The Effect of Low Ionic Strength on the Radiation Chemistry and Physical Properties of Calf Thymus DNA

Summary

Studies of ultraviolet and circular dichroism spectra of aqueous solutions of calf thymus (CT) DNA confirm the tendency of DNA to change conformation at low ionic strength. The qualitative shape and transition width of 260 nm melting curves below 1 mM NaCl differed significantly from those previously published for DNA solutions containing 1 mM NaCl and above. Neutral aqueous solutions of CT DNA at low ionic strengths (0·1 mM–10 mM NaCl) were irradiated with low doses of γ-rays. The melting temperature, T_m, of irradiated DNA samples increased below 1 mM NaCl suggesting interstrand crosslinking of the denatured DNA or formation of regions of more thermally stable DNA conformation. The magnitudes of these radiation reponses were found to be a function of the time elapsed between salt concentration changes and irradiation as well as time after irradiation. These results are consistent with the hypothesis that the purine and pyrimidine base chromophores in double stranded DNA are sheltered from radical attack by the sugar phosphate backbone. Low dose radiation studies (0·8–8·0 Gy) of CT DNA in 1 mM NaCl and below showed a split dose and dose rate dependence for the sample melting curves.

• calf thymus DNA
• ultraviolet spectra
• circular dichroism spectra
• low ionic strength