Effect of locked nucleic acid modifications on the thermal stability of noncanonical DNA structure.

We studied the kinetic and thermodynamic effects of locked nucleic acid (LNA) modifications on parallel and antiparallel DNA duplexes. The LNA modifications were introduced at cytosine bases of the pyrimidine strand. Kinetic parameters evaluated from melting and annealing curves showed that the association and dissociation rate constants for the formation of the LNA-modified parallel duplex at 25.0 °C were 3 orders of magnitude larger and 6 orders of magnitude smaller, respectively, than that of the unmodified parallel duplex. The activation energy evaluated from the temperature-dependent rate constants was largely altered by the LNA modifications, suggesting that the LNA modifications affected a prenucleation event in the folding process. Moreover, thermodynamic parameters showed that the extent of stabilization by the LNA modification for parallel duplexes (3.6 kcal mol(-1) per one modification) was much more significant than that of antiparallel duplexes (1.6 kcal mol(-1)). This large stabilization was due to the decrease in ΔH° that was more favorable than the decrease in TΔS°. These quantitative parameters demonstrated that LNA modification specifically stabilized the noncanonical parallel duplex. On the basis of these observations, we succeeded to stabilize the parallel duplex by LNA modification at the physiological pH. These results can be useful in the rational design of functional molecules such as more effective antisense and antigene strands, more sensitive strands for detection of target DNA and RNA strands, and molecular switches responding to solution pH.