PubMed1975
The enthalpies of hydrolysis of acyclic, monocyclic, and glycoside cyclic phosphate diesters have been measured by flow microcalorimetry using a phosphohydrolase isolated from Enterobacter aerogenes as catalyst. The values so obtained (kilocalories per mol) (at 25 degrees) for sodium salts are: diethyl phosphate, minus 1.8 plus or minus 0.5; ethylene phosphate, minus 6.4 plus or minus 0.2; trimethylene phosphate, minus 3.0 plus or minus 0.2; tetramethylene phosphate, minus 2.2 plus or minus 0.1; methyl beta-D-ribofuranoside cyclic 3:5-phosphate, minus 11.1 plus or minus 0.2; methyl alpha-D-glucopyranoside cyclic 4:6-phosphate, minus 6.3 plus or minus 0.1; and cyclic adenosine 3:5-monophosphate (5-ester bond), minus 11.1 plus or minus 0.4 (10-minus 3 M Mg-2+). The enthalpy of hydrolysis of the 3-ester bond of cyclic adenosine 3:5-monophosphate (10-minus 3 M Mg-2+) has been revised to minus 11.1 plus or minus 0.2 kcal/mol from the value of minus 13.2 plus or minus 0.4 kcal/mol reported previously (greengard, p., rudolph, s.a., and sturtevant, j. m. (1969) j. biol. Chem. 244, 4798). All these values pertain to the hydrolysis of singly charged diesters to form singly charged monoesters. The data for the acyclic and monocyclic phosphodiesters are in qualitative agreement with their hydrolytic reactivities. The enthalpies measured for the hydrolysis of the glycoside cyclic phosphates cannot now be explained on the basis of their structures or reactivities; perhaps a contribution to the enthalpies by solvation or a previously unrecognized geometric strain effect may be responsible for the large exothermic enthalpies of these cyclic phosphate diesters. Changes in the heat capacity, increment Cp, for some of the hydrolytic reactions were also measured.