Direct measurement of specific interactions between nucleic acid base pairing

Surface forces between apposed surfaces modified with thymine (T) or adenine (A) monolayers have been determined by surface forces measurement at various pH's. For a complementary base pair (T-A), a long ranged attraction was always observed, which can be explained by the so-called "long ranged hydrophobic attraction" and electric double layer attraction. On the other hand, the forces between non-complementary base pairs (T-T, A-A) changed from attraction ro repulsion depending on pH's, which reflects the acid dissociation of the nucleic acid bases. The acid dissociation constants of T and A on the surfaces were estimated to be ca. 7 and 4, respectively. Adhesive (pull-off) forces have also been measured. The complementary base pair (T-A) showed a larger adhesive force than the non-complementary ones and had a maximum at the neutral pH region, which means that the complementary base pair exhibits the specific molecular recognition ability effectively at the physiological pH region.