The Reaction of Thiol and Disulphide Groups with Mercuric Chloride and Methylmercuric Iodide. II. Fibrous Keratins

The reaction of HgCl2 with wool has been studied polarographically over the pH range 1 to 9 under various conditions. At pH 1 to 6 the salt appears to be bound in the un-ionized form probably at amide and peptide group sites. The binding is greatly suppressed in the presence of complexing anions such as Cl- and S2O3=. SO3= also has this complexing effect but in addition produces new -SH binding sites by reversibly splitting -SS- bonds. This splitting proceeds to completion at pH 7 to 10 in the presence of an excess of mercurial, HgCl2 requiring several days and MeHgI several hours. The reaction of keratins with SO3= and mercurials may be followed polarographically and has provided a convenient method of preparing keratins with zero or decreased -SS- bond contents. The reaction rate increases in the presence of 8~ urea and then forms the basis for new analytical methods of measuring the -8s- content of intact keratins. The keratin (7.5 mg or more) is reacted with Na2SO3 in 8M urea (5 ml) for at least 24 hr (with excess HgCl2) or 2 hr (with excess MeHgI) at 20 °C. Single current readings at -1 0 and -0.8 V (v. S.C.E.) respectively at the dropping mercury electrode indicate the residual mercurial and hence by difference the (-SS- plus -SH) content. The -SH content of intact keratins is estimated by reacting say 25 mg with excess MeHgI at pH 7 for at least 4 hr at 20 °C in the absence of SO3=. This mercurial reacts much more rapidly with the -SH sites in keratin than do other mercurials.