Expedient Synthesis of Difficult Proteins via Fmoc Chemistry by Embedding 2-(2-Hydroxyphenyl)thiazolidine Dipeptides (HTDs) as Aggregation Disruptors

Although the chemical protein synthesis offers an effective method for access to customized proteins and their derivatives, the synthesis of water-insoluble polypeptides with difficult sequences is still challenging for conventional solid-phase peptide synthesis (SPPS) and restricts the subsequent chemical ligations. In this study, 2-(2-hydroxyphenyl)thiazolidine dipeptides (HTDs) were developed as effective and robust building blocks to overcome these obstacles. The facilely obtained HTDs can be directly installed in difficult sequences with or without solubilizing tags through the standard Fmoc-SPPS. The switchable tolerance against trifluoroacetic acid also makes HTDs potential protecting groups, which could be readily removed according to need. The effectiveness and applicability of this new method have been demonstrated by the successful synthesis of a small protein glutamine synthetase translational inhibitor (GstI), a membrane-associated protein [Nle1,37, Gly19]BM2(1–51), and a multicyclic protein calcicludine, which plays a role in excitatory synaptic transmission.