Thermochemical Identification of the Structural Factors Responsible for the Thermodynamic Instability of 3’,5'-Cyclic Nucleotides

The enthalpies of hydrolysis of several cyclic phosphate diesters which can be considered to be structural analogues of the trans-fused trimethylene phosphate-ribofuranoside ring system of adenosine 3’,5'-cyclic phosphate have been determined by microcalorimetric techniques using the metal-dependent phosphohydrolase from Enterobacter aerogenes as catalyst. At pH 7.3 and 25 °C we have obtained the following values (kcal/mol) for sodium salts: trans-2-hydroxytetrahydrofuranmethanol cyclic phosphate, -10.6; trans-2-hydroxycyclopentanemethanol cyclic phosphate, -7.9; cw-2-hydroxycyclopentanemethanol cyclic phosphate, -2.5; 5-methoxytrimethylene phosphate, -4.9; 5-methyltrimethylene phosphate, -3.8. From these values and those determined previously, we can make the following conclusions: (1) the trans-fused trimethylene phosphate- tetrahydrofuran structure is responsible for the 8 kcal/mol more exothermic enthalpy of hydrolysis which cyclic AMP displays relative to trimethylene phosphate; (2) about 5 kcal/mol of the excess enthalpy of hydrolysis of cyclic AMP is the result of geometric distortion due to the trans-ring fusion; (3) about 3 kcal/mol of the excess enthalpy of hydrolysis of cyclic AMP cannot be accounted for by intramolecular effects, suggesting that solvation effects play an important role in the thermodynamic stability of cyclic AMP.