Effect of tetramethylammonium ions on conformational changes of DNA in the premelting temperature range

The reversible conformational change of DNAs and polydeoxyribonucleotides occurring before melting was followed by circular dichroism. Δθ/δT, the rate of change of ellipticity θ with temperature, was used mainly as a measure of this premelting phenomenon. If sodium ions were replaced by tetramethylammonium ions Δθ/δT decreased for poly (dA) poly (dT) and poly (dA.dT) poly (dT.dA), but increased for poly (dG.dC) poly (dC.dG). DNAs of different base composition showed no more premelting (Δθ/ΔT ~ 0) even at low molarities of TMACl provided the Na/TMA ratio was very small. For all cases studied the θ values at 0°C and at a given ionic strength were smaller in NaCl than in TMACl. When studying the series of ammonium ions from NH⁺₄ to (C₂H₅)₄,N⁺ the Δθ/ΔT values first decreased, going through zero with TMA⁺ io and then increased again. A tentative and qualitative explanation of our results can be given: (a) Hydration of the polymers increases in presence of TMA ions and their average stability decreases; locally, however, (AT) pairs are preferentially stabilized by TMA ions owing to a specific interaction at the level of O₂ of thymine. (b) In order to explain the different behaviour of (AT) polymers and DNA, it is assumed that only the B structure is able to accommodate TMA ions in the small groove of the double stranded helix.