MEASUREMENT OF INTERCOLUMNAR FORCES BETWEEN PARALLEL GUANOSINE 4-STRANDED HELICES

The deoxyguanosine-5'-monophosphate in aqueous solution self-associate s into stable structures, which include hexagonal and cholesteric colu mnar phases. The structural unit is a four-stranded helix, composed of a slacked array of Hoogsteen-bonded guanosine quartets. We have measu red by osmotic stress method the force per unit length versus interaxi al 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 str ong dependence on the salt concentration of the solution. The second c ontribution is short range, dominates at interaxial separations smalle r than about 30-32 Angstrom, 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 in sensitive to the ionic strength of the medium. Because these features are distinctive of the hydration force detected between phospholipid b ilayers or between several linear macromolecules (DNA, polysaccharides , collagen), we conclude that the dominant force experienced by deoxyg uanosine helices approaching contact is hydration repulsion. The obser ved decay length of about 0.7 Angstrom has been rationalized to emerge from the coupling between the 3-Angstrom decay length of water solven t 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 stru cture of interacting surfaces and confirm the correlations between for ce and structure.