The characterization of the initial degradative alterations of Bombyx mori silk fibroin: A comparative study of artificially aged silk fabric and silk fabrics from The First Ladies Collection, Smithsonian Institution, Washington, D.C.

摘要

The destructive effect of light on silk fibroin is evaluated in the context of preservation of museum textile and costume collections. Three biochemical techniques: amino acid analysis, solubility and N-terminal analysis were used to detect and quantify microchemical alterations in silk fibroin resulting from exposure to light. A set of artificially aged silk samples was prepared by irradiation in a Weather-ometer{dollar}spcircler{dollar} with increasing exposure and three levels of ultraviolet content. Naturally aged silk fabric samples were obtained from the gowns of the First Ladies in the Smithsonian Institution's First Ladies Collection at the National Museum of American History.; The degradative changes seen in naturally aged silk fabric samples were correlated to the chemical alterations of Bombyx mori silk habutae artificially aged by irradiation using color difference measurements, scanning electron microscopy and biochemical alterations. The artificially aged silk fabrics showed increased yellowing upon irradiation. Morphological alterations were observed only in the highest UV content exposures where the fibers had a "melted" appearance.; Amino acid compositions of artificially aged silk fabric showed a decrease in the hydroxylated amino acids serine and tyrosine. The amounts of hydrolyzable ammonia increase as exposure times and UV content increase, suggesting deamination was also taking place. The increased solubility of silk fabric under denaturing conditions (7 M urea) indicated cleavage of the peptide bonds in the fibroin molecule. The amino acid compositions of the extracted degradation products suggested the amorphous regions of the fibroin molecule degrade first. Molecular weight distributions of the soluble degradation products determined by Fast Pressure Liquid Chromatography (FPLC) support random cleavage of the peptide backbone. N-terminal analysis suggested selective cleavage of the peptide bond initially, which then became random cleavage as the exposure was increased.; Of the three biochemical techniques used in this study, the ratio of glycine to hydrolyzable ammonia, Gly/NH{dollar}sb3{dollar} provided the best approach for placing the naturally aged silks within the context of artificial aging conditions. The presence of sericin in the naturally aged silk fabrics was also estimated from the amino acid compositions.