Theory of DNA condensation: Collapse versus aggregation

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Abstract
In an unfavorable solvent environment, DNA (and other polymers) undergo a conformational transition to a collapsed form, accompanied by a dramatic reduction in the effective volume of the molecule. Solvent conditions leading to the collapse are the same as those that cause aggregation. We give here a thermodynamic description of the collapse and its relations to aggregation (or precipitation). This is formulated in terms of the Flory‐Huggins theory of the thermodynamics of polymer solutions. The results show that it is possible for three different states of DNA to be stable under different conditions: (1) the extended random coil, (2) the collapsed coil, and (3) a concentrated phase of aggregated random coils. The collapsed coil is predicted to be stable against aggregation only at high dilutions, of the order of parts per million. For DNA the transition between the extended coil and the collapsed coil is predicted to be discontinuous, in the sense that intermediate states are not present, because of the relatively high stiffness of the chain. The transition should appear diffuse because of the small size of the single molecule in comparison to macroscopic systems.