Studies of the rates of thermal decomposition of glycine, alanine, and serine

Article abstract of DOI:10.1134/S1070363209080209

Rates of thermal decomposition of glycine, alanine, and serine are described by the equation of first order reaction in the temperature range 200-300°C. Apparent rate constants and apparent activation energies of decomposition of α-amino acids were evaluated. It was found that the main gaseos reaction product is carbon dioxide.

Full text of DOI:10.1134/S1070363209080209

ISSN 1070-3632, Russian Journal of General Chemistry, 2009, Vol. 79, No. 8, pp. 1704–1706. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © V.Ya. Yablokov, I.L. Smel’tsova, I.A. Zelyaev, S.V. Mitrofanova, 2009, published in Zhurnal Obshchei Khimii, 2009, Vol. 79, No. 8,
pp. 1344–1346.
Studies of the Rates of Thermal Decomposition
of Glycine, Alanine, and Serine
V. Ya. Yablokov, I. L. Smel’tsova, I. A. Zelyaev, and S. V. Mitrofanova
Nizhnii Novgorod State Architectural and Construction University,
ul. Il’inskaya 65, Nizhnii Novgorod, 603950 Russia
e-mail: Irina-smelcova@yandex.ru
Received December 30, 2008
Abstract—Rates of thermal decomposition of glycine, alanine, and serine are described by the equation of first
order reaction in the temperature range 200–300ºC. Apparent rate constants and apparent activation energies of
decomposition of α-amino acids were evaluated. It was found that the main gaseos reaction product is carbon
dioxide.
DOI: 10.1134/S1070363209080209
Glycine, alanine, and serine participate in various
metabolic processes taking place in living organism.
Glycine exhibits the retarding action upon the nervous
system and also neutralizes the ill effect of benzoic
acid. It is widely used in manufacturing of fertilizers
[1], explosive mixtures [2], and in the food industry
[3]. Alanine is used for preparing carnosine, the
anticataractive ophthalmologic remedy. It is also used
in manufacturing of fertilizers [1, 4]. Serine takes part
in formation of the monocarbon enzyme [5].
228ºC). Results presented in Fig. 1 permitted to choose
the optimal temperature range for the measurements of
rates of decomposition of the amino acids.
Decomposition of glycine proceeded with the
clearly expressed induction period (Fig. 2). The time of
heating the reagents to the given temperature was 8–
15 min (average), and the pressure of gases
corresponding to the given temperature was accepted
as the initial pressure p0. The character of curves
reflecting the alteration in pressure with the time of
heating shows that decomposition of the crystalline
glycine corresponds to the class of topochemical
reactions. Inductive period corresponds to the growth
of active centers (nuclei). Time of the inductive period
at 212ºC is 30 min.
During the thermal treatment of the protein-
containing food stuffs the amino acids formed in the
course of hydrolysis are exposed to the action of
temperature. The aim of this work is the investigation
of thermal decomposition of the amino acids (glycine,
alanine, and serine), and establishing of the
composition of products formed in the course of
thermal degradation of compounds under study.
Joining of separate nuclei leads to the formation of
the common reaction front. Under the conditions of the
stationary reaction front linear dependence of pressure
on time is observed. After that the rate of liberation of
the volatile product sharply decreases and stops after
the completion of the process (Fig. 2).
For establishing the temperature corresponding to
the beginning of thermal decomposition of amino acid
the sample under investigation was placed in the
reactor which was evacuated to the residual pressure
2×10–2 Torr. The sample was heated slowly at a rate
4ºC/min.It turned out that noticeable decomposition of
the amino acids accompanied by formation of the
volatile products was observed at the temperatures
considerably lower than their melting points. For
instance, gaseous products were formed at 210ºC
while heating of glycine (mp 262ºC ), at 240ºC for
alanine (mp 297ºC ), and at 200ºC for serine (mp
The character of variation in the pressure of the
volatile products of decomposition of glycine
presented in the pressure (p)–time (t) coordinates
permits to establish the range of the intense gas
liberation (middle part on the curves excluding the
induction period of decomposition), and on its basis to
evaluate the apparent rate constants. In this case the
calculation of rate constants of the general process may
1704

STUDIES OF THE RATES OF THERMAL DECOMPOSITION
60
1705
90
80
70
60
50
40
30
20
10
0
1
50
2
1
3
4
5
40
2
30
3
20
10
0
0
50 100 150 200 250 300 350 400
0
50
100
t, min
150
200
250
T, °C
Fig. 1. Dependence of pressure of the decomposition
products on temperature; (1) glycine, (2) alanine, and
(3) serine.
Fig. 2. Dependence of pressure (p) on time (t) for glycine,
ºC: (1) 239.5 , (2) 231 , (3) 226, (4) 220, and (5) 212.
be formally carried out according to the first order
equation.
In the series of the amino acids under investigation
serine contains the additional hydroxymethyl group.
k = (1/t)ln[(p∞ – p0)/(p∞ – pt)].
H2N–CH–COOH
H2N–CH–COOH H2N–CH–COOH
Here p0 and p∞ are the initial and the final gas pressure
in the range of measurement of the intense gas
evolution; pt is the gas pressure at the time t.
H
CH3
alanine
CH2OH
serine
glycine
Just this circumstance leads to the sharp decrease in
thermal stability of serine as compared to glycine and
alanine. The character of variations in pressure of the
gaseous reaction products on time is presented in the
Fig. 4. The plot in the pressure (p)–time (t) coordinates
has two clear parts. The first part corresponds
evidently to decomposition of the amino acid while the
second one reflects the increase in pressure caused by
decomposition of the reaction products. We used the
first plot for the calculation of the apparent rate
constants of the first order reaction.
Rate of the overall process is regarded as the assembly
of all parallel and consecutive elementary reaction on
the solid-gas border as well as in the gas phase.
No induction period is observed for alanine (Fig. 3).
Alanine sublimes easily, and depending on
temperature of the experiment the initial pressure p0
varies from 18 to 157 mm. Methyl group of alanine
increases thermal stability of this compound as
compared to glycine. Similarly to glycine, the apparent
rate constants of the process of decomposition of
alanine were calculated according to the equation of
the first order reaction.
Dependence of rate constants of thermal decom-
position of the amino acids in the ln k–1/T coordinates
80
70
1
1
70
60
50
40
30
20
10
0
3
60
50
40
30
20
10
0
2
2
3
4
4
5
0
500
1000
1500
2000
0
50
100
150
200
250
t, min
t, min
Fig. 3. Dependence of pressure (p) on time (t) for alanine,
ºC: (1) 309 , (2) 277, (3) 264, and (4) 241.
Fig. 4. Dependence of pressure (p) on time (t) for serine,
ºC: (1) 222, (2) 216, (3) 208, (4) 203, and (5) 198.
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YABLOKOV et al.
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Studies of rate of decomposition of the amino acids
were carried out in a thermostate using the evacuated
30 cm3 glass ampules having thin membrane. About
20 mg samples of substances under investigation were
used, and the reaction rate was monitored by the
increase in the pressure of the volatile decomposition
products at the given temperature of experiment.
Increase in the pressure was measured by means of
compensational method maintaining the zero position
of membrane by means of the outer pressure. Scheme
of the installation is described in [7]. Analysis of liquid
phase after decomposition of the amino acids was
carried out on the FOCUS DSO chromatomass
spectrometer using the TR5 capillary column
(60000×0.25 mm). Samples of the liquid under study,
0.05 mcl, were loaded in the injector of the
chromatograph heated to 250ºC . Carrier gas (helium
M 60 grade) flow rate was 1 ml min–1. Temperature of
column varied from 50 to 250ºC . Mass spectra were
measured in the range 29–500 atomic units.
Identification of the mixture components was carried
out using the NIST 2003 electronic library of mass
spectra. Analysis of gas phase was carried out on a
MI1202 mass spectrometer at the temperature of
source and the admission system 25ºC. Energy of the
ionizing electrons 70 eV. Identification of components
was carried out by comparison of the data obtained
with the mass spectra of the NIST electronic library.
2
3
1
1.7
1.8
1.9
2.0
2.1
2.2
(1/T)×103
Fig. 5. Temperature dependences of the apparent rate
constants: (1) alanine, (2) glycine, and (3) serine.
is presented in Fig. 5. In these coordinates the points
of corresponding amino acid lie satisfactory on the
straight lines of the Arrhenius dependences. Values of
the apparent activation energies and the logarithms of
the preexponential factors of the Arrhenius equations
are listed below.
Amino acid
Glycine Alanine Serine
Temperature range of
decomposition , T, °С
212–240 241–309 198–222
ЕА, kJ mol–1
157±7
30.80
262
101±19
14.73
297
344±29
77.84
228
Ln k0
mp of the amino acid [6]
REFERENCES
Analysis of the gas phase obtained after decom-
position of glycine showed that the main reaction
products are CO2 and water. Small amounts of
ammonia and methylamine were also found. In the
case of alanine together with carbon dioxide ammonia
is present. Decomposition of serine yields only carbon
dioxide. Analysis of liquid phase obtained after
decomposition of glycine at 220ºC showed the
presence of acetic acid, of formamide, acetamide, and
N-methylacetamide, of n-butyramide and propion-
amide. It may be concluded that prolonged heating of
glycine, alanine, and serine at the temperatures about
200ºC that is 30–50ºC below their melting points
causes their decomposition with the formation of
volatile and liquid substances. Some of them, amides,
are probably cancerogenic for humans.
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EXPERIMENTAL
7. Yablokov, V.A., Smeltsova, I.L., Zelyaev, I.A., and
Mitrofanova, S.V., Privolzhskii Nauchn. Zh., 2007,
no. 4, p. 115.
Crystalline glycine and l-serine of “pure” grade and
L-alanine of “chemically pure” grade were used.
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