Thermal decomposition of the amino acids glycine, cysteine, aspartic acid, asparagine, glutamic acid, glutamine, arginine and histidine

Abstract

Background: The pathways of thermal instability of amino acids have been unknown. New mass spectrometric data allow unequivocal quantitative identification of the decomposition products.

Results: Calorimetry, thermogravimetry and mass spectrometry were used to follow the thermal decomposition of the eight amino acids G, C, D, N, E, Q, R and H between 185 °C and 280 °C. Endothermic heats of decomposition between 72 and 151 kJ/mol are needed to form 12 to 70% volatile products. This process is neither melting nor sublimation. With exception of cysteine they emit mainly H₂O, some NH₃ and no CO₂. Cysteine produces CO₂ and little else. The reactions are described by polynomials, AA→a NH₃+b H₂O+c CO₂+d H₂S+e residue, with integer or half integer coefficients. The solid monomolecular residues are rich in peptide bonds.

Conclusions: Eight of the 20 standard amino acids decompose at well-defined, characteristic temperatures, in contrast to commonly accepted knowledge. Products of decomposition are simple. The novel quantitative results emphasize the impact of water and cyclic condensates with peptide bonds and put constraints on hypotheses of the origin, state and stability of amino acids in the range between 200 °C and 300 °C.

Keywords: Amino acid; Quantitative mass spectrometry; Thermal analysis.

Fig. 1

Glycine data. C₂H₅NO₂, 75 Da, H_f=−528 kJ/mol

Fig. 2

Cysteine data. C₃H₇NO₂S, 121 Da, H_f=−534 kJ/mol

Fig. 3

Aspartate data. C₄H₇NO₄, 133 Da, H_f=−973 kJ/ mol

Fig. 4

Asparagine data. C₄H₈N₂O₃, 132 Da, H_f=−789 kJ/mol

Fig. 5

Glutamate data. C₅H₉NO₄, 147 Da, H_f=−1097 kJ/mol

Fig. 6

Glutamine data. C₅H₁₀0N₂O₃, 146 Da, H_f=−826 kJ/mol

Fig. 7

Arginine data. C₆H₁₄N₄O₂, 174 Da, H_f=−623 kJ/mol

Fig. 8

Histidine data. C₆H₉N₃O₂, 155 Da, H_f=−467 kJ/mol

Fig. 9

QMS data for the 17 Da channel. Signals in the 17 Da, the NH₃ channel, for each of the amino acids. Ionic charges in the peaks on the left, mol NH₃/mol amino acid on the right. The clustering of G, C, D, Q around $\frac{1}{2}$ mol NH₃ per mol AA and of N and R around 1 mol NH₃ per mol AA is striking

Fig. 10

QMS data for the 18 Da channel. Signals in the 18 Da, the H₂O channel, for each of the amino acids. Ionic charges in the peaks on the left, mol H₂O/mol amino acid on the right. The clustering of C and Q around the $\frac{1}{2}$ mol H₂O level, of N, E, R, H around the 1 mol H₂O level, and the 2 mol point for D are striking

Fig. 11

QMS data for the 44 Da channel. Signals in the 44 Da, the CO₂ channel, for each of the amino acids. Ionic charges in the peaks on the left, mol CO₂/mol amino acid on the right. Only C produces 1 mol CO₂, the level of the others is negligible

Fig. 12

Comparison of mass balances registered by TGA and QMS experiments. The difference between the mass loss registered by TGA, ΔM and the volatile mass found as NH₃, H₂O, CO₂ and H₂S, ΔM − M _gas remains below |9| Da. This is confirmation that no other gases are produced

Fig. 13

Interpretation of Glycine data. a, Residue of Gly, C₂H₂N₂O₂, 1,3-Diazetine-2,4,dione, 86 Da. b, Isomer of Fig. 13a, 1,2-Diazetine-3,4-dione, 86 Da. c, 2-Aziridinone, C₂H₃NO, 57 Da. d, Intermediate dimer, glygly, C₄H₈N₂O₃, 132 Da

Fig. 14

Interpretation of Cysteine data. a, Intermediate compound: 3-pyrrolidinamine, chemspider 144134, 86 Da. b, Residue of Cys, C₄H₇N, 2,5-Dihydro-1H-pyrrole, chemspider 13870958, 69 Da

Fig. 15

Interpretation of Aspartate data. The pathway from Aspartic acid (D) to polysuccimide (PSI). Compared with succinimide, the N-C bond in polysuccinimide economizes two hydrogen atoms

Fig. 16

Interpretation of Asparagine data. Two pathways from asparagine (N) to polysuccinimide (PSI): either through polyasparagine (poly-N) or polyaspartic acid (poly-D). Compared with succiminide, the N-C bond in polysuccinimide economizes two hydrogen atoms

Fig. 17

Interpretation of Glutamate data. a, The final residue of Glu, pyroglutamic acid, C₅H₇NO₃, 129 Da. b, Succinimide, C₄H₅NO₂, 99 Da. c, Pyrrolidone, C₄H₇NO, 85 Da

Fig. 18

Interpretation of Glutamine data. a, Intermediate step, gamma-glutamylglutamine, C₁₀H₁₇N₃O₆, 275 Da. b, The residue of Gln: 5-Oxo-L-prolyl-L-glutamine, C₁0H₁5N₃O₅, 257 Da

Fig. 19

Interpretation of Arginine data. a, 1-Carbamimidoylproline, 157 Da, representing the intermediate step after ejection of NH₃ from Arg. b, The final residue of Arg, C₆H₉N₃O, 139 Da, “creatine-proline”. The creatine ring on top joins the proline ring

Fig. 20

Interpretation of Histidine data. Final residue of His, C₆H₉N₃O, 139 Da, consisting of two 5-rings: 2-amino-2,4-cyclopentadien-1-one (C₅H₅NO, chemspider 28719770) and imidazole

Fig. 21

Overview of the residues with respect to H₂O or NH₃ contents. All residues are obtained by ejection of 0, $\frac{1}{2}$, 1, $1\frac{1}{2}$ or 2 mols of H₂O or NH₃, placing them on two axes. All residues contain either 0, $\frac{1}{2}$ or 1 mol NH₃ or H₂O, placing them on two axes. Most of them contain peptide bonds. The polysuccinimide of D and N is an exception, cysteine, for lack of oxygen, the other