Abstract
We have mapped the thermodynamic potentials that drive transitions of DNA in uni- and di-valent salt solutions. The successive mesophases, with measured free energies of deformation and transition, allow computation of interaction potentials as well as transition entropies and enthalpies. We have been able to measure transitions of DNA tetraplexes and duplexes and to compare entropic and enthalpic contributions. Changes in fluctuation free energies are much greater at DNA-ordering transitions for tetraplexes than for duplexes, indicating strong entropic contributions. Disordering due to fluctuations is much greater in the less-ordered (cholesteric) phase, seen in broadening of x-ray scattering peaks. This indicates attraction in the more-ordered phase, where packing is stricter, and the effect of fluctuations is much smaller. This attraction is stronger for quadruplexes than for duplexes.We can read two kinds of information from the x-ray data: the degree of ordering, and the change in density. These changes are much bigger in tetraplexes than in double helical DNA.In addition, we also observe that upon decreasing the applied osmotic stress on the less-ordered phase, there is spontaneous disassembly of tetraplexes. This second transition, from a stack of tetramers to monomers, also depends on temperature, allowing us again to measure transition entropy and free energy.Lowering the temperature in the cholesteric phase favors tetraplex formation. The critical osmotic pressure for the formation of tetraplexes, just as the critical osmotic pressure for inducing the higher-density packing, depends strongly on the temperature.Out next goal is to compare tetraplexes, which lack a linking backbone, with quadruplexes (where the bases are linked) so as to see the stabilizing contributions of a polymer backbone.
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