Studies on the degraded state of excavated archaeological silk fibers using infrared micro-spectroscopy and curve fitting analysis

Abstract

The almost archaeological silk fibers are heavily degraded due to the long-term preservation at underground. The authors have been continuing the scientific research on the degraded state of silk fibers using infrared microspectroscopy. Though we have previously reported results about the characteristics of deformed infrared spectra by curve fitting procedure of the secondary components of fibroin molecules, obtained results were not completed. In the present report, we used the modified procedure to get more reasonable peak top wavenumbers for curve fitting using the second and fourth derivative spectrum of sample infrared spectrum. The refined silk fiber (Bombyx mori) was used as a modern reference material. The samples of degraded silk fibers were excavated from Fujinoki tumulus (6th C. AD) and Shimoikeyama tumulus (3rd C AD) in Nara Pref. in Japan. As a result of curve fitting analysis, it was found that the Amide I peak of the reference silk fiber is consisted of eleven component peaks. The strong component peaks are at 1659 cm-1 (amorphous) and 1650 cm-1(β-sheet). The infrared spectrum of Fujinoki sample showed thirteen component peaks in Amide I peak. Among them, peak at 1672 cm-1 (β-sheet) showed strong intensity compared with that of corresponding reference spectrum. On the other hand, the component peak intensity at 1667-1655 cm-1 (amorphous) is evidently decreased compared with that of corresponding reference spectrum. Namely it was shown that the crystallinity of fibroin molecules increases in the degraded state.