Convergent Hybrid Phase Ligation Strategy for Efficient Total Synthesis of Large Proteins Demonstrated for 212-residue Linker Histone H1.2

Simple and efficient total chemical synthesis of large proteins remains a significant challenge. Here, we report development of a convergent hybrid phase native chemical ligation (CHP-NCL) strategy that should be generally applicable for facile preparation of large proteins. Key to the strategy is the use of sequential ligation on the solid phase for the directed assembly of ~100-residue segments from short, synthetically accessible peptide components. These segments can then be assembled via convergent solution phase ligation, exploiting o-aminoaniline as a chemically flexible cryptic thioester with multiple activation modalitiies on resin and in situ. We demonstrate the feasibility of our approach through the total synthesis of 212-residue linker histone H1.2 in unmodified, phosphorylated, and citrullinated forms, each from eight component peptide segments. We further demonstrate that fully synthetic H1.2 replicates the binding interactions of linker histones to intact mononucleosomes, as a proxy for the essential function of linker histones in the formation and regulation of higher order chromatin structure.