Water-Based Solid-Phase Peptide Synthesis without Hydroxy Side Chain Protection

The development of protecting group-free synthesis has come to the forefront this century, as there is an increasing need to switch to greener synthetic methods. In peptide synthesis, a strategy of maximum protection offers the most efficient synthetic pathway, but minimal side chain protection is more favorable in terms of green chemistry. Here, we describe solid-phase peptide synthesis (SPPS) without hydroxy side chain protection based on an aqueous microwave (MW)-assisted method. First, we investigated the extent of O-acylation of the hydroxy side chain of Ser, Thr, and Tyr occurring in our method, which uses 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride. Under aqueous MW-assisted conditions, the coupling reaction proceeded efficiently without substantial O-acylation. Next, we applied the aqueous synthetic protocol without hydroxy side chain protection to synthesis of a laminin-related peptide, H-Tyr-Ile-Gly-Ser-Arg-NH₂. HPLC analysis of the crude peptide revealed a single peak, suggesting the absence of side reactions including O-acylation and racemization. We also succeeded in synthesizing a difficult peptide sequence, acyl carrier protein (65-74) peptide, by aqueous SPPS without hydroxy or carboxamide side chain protection. Based on the eighth criterion of the 12 principles of green chemistry, namely, "reduce derivatives", our approach without hydroxy side chain protection will provide a greener peptide synthesis.