Binary and Ternary Metal Complexes of Adenosine and Thermodynamic Parameters Associated With the Interaction of Adenosine With Bivalent Metal Ions in Solution

Badar T. Khan and Ch. Abraham Lincoln

Department of Chemistry, P.G. College of Science, Osmania University, Saifabad, Hyderabad - 500 004 (India)

The stability constants of the binary and ternary complexes of Cu(II), Ni(II), Co(II), Zn(II), Mg(II) and Ca(II) with adenosine as a primary ligand and cytidine, uridine and thymidine as secondary ligands were determined by potentiometric titrations at 15°, 25°, 35°, and 45°C and 0.10M (KNO3) ionic strength. The stability of 1:2 metal-adenosine and ternary metal complexes was characterized by the D log k values (difference in stabilities of the 1:2 and 1:1 complexes and difference between the 1:1:1 and the corresponding binary systems). These values are positive for the 1:2 metal adenosine and ternary systems, this is attributed to stacking interactions. The thermodynamic parameters DH°f, DS°f and DG°f, are reported for all the systems. The enhanced stabilities of ternary systems have been measured in terms of their DDH, = DH2 - DH, where DH2, is DH°f of 1:1:1 system and DH1 is DH°f for 1:1 systems and DDS = DS2 - DS1 where DS2 and DS1 are the DS°f for 1:1:1 and 1:1 systems respectively. The DDH values are highly negative and DDS values are negative for their systems. The exothermic enthalpy in the formation of 1:1:1 ternary complexes may be attributed to the energy of stacking in these systems.

Binary; ternary metal complexes; adenosine; thermodynamic parameters