The thiol (-SH) containing side chain of cysteine is the most reactive species present in naturally occurring peptides and consequently cysteine containing peptides must be handled carefully to prevent side reactions. Peptides containing a single cysteine residue can form dimers by oxidation of the cysteine side chain thiols, linking two chains together by a disulphide bridge. Peptides containing two or more cysteines can form intramolecular disulphide bridges giving cyclic peptides. These oxidations normally take place at neutral or higher pH. Therefore, cysteine containing peptides that have been purified in aqueous systems buffered with trifluoroacetic acid are usually protected from oxidation during the purification and lyophilisation process by the low pH of the solvent. However, care must be taken when redissolving the peptide to avoid unwanted oxidation, for example by using acidic solvents or by adding dithiothreitol (DTT). DTT can also be used to reduce disulphide bridges to obtain the original thiols.
Peptides containing two or more cysteine residues may be required as the cyclic disulphide derivative. This cyclisation is usually achieved by air oxidation of a dilute aqueous solution of the peptide at neutral pH.
The unique reactivity of the cysteine side chain allows various useful chemistries to be undertaken.
- When peptides are required for antibody production, a cysteine can be added to the N or C terminus of the sequence allowing the selective conjugation of the peptide to a carrier protein prior to immunization
- Biotinylated peptides can be obtained by reaction of iodoacetyl-biotin with the cysteine side chain thiol to give a stable thioether bond
- A variety of fluorescent dyes can be coupled to peptides through the cysteine side chain