Raman Spectroscopic Characterization of Carotenoid and Phenolic Co-Occurrence in Euphorbia pulcherrima Bracts and Inflorescences.

Abstract

The present study investigated the carotenoid content in colored bracts, nectar gland, flowers, and pollen of Euphorbia pulcherrima (poinsettia) using Raman spectroscopy. Raman spectra obtained from in situ and crude extracts of the structural components were attributed to carotenoids, phenolic acids, flavonoids, and anthocyanins. Significant changes in the spectral profile were observed according to the bract´s color variation. In the green leaves, bands assigned to carotenoids presented a higher intensity when compared to the main bands ascribed to phenolic acids/polyphenols. Intensity ratios involving ν(C=C) mode at 1527 cm^-1, from carotenoids and ν(C=C)_ar at 1600 cm^-1, from polyphenols decreases in the following order: green leaves (I_car/I_pol = 1.89), yellow (I_car/I_pol = 0.72), red (I_car/I_pol = 0.4), and pink bracts (I_car/I_pol = 0.3). No carotenoid band was detected in white bracts. Raman mapping performed with bracts in transition of the coloration from green to red revealed that both carotenoids and phenolic compounds occur throughout the adaxial surface, but with different color gradients according to the development of the bract. The analysis of the inflorescences showed that carotenoids were the main pigments responsible for the coloration of the yellow nectar gland and pollen grains, while bracts and flowers (pedicel and style) had a predominance of polyphenols. We also used Raman spectroscopy to characterize the efficiency of crude extracts of green leaves and red bract as a reducing agent in the synthesis of silver nanoparticles (AgNPs). The red bract acetone crude extract yielded the most intense localized surface plasmon resonance (LSPR) signals. This work demonstrates the dual utility of E. pulcherrima bracts: as a source of diverse phytochemicals and as a sustainable route to biologically active nanostructured materials.