DMF-Assisted Iodination Side Reaction during the Preparation of Disulfide Peptides, Its Substrate/Solvent/pH Dependence, and Implications on Disulfide-Peptide Production

Abstract

Severe iodination of a disulfide peptide molecule (peptide D) was detected from the on-resin disulfide cyclization process. LC/MS/MS results indicated that iodination occurred on the Thi (thienylalanine) residue. Systematic investigation revealed that the iodination reaction occurred predominantly in the I2/DMF-mediated on-resin disulfide formation reaction. The iodination side reaction was strongly solvent-dependent and favored in DMF and NMP. A tentative mechanism similar to that of the Vilsmeier–Haack reaction was proposed. A variety of amino acid/peptide substrates were tested as iodination substrates. The results indicated the susceptibility of amino acids with electron-rich aromatic groups to iodination. Additionally, various solutions were proposed for preventing peptide iodination based on the findings in this study. This study demonstrates the applicability of the on-resin disulfide production of peptides bearing electron-rich aromatic residues.