| Abstract [eng] |
The aim of this work is to investigate Raman spectral shifts induced by the complexation of the apocarotenoid β-apo-8'-carotenal with β-cyclodextrin. The calculations were performed using density functional theory (DFT) with the BHandHLYP functional and the cc-pVDZ basis set. The main objectives were to identify probable molecular complex geometries, calculate Raman spectra of the monomer and its complexes, determine the possible origin of the observed spectral shifts, and compare the results with related effects reported in the literature. Several initial configurations were tested by placing β-cyclodextrin at different positions relative to the apocarotenoid molecule. The inclusion complexes were divided into two groups, denoted as “OF” and “CF”, depending on the orientation of β-cyclodextrin relative to the carbonyl-containing end of β-apo-8'-carotenal. Additional non-inclusion structures were also considered. All candidate structures were optimized, and their binding energies were evaluated using the basis set superposition error (BSSE) correction. The calculated Raman spectra show that complexation with β-cyclodextrin clearly affects the Raman spectrum of β-apo-8'-carotenal. The most pronounced changes are observed in the ν1 band, which is mainly associated with C=C stretching vibrations in the conjugated polyene chain. For the most important structures, the ν1 band shift ranges from approximately −7.7 to −17.2 cm⁻¹. The analysis of optimized structures shows that the main static geometrical parameters of β-apo-8'-carotenal, such as bond lengths and β-ring orientation, change only slightly upon complexation. Therefore, these structural changes alone cannot explain the calculated Raman shifts. However, complexation changes the distribution of the ν1 vibrational mode along the polyene chain. The vibrational contribution near the carbonyl-containing end of the molecule becomes more pronounced, indicating that the Raman spectral changes are most likely related to perturbation of this molecular region and redistribution of the ν1 vibrational mode. The main conclusions of this work are: 1. Complexation with β-cyclodextrin significantly affects the calculated Raman spectrum of β-apo-8'-carotenal, especially the ν1 band. 2. β-Cyclodextrin does not significantly change the equilibrium geometry of β-apo-8'-carotenal, but it affects the distribution of the ν1 vibrational mode along the polyene chain. 3. The calculated Raman spectral changes are most likely related to perturbation of the carbonyl-containing end of β-apo-8'-carotenal and to redistribution of the ν1 vibrational mode caused by interaction with β-cyclodextrin. |
