Determination of DNA base compositions from melting profiles in dilute buffers

Equations were determined for the dependency of the melting temperature ( T m ) of DNA upon the logarithm of the sodium ion concentration, for four DNA samples of widely different base compositions (θ GC ). The slopes of these T m versus log M equations wore found to decrease with increasing θ G C of the samples. An empirical equation relating T m , log M (Na + ) and θ G C was derived, which also accounts for differences in T m versus log M slopes. Data from the literature for some synthetic polynucleotides and for the crab( Cancer pagarus ) satellite poly AT are discussed in relation to the above finding. The changes in T m versus log M slopes with θ G C are interpreted in terms of changes in the thermodynamic parameters Δ S and Δ H with base composition.