The self-assembly of dideoxyguanosine (3-3') and (5-5') monophosphates.

The title compounds show a pronounced cation-directed ability to self-assemble in water and to gives columnar structures similar to four-stranded helices; for compound (5'-->5')-d(GpG), this leads to the formation of cholesteric and hexagonal liquid crystalline phases. Both phases are columnar and the cholesteric phase is left handed. This behaviour is a further confirmation of the tendency of guanine derivatives to self-assemble to give stacked columnar structures whenever not impossible for structural reasons. The CD spectra of the aggregates in isotropic solutions are dominated by a negative exciton couplet centred around 250 nn associated to a left-handed columnar chirality. The shapes of the profiles, in the 220-300-nm region, for (5'-->5')-d(GpG) (in water or in saline solutions) and for (3'-->3')-d(GpG) (in KCI solution) are quasi-mirror images of those of poly(G) and (3'-->5')-d(GpG). The appearance of relatively intense CD signals around 280-300 nm in solution of (3'-->3')-d(GpG) in the presence of NaCl resembles that of (3'-->5')-d(GpG) in the presence of Rb+ or Na+. In the compounds investigated in this work, which present two equivalent ends, one observes the two CD features that have been associated, in the current literature, with the signature of four-stranded parallel and antiparallel structures: hence the origin of these CD bands cannot be found in the polarity of the strands. Self-assembly is favoured by the addition of extra salt and the stabilising effect of K+ is greater than that of Na+ in the case of (3'-->3')-d(GpG), an assembled species could be detected by CD only in the presence of extra salt.