STRUCTURAL-ANALYSIS OF THE LYOTROPIC POLYMORPHISM OF 4-STRANDED AGGREGATES OF 2'-DEOXYGUANOSINE 3'-MONOPHOSPHATE DERIVATIVES

Guanosine derivatives, dissolved in water, can form chromonic choleste ric and hexagonal phases. The common structural unit is a chiral stack of Hoogsteen-bonded guanosine tetramers. Using optical microscopy, ci rcular dichroism and X-ray diffraction techniques, we have analysed th e complex columnar polymorphism of two derivatives of 2'-deoxyguanosin e the 3'-monophosphate d(Gp) and the 3'-monophosphate monoisobutyl est er d(Gp)iBu. As a function of water concentration, both compounds exhi bit four different columnar phases: in addition to the well-known chol esteric (Ch) and hexagonal (H) phases, a 2-dimensional square (Sq) lat tice and a new hexagonal packing have been observed. In such a structu re (H(b)), two columns are packed in the 3m special positions of co-or dinates 1/3, 2/3 and 2/3, 1/3 of the two-dimensional hexagonal unit ce ll. The unusual phase sequence appears to be related to the presence o f strong short range intercolumnar interactions. Moreover, the easing of stacking constraints on increasing dilution seems to induce a tilti ng of the guanosine residues which form the tetramers.