Vibrational study of sulphated wool, bombix mori and tussah silk fibres.

There is a growing interest in exploiting biotechnological tools for the functionalization of natural polymers with the aim to create a new range of high-performing, environmental friendly processes and materials for traditional and innovative applications. As proteins, fibroin and wool can be chemically modified at side chain groups of constituent amino acids, thus improving their properties according to the desired function. Recently Tamada reported that sulphated silk fibroin and sericin expressed anticoagulant activity like heparin. Moreover, Gotoh et al. observed that sulphated fibroin inhibits HIV replication in vitro [3]. Here we present a comparative IR and Raman study on wool, Bombyx mori and Tussah silk fibroin sulphated with chlorosulphonic acid in pyridine in order to elucidate the affinity for sulphate groups, the mechanism and the mode of linkage, the amino acid side chains involved, and the possible conformational changes caused by sulphation. All the fibres were analyzed by ATR/FT-IR and FT-Raman spectroscopies after a 3 hour treatment. The spectra showed that all the samples underwent conformational rearrangements upon sulphation, as revealed by the positions and relative intensities of the Amide I, II and III modes. Moreover, the samples degraded to different extents, Tussah silk fibres being the least degraded. The IR spectra showed more pronounced changes if compared with the Raman ones. The main IR bands attributable to sulphated groups in sulphated fibres were identified and their tentative assignments are reported. The spectrum of sulphated B. mori silk fibres was characterized by new strong absorptions appearing at 1049 and 1014 cm-1, attributable to vibrations of organic sulphate salts. Sulphation also resulted in a progressive increase in intensity and broadening of the bands in the 1300-1180 cm-1 range, where asSO2 of alkyl sulphate salts, asSO2 of sulphonamides, and sSO2 of organic covalent sulphates fall and overlap the Amide III band of silk. The Raman spectrum of the same sample showed new components due to sulphate groups at 575 and 622 cm-1 (SO3), and 1065, 1055, and 1025 cm-1 (sSO2 of different sulphate residues [6]), 1015 cm-1 (CO of organic sulphated salts). The Raman intensity ratio between the two Tyr bands at 850-830 cm-1 (I850/I830) is sensitive to the hydrogen bonding state of the Tyr phenoxyl group and has been extensively used as an indicator of Tyr interactions in globular proteins, their assemblies and their degree of exposure to water. The I850/I830 intensity ratio increased from 1.41 to 1.52 upon sulphation, suggesting a change towards a more exposed state of Tyr residues in sulphated B. mori fibroin. This feature is in agreement with a more disordered conformation detected by both IR and Raman spectroscopy. The trend of both IR and Raman spectra revealed that B. mori fibroin covalently bound sulphate groups mainly through the hydroxyl groups of Tyr and Ser residues leading to the formation of sulphate salts. The spectra of Tussah fibres showed minor changes with respect to B. mori silk fibroin; they showed only weak shoulders at 1015 cm-1 (IR, CO of organic sulphated salts) and 1029 cm-1 (Raman, sSO2 of different sulphate residues). The I850/I830 intensity ratio decreased from 1.55 to 1.22 upon sulphation, suggesting a change towards a less exposed state of Tyr residues in sulphated fibres. These trends suggest that Tussah silk fibres poorly reacted, in agreement with the low weight gain measured for these samples. Wool fibres were the most reactive towards sulphation. Among the analysed samples, the vibrational spectra of sulphated wool fibres showed the most pronounced changes, in agree with the weight gain measurements, suggesting the highest affinity for sulphated groups. The IR spectrum was charactrized by strong absorptions at about 1200 and 1000 cm-1 ; the Raman spectrum showed strong b...