The cyclol hypothesis is the now discredited first structural model of a folded, globular protein, formulated in the 1930s. It was based on the cyclol reaction of peptide bonds proposed by physicist Frederick Frank in 1936, in which two peptide groups are chemically crosslinked. These crosslinks are covalent analogs of the non-covalent hydrogen bonds between peptide groups and have been observed in rare cases, such as the ergopeptides.