Chemical modification of silk fibroin with cyanuric chloride-activated poly(ethylene glycol): Analyses of reaction site by proton NMR spectroscopy and conformation of the conjugates

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Abstract
Solubilized silk fibroin (SF) in 0.1 M borate buffer (pH 9.4) was modified with 2-O-[methoxy(polyethylene glycol)]-4,6-dichloro-s-triazine (actPEG1) at 4 degrees C. The weight of the modified SF (PEG1-SF) was at least 3.2 times that of the starting material SF. Amino acid analysis of PEG1-SF suggested that the nucleophilic epsilon-amino group of the lysine residue and the nucleophilic imidazole group of the histidine residue in SF reacted with actPEG1. The 1H-NMR spectrum of PEG1-SF showed a downfield shift of the aromatic protons of the tyrosine residue from the corresponding protons of SF. The 1H-NMR spectrum of the SF reacted with cyanuric fluoride (CyF), whose fluorine atoms are known to react with the phenolic hydroxyl group of the tyrosine residue, also showed the downfield shift. These results suggested that the reaction site of SF with actPEG1 was the phenolic hydroxyl group of the tyrosine residue in addition to the lysine and histidine residues. The conformation of PEG1-SF in a solid state was examined by means of IR and X-ray measurement. The IR spectrum of PEG1-SF revealed a change in secondary structure from random coil to beta-sheet due to the coexistence of PEG molecules. The X-ray diffraction pattern of PEG1-SF indicated that the PEG molecules covalently bonding to SF narrowed the spacing of the interchain periodicity and promoted the formation of the interchain beta-sheet.

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