The Denaturation Transition of DNA in Mixed Solvents
Boualem Hammouda 1* and David Worcester 2
1 National Institute of Standards and Technology
2 U of Missouri
* To whom correspondence should be addressed. E-mail: hammouda{at}nist.gov.
Submitted on March 23, 2006
Revised on April 17, 2006
Accepted on 13 June 2006
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Abstract |
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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). Two solvents have been used: water and ethylene glycol. The "melting" transition temperature was found to be 94 °C for 4 % mass fraction DNA/d-water and 38 °C for 4 % mass fraction NA/d-ethylene glycol. Deuterated solvents (d-water and d-ethylene glycol) were used to enhance the SANS signal and 0.1 M NaCl (or 0.0058 g/g mass fraction) salt concentration was added to screen charge interactions in all cases. The DNA melting transition temperature was found to vary linearly with the solvent fraction in the mixed solvents case. 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 8.5 Å to 12.3 Å across the melting transition. Ethylene glycol and water mixed solvents were found to mix randomly inside 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.
Key Words:
DNA, DNA structure, SANS, denaturation transition, neutron scattering
