Base stacking of simple mRNA cap analogues. Association of 7,9-dimethylguanine, 7-methylguanosine and 7-methylguanosine 5'-monophosphate with indole and purine derivatives in aqueous solution

Equilibrium constants for the association of different ionic forms of 7,9-dimethylguanine, 7-methylguanosine and 7-methylguanosine 5'-monophosphate with indole, caffeine and various methylated adenines have been determined by distributing the latter compounds between an organic solvent and aqueous solutions of the 7-methylguanine derivatives. The data are compared to those obtained for the association of unsubstituted purine with the same cosolutes. The stacking affinity of both cationic and zwitterionic forms of the 7-methylguanine ring correlates with the ring polarizability rather than the polarizing power of the cosolute. The cationic species stacks usually more efficiently. The chemical nature of the N9-substituent has only a moderate influence on the base-stacking properties.