Structural elucidation of the main coloured compounds of Justicia spicigera based on experimental and computational electronic features

Abstract

In continued efforts to characterise the dye extracted from Justicia spicigera leaves, the two main coloured compounds were isolated. In this study, a multi-analytical techniques characterisation of their structures was carried out by HRMS, NMR, vibrational (IR and Raman) and electronic (UV–Vis absorption and fluorescence) spectroscopies, completed by computational studies. Two unreported compounds from the phenoxazone (aka phenoxazine-3-one) family were identified, the 2-amino-7‑hydroxy-8‑methoxy-3H-phenoxazin-3-one, a positional isomer of the already described perisbivalvin B and the 2-N-(4‑hydroxy-γ-glutamyl)-7‑hydroxy-8‑methoxy-3H-phenoxazin-3-one. The simulated UV–Vis absorption and fluorescence spectra of the suggested structures showed good agreement with the experimental ones, supporting the structural elucidation and giving more insights into the chemistry of these compounds.