Sequence and three-dimensional structure of cycloreticulins A and B, new cyclooctapeptides from the seeds of Annona reticulata

Two cyclooctapeptides, cycloreticulin A, cyclo(Pro1-Gly2-Asp3-Ile4-Ser5-Ile6-Tyr7-Tyr8) (1) and cycloreticulin B, cyclo(Pro1-Mso2-Tyr3-Gly4-Thr5-Val6-Ala7-Val8) (2), have been isolated from the methanol extract of the seeds of Annona reticulata L. The sequences were elucidated on the basis of the MS/MS fragmentation using a QTOF mass spectrometer equipped with an ESI source, chemical degradation and extensive 2D-NMR. The solid state conformation of cycloreticulin A, carried out by X-ray study, is characterised by the presence of two β-turns (types II and III) and an inversed γ-turn. Its solution structure appeared quite similar to the crystal one. The cyclic backbone solution structure of cycloreticulin B, close to that of the cyclooctapeptide squamin A, from which its sequence only differs by a Val8/Ile8 substitution, involves three β-turns, two of type I and one of type III, being similar to the crystal structure of squamin A.