2-Sulphobenzyl, a new solubilizing and reversible protecting group for cysteine in proteins. Its scope and limitations

S-2-Sulphobenzylcysteine and S-2-(sulphomethyl)benzylcysteine are prepared by alkylation of cysteine with omega-toluenesultone and 2,3-benzo-1,4-butanesultone respectively. Owing to the presence of the sulphonic acid group, these protected cysteine derivatives are extremely water-soluble and are stable to acid hydrolysis. The groups can be removed by treatment with sodium in liquid NH3. Reduction with tributylphosphine and simultaneous alkylation of insulin with toluenesultone under mild conditions (pH 8.3, aq. 50% propanol) gives the fully S-substituted derivatives in excellent yield; they can be separated by isoelectric precipitation of the S-sulphobenzylated B-chain. Treatment of the latter with sodium in liquid NH3 led simultaneously to the removal of the protecting groups and to the well-documented cleavage at the threonine-proline bond which can be prevented by addition of sodium amide. When deprotected A-chain was recombined with B-chain, insulin was isolated in the same yield and with the same degree of biological activity as that in the control experiment.

Diffusion of long-chain sulphonic acids*

The straight paraffin chain sulphonic acids are of extraordinary interest on account of their simple chemical nature combined with the whole manifold of soap-like behaviour which their solutions exhibit. They are like saturated fatty acids in which the carboxy group at the end has been replaced by the true sulphonic acid group SOsH. Being free acids already, they cannot hydrolyse. They are typical colloidal electrolytes like all soaps and many modern detergents. There are at present two chief conflicting interpretations of the behaviour of colloidal electrolytes, that of McBain et al. and that of Hartley et al. McBain and Salmon (1920), in defining the class of colloidal electrolytes and reviewing the osmotic as well as the electrical behaviour together with E.M.F. data, migration, and ultrafiltration, found it necessary to postulate the existence of two kinds of colloidal particles which he names neutral micelle and ionic micelle. Hartley (1936 a, b ) and many others assume only one kind of colloidal particle with properties intermediate between McBain’s neutral and ionic micelles. However, their interpretation has been based almost solely upon conductivity data neglecting thermodynamic data. The diffusion data here to be presented cannot be explained on the basis of one kind of colloidal particle.