Cap analogs with a hydrophobic photocleavable tag enable facile purification of fully capped mRNA with various cap structures.

Removing immunogenic uncapped mRNA from in vitro transcribed mRNA is critical in mRNA research and clinical applications. Commonly used capping methods provide a maximum capping efficiency of around 80-90% for widely used Cap-0- and Cap-1-type mRNAs. However, uncapped and capped mRNA possesses almost identical physicochemical properties, posing challenges to their physical separation. Herein, we developed hydrophobic photocaged tag-modified cap analogs, which separated capped mRNA from uncapped mRNA by reversed-phase HPLC. Subsequent photo-irradiation recovers footprint-free native capped mRNA. This approach provided 100% capping efficiency even in Cap-2-type mRNA with versatility applicable to 650 nt and 4,247 nt mRNA. The Cap-2-type mRNA showed up to 3 to 4-fold higher translational activity in cultured cells and animals than mRNA prepared by the standard capping method. Notably, the purification process simultaneously removed immunogenic double-stranded mRNA, another major contaminant of in vitro transcribed mRNA, drastically reducing mRNA immunogenicity in cultured cells.