Affinity towards sulphation of wool and silk fibres.

Wool keratin and silk fibroin are excellent biopolymers with outstanding properties that make them extremely valuable in biomedicai field. Coatings are commonly applied to the surface of materials to improve their surface properties. The biocompatibility and non-immunogenicity of silk proteins should allow their application as coatings for biomedical implants, potentially as anticoagulants, and either promoters or inhibitors of cell adhesion. Incorporation of sulphate and sulphonate groups confers anticoagulant and anti-thrombogenic properties to polymers. In view of widening the biomedicai utility of natural polymers as biomaterials, here we present a comparative vibrational study on wool, B. morì and A. pernyi silk fibroin fibres sulphated with chlorosulphonic acid in pyridine, which is known to enhance the yield of sulphation. Our aim is to prepare sulphated fibres by keeping the intrinsic fibre properties and texture unchanged, using short reaction times (i.e. 3h). The fibres were analyzed by Attenuated Total Reflectance, ATR/FT-IR and FT-Raman spectroscopy to comparatively 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. Among the analysed samples, the vibrational spectra of sulphated wool fibres showed the most pronounced changes, suggesting the highest affinity towards sulphation. New bands in the 1300-1200 and 1100-900 cm-1 ranges were assigned to the formation of alkyl and aryl sulphate salts, sulphonamides, sulphoamines and covalent aryl-alkyl sulphates. Vibrational spectra revealed the occurrence of a certain fibre degradation as well as rearrangements with consequent changes in secondary structure, conformation of disulphide bridges and tyrosine environment. The amino acid residues mainly involved in sulphation were identified as serine, threonine, tyrosine and tryptophan. Upon sulphation, the IR and Raman spectra of B. mori silk fibroin fibres showed analogous changes although less pronounced than for wool fibres. No significant changes were detected for A. pernyi silk fibroin fibres: only slight conformational rearrangements were observed. The reactivity towards sulphation was found to decrease along the series: wool > B. mori silk fibroin > A. pernyi silk fibroin, in agreement with the weight gain measurements which decreased along the same series. These results can be explained in relation to the different composition of the analysed fibres. Wool fibres were characterised by the highest content of the potentially reactive sites; evidently, these groups had also a good accessibility for the sulphating agent.