triacetylguanosine self-assembly in the presence of alkaline earth metal ions (Ca 2+ ,S r 2+ ,B a 2+ )

We report structural determination of cation-templated self-assembly of a guanosine derivative, 2 0 ,3 0 ,5 0 -O-triace- tylguanosine (TAG), in the presence of three alkaline earth metal ions (Ca 2+ ,S r 2+ , and Ba 2+ ) in CDCl3. Using a combina- tion of nuclear magnetic resonance (NMR) and electrospray ionization mass spectrometry (ESI-MS) methods, we have found that TAG molecules form discrete octamers in the form of (TAG)8M 2+ (M 2+ =C a 2+ ,S r 2+ , and Ba 2+ ), which is com- posed of two G-quartets and a sandwiched metal ion. We have determined the ability of the three alkaline earth metal ions to promote TAG self-assembly (relative binding affinity) to be Sr 2+ ≫ Ba 2+ >C a 2+ . More importantly, we have used two- dimensional (2D) NMR methods to determine the structural details of (TAG)8Sr 2+ . In particular, we found that each octamer consists of an all-anti G-quartet stacking on top of an all-syn G-quartet in a tail-to-head fashion with a twist angle of 45° be- tween the two G-quartets. This TAG octamer structure represents a unique case quite different from other lipophilic guano- sine octamers reported in the literature.