SPECIFIC FORCES BETWEEN DNA BASES

Molecular recognition occurs at all levels of living matter but the me chanisms are not understood in physical terms. One striking example is that of DNA whose properties are intimately related to the specific m olecular interactions of four nucleosides, based on hydrogen-bonds and size complementarities. We have directly measured the interaction bet ween two of them, adenosine and thymidine, using a surface force appar atus. In these experiments, lipids functionalised with nucleosides wer e synthesised, and used to coat the surfaces between which forces were measured. The interactions of complementary molecules were compared t o those, markedly different, for which the complementarity was hindere d by a small modification of one of the molecules. The distance range of the specific forces, deduced from this comparison, was surprisingly long. The adhesion energy of the surfaces covered by these nucleoside s were highly specific. Binding energies obtained from these measureme nts were in good agreement with values from the literature. The result s also show that without tile size effect existing in DNA, H-bonds alo ne can generate the specificity. An unusual behaviour, attributed to t he sticky and fluid character of the layers, was pointed out. A long-r ange non specific interaction, also unexpected, was found. These featu res observed on surfaces coated with chemical functional groups may pa rtly result from a collective behaviour. They illustrate the variety o f physical effects one can obtain by playing oil the chemistry of a su rface.