THE DECOMPOSITION OF SILK FIBROIN BY SUNLIGHT

(III) ON THE MECHANISM OF THE YELLOWING OF SILK

Abstract

On the mechanism of the yellowing of silk by the sunlight, it was reported recently by Chujo that tyrosine contained 13-14 per cent in silk fibroin was changed into the indole derivatives and then into melanin-like pigments photochemically. This theory however deserves further investigations.
We recognized that tyrosine was easily decomposed in the diluted hydrogen peroxide solution at room temperature, while tryptophan, especialy its indole-ring, was more stable for it. So the silk cloth was dipped in the diluted hydrogen peroxide solution to make tyrosine in it decompose, and was exposed to ultraviolet radiation. The exposed cloth was sifted by means of the rate of yellowing and the reactions for the colouring matters such as nitric acid or Ehrlich's reagent.
These changes were compared with those of the oxidized wool as well as the formalized silk or wool. While, tyrosine and tryptophan fixed on the rayon cloth by polyvinylalcohol were also exposed to light and their changes were detected by paper chromatographic method.
It may be conclueded that the changing process proposed by Chujo does not coincide with the various results obtained in this experiments, and that the leading cause of the yellowing is not attributed to the changes of tryosine, but to tryptophan although the amount contained in silk fibroin is very little.