Cysteins and homocysteins are found in many natural and artificial peptide sequences. Mercaptoproline is a rigid analogue of homocysteine offering several options to the medicinal chemist.

The distance between S and carboxylic acid function is the same as in homocysteine, however, flexibility of the side chain is reduced and thus rigidity increased. One interesting additional advantage is that this building block provides a new possibility to form disulfide bridges at the positions of prolines or derivatizing this particular position with reagents bearing mercapto reactive groups like for example maleinimides of dyes or PEG-iodides.

Derivatives are available with both optical configurations of the mercapto position.