Stability of Glutamine and Pyroglutamic Acid under Model System Conditions: Influence of Physical and Technological Factors

Abstract

Storage and processing conditionsThe stability of glutamine and pyroglutamic acid as regards pH, temperature and oxygen was studied during storage and two thermal processings, the residual products and those which appeared being determined. Contrary to glutamic acid, the two derivatives are labile below pH 2 and above pH 13 and alteration occurs much faster. Temperature increase promotes this phenomenon. Glutamic and pyroglutamic acids convert reversibly into one another, more or less quickly and completely according to the conditions. Glutamine changes during heating in two steps, first into glutamic acid, then into pyroglutamic acid if the pH values are not extreme. The influence of pH conditions on the direction and the rate of the reaction are indicated.The solutions were stored for 15 days at about 20 "C in the dark in sterilized ground-glass stoppered bottles, which were completely filled in order to study storage without atmospheric contact. Each determination of residual glutamine and pyroglutamic acid concentration after a given time necessitated the use of an individual bottle to avoid the entry of air resulting from sampling. The solutions were therefore not entirely oxygen-free, but the influence of air was limited to that of the air dissolved in them. This storage was termed, for convenience sake, "without atmospheric contact."Solutions were boiled under reflux for 60 min and residual and conversion products were determined as soon as the solutions had cooled. In the autoclaving processing, the solutions were heated for 35 min to 135 "C under 2.7 k&m2 pressure in screw-top glass bottles with bakelite caps and Teflon@' seals ("SVL"@, France).

Compound‐specific δ13C and δ15N analysis of amino acids: a rapid, chloroformate‐based method for ecological studies

“…The derivatization of glutamic acid resulted in two peaks because a portion of glutamic acid is cyclized into pyroglutamic acid during derivatization. The proportion of glutamic acid that is converted into pyroglutamic acid is dependent on reaction conditions, especially pH and temperature . To avoid fractionation that might arise during conversions between forms, it would be ideal to convert all Glx to one form.…”

“…The proportion of glutamic acid that is converted into pyroglutamic acid is dependent on reaction conditions, especially pH and temperature. [48][49][50][51] To avoid fractionation that might arise during conversions between forms, it would be ideal to convert all Glx to one form.…”

Occurrence and evolution of amino acids during grape must cooking

“…Under the experimental conditions, glutamine was probably converted into pyroglutamic acid. When heated, in fact, glutamine releases ammonia on a molar basis mainly due to the deamidation of the amide group (Airaudo, Gayte-Sorbier, & Armand, 1987). Unfortunately, pyroglutamic acid does not react with DEEMM, and its presence could not be confirmed.…”

Massive Glutamine Cyclization to Pyroglutamic Acid in Human Serum Discovered Using NMR Spectroscopy

“…10–14 At pH < 2 and pH > 13, both glutamine and pyroglutamic acid interconvert reversibly; on heating, glutamine converts into glutamic acid, which then converts into pyroglutamic acid. 15 Interestingly, more recent investigations show that cyclization of glutamine to pyroglutamic acid also occurs in situ during mass spectrometric analysis. 10,16 These studies, based on liquid chromatography-mass spectrometry (LC-MS) methods, have shown that the magnitude of glutamine cyclization to pyroglutamic acid is significant, which highlights the magnitude of error introduced in the analysis of these metabolites if not accounted for appropriately.…”