You look at norcysteine and you go “How in the world is this stable?”. But apparently it is stable… Jean Rivier of the Salk Institute in La Jolla has done really nice work that demonstrates some of the unusual properties of this amino acid. Here is a representative paper:

http://pubs.acs.org/doi/abs/10.1021/ol0606740

You might imagine that the corresponding SS bonds would be really different (shorter or longer SS bond?) from typical Cys-Cys connections. The thing for me is that the aldehyde oxidation state of the alpha-carbon might not mesh well with a reasonable expectation for configurational stability of this particular amino acid. However, norcysteine-containing peptides are configurationally stable at the alpha methine position. One can imagine a ton of applications for this molecule. I doubt we will see a variant of the native chemical ligation, but who knows?