Poly(ethylene glycol)-induced DNA condensation in aqueous/methanol containing low-molecular-weight electrolyte solutions Part II. Comparison between experiment and theory

Viscometry, UV/Vis-centrifugation and dynamic light scattering (DLS) were used to monitor the coil-globule transition of calf-thymus DNA by poly(ethylene glycol) (PEG) in aqueous/methanolic NaCl solutions. All three methods confirm that methanol and PEG promote the transition synergistically. The PEG concentration at which the DNA collapses decreases as the methanol fraction of the solvent is increased. The values found for the critical PEG and methanol concentrations agree quite well with those predicted by the modified Flory-Huggins theory. This is rather surprising, because effects such as selective solvent adsorption or intramolecular charge repulsion are neglected. The most informative experimental technique for the present investigations is DLS. The DNA molecules are not affected by outside forces and DLS allows the measurement of molecular size as well as the distribution of the conformational states. It was observed that there are no intermediate conformational states. A DNA molecule is either in the expanded coil state or in the collapsed state. (C) 1999 Elsevier Science Ltd. All rights reserved.