Abstract
The relative order of denaturation of four transforming activities in Diplococcus pneumoniae DNA is different in the two solvents, 0.15 m-NaCl-0.015 m-sodiumcitrate and 7.2 m-NaClO 4-0.1 m-sodium-EDTA (pH 7.0), in both of which stability increases with the percentage of guanine-cystosine base pairs. For example, at low molecular weight, the streptomycin resistance marker is the least stable of the four in NaClO 4, whereas novobiocin resistance is least stable in the sodium chloride-citrate solution. There is therefore a second-order effect on the relative stabilities of the DNA molecular fragments against irreversible denaturation (strand separation). If interpreted in terms of ΔH per mole of base pairs, the differential change is only 20 to 30 cal./mole between the two solvents. These effects are small, therefore, and yet readily detectable in this system. They may be due to nearest-neighbor effects on the stacking interactions, to a change in the degree of co-operativeness, to the known difference in the relative stability of adenine-thymine and guanine-cytosine pairs in the two solvents, or to some combination of these factors.
Published Version
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