Abstract
Abstract The synthesis of N-acetyl-l-isoasparagine and of N-acetyl-l-isoglutamine is reported. The acid dissociation constants of these compounds, and those of their corresponding N-carbobenzoxy derivatives, have been measured. Since these compounds are uncharged molecules of the type —NH—CH(—CH2COOH)—CO— or the corresponding glutamyl homologue, their dissociation constants represent the intrinsic dissociation constants of aspartyl and glutamyl side chains in polypeptide chains. Additional values for these constants were obtained from pK values of charged molecules of the same type, after calculating corrections for the effects of the charges. All results obtained in this way are consistent, and indicate that aspartyl carboxyl groups are significantly more acidic than glutamyl carboxyl groups. Intrinsic pK values at 25° and zero ionic strength are 4.08 ± 0.05 for aspartyl groups and 4.50 ± 0.06 for glutamyl groups.
Highlights
The corresponding glutamyl homologue, their dissociation constants represent the intrinsic dissociation constants of aspartyl and glutamyl side chains in polypeptide chains. Additional values for these constants were obtained from pK values of charged molecules of the same type, after calculating corrections for the effects of the charges
All results obtained in this way are consistent, and indicate that aspartyl carboxyl groups are significantly more acidic than glutamyl carboxyl groups
The most important aspect of the results is that they show that aspartyl carboxyl groups are significantly more acidic than glutamyl groups
Summary
CBZ-isoaspuragine*-N-CBZ-L-aspartic acid (Cycle Chemical) was converted to the anhydride by the method of Yamamoto [4], and to CBZ-isoasparagine by the method of Ressler, Malodeczky, and Kashelikar [5]. Our sample was found to be identical with an authentic sample provided by Dr C Ressler, both by observation of melting behavior and by thin layer chromatography (see below). The mixture was shaken with hydrogen under a pressure of 24 p.s.i. for 1 hour. One or two additional recrystallizations from ethanol were required to obtain Ac-isoasparagine with a sharp melting point.
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