The Enthalpy of Hydrolysis of Acetylcholine

The enthalpy of hydrolysis of acetylcholine at pH 7 and 25° is found to be ΔH = +280 ± 20 cal mole-1 after correction for buffer ionization heat. Under physiological conditions, the hydrolysis, which is accompanied by the liberation of 1 proton per molecule hydrolyzed, will have an enthalpy change depending almost entirely on the effective heat of protonation of the complex buffering system present. At neutral pH the buffering in vivo is presumably largely by imidazole and amino groups on protein side chains, with protonation heats of -7 to -12 kcal mole-1.