Cyclic peptides nomenclature

Cyclic peptides are analyzed less frequently then the linear ones but their sequencing and nomenclature are much more complicated. There is no generally accepted nomenclature scheme but we describe here the one proposed by Ngoka and Gross [4]. It takes account of Roepstorff-Fohlmann s and Biemann s conventions but introduces amendments to address cyclic peptide fragmentation. 

Figure 6.10. Nomenclature of product ions of cyclic peptides.

When molecules are composed of several peptide units, they may adopt the extended conformation, in which the entire backbone is nearly planar and each peptide unit is in the trans form. More often, however, peptides, whether they are linear or cyclic, are folded. The folding is accomplished by rotations about the N—C bonds and the C —C bonds. An international commission has established a convention for the nomenclature (lUPAC-IUB Commission on Biochemical Nomenclature, 1970) for torsional angles (Fig. 1). According to this convention, the torsional angles f, and a>i equal 180° for a fully extended chain. For a planar trans peptide unit, a>i = 180°, whereas... 

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