Sulphydryl group protection

The production of sulphur compounds is believed to be very important in the development of Cheddar cheese flavour. Residual sulphydryl oxidase activity may play a role in initially reoxidizing sulphydryl groups exposed upon heating cheesemilk the sulphydryl groups thus protected may be reformed during the ripening process. 

Another common property of synthetases is their sensitivity to low concentrations of sulphydryl-group reagents. The protective action of the substrate(s) reinforces the conclusion that these SH groups are essential for catalytic activity. 

Erythrocytes (red blood cells) require NADPH for a different reason, i.e. to protect indirectly against (i) the oxidation of the sulphydryl groups of haemoglobin (Section 4.7) with concomitant impairment of its performance in oxygen transport and (ii) cell lysis due to the oxidation of unsaturated lipids of the cell membrane. 

The best known, and most widely used sulphydryl protecting group is the benzyl group. Benzylation takes place by reacting benzyl chloride in the presence of base with the thiol in aqueous or non-aqueous media (reaction 4 R = H) > . The reaction could take place also with the sodium... 

Instead of using Cys-containing peptides it is convenient to use S-acetylmercaptoacetyl (SAMA) peptides for conjugation purposes. Unlike Cys, SAMA is not sensitive towards (air) oxidation during purification of the peptide. SAMA can be introduced into deprotected peptides in solution by reaction with N-succinimidyl S-acetylmercaptoacetate (18, 24). Selective introduction of SAMA can be achieved at the end of solid phase synthesis by N-terminal modification (Chapter 6) of the side-chain protected and resin-bound peptide with pentafluorophenyl 5-acetylmercaptoacetate (SAMA-OPfp) and 1-hydroxybenzotriazoIe (25). The thiol group of SAMA-peptides can be liberated by reaction with hydroxylamine. This S-deacetylation can be performed in the conjugation reaction mixture (25,26), i.e. in the presence of the sulphydryl-reactive protein Protocol 2). 

---