Structural organization of guanosine derivatives in dilute solutions: small angle neutron scattering analysis.

Guanosine derivatives, dissolved in water, can form cholesteric and hexagonal mesophases. The common structural unit is a chiral rod-shaped aggregate consisting of a stack of Hoogsten-bonded guanosine tetrameric disks. In order to elucidate the self-association process, we decided to investigate, by small-angle neutron scattering, the structural properties of d(pG), d(GpG), d(GpGpG), d(GpGpGpG) and d(GpGpGpG pGpG) derivatives in very dilute solutions. Under our experimental conditions only d(pG) seems not to form detectable particles. On the other hand, the results for the other derivatives indicate that cylindrical aggregates, having a 10 A cross-section gyration radius and a length of about 70 A, exist in the isotropic phase. According to the structure of the hexagonal and cholesteric phases, we fitted the experimental data by using a model of rod-shaped aggregates formed by stacking about 18 to 20 guanosine tetramers. Moreover, from the measurement of the concentration of scattering particles, we deduced that guanosine derivatives are only partially aggregated, depending on their ability to form mesophases.