In extending a previous project that examined the Raman and SERS spectra of berberine in combination with density functional theory (DFT) calculations that indicated a flat adsorption geometry of the molecule on the Ag surface, the current article reports on the study of related protoberberine alkaloids, palmatine, jatrorrhizine, and coptisine.
Quaternary protoberberine alkaloids are a class of natural dyes characterized by bright colors ranging from yellow to orange. Since they present a strong fluorescence emission, their analysis by Raman spectroscopy is limited to specific techniques, such as Fourier transform (FT)‐Raman and spectral shift Raman techniques such as shifted subtracted Raman difference spectroscopy (SSRDS) and shifted excitation Raman difference spectroscopy (SERDS). The previous study by the authors successfully used surface‐enhanced Raman scattering (SERS) in the analysis of the alkaloid dye berberine in an ancient textile. The same adsorption geometry as in berberine was deduced. The authors found that the four alkaloids, although minimally different in their chemical structures, could be differentiated by the position of marker bands. Those bands are the most enhanced ones in the SERS spectra, which appear in the 700–800 cm−1 region. In the current project, the same adsorption geometry as in berberine was deduced. It found that the four alkaloids, although minimally different in their chemical structures, could be differentiated by the position of marker bands. Those bands are the most enhanced ones in the SERS spectra, which appear in the 700–800 cm−1 region. (publisher abstract modified)
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