Calorimetric study of the complexes poly(A)center dot poly(U)-phosphatidylcholine liposomes-Mg2+

Differential scanning microcalorimetry was used to study-triple complexes o f poly(A).poly(U) with dimyristoylphosphatidylcholine or dipalmitoylphospha tidylcholine liposomes in the presence and absence of Mg2+ cations. The mel ting enthalpies for the polynucleotides and phospholipid bilayer of liposom es in control and complexes were calculated. It is found that the enthalpy of poly(A).poly(U) in triple complexes decreases with increasing concentrat ion of Mg2+ and the lipid:polynucleotide ratio, which is accompanied by the splitting of the melting peak of poly(A).poly(U) into two or three peaks. It was shown that poly(A).poly(U) interact with liposomes also in the absen ce of Mg2+, which leads to changes in enthalpy of the components and insign ificant shifts in their temperature transitions. The results of the calorim etric study and the electron microscopy data are consistent with the model of triple complexes proposed previously.