EFFECT OF OSMOREGULATORY SOLUTES ON THE THERMAL-STABILITY OF CALF-THYMUS DNA

The thermal denaturation of calf-thymus DNA (ct-DNA) in aqueous soluti on of osmoprotectant solutes, namely glycine, sarcosine (N-methylglyci ne), N,N-dimethylglycine and betaine (N,N,N-trimethylglycine), and of T4 bacteriophage DNA (T4-DNA) in aqueous solutions of betaine, has bee n investigated by differential scanning calorimetry (DSC). We show tha t these solutes are able to reduce or eliminate the base-pair composit ion dependence of DNA thermal denaturation. By raising the concentrati on of these additives, the broad, asymmetric and characteristic DSC pr ofile of calf-thymus DNA becomes sharper, with the 'satellite' peaks b eing shifted down the temperature scale. The term 'isostabilization' w as used to described the critical concentration of additives at which the dA-dT and dG-dC base pairs show the same melting temperature. Thes e zwitterionic solutes are able to isostabilize DNA without altering t he conformation of double stranded DNA in the B-form, as shown by CD m easurements and polyelectrolyte properties. Both the overall destabili zation and isostabilization of DNA in the presence of these solutes ar e discussed. We compare this behaviour with the different behaviours o f tetraalkylammonium salts.