Effect of osmoregulatory solutes on the thermal stability of calf-thymus DNA.

The thermal denaturation of calf-thymus DNA (ct-DNA) in aqueous solution of osmoprotectant solutes, namely glycine, sarcosine (N-methylglycine), N,N-dimethylglycine and betaine (N,N,N-trimethylglycine), and of T4 bacteriophage DNA (T4-DNA) in aqueous solutions of betaine, has been investigated by differential scanning calorimetry (DSC). We show that these solutes are able to reduce or eliminate the base-pair composition dependence of DNA thermal denaturation. By raising the concentration of these additives, the broad, asymmetric and characteristic DSC profile of calf-thymus DNA becomes sharper, with the ‘satellite’ peaks being shifted down the temperature scale. The term ‘isostabilization’ was used to described the critical concentration of additives at which the dA–dT and dG–dC base pairs show the same melting temperature. These zwitterionic solutes are able to isostabilize DNA without altering the conformation of double stranded DNA in the B-form, as shown by CD measurements and polyelectrolyte properties. Both the overall destabilization and isostabilization of DNA in the presence of these solutes are discussed. We compare this behaviour with the different behaviours of tetraalkylammonium salts.