Trimethylaminoethylation of cysteine

The cysteinyl residue may be converted to a strongly basic derivative, 4-thialaminine, by alkylation with (2-bromoethyl) trimethylammonium 

The virtues of this modification reaction lie in the very high stability of 4-thialaminine under conditions of acid hydrolysis, and the ready solubility of a number of trimethylaminoethylated proteins in aqueous solution. 

For complete alkylation of half-cystine residues, after the DTT reduction procedure described in 3.8.2.1, (2-bromoethyl) trimethyl-ammonium bromide is added in a 3-fold molar excess over the DTT concentration. After reaction for 24 to 48 hr under nitrogen, at room temperature, the protein derivative is desalted either by gel filtration, or exhaustive dialysis, and lyophilized. 

The 4-thialaminine content of the protein derivative is determined by amino acid analysis after acid hydrolysis ( 2.5.6). 

---