Structural analysis of the lyotropic polymorphism of four-stranded aggregates of 2′-deoxyguanosine 3′-monophosphate derivatives

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Abstract
Guanosine derivatives, dissolved in water, can form chromonic cholesteric and hexagonal phases. The common structural unit is a chiral stack of Hoogsteen-bonded guanosine tetramers. Using optical microscopy, circular dichroism and X-ray diffraction techniques, we have analysed the complex columnar polymorphism of two derivatives of 2′-deoxyguanosine—the 3′-monophosphate d(Gp) and the 3′-monophosphate monoisobutyl ester d(Gp)iBu. As a function of water concentration, both compounds exhibit four different columnar phases: in addition to the well-known cholesteric (Ch) and hexagonal (H) phaseg, a 2-dimensional square (Sq) lattice and a new hexagonal packing have been observed. In such a structure (Hb), two columns are packed in the 3m special positions of co-ordinates 1/3,2/3 and 2/3, 1/3 of the two-dimensional hexagonal unit cell. The unusual phase sequence appears to be related to the presence of strong short range intercolumnar interactions. Moreover, the easing of stacking constraints on increasing dilution seems to induce a tilting of the guanosine residues which form the tetramers.