Unprecedented Occurrence of Isoaspartic Acid in a Plant Cyclopeptide

Three structurally related cycloheptapeptides, cyclocitropsides A–C, have been isolated from a MeOH extract of the root bark of Citropsis articulata, a medicinal plant in Uganda. Their sequences were elucidated on the basis of their MS/MS fragmentation, extensive 2D-NMR, chemical degradation, and biochemical modifications. Surprisingly, the sequence of cyclocitropside C differed from that of cyclocitropside B only by an Asp5/isoAsp5 substitution. This is the first report of an isoAsp residue in a plant cyclic peptide.