the Raman 1 position (42). Bringing one of these end cycles back into the plane of the CϭC conjugated chain should accordingly result in a net increase of the effective conjugation length of these molecules of approximately 0.6 -0.7, exactly as observed here for the redabsorbing lutein and ␤-carotene in LHCII and PSII-RC.…”

Modeling Dynamic Conformations of Organic Molecules: Alkyne Carotenoids in Solution

“… 5 , 31 , 44 A polar environment can cause the shift of Raman frequencies, 31 which can be determined for specific solvents by the PCM method, with a proportional shift between experiment and calculations. 6 , 31 , 37 We chose the B3LYP/cc-pVDZ method for the present study, available in the Gaussian 09 package (Rev D.01). 50 The calculations were performed with the “nosymm” keyword, disabling attempts to identify the point group of the molecule.…”

Comparison of Four Types of Raman Spectroscopy for Noninvasive Determination of Carotenoids in Agricultural Products

“…The difference before and after are measured by Raman spectroscopy and HPLC [43]. Combined with density functional theory, the molecular structure are also analyzed by comparing the calculated and determination results [44].…”