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.
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