Passage: The cyclol hypothesis is the first structural model of a folded, globular protein.  It was developed by Dorothy Wrinch in the late 1930s, and was based on three assumptions.  Firstly, the hypothesis assumes that two peptide groups can be crosslinked by a cyclol reaction (Figure 1); these crosslinks are covalent analogs of non-covalent hydrogen bonds between peptide groups.  These reactions have been observed in the ergopeptides and other compounds.  Secondly, it assumes that, under some conditions, amino acids will naturally make the maximum possible number of cyclol crosslinks, resulting in cyclol molecules (Figure 2) and cyclol fabrics (Figure 3).  These cyclol molecules and fabrics have never been observed.  Finally, the hypothesis assumes that globular proteins have a tertiary structure corresponding to Platonic solids and semiregular polyhedra formed of cyclol fabrics with no free edges.  Such "closed cyclol" molecules have not been observed either.

Question: What kind of reaction crosslinks the two peptide groups?
cyclol