The heats of mixing of an equimolar mixture of poly G and poly C in solutions of various pH values were measured using a LKB batch type of microcalorimeter at 298±0.002 K. The interaction between poly G and poly C in neutral medium seems to lead to a complex such as poly(G+C). In order to obtain further information about the interaction between poly G and poly C, the CD spectra for poly G, poly C, and an equimolar mixture of poly G and poly C in solutions with various pH were also measured. From the results of the heats of mixing and the spectra measurements, it was concluded that poly G and poly C in neutral medium leads to a weak complex formation and the enthalpy of complex formation was stoichiometrically estimated to be about-9.3 kJ per mol of base pair.

SynopsisThe thermal effect of magnesium ions on the helix-coil transition of DNA was studied calorimetrically by a modified differential scanning calorimeter (DSC). It was found that the transition temperature of DNA depends on both the DNA and magnesium ion concentrations. The dependence of the helix-coil transition of DNA on the mole ratio of magnesium ions to DNA(P) can be classified into two groups. When this mole ratio is less than 1, magnesium ions tend to stabilize the double-helix DNA, so that the transition temperature increases linearly and the heat of transition increases significantly with increasing mole ratio. When the mole ratio is more than 1, magnesium ions tend to destabilize the double-helix DNA, so that DNA precipitates when the temperature is raised above the transition temperature. In this case, both the transition temperature and the heat of transition decrease with increasing mole ratio.

A DTA apparatus suitable for measuring polymer solutions has been constructed. By using this apparatus the phase equilibrium of the system methylceIlulose/water was studied. The precipitation temperature was determined from the DTA curve which shows a sigmoidal shape. The thermodynamic parameters for entropyYl= 2.11 and enthalpy x1 = 2.13 (at 328.8'K) and the theta temperature 0 = 59.5OC were estimated. ZUSAMMENFASSUNG:Ein zur Vermessung von Polymeren in Losung geeigneter DTA-Apparat wurde entwickelt. Mit Hilfe dieses Apparates wurde das Phasengleichgewicht des Systems Methyl-celIulose/Wasser untersucht. Aus der DTA-Kurve, die einen sigmoidalen Verlauf aufwies, wurde die Triibungstemperatur bestirnrnt.Die thermodynamischen Grof3en der Entropie,Y'1 = 2J1, und Enthalpie, x1 = 2,13 (bei 328,8'K), als auch die Theta-Ternperatur, 0 = 59,9OC, wurden abgeschitzt.

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