Poly(ethylene glycol)-induced DNA condensation in aqueous/methanol containing low-molecular-weight electrolyte solutions Part I. Theoretical considerations

In a certain unfavourable environment, a single DNA molecule undergoes a co nformational transition from an expanded coil state to a collapsed form. He re, this transition is induced by poly(ethylene glycol) (PEG) in a solvent mixture composed of an aqueous salt buffer and methanol. A theoretical desc ription is presented in terms of the free energy of mixing DNA, PEG, and so lvent, the elastic part of the free energy for DNA chains, and the translat ional entropy of the low molecular ions. Further effects taken into account are DNA-counterion binding and solvent quality. The theoretical prediction s are: (1)The transition between the coil and the collapsed state is discon tinuous. There exists a coexistence region where both states coexist side b y side, but its width is very small. (2) The collapse takes place at a cert ain critical PEG concentration C-PEG,C-c. The value of this PEG-concentrati on depends on the degree of PEG polymerisation, P-w,P-PEG, the molar fracti on, X-methanol, Of methanol, and on the concentration, c(salt), of the adde d salt. For given values of X-methanol and c(salt), C-PEG,C-c, decreases wi th increasing P-w,P-PEG. That is, it is easier to induce the DNA collapse w ith PEG of high molar mass than with PEG of low molar mass. If both P-w,P-P EG and c(salt) are constant, C-PEG,C-c increases as the methanol concentrat ion decreases. This means that addition of methanol promotes DNA condensati on. If finally P-w,P-PEG and X-methanol are chosen constant, C-PEG,C-c decr eases as c(salt) increases. Thus we can say, the collapsed DNA state is the more stabile the higher are P-w,P-PEG, X-methanol as well as c(salt). That is, these three parameters act synergistically. (3) Theory gives some info rmation about the DNA-molecule size. While in the coil state the expansion factor, ct, is of the order of 1.8-2.4, it is of the order of 0.3 in the co mpact state. Results of measurements presented in the companion paper affir m these results. (C) 1999 Published by Elsevier Science Ltd. All rights res erved.