Structure,Kinetics and Thermodynamics of DNA Hairpin Fragments in Solution

Abstract

The hairpin-to-coil equilibrium of the hexadecadeoxynucleotide d(ATCCTATTTTTAGGAT) was extensively studied by means of NMR, T-jump and UV. The thermodynamic and kinetic parameters for this equilibrium were determined, yielding a consistent picture of the dynamical behavior of this hairpin structure, which is shown to be a clear example of a situation in which the linebroadening of the imino proton resonances is not determined by the lifetime of the double helix. A comparative study of the homologous hairpins in which the size of the loop was elongated from 4 to 7 thymidine residues shows a monotonous decrease in T_m for the hairpin-to-coil transitions. This finding is in contrast with the view that the stability of hairpins reaches a maximum with a loop size of 6–7 residues. The NMR results indicate that the accessibility of the thymine bases in the loop towards solvent molecules or complementary nucleotides greatly depends on the size of the loop.