Unwinding of double-stranded DNA helix by dehydration.

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Abstract
Conformational changes of the double-stranded DNA helix in response to dehydration were investigated by monitoring, by agarose gel electrophoresis, the linking number of covalently closed circular DNA generated by ligation of linear DNA in the presence of different organic solvents or at different temperatures. The DNA helix unwinds upon addition of certain organic solvents or elevation of temperature. The conformational change observed under the experimental conditions is a continuous process in response to the organic solvent concentration. The .DELTA.H of unwinding 1 linking of the DNA helix is constant at .apprx. 12.2 .+-. 0.4 kcal/mol (1 kcal = 4.184 kJ); the corresponding .DELTA.S and d(.DELTA.S)/dn are 2nkR and 2kR, in which n is the relative equivalent linking number (referred to the state of .DELTA.S = 0 for unwinding) of the DNA, R is the gas constant, and k is equal to 1117/number of base pairs. The .DELTA.H, .DELTA.S, and d(.DELTA.S)/dn for unwinding i linkings are i .times. 12.2 kcal/mol, 2inkR, and 2ikR, respectively. d(.DELTA.S)/dn, like k, is inversely proportional to the number of base pairs in DNA. Double-stranded DNA of different chain lengths have average .DELTA.S = 35 cal/mol .cntdot. K for unwinding 1 linking under the experimental conditions; this corresponds to 127 .+-. 14 base pairs/relative linking.