Binding of Amino Acids and Peptides

The chemical reactions to conjugate amino acids or peptides with polystyrenes and other supports naturally depend on the type of functional sites on the gel phase. In most cases, chloromethyl groups are used to bind the starting N-protected amino acid - which is the C-terminal one of the target sequence to be synthesized — to the insoluble but swollen polymer. To date this is most efficiently done by utilizing caesium salts of the carboxylic components [84], which are simply prepared with caesium bicarbonate or hydroxide. To prevent base catalyzed racemization and side reactions on the polymer like saponification of ester bonds or of chloromethyl functions by caesium hydroxide not neutralized, N-pro-tected amino acids are used in a slight excess. 

Two milliequivalents of the lipophilic amino acid caesium salts, so formed in dimethyl-formamide, completely transform chloromethyl functions up to 0.8 milliequivalents/g of 0.5% cross-linked polystyrene at elevated temperature into benzyl ester bonds (Fig. 28) [85]. 

However, already in a minor excess of 50% the caesium salts not only of N-protected amino acids but di- and tripeptides under identical conditions completely react with bro-moacetyl groups on polymer, because of the enhanced electrophilic reactivity of those sites (Fig. 29) [85]. The phenacyl ester so formed are particularly suitable in experiments to synthesize fully protected peptide fragments, an aspect which will be discussed in Sect. 3.5. 

To circumvent this uncertainty without use of the caesium salt procedure, hydroxymethyl [87] or aminomethyl functions on the polymer can be reacted with carboxylic partners in an esterification reaction [207] or peptide bond formation with the aid of condensing agents like carbonylbisimidazole [88], dicyclohexylcarbodiimide [89] or others. For this purpose in the author s laboratory the use of symmetric anhydrides [3] of the N-protected amino acids to be attached to the support was found to be most effective [90], especially in the formation of activated esters on the gel phase with phenolic hydroxyl functions [38]. By this procedure, on 0.5% cross-linked polystyrenes, load levels up to 1.5 millimoles/g of the support are reached. 

The subsequent binding of peptides, however, is not possible via symmetric anhydride formation. Even by dicyclohexylcarbodiimide and carbonylbisimidazol condensation with lipophilic tripeptides it is scarcely possible to bind more than 0.2 mmole/g of support. In those cases the attachment to bromoacetyl sites on polystyrene via caesium salts is highly recommended [85]. 

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