Optical studies of a conformational change in DNA before melting

The circular dichroism of double-stranded DNA is temperature dependent prior to its melting. As the temperature is increased the spectrum becomes more nonconservative. This is certainly due to a conformational change within the framework of the double helix. To ascertain the nature of the conformational change, a series of synthetic and natural DNA's from a variety of sources was investigated. The same qualitative changes were seen for all the DNA samples, independent of base composition. However, there were definite quantitative differences, with poly [d(A-T)] manifesting the largest effect. Oligomers of the form [d(A-T)]_n with n = 10 to 21 behaved in a manner similar to the polymer. There is no observed chain-length dependence. The breadth of the pre-melt transition indicates a low ΔH (less than 5 kcal./mole); the lack of dependence on chain length indicates that the co-operative unit is smaller than eight base pairs. Varying the salt concentration (NaCl) revealed a complete lack of dependence on ionic strength. It has been suggested previously that this pre-melt behavior represents a transition from B-DNA to A-DNA. Our data tend specifically to rule out this possibility. Debye-Hückel calculations on the electrostatic interaction within the three known types of DNA helices indicate that the B-DNA to A-DNA transition should be sensitive to salt. Comparison with recently available circular dichroism spectra from studies on DNA films also tends to discourage this idea. Furthermore, a sample-of T2r‡DNA which is 70% glucosylated and unable to go into the A form shows the same pre-melt transition. Other possible conformational changes are discussed in view of published data from other physical and chemical studies on DNA structure.