ABSTRACT
A number of naturally occurring cyclic peptides, such as antibiotics and toxins, have been described in the past 50 years. They can be divided into two general classes: homodetic and heterodetic. In the first case, ring formation occurs only through usual peptide (amide) linkages connecting amino and carboxyl functions, whereas heterodetic cyclic peptides include any other linkages such as lactone, ether, thioether, and, most commonly, disulfide bridge linkages. Investigation of the biological properties of these molecules showed that cyclic structures may exhibit improved metabolic stability and increased potency, receptor selectivity, and bioavailability. Moreover, the constrained geometry of a cyclic peptide allows conformational investigations.
