PEGylation by Alkylation

It has later been shown that reagent 31, which can be obtained by the reaction of mPEG or mPEG alkoxide with cyanuric chloride, reacts in a nonselective fashion with the nucleophilic groups present in proteins, such as lysine, tyrosine, histidine, cysteine and serine. PEGylation reactions with 31 and related branched compound 32 are performed at neutral or basic pH (7.4-10.0). 

After the first substitution, the remaining chloride of 31 is reactive enough for a second substitution, and this can lead to undesirable crosslinking of the proteins. Several examples of the use of 31 have been published, mainly in early studies, including the PEGylation of ovoalbumin [137,138], bovine serum albumin [20], superoxide dismutase [139], elastase [140], acyl-plasmin-streptokinase complex [141], trypsin [142], silk fibroin [143] and immunoglobulin [144], catalase [145,146], amongst others. 

Epoxide 34 can be prepared by nucleophilic reaction of mPEG with the oxirane ring of epichlorohydrin in the presence of BFj.Et O complex. The intermediate alcohol product is then dehalogenated with sodium aluminate to afford the required mPEG reagent 34 [170]. 

---