Amino acid side chains participate in a number of reactions that result in peptide modification. Some of these reactions are of fundamental significance and play an important role in folding processes. Disulfide formation by oxidation of two proximal cysteine residues is one such case. Apart from its relevance in protein biochemistry, this reversible process also ensures structural integrity of fairly small cyclic peptide systems. Ziconotide is an example of a disulphide-rich analgesic agent derived from a conotoxin.
There are also amino acid reactions one would want to avoid. Today I want to talk about threonine. I always viewed this amino acid with caution (and serine too). If you pay close attention, you will note that threonine looks like the product of an aldol reaction, in which the amide functionality acts as the enolate component, whereas acetaldehyde corresponds to the aldehyde partner. Whenever you see an adol process, you can be sure that its microscopic reverse is feasible. Writing in Organic Process Research and Development, a group from Astellas report a retro-aldol by-product in their synthesis of a cyclic peptide drug candidate. I think this is an important warning for those who work with serine and threonine containing peptides. Be careful with the base you use!