Side Reaction During the Deprotection of Cys(Acm)-Containing Peptides with Iodine. Synthesis of Disulfide Fragments from Cathepsin D Structure

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Abstract
Peptides H-TPPQC(Acm)FTV-NH2 (I) and H-VSVPC(Acm)QSASSAS-NH2 (III) were prepared by the solid phase method. Their oxidation with iodine afforded hexadecapeptide II, tetracosapeptide IV and eicosapeptide VI. The disulfide peptides II, IV and VI are designed according to the sequence of the processing loop in human cathepsin D. The purity of the peptides was determined by analytical HPLC and capillary zone electrophoresis. In addition to the expected [M + H]+ ion, FAB MS of HPLC-pure tetracosapeptide IV exhibited a molecular ion with the same relative molecular mass as the starting dodecapeptide III, in spite of clean HPLC separation of III and IV. Free-flow zone electrophoresis of IV separated peptide V, isomeric with III. Mass spectra, amino acid analysis and Edman sequencing revealed that the peptide V is a product of iodine-mediated S-->O shift of Acm group in the serine-rich peptide III. Daughter-ion spectra of protonated molecules, recorded after collision-induced dissociation, have shown that the Acm moiety is bonded to Ser 9 or Ser 10.

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