A method for the determination of the pKa of α-n-benzoyl-l-arginine in the presence of an impurity

✍ Scribed by Bernard R. Glick; David J. Leggett

• Publisher: Elsevier Science
• Year: 1976
• Tongue: English
• Weight: 379 KB
• Volume: 70
• Category: Article
• ISSN: 0003-2697
• DOI: 10.1016/0003-2697(76)90483-8

✦ Synopsis

A method is proposed for the simultaneous detection and determination of minor impurities in weak acid (or bases) along with the evaluation of all relevant pK's. By use of the method described, the pK's of c~-N-benzoyl-Larginine and c~,~dibenzoyl-L-arginine have been accurately determined in the same solution. These values are 3.31 for a-N-benzoyl-e-arginine and 2.24 for ~,~dibenzoyl-k-arginine. This latter compound was found to exist as a 5.3% impurity in a commercial preparation of a-N-benzoyl-k-arginine.

The quantitative determination in the pH-stat of free carboxyl groups liberated by the enzymatic hydrolysis of ester substrates is complicated at pH values near and below the carboxyl pK a since it is necessary to correct for the incomplete ionization of the carboxyl group. As a part of a study of the hydrolysis of ~-Y-benzoyl-L-arginine ethyl ester by Y-bromosuccinimide-modified papain (1), we desired to make such a correction for the incomplete ionisation of a-N-benzoyl-L-arginine (Bz-Arg) at low pH values. It was therefore necessary to determine as accurately as possible the pK of Bz-Arg.

Potentiometric titration of the dissociable protons is the most convenient experimental technique capable of yielding equilibrium constants of high precision. However, unless the method of data processing is judiciously chosen, vital information may readily be obscured.

Computer analysis of potentiometric titration data of Bz-Arg revealed that the compound was not pure, a point easily overlooked by a simpler calculation procedure. This would naturally lead to an incorrect value for the pK of Bz-Arg. The technique described here, represents a simple method for the simultaneous detection, determination, and, in favorable instances, characterization of an impurity in weak acids (or bases).