Effect of various anions on the stepwise melting of plasmid DNA.

Abstract

Differential scanning calorimetry (DSC) was carried out for the study on the effect of various anions on the melting of plasmid pJL3-TB5 DNA (5, 277 base pairs in length). In the presence of halide ions such as Cl-, Br-, or I- in citrate buffer solutions, seven separate peaks caused by the stepwise melting of the DNA molecule were found in the DSC curve. Similar profile of the DSC curve was also found in the presence of SCN-. Such a characteristic melting profile was not observed, when some other buffer solutions such as phosphate, borate, and acetate buffers were used. These ions affected significantly the melting profile of DNA; the height of the peaks seen in the high temperature range decreased and broadened. The presence of the anions such as NO3-, ClO4-, SO42- and IO3- instead of Cl- in the saline-sodium citrate (SSC) buffer solution also affected the DSC curves in the high temperature range in the similar manner to phosphate, borate, and acetate buffers. These results indicate that the type of anions in the solution as well as that of cations is very important for the melting of DNA. Since anions are known to alter the structure of water, it is suggested that these results are closely related to alteration of the structure of water caused by the interaction between anions and water, rather than the direct interaction between anions and DNA. The borate buffer solutions also showed not only clearly separated peaks, but also excellent reversibility for the melting of the DNA.