Insight Into The Helix-To-Coil Transition in DNA ... And More
Boualem Hammouda,
NCNR, NIST
The helix-to-coil denaturation transition in DNA has been investigated in mixed
solvents at high concentration using ultra-violet (UV) light absorption spectroscopy
and small-angle neutron scattering (SANS). This transition is driven by the
unstacking of the amine bases and the breaking of hydrogen bonds between them.
Two solvents have been used: water and ethylene glycol. The `melting' transition
temperature was found to be lower by 56oC in ethylene glycol than in water.
This can be understood in simple terms of hydrophobic/hydrophilic interactions.
Moreover, the DNA melting transition temperature was found to vary linearly with
the solvent fraction for mixed solvents. DNA structural information was obtained
by SANS including a correlation length characteristic of the inter-distance
between the hydrogen-containing (desoxyribose sugar-amine base) groups. This
correlation length was found to increase from 0.85 nm to 1.23 nm across the
melting transition. Ethylene glycol and water mixed solvents were found
to mix randomly in the solvation region in the helix phase, but non-ideal
solvent mixing was found in the melted coil phase. In the coil phase, solvent
mixtures are more effective solvating agents than either of the individual
solvents. Once melted, DNA coils behave like swollen water-soluble synthetic
polymer chains. SANS data taken from other biopolymers will also be presented.
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