Kinetic studies of the alkaline hydrolysis of some choline and carnitine esters

Abstract

The rates of alkaline hydrolysis of the butyryl and benzoyl esters of choline, and the butyryl ester of carnitine were measured. Reactions were conducted at 25.0°C in aqueous solutions buffered with sodium glycinate-glycine or sodium carbonate-sodium bicarbonate at ionic strengths varying from 0.2-1.2.

Changes were observed in the calculated psuedo-first order rate constants due to production of acid during the run. Second order rate constants were calculated by dividing the first order constant by the hydroxide ion activity. General base catalysis was not observed because the rates of hydrolysis were found to be independent of butter concentration for all three esters studies. The rates for the choline esters were found to be independent of ionic strength changes within experimental error, but butyrylcarnitine showed an increase in rate with increasing ionic strength. Average values of the second order constants (benzoylcholine 0.58 sec -1 M-1, butyrylcholine 0.73 sec-1 M-1, and butyrylcarnitine 0.070 sec -1 M-1) are greater than values reported for analogous neutral esters but less than second order constants reported for the activated p-nitrophenyl esters. Second order constants found using the Sargent-Welch Recording pH Stat confirmed the findings in buffer solutions, though results from this instrument showed greater experimental errors. The aminolysis reaction of benzoylcholine with cyclo-hexylamine in dimethylsufoxide was much slower than hydrolysis of the same ester.