Raman, SERS, and DFT Analysis of the Main Alkaloids Contained in Syrian Rue

Abstract

Syrian Rue, obtained from the seeds of the Peganum harmala plant, is a reddish colorant especially popular for dyeing carpets and wool fabrics in Western Asia and traditionally used as an alternative source for the madder-derived Turkey Red dye. The main constituents of the Syrian Rue dye are harmane, harmine, harmaline and harmalol, which belong to a group of substances collectively known as harmala alkaloids. These compounds are also called β-carboline alkaloids, as they display very similar molecular structures that consist of either a pyridine ring or a nitrogen-containing six-atom ring with two conjugated double bonds, each fused to an indole skeleton. In the present study, density functional theory was employed to perform a vibrational analysis of the four constituents of Syrian Rue by means of the Gaussian package. First, the molecular geometry of each compound was optimized and its Raman spectrum was calculated. Then, all the observed Raman bands were assigned to specific vibrational normal modes of the corresponding molecule. As a final step, the FT-Raman and surface-enhanced Raman spectroscopy (SERS) spectra of the four structurally related alkaloids were examined and compared, with the ultimate aim to study the adsorption mechanism of these molecules on silver nanoparticles used as SERS substrates.