Synthesis and Evaluation of Diguanosine Cap Analogs Modified at the C8-Position by Suzuki–Miyaura Cross-Coupling: Discovery of 7-Methylguanosine-Based Molecular Rotors

Chemical modifications of the mRNA cap structure can enhance the stability, translational properties, and half-life of mRNAs, thereby altering the therapeutic properties of synthetic mRNA. However, cap structure modification is challenging because of the instability of the 5′-5′-triphosphate bridge and N7-methylguanosine. The Suzuki–Miyaura cross-coupling reaction between boronic acid and halogen compound is a mild, convenient, and potentially applicable approach for modifying biomolecules. Herein, we describe two methods to synthesize C8-modified cap structures using the Suzuki–Miyaura cross-coupling reaction. Both methods employed phosphorimidazolide chemistry to form the 5′,5′-triphosphate bridge. However, in the first method, the introduction of the modification via the Suzuki–Miyaura cross-coupling reaction at the C8 position occurs postsynthetically, at the dinucleotide level, whereas in the second method, the modification was introduced at the level of the nucleoside 5′-monophosphate, and later, the triphosphate bridge was formed. Both methods were successfully applied to incorporate six different groups (methyl, cyclopropyl, phenyl, 4-dimethylaminophenyl, 4-cyanophenyl, and 1-pyrene) into either the m7G or G moieties of the cap structure. Aromatic substituents at the C8-position of guanosine form a push–pull system that exhibits environment-sensitive fluorescence. We demonstrated that this phenomenon can be harnessed to study the interaction with cap-binding proteins, e.g., eIF4E, DcpS, Nudt16, and snurportin.