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What happens when you try to dissolve ʟ-glutamine in hot water (above 80 °C)? Does the heat destroy the ʟ-glutamine?

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  • $\begingroup$ Thermophilic proteins are depleted in glutamine compared to their mesophilic counterparts: pubs.acs.org/doi/abs/10.1021/bi027435e. This is a hint that maybe they tend to hydrolyze at higher temperature. $\endgroup$ Aug 3 '20 at 10:40
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Glutamine in aqueous solutions degrades slowly when stored in room temperature. Therefore, we can expect that rate of degradation is faster at higher temperatures. It is evident that the hydrolysis product of glutamine is 5-pyrrolidone-2-carboxylic acid (not glutaric acid as shown for enzymatic degradation) and ammonia (Ref.1):

L-Glutamine degradation

The Ref.1 states that:

The degradation kinetics of ʟ-glutamine (Gln) in aqueous solution was studied as a function of buffer concentration, $\mathrm{pH}$ and temperature. Stability tests were performed using a stability-indicating high-performance liquid chromatographic assay. The degradation product of Gln was 5-pyrrolidone-2-carboxylic acid. The reaction order for Gln in aqueous solution followed pseudo-first-order kinetics under all experimental conditions. The maximum stability of Gln was observed in the $\mathrm{pH}$ range from 5.0 to 7.5. The $\mathrm{pH}$–rate profile described by specific acid–base catalysis and hydrolysis by water molecules agreed with the experimental results. Arrhenius plots showed the temperature dependence of Gln degradation, and the apparent activation energy at $\mathrm{pH}$ 6.41 was determined to be $\pu{9.87 \times 10^4 J mol−1}$.

Factors affecting the stability of ʟ-glutamine in solution has also been studied (Ref.2). They also found that glutamine degradation resulted in the equimolar formation of ammonia and no associated formation of glutamate. At $\pu{22–24 ^\circ C}$, the degradation rate of glutamine was variable depending on the type of solution used. It was 0.23% in water $\mathrm{pH}$ 6.5; 0.22% in dextrose/water (15% $w/v$); 0.8% in mixed total parenteral nutrition (TPN) solution; and on the $\mathrm{pH}$, molarity and type of buffer used. The degradation rate was essentially unaffected by light and $\ce{O2}$.

Nonetheless, the solubility of ʟ-glutamine in water increases with temperature (Ref.3).

References:

  1. Kanji Arii, Hideyuki Kobayashi, Toshiya Kai, Yukifumi Kokuba, "Degradation kinetics of L-glutamine in aqueous solution," European Journal of Pharmaceutical Sciences 1999, 9(1), 75-78 (https://doi.org/10.1016/S0928-0987(99)00047-0).
  2. K. Khan, M. Elia, "Factors affecting the stability of ʟ-glutamine in solution," Clinical Nutrition 1991, 10(4), 186-192 (https://doi.org/10.1016/0261-5614(91)90037-D).
  3. Jan P. Amend, Harold C. Helgeson, “Solubilities of the common ʟ-$\alpha$-amino acids as a function of temperature and solution $\mathrm{pH}$,” Pure and Applied Chemistry 1997, 69(5), 935-942 (https://doi.org/10.1351/pac199769050935).
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This paper here which describes the isolation of Glutamine from beets, contains a note that Glutamine is rapidly hydrolysed in water at high temperatures. The paper includes procedures in water at 60C.

edit: This paper here describes the degradation kinetics of L-Glutamine in aqueous solution to 5-pyrrolidine-2-carboxylic acid.

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  • $\begingroup$ Thank you for your answer! What does glutamine hydrolyse into? $\endgroup$
    – iamarhino
    Aug 3 '20 at 8:19
  • $\begingroup$ Do you have an idea of the order of magnitude that could be. For instance, if left an aqueous solution at saturation point for 10 hours at 60, 80, 90 Celcius degrees, what respective order of degradation can I expect to have? 0.1% / 1% / 10% / 50%? $\endgroup$
    – iamarhino
    Aug 3 '20 at 8:22
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    $\begingroup$ I have added a second reference that may answer that $\endgroup$
    – Waylander
    Aug 3 '20 at 8:42
  • $\begingroup$ Thank you, but I can't manage to access the full text, it's asking me for money :'( $\endgroup$
    – iamarhino
    Aug 3 '20 at 9:08
  • $\begingroup$ I do not have access either. $\endgroup$
    – Waylander
    Aug 3 '20 at 9:23
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Apart from normal hydrolysis by applying heat, this reaction is also enzyme catalyzed as it is discussed in this paper1. ʟ-Glutamine hydrolyze to glutamic acid and ammonia with the help of glutaminase enzyme. This reaction is also catalysed by PabA subunit of p-aminobenzoate synthase. Another enzyme called glutamin-(asparagin-)ase (EC 3.5.1.38) is also known to catalyse hydrolysis of both ᴅ- and ʟ-isomers glutamine and asparagine.

ʟ-glutamine to ʟ-glutamic acid

Reference

  1. Kishore, N.; Tewari, Y. B.; Goldberg, R. N. A Thermodynamic Study of the Hydrolysis of ʟ-Glutamine to (ʟ-Glutamate + Ammonia) and of ʟ-Asparagine to (ʟ-Aspartate + Ammonia). The Journal of Chemical Thermodynamics 2000, 32 (9), 1077–1090. DOI: 10.1006/jcht.1999.0496.
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    $\begingroup$ Is this relevant to the OP's question? I interpreted the question to mean that they made a solution of Gln in water, so there is no reason to believe that degradation enzymes would be present. $\endgroup$
    – Andrew
    Aug 3 '20 at 11:22
  • $\begingroup$ @Andrew Yes, glutamine solutions can be hydrolysed by applying heat as the other answerers noted. My answer points out that hydrolysis can also be catalysed with the help of enzymes to give a different product, It might not to be relevant to you but I think it is still relevant to OP's question if not directly. Heat is not the only factor in hydrolysis reactions. $\endgroup$ Aug 3 '20 at 11:43

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