Lanasol dyes and wool fibres. Part I: Model studies on the mechanism of dye fixation in a mixed solvent system

The mechanism of fixation of Lanasol dyes, which are distinguished by their α-bromoacrylamido reactive group, to the various amino acid side chain sites within wool protein is quite complex. It has been proposed that the reaction could proceed by either a nucleophilic substitution or Michael addition pathway. We have investigated this mechanism through the reaction between model dye compounds which possess either an α-bromoacrylamido or α, β-dibromopropionamido group and wool mimetics that contain amine, thiol and hydroxyl groups. These latter groups are typical of the reactive sites found in wool proteins. The reactions were carried out in an acetone-water solvent system. The reactions were monitored by HPLC and the products were isolated and then characterised by proton and carbon-13 NMR, and mass spectroscopy. The results of the study confirmed that the dibromo form of the dye reactive group is only converted to the monobromo form in the presence of model wool compounds and that both forms react with these model wool compounds to yield the same products. When the nucleophilic moiety was an amine, the reaction terminated with a product containing an aziridine ring. No evidence for the proposed reaction of this aziridine ring with a second nucleophilic moiety to form a cross-link could be detected. The thiol of cystiene and imidazole of histidine were also found to reaction with the model dyes.