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-str anded helices; for compound (5'-->5')-d(GpG), this leads to the format ion of cholesteric and hexagonal liquid crystalline phases. Both phase s are columnar and the cholesteric phase is left handed. This behaviou r is a further confirmation of the tendency of guanine derivatives to self-assemble to give stacked columnar structures whenever not impossi ble for structural reasons. The CD spectra of the aggregates in isotro pic solutions are dominated by a negative exciton couplet centred arou nd 250 nn associated to a left-handed columnar chirality. The shapes o f the profiles, in the 220-300-nm region, for (5'-->5')-d(GpG) (in wat er 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 solut ion 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 investiga ted 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 structur es: 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 sa lt 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 C D only in the presence of extra salt. (C) 1998 Wiley-Liss, Inc.