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Cautionary note: The thermal decomposition of amino acids in the solid state
Thermochimica Acta 555 (2013) 89–90
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short communication
Cautionary note: The thermal decomposition of amino acids in the solid state David S. Ross ∗,1 SRI International, 333 Ravenswood Ave. Menlo Park, CA 94025, United States
a r t i c l e
i n f o
Article history: Received 15 December 2012 Received in revised form 21 December 2012 Accepted 21 December 2012 Available online 14 January 2013 Keywords: Kinetics Cautionary note Amino acids
a b s t r a c t The Arrhenius parameters governing the pyrolysis rates for 19 amino acids in the solid state presented in the paper listed as reference 1 were developed in a dynamic TGA–DSC procedure employing single heating rate techniques and appear to be untrustworthy. The incongruities resulting from the parameters include clearly implausible half-lives for glycine and alanine at 25 ◦ C of respectively 8 min and 2.5 years. A simulation of an experimental run for glycine employing the recorded parameters and the procedure described in the paper reveals that the amino acid should be fully consumed at temperatures well below the recorded region of thermolysis. The recorded parameters for alanine are significantly different from those reported 3 decades earlier in a kinetic study that utilized conventional isothermal techniques and provided acceptable estimates of alanine’s stability. The source of the problem is the use of a single heating rate rather than multiple rates in the determinations, and it is suggested that the parameters listed in reference 1 be avoided. © 2013 Elsevier B.V. All rights reserved.
The Arrhenius parameters for the thermal decomposition of 19 ␣-amino acids in the solid state were developed in studies employing dynamic TG–DSC techniques and temperatures ramped at 10 ◦ C/min up to 600 ◦ C [1]. Over the 2 decades since its publication the work has been widely cited [2–11], but the extreme breadth and extent of the recorded parameters for a single compound class raise alerts as to their legitimacy. An examination of the rate constants developed from the parameters (A (s−1 ), Ea (kcal/mol)) for glycine (24.5, 5.9) and alanine (4.0 × 108 , 23.0), for example, yields unrealistic half-lives at 25 ◦ C of about 8 min and 2.5 years respectively. At 100 ◦ C the half-lives for both glycine and alanine fall in the seconds range while that for methionine (7.5 × 1031 , 81.3) is a stunning 109 years. These issues notwithstanding, it is conceivable that the parameters could nonetheless reliably reflect the phenomenology at the temperatures at which they were generated. This prospect cannot be the case, however, as was demonstrated in a simulated experimental determination using the recorded parameters for glycine in a TG–DSC run initiated at 25 ◦ C and ramped at 10◦ /min. The results showed that the amino acid would have been fully spent at 170◦ in the face of the recorded zone of decomposition of 226–573 ◦ C. (The simulation was conducted with Kintecus, a software package for numerical simulations of complex chemical systems [12].)
∗ Corresponding author. Current address: 149 Walter Hays Dr., Palo Alto, CA 94303, United States. Tel.: +1 6503273842; fax: +1 6503219608. E-mail address: [email protected] 1 Retired. 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.12.021
It is noteworthy that an earlier kinetic study of solid alanine decarboxylation by Conway and Libby employing conventional isothermal procedures yielded A = 6.6 × 1011 s−1 and Ea = 40.2 kcal/mol [13]. With these values it evolves that alanine is more stable than would be expected from the reference 1 parameters by almost 7 orders of magnitude at 100 ◦ C, and remains more stable by a factor of 10 at 600 ◦ C. It is of interest to note further that for temperatures up to 600 ◦ C the Conway and Libby rates for alanine fall to within an order of magnitude to those for glycine developed in studies conducted in hydrothermal media [14]. The source of the problem is the use of a single heating rate in the determinations, which practice introduces compensating errors and ultimately unreliable Arrhenius parameters [15]. The Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry has recommended that practice not be used, and advocates procedures employing a series of multiple rates [16].2 It is accordingly prudent to presume these issues apply to all of the amino acids listed in reference 1, and it is suggested that the parameters recorded there be avoided. References [1] F. Rodante, Thermodynamics and kinetics of decomposition processes for standard ␣-amino acids and some of their dipeptides in the solid state, Thermochim. Acta 200 (1992) 47–61. [2] B. Cohen, C. Chyba, Racemization of meteoritic amino acids, Icarus 145 (2000) 272–281.
2 The author is grateful to a referee for constructive comment and counsel on the single heating rate issue.
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D.S. Ross / Thermochimica Acta 555 (2013) 89–90
[3] D. Glavin, J. Bada, Survival of amino acids in micrometeorites during atmospheric entry, Astrobiology 1 (2001) 259–269. [4] D. Glavin, G. Matrajt, J. Bada, Re-examination of amino acids in Antarctic micrometeorites, Adv. Space Res. 33 (2004) 106–113. [5] A. Burton, D. Glavin, M. Callahan, J. Dworkin, P. Jenniskens, M. Shaddad, Heterogeneous distributions of amino acids provide evidence of multiple sources within the Almahata Sitta parent body, asteroid 2008 TC3, Meteorit. Planet. Sci. 46 (2011) 1703–1712. [6] O. Abramov, S. Mojzsis, Abodes for life in carbonaceous asteroids? Icarus 213 (2011) 273–279. [7] L. Jie, L. Yuwen, S. Jingyan, W. Zhiyong, H. Ling, Y. Xi, W. Cunxin, The investigation of thermal decomposition pathways of phenylalanine and tyrosine by TG–FTIR, Thermochim. Acta 467 (2008) 20–29. [8] H. Abdoul-Carime, S. Gohlke, E. Illenberger, Fragmentation of tryptophan by low-energy electrons, Chem. Phys. Lett. 401 (2005) 497–502. [9] E. Pierazzo, C. Chyba, Amino acid survival in large cometary impacts, Meteorit. Planet. Sci. 34 (1999) 909–918. [10] T. Otake, T. Taniguchi, Y. Furukawa, F. Kawamura, H. Nakazawa, T. Kakegawa, Stability of amino acids and their oligomerization under high-pressure conditions: implications for prebiotic chemistry, Astrobiology 11 (2011) 799–813.
[11] R. Wilson, V. Pearson, G. Morgan, I. Franchi, D. Turner, I. Wright, I. Gilmour, Experimental simulation of volatile organic contributions to planetary atmospheres and surfaces, in: 38th Lunar and Planetary Science Conference (Lunar and Planetary Science XXXVIII), Houston, TX, USA, 12–16 March, 2007. [12] J. Ianni, Comparison of the Bader–Deuflhard and the Cash–Karp Runge–Kutta integrators for the GRI-MECH 3.0 model based on the chemical kinetics code kintecus, in: K. Bathe (Ed.), Computational Fluid and Solid Mechanics, Elsevier Science Ltd., Oxford, UK, 2003, pp. 1368–1372. [13] D. Conway, W. Libby, The measurement of very slow reaction rates; decarboxylation of alanine, J. Am. Chem. Soc. 80 (1958) 1077–1084. [14] M.J. Snider, R. Wolfenden, The rate of spontaneous decarboxylation of amino acids, J. Am. Chem. Soc. 122 (2000) 11507–11508. [15] S. Vyazovkin, C.A. Wight, Model-free and model-ftting approaches to kinetic analysis of isothermal and nonisothermal data, Thermochim. Acta 340–341 (1999) 53–68. [16] S. Vyazovkin, A. Burnham, J. Criado, L. Pérez-Maqueda, P. Popescu, N. Sbirrazzuoli, 2000. ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data, Thermochimica Acta 520 (2011) 1–19.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Short communication
Cautionary note: The thermal decomposition of amino acids in the solid state David S. Ross ∗,1 SRI International, 333 Ravenswood Ave. Menlo Park, CA 94025, United States
a r t i c l e
i n f o
Article history: Received 15 December 2012 Received in revised form 21 December 2012 Accepted 21 December 2012 Available online 14 January 2013 Keywords: Kinetics Cautionary note Amino acids
a b s t r a c t The Arrhenius parameters governing the pyrolysis rates for 19 amino acids in the solid state presented in the paper listed as reference 1 were developed in a dynamic TGA–DSC procedure employing single heating rate techniques and appear to be untrustworthy. The incongruities resulting from the parameters include clearly implausible half-lives for glycine and alanine at 25 ◦ C of respectively 8 min and 2.5 years. A simulation of an experimental run for glycine employing the recorded parameters and the procedure described in the paper reveals that the amino acid should be fully consumed at temperatures well below the recorded region of thermolysis. The recorded parameters for alanine are significantly different from those reported 3 decades earlier in a kinetic study that utilized conventional isothermal techniques and provided acceptable estimates of alanine’s stability. The source of the problem is the use of a single heating rate rather than multiple rates in the determinations, and it is suggested that the parameters listed in reference 1 be avoided. © 2013 Elsevier B.V. All rights reserved.
The Arrhenius parameters for the thermal decomposition of 19 ␣-amino acids in the solid state were developed in studies employing dynamic TG–DSC techniques and temperatures ramped at 10 ◦ C/min up to 600 ◦ C [1]. Over the 2 decades since its publication the work has been widely cited [2–11], but the extreme breadth and extent of the recorded parameters for a single compound class raise alerts as to their legitimacy. An examination of the rate constants developed from the parameters (A (s−1 ), Ea (kcal/mol)) for glycine (24.5, 5.9) and alanine (4.0 × 108 , 23.0), for example, yields unrealistic half-lives at 25 ◦ C of about 8 min and 2.5 years respectively. At 100 ◦ C the half-lives for both glycine and alanine fall in the seconds range while that for methionine (7.5 × 1031 , 81.3) is a stunning 109 years. These issues notwithstanding, it is conceivable that the parameters could nonetheless reliably reflect the phenomenology at the temperatures at which they were generated. This prospect cannot be the case, however, as was demonstrated in a simulated experimental determination using the recorded parameters for glycine in a TG–DSC run initiated at 25 ◦ C and ramped at 10◦ /min. The results showed that the amino acid would have been fully spent at 170◦ in the face of the recorded zone of decomposition of 226–573 ◦ C. (The simulation was conducted with Kintecus, a software package for numerical simulations of complex chemical systems [12].)
∗ Corresponding author. Current address: 149 Walter Hays Dr., Palo Alto, CA 94303, United States. Tel.: +1 6503273842; fax: +1 6503219608. E-mail address: [email protected] 1 Retired. 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.12.021
It is noteworthy that an earlier kinetic study of solid alanine decarboxylation by Conway and Libby employing conventional isothermal procedures yielded A = 6.6 × 1011 s−1 and Ea = 40.2 kcal/mol [13]. With these values it evolves that alanine is more stable than would be expected from the reference 1 parameters by almost 7 orders of magnitude at 100 ◦ C, and remains more stable by a factor of 10 at 600 ◦ C. It is of interest to note further that for temperatures up to 600 ◦ C the Conway and Libby rates for alanine fall to within an order of magnitude to those for glycine developed in studies conducted in hydrothermal media [14]. The source of the problem is the use of a single heating rate in the determinations, which practice introduces compensating errors and ultimately unreliable Arrhenius parameters [15]. The Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry has recommended that practice not be used, and advocates procedures employing a series of multiple rates [16].2 It is accordingly prudent to presume these issues apply to all of the amino acids listed in reference 1, and it is suggested that the parameters recorded there be avoided. References [1] F. Rodante, Thermodynamics and kinetics of decomposition processes for standard ␣-amino acids and some of their dipeptides in the solid state, Thermochim. Acta 200 (1992) 47–61. [2] B. Cohen, C. Chyba, Racemization of meteoritic amino acids, Icarus 145 (2000) 272–281.
2 The author is grateful to a referee for constructive comment and counsel on the single heating rate issue.
90
D.S. Ross / Thermochimica Acta 555 (2013) 89–90
[3] D. Glavin, J. Bada, Survival of amino acids in micrometeorites during atmospheric entry, Astrobiology 1 (2001) 259–269. [4] D. Glavin, G. Matrajt, J. Bada, Re-examination of amino acids in Antarctic micrometeorites, Adv. Space Res. 33 (2004) 106–113. [5] A. Burton, D. Glavin, M. Callahan, J. Dworkin, P. Jenniskens, M. Shaddad, Heterogeneous distributions of amino acids provide evidence of multiple sources within the Almahata Sitta parent body, asteroid 2008 TC3, Meteorit. Planet. Sci. 46 (2011) 1703–1712. [6] O. Abramov, S. Mojzsis, Abodes for life in carbonaceous asteroids? Icarus 213 (2011) 273–279. [7] L. Jie, L. Yuwen, S. Jingyan, W. Zhiyong, H. Ling, Y. Xi, W. Cunxin, The investigation of thermal decomposition pathways of phenylalanine and tyrosine by TG–FTIR, Thermochim. Acta 467 (2008) 20–29. [8] H. Abdoul-Carime, S. Gohlke, E. Illenberger, Fragmentation of tryptophan by low-energy electrons, Chem. Phys. Lett. 401 (2005) 497–502. [9] E. Pierazzo, C. Chyba, Amino acid survival in large cometary impacts, Meteorit. Planet. Sci. 34 (1999) 909–918. [10] T. Otake, T. Taniguchi, Y. Furukawa, F. Kawamura, H. Nakazawa, T. Kakegawa, Stability of amino acids and their oligomerization under high-pressure conditions: implications for prebiotic chemistry, Astrobiology 11 (2011) 799–813.
[11] R. Wilson, V. Pearson, G. Morgan, I. Franchi, D. Turner, I. Wright, I. Gilmour, Experimental simulation of volatile organic contributions to planetary atmospheres and surfaces, in: 38th Lunar and Planetary Science Conference (Lunar and Planetary Science XXXVIII), Houston, TX, USA, 12–16 March, 2007. [12] J. Ianni, Comparison of the Bader–Deuflhard and the Cash–Karp Runge–Kutta integrators for the GRI-MECH 3.0 model based on the chemical kinetics code kintecus, in: K. Bathe (Ed.), Computational Fluid and Solid Mechanics, Elsevier Science Ltd., Oxford, UK, 2003, pp. 1368–1372. [13] D. Conway, W. Libby, The measurement of very slow reaction rates; decarboxylation of alanine, J. Am. Chem. Soc. 80 (1958) 1077–1084. [14] M.J. Snider, R. Wolfenden, The rate of spontaneous decarboxylation of amino acids, J. Am. Chem. Soc. 122 (2000) 11507–11508. [15] S. Vyazovkin, C.A. Wight, Model-free and model-ftting approaches to kinetic analysis of isothermal and nonisothermal data, Thermochim. Acta 340–341 (1999) 53–68. [16] S. Vyazovkin, A. Burnham, J. Criado, L. Pérez-Maqueda, P. Popescu, N. Sbirrazzuoli, 2000. ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data, Thermochimica Acta 520 (2011) 1–19.