MEASUREMENT OF FORCES BETWEEN SURFACES COMPOSED OF 2-DIMENSIONALLY ORGANIZED, COMPLEMENTARY AND NONCOMPLEMENTARY NUCLEOBASES

Langmuir-Blodgett films of amphiphiles with an erotic acid (Oro, uraci l-like) headgroup or a complementary adenine (Ade) headgroup were depo sited onto mica surfaces and investigated by surface force measurement . Interactions between both complementary (Oro-Ade) and noncomplementa ry (Ade-Ade, Oro-Oro) nucleobase pairs were studied. The LB films show ed functional features of the nucleobase headgroups. Electrolyte-depen dent electrostatic repulsion was found at pH's close to the pK(a) of t he headgroups. In the absence of strong electrostatic forces, i.e., Or o-Ade and Oro-Oro combinations at pH 5.6, weak long-range attraction w as detected. At close distances strong attractive forces made the surf aces jump into a contact position. The adhesive force between the comp lementary layers was found to be 100 mN/m, while the values between th e noncomplementary pairs were ca. 50 mN/m and 250 +/- 50 mN/m for Ade- Ade and Oro-Oro, respectively. The force profiles found between nucleo base surfaces are made of two components. One is observed at ca. 100-2 0 nm and changes its intensity drastically depending on conditions. Th is force is found both for complementary and for noncomplementary surf aces and is similar to the very long range attraction reported for hyd rophobic surfaces (Claesson et al., 1988, 1994; Kurihara et al., 1990, 1992; Evans, 1993). The other, appearing at around 20 nm, is characte ristic of complementary pairs and remains unchanged for pH changes and by salt (KBr) addition. These forces may have an important bearing on interaction mechanisms involved in molecular recognition and supramol ecular assembling in biological systems.