Cross-linking Staining of Fibroin by Genipin in Solution*

Abstract

Genipin,a natural iridoid compound,was transformed to blue pigment by the reaction with amino acids. Amethyst solution was formed when colorless fibroin abstracted from natural silk was mixed with genipin. The effects of temperature and acidity or alkalinity on genipin-fibroin reaction and the product stability against light were studied with UV-vis spectra analysis. The mechanism of the reaction was primarily investigated through fluorescence spetra. The results indicated that the reaction products had a characteristic absorbance at 590 nm,while high temperature and pH accelerated the color reaction and the color of the reaction products was correlated with pH changing,tending to be blue at acidic pH and purple at alkaline pH. The reaction products possessed a good stability against light because only a small decrease of A590 nm value was observed under natural light for one month or illumination at 5 000 lx for 6 h. The influorescence intensity of fibroin was obviously decreased after the reaction,suggesting that a drastic change of fibroin conformation happened. The results showed that genipin was cross-linked with some amino acid side chains of fibroin and produced a product with stable color. This study would be helpful for staining silk with natural iridoid compound. Fig 5,Tab 1,Ref 18