Empty G-quartets

Without a templating cation, most guanosine analogs form hydrogen-bonded dimers or ribbons. But, this is not always the case. Thus, Sessler and colleagues 

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Figure 16 (a) An empty G-quartet formed by guanine 10. (b) A hydrogen bound network... 

It is clear that, if available, monocations such as potassium and sodium will be incorporated into a quadruplex. The potassium and ammonium ions are too large to be coordinated by a single G-quartet in a coplanar fashion. As a result, coordination of these ions occurs between two G-quartets planes. Each quadruplex involving n quartets will then accommodate n — 1) of these specific ions. For example, quantitative determination of ammonium peak intensity revealed that three NH4 ions are placed between four quartets. In contrast, the smaller Na" ion (ionic radius of 1.18 A) allows for in-plane coordination. Multiple Na" ions are therefore not restricted to the spacing between G-quartets, and can move further away form each other to reduce electrostatic repulsions. In any case, empty sites between quartets are probably very rare. In fact, although vacant coordination sites are likely to exist (as ions move between sites, see below), their lifetime must be very short as demonstrated, for example, by Federiconi et who determined a site occupancy of 0.97 K " ions per tetramer in GMP quadruplexes prepared in 0.5 M KCl. 

Some evidence favoring the quartet state was consistent population of the Cg orbital would promote a dissociative mechanism, but the empty t g orbital would be expected to favor an associative mechanism. 

A bioorganometallic conjugate GAu(III) composed of the guanosine and cyclo-metalated gold(III) moiety is performed to form the empty quartet, octamer, and polymeric columnar aggregate depending on the amount of KPF wherein the formation of the G-quarteruplex via self-assembly induces a k-k interaction (Fig. 4.44) [115]. 

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