ACCORDING to the cyclol theory of the structure of proteins1—a working hypothesis recently put forward in these columns—the polypeptide, the essential unit in the molecule on the classical theory, is replaced by the cyclized polypeptide (Fig. 1): correspondingly the classical peptide links (joining one pair of C,N atoms) are replaced either by the double peptide links (joining two pairs of C,N atoms) or by the triple peptide link (joining three pairs of C,N atoms) (Fig. 2). The reaction polypeptides r proteins is thus regarded as a ring chain tautomerism2, which takes the form if the appropriate groups are in the imide form, and the form if they are in the amide form.