Color Forming Property of Derivative and Modification Products of Natural Iridoids with Methylamine

Abstract

Derivatives and modified products of five natural iridoids (geniposide, gentiopicroside, swertiamarin, loganin and morroniside) were obtained by derivatization or modification. Compounds before and after derivatization or modification were adopted to react with methylamine to investigate their color-forming property. The relationship between the color of the pigments and the structure was analyzed using ultraviolet-visible (UV-vis) spectroscopy. The results indicated that: (1) when the backbone of the iridoids was the same, the position of the modified substituent groups had the greatest effect on the color of the pigments formed. For example, methygentiopicroside aglycone and methylswertimarin aglycone have a similar skeleton structure, differing only at positions C-5 and C-6, and the pigments obtained from their reaction with methylamine were also similar i.e. bright yellow. However, geniposidic acid aglycone and genipin, which also have a similar skeleton structure, differing only in the substituent at position C-4, produce pigments of different color upon reaction with methylamine, one being dark blue and the other purple red. (2) In different buffer systems, i.e., phosphate buffered saline (PBS, pH=8.0) and acetate buffer (ACE, pH=5.0), the same iridoid formed pigments with different colors. (3) After the ð-ð conjugation of benzoyl was introduced to the skeleton of loganin, genipin and morronin as a substituent group, the characteristic absorption wavelength of pigments changed. Taking loganin as an example, the characteristic absorption wavelength of pigments changed from 430 nm and 650, to 465 nm and 670 nm when reacted in PBS, and from 560 nm to 610 nm when reacted in ACE. The reason might be that the conjugation system became longer and the molecular structure became more stable.