2-Methoxy-4-methylsulfinylbenzyl: A Backbone Amide Safety-Catch Protecting Group for the Synthesis and Purification of Difficult Peptide Sequences

The use of 2-methoxy-4-methylsulfinylbenzyl (Mmsb) as a new backbone amide-protecting group that acts as a safety-catch structure is proposed. Mmsb, which is stable during the elongation of the sequence and trifluoroacetic acid-mediated cleavage from the resin, improves the synthetic process as well as the properties of the quasi-unprotected peptide. Mmsb offers the possibility of purifying and characterizing complex peptide sequences, and renders the target peptide after NH4I/TFA treatment and subsequent ether precipitation to remove the cleaved Mmsb moiety. First, the difficult peptide sequence H-(Ala)(10)-NH2 was selected as a model to optimize the new protecting group strategy. Second, the complex, bioactive Ac-(RADA)(4)-NH2 sequence was chosen to validate this methodology. The improvements in solid-phase peptide synthesis combined with the enhanced solubility of the quasi-unprotected peptides, as compared with standard sequences, made it possible to obtain purified Ac-(RADA)(4)-NH2. To extend the scope of the approach, the challenging A(1-42) peptide was synthesized and purified in a similar manner. The proposed Mmsb strategy opens up the possibility of synthesizing other challenging small proteins.