Question

Q: DNA is negatively charged because of the presence of phosphate groups. But these negative charges repel each other as well. Cations are needed for charge shielding and hence increasing the stability of double stranded DNA. More stable a double helix is, greater is its melting temperature (Tm). However it is observed that high salt concentration ( [Na⁺] > 5 M), the Tm of DNA decreases with increasing [Na⁺]. What could a possible reason for this observation?

Answer 1

Tm value increasing with the increasing salt concentration because positive charge on salt neutralize the negative charge present on phosphate group in DNA . But at very high salt concentration after a limit ([Na+]>5 M) the reverse happens ie, the Tm value starts decreasing because the free water molecules which make hydrophobic force between nucleotides starts binding with extra positive(Na+) ions and there is no free water to make hydrophobic interaction .