Espectroscopia Raman aplicada ao estudo de carotenos do tomate

Abstract

In this study, we used Raman spectroscopy to characterize carotenoids composition during tomato (Lycopersicon esculentum) ripening and to identify the biosynthetic precursors phytoene and phytofluene. The analysis of maturity stages was carried out directly on the exocarp, without any prior preparation of the sample. The Raman spectra recorded in situ suggested the presence of molecules containing 7, 9 and 11 conjugated double bonds along the maturation process. The number of conjugated double bonds was correlated with the wavenumbers referring to the stretch vibrations C=C. Spectral analyzes also showed variations in the composition and distribution of carotenoids throughout the maturation process. The distribution of carotenoids on the surface (exocarp) of the tomato is not homogeneous and presents alternation of pigments regardless of the stage of maturation. In order to isolate and identify by Raman spectroscopy the carotenes produced during the maturation process we performed extraction of the exocarp by different methodologies using ethanol, acetone and petroleum ether as solvents. The petroleum ether extract was purified in an open column using Hyflosupercel and magnesium oxide (2:1) as a stationary phase. The eluents used were ethyl ether in petroleum ether and acetone in petroleum ether. The spectroscopic characterization of the fractions obtained from the open column was performed on an FT-Raman and on a UV / Vis spectrophotometer in the region between 200 to 1100 nm. The results obtained from the spectroscopic techniques indicated the presence of carotenes phytoene and phytofluene in fractions F1 and F2, respectively, β-carotene in fraction F5 and lycopene in mixture with neurosporene in fraction F10. The unambiguous attribution of the Raman bands obtained from colorless carotenes was performed by comparison of spectral data from the standard sample cis/trans-phytoene and spectra obtained from vibrational frequency calculations using the DFT method. The results showed that Raman spectroscopies associated with UV-Vis are suitable to characterize the main carotenes biosynthesized during the maturation process.