The deoxyguanosine-5'-monophosphate in aqueous solution self-associates into stable structures, which include hexagonal and cholesteric columnar phases. The structural unit is a four-stranded helix, composed of a stacked array of Hoogsteen-bonded guanosine quartets. We have measured by osmotic stress method the force per unit length versus interaxial distance between helices in the hexagonal phase under various ionic conditions. Two contributions have been recognized: the first one is purely electrostatic, is effective at large distances, and shows a strong dependence on the salt concentration of the solution. The second contribution is short range, dominates at interaxial separations smaller than about 30-32 A, and rises steeply as the columns approach each other, preventing the coalescence of the helices. This repulsion has an exponential nature and shows a magnitude and a decay length insensitive to the ionic strength of the medium. Because these features are distinctive of the hydration force detected between phospholipid bilayers or between several linear macromolecules (DNA, polysaccharides, collagen), we conclude that the dominant force experienced by deoxyguanosine helices approaching contact is hydration repulsion. The observed decay length of about 0.7 A has been rationalized to emerge from the coupling between the 3-A decay length of water solvent and the helically ordered structure of the hydrophilic groups on the opposing surfaces. The present results agree with recent measurements, also showing the dependence of the hydration force decay on the structure of interacting surfaces and confirm the correlations between force and structure.
Deoxyguanosine-5'-monophosphate in water self-associates into stable structures, which include liquid-crystalline hexagonal and cholesteric phases. The structural unit is a four-stranded helix, composed of stacked Hoogsteen-bonded guanosine quartets. By using the osmotic stress method, we recently measured the force between helices in KCl solutions up to 2 M. In addition to the long-range electrostatic force, a short-range hydration repulsive contribution was recognized. The hydration repulsion is exponential, and shows a decay length independent from the ionic strength of the solution. Here, we report that more concentrated KCl solutions cause condensation of the guanosine helix in a hexagonal phase with constant equilibrium separation of approximately 7 A between helix surfaces. Long-range attraction, which induces the self-assembly, and short-range repulsion, which prevents the contact between the helices, are implied. By using osmotic stress, the force needed to push helices closer from the spontaneously assumed position has been measured. The attractive force was then estimated as a difference between the net force and the repulsive contribution, revealing an exponential decay length about two times larger than that of the short-range repulsion. The agreement with the helix interaction theory introduced recently by Kornyshev and Leikin (Kornyshev, A. A., and S. Leikin, 1997. Theory of interaction between helical molecules. J. Phys. Chem. 107:3656-3674) suggests that the repulsive and attractive forces originate from helix-specific interactions.
The lipophilic guanosine derivative 1 acts as a self-assembled ionophore and, in the presence of alkali metal ions, forms chiral polymeric structures in organic solvents. These polymeric columnar aggregates are comprised of G-quartets held together by alkali metal ions which are located inside the tubular structure; the quartets are surrounded by hydrocarbon chains. In hydrocarbon solvents, these columnar aggregates form lyomesophases of the cholesteric and hexagonal type. Copyright 2000 Wiley-Liss, Inc.
To explore the molecular mechanism of the protective function of sugars on cubic lipidic systems, the mesomorphic properties of the monoolein-water system, dehydrated in the presence of a series of sugars, have been studied by osmotic stress experiments. Two bicontinuous inverse cubic structures (Pn3m and Ia3d) and a lamellar L(alpha) phase form under dehydration in pure water. In sugar solutions, the Pn3m phase shows an extraordinary stability: as a function of sugar concentration, the lattice parameter decreases to very low values, but no phase transitions occur. Instead, the Pn3m to Ia3d phase transition is obtained by equilibrating the lipid phase with aqueous polymer solutions of increasing osmotic pressure. As a result, the pressure at which the phase transition occurs strongly depends on sugar concentration. The free-energy curves obtained from the osmotic-pressure unit-cell data show that the sugar exerts an additional stabilization on both the cubic phases. The analysis of the structural parameters indicates that sugars alter the interface geometry. We suggest that a consequent release of stretching contributions in the chain packing or a reduction of the inhomogeneity in molecular splay mainly stabilize the Pn3m phase and prevent the transition to the Ia3d phase on dehydration.
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