Cysteine added during peptide

Cysteine-containing peptides can be conjugated to sulphydryl-reactive proteins, provided that the cysteine is not essential for the biological activity. If the peptide is devoid of Cys, this amino acid can be added N- or C-terminally during synthesis. Depending on the linker attached to the solid phase, C-terminal incorporation is not recommended to avoid possible racemization of Cys (17). 

Special chemical derivatives free thiol functions of cysteine can be problematic because of post-synthetic uncontrolled oxidation. To avoid this, you can replace Cys by serine (Ser), alanine (Ala), or u-aminobutyric acid (Abu). Alternatively, choose the hydrophilic Cys(Acm) and leave protected. For the simultaneous preparation of peptides of different size with free amino terminus, couple the terminal amino acid as /V-Boc derivatives so that they will not become acetylated during the normal elongation cycle. Boc is removed during the final side chain deprotection procedure (Protocol 4). Special labels can be attached to the N-termini by spotting respective derivatives in an additional coupling cycle. We have succes lly added biotin via the in situ formed HOBt-ester (normal activation procedure) or fluorescein via the isothiocyanate (FITC) dissolved in NMP. 

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