Threonine transformation under hydrothermal conditions

Article abstract of DOI:10.1246/cl.2005.158

In order to revisit the kinetics of amino acid transformation in the chemical evolution, a series of hydrothermal experiments has been conducted for threonine (Thr) solution. Thr transformed into glycine (Gly) and other products including peptides and polymers at 121 to 160°C. The Thr → Gly decomposition rate (k₁ ) is different from apparent Thr decreasing rate (k), because of the presence of other side reactions. Copyright

Full text of DOI:10.1246/cl.2005.158

158
Chemistry Letters Vol.34, No.2 (2005)
Threonine Transformation under Hydrothermal Conditions
Dai Shiota and Satoru Nakashima
Interactive Research Center of Science and the Department of Earth and Planetary Sciences,
Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-Ku, Tokyo 152-8551
(Received October 18, 2004; CL-041218)
In order to revisit the kinetics of amino acid transformation
1 Asp
2 Thr
3 Ser
4 Glu
5 Pro
6 Gly
7 Ala
8 Cys
9 Val
10 Met
11 Ile
in the chemical evolution, a series of hydrothermal experiments
has been conducted for threonine (Thr) solution. Thr trans-
formed into glycine (Gly) and other products including peptides
and polymers at 121 to 160 ꢀC. The Thr ! Gly decomposition
rate (k1) is different from apparent Thr decreasing rate (k),
because of the presence of other side reactions.
product
9
10
6
7
3
11
12 Leu
The emergence of the first life on the earth is the result of a
long chemical evolution.1,2 One of the important steps in the
chemical evolution is a polymerization of organic matter. The
polymerization of amino acids through oligomers to proteins is
considered to have a thermodynamic difficulty.3,4 That is, organ-
ic compounds cannot be polymerized spontaneously. One of the
energy sources to overcome this difficulty is heat. However, the
heating processes also cause the breaking down of amino acids
and oligomers.5–7 Thus it is important to evaluate the competi-
tion of the decomposition of starting materials against the poly-
merization. This study aims to evaluate the rates of amino acid
decomposition processes by kinetic experiments on an OH-bear-
ing amino acid (Threonine: Thr), often found in proteins of
marine phytoplankton such as diatom.8
40 mL of the 0.1 mol/L L-threonine (Thr) aq solution
(pH ¼ 5:8 ꢁ 0:1) was put in a PTFE (Teflon) container, which
is placed in a screw-capped stainless outer vessel. Four sets of
these reaction vessels were heated on the hot plate at 121, 131,
141, and 160 ꢀC covered with aluminum foil for up to 7 days.
All temperatures were measured with thermocouples inserted
at the side of Teflon container. The temperature precision was
within about ꢁ4 ꢀC and the accuracy of the heating duration is
within about 4 h considering the warming periods to the desired
temperature (50 to 220 min) and cooling periods after the heating
(less than 3 h). After cooling at ambient condition, heated solu-
tions were filtered with 0.45 mm-size disposable filter units and
analyzed by high performance liquid chromatography (HPLC)
system.
13 Tyr
4
12
2
13
5
1
8
standard
0
5
10
15
20
Retention time / min
Figure 1. Typical chromatogram of the heated solution (500
times diluted).
ed in trace amounts. Glycylglycylglycine was not detected in
these conditions.
At all the temperatures, Thr concentration decreased with
time, while Gly concentrations increased. pH increased with
time and heating temperatures (the highest pH was 9:3 ꢁ 0:1).
The decreasing rates of Thr and the increasing rates of Gly were
faster for higher temperatures. Theses results suggest that the
transformation of Thr into Gly occurs under these conditions.
Vallentyne showed the nearly complete conversion of Thr to
Gly during the course of pyrolysis in the absence of oxygen.5
The Thr to Gly transformation observed here in the presence
of oxygen is mostly consistent with the Vallentyne’s results.
However, the Gly yield was much less than was expected by
the total conversion from Thr to Gly. Other products such as
GlyGly, brown stuff in soluton and film-like products were
formed in these conditions. Therefore, side reactions other than
the Thr to Gly conversion are considered to be operating in our
system. Characterization and quantification of other products
such as GlyGly and film-like products will be discussed in a sep-
arate paper.
Dissolved organic compounds were analyzed by a HPLC
apparatus (Jasco LC-Series) using AApak Na2–S2 column
(Jasco Corp.) as described previously.9 Fluorescence at 455 nm
after post-column derivatization with phthalaldehyde was moni-
tored as a function of the retention time. After heating, some of
the solutions changed its color from colorless to brown while
keeping their transparency. The film-like products were formed
on the solution surface for some samples. All the solutions,
which had film-like products, changed its color from colorless
to brown.
Based on the above arguments on reaction schema, the
whole system of the Thr transformation reaction can be schema-
tized as follows, with corresponding rate constants k1 and k2.
k1
!
Gly
Thr
k2
HPLC analyses on the solution products showed that re-
maining Thr and glycine (Gly) are the major products
(Figure 1). For higher temperatures and longer durations, glycyl-
glycine (GlyGly), alanine (Ala), and some products were detect-
!
It should be noted that Thr has both the decomposition
(Thr ! Gly) and the transformation (Thr ! Others) pathways.
Others
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.2 (2005)
159
are 6:4 ꢅ 10ꢃ8 sꢃ1 (4:39 ꢅ 10ꢃ8 to 8:90 ꢅ 10ꢃ8 sꢃ1) at 121 ꢀC,
−2
−4
Li and Brill, 2003
Vallentyne,1964
k
k1
ꢃ7
3:4 ꢅ 10ꢃ7 sꢃ1 (1:31 ꢅ 10ꢃ7 to 4:42 ꢅ 10
s
ꢃ1) at 131 ꢀC,
1:1 ꢅ 10ꢃ6 sꢃ1 (5:71 ꢅ 10ꢃ7 to 1:34 ꢅ 10ꢃ6 sꢃ1) at 141 ꢀC and
1:2 ꢅ 10ꢃ5 sꢃ1 (1:18 ꢅ 10ꢃ5 to 1:27 ꢅ 10ꢃ5 sꢃ1) at 160 ꢀC.
Then we can calculate k2 values by subtracting the above aver-
age k1 values from k (Eq 2): k2 ¼ k ꢃ k1. The obtained k2 values
are 2:4 ꢅ 10ꢃ7, 5:8 ꢅ 10ꢃ7, 1:0 ꢅ 10ꢃ6, and 6:0 ꢅ 10ꢃ6 sꢃ1 at
121, 131, 141, and 160 ꢀC, respectively.
−6
k2
−8
−10
−12
−14
−16
−18
In conclusion, the obtained apparent rate constants k
(¼ k1 þ k2) and the apparent activation energy (147 kJ/mol)
showed a good agreement with the literature data for the de-
crease rate of Thr.5,10 (Figure 2). However the k1 and k2 values
showed different slopes in an Arrhenius diagram. These two
slopes for k1 and k2 have a crossing points around 140 ꢀC
(Figure 2). At temperatures under 140 ꢀC, the transformation re-
action has larger rate constants (k2) than the decomposition (k1).
This study demonstrated that the intrinsic amino acid decompo-
sition rate such as that for Thr ! Gly decomposition might be
different from the apparent decrease rate of amino acids, because
of the presence of other side reactions. This difference will be
large at lower temperatures (Figure 2). Therefore, kinetic consid-
eration on the chemical evolution of amino acids against their
decomposition needs to be re-evaluated by taking into account
the present results.
1.6
1.8
2
2.2
2.4
2.6
1000/T / K−1
Figure 2. The Arrhenius diagram for the apparent decrease rate
constants k of threonine together with the literature data.5,10
Let us now consider kinetic aspects of the Thr transformation
reactions described as above with corresponding rate constants
(k1 and k2). If we assume that, all of these reactions are regarded
as the 1st order reaction, the following rate equations can be
formulated.
We would like to thank Dr. M. Ikoma for his discussions on
the kinetic treatments of our data.
½Thrꢂ ¼ ½Thrꢂ0eꢃkt
k ꢄ k1 þ k2
ð1Þ
ð2Þ
References
1
A. I. Oparin, Reproduced in ‘‘The origin of life,’’ ed. by
J. D. Bernal, Weidenfeld and Nicolson, London (1924),
p 199.
k1
½Glyꢂ ¼ ð½Thrꢂ0 ꢃ ½ThrꢂÞ
ð3Þ
k
[Thr] denotes the concentration of Thr in the product solu-
tion, [Thr]0 the initial Thr concentration and [Gly] the Gly con-
centration.
2
3
M.-C. Maurel and J.-L. Decout, Tetrahedron, 55, 3141
(1999).
A. Brack, B. Barbier, F. Boillot, and A. Chabin, in ‘‘Geo-
chemistry and the Origin of Life,’’ ed. by S. Nakashima S.
Maruyama, A. Brack, and B. F. Windley, Universal Acade-
my Press, Tokyo (2001).
S. Nakashima and D. Shiota, in ‘‘Geochemistry and the
Origin of Life,’’ ed. by S. Nakashima S. Maruyama, A.
Brack, and B. F. Windley, Universal Academy Press, Tokyo
(2001).
J. R. Vallentyne, Geochim. Cosmochim. Acta, 28, 157
(1964).
R. H. White, Nature, 310, 430 (1984).
J. L. Bada, S. L. Miller, and M. Zhao, Origins Life Evol.
Biosphere, 25, 111 (1995).
R. E. Hecky, K. Mopper, P. Kilham, and E. T. Degens, Mar.
Biol., 19, 323 (1973).
D. Shiota and S. Nakashima, in ‘‘Geochemistry and the
Origin of Life,’’ ed. by S. Nakashima S. Maruyama, A.
Brack, and B. F. Windley, Universal Academy Press, Tokyo
(2001).
In order to determine the rate constants from our experimen-
tal data sets, the following graphical method was used. The
slopes for linear regression lines in natural logarithm of Thr con-
centration—time diagrams correspond to the first order Thr de-
creasing rate constants k (¼ k1 þ k2). The obtained values are
3:0 ꢅ 10ꢃ7 sꢃ1, 9:2 ꢅ 10ꢃ7 sꢃ1, 2:1 ꢅ 10ꢃ6 sꢃ1, and 1:8 ꢅ 10ꢃ5
sꢃ1 at 121, 131, 141, and 160 ꢀC, respectively. These apparent
decrease rate constants k (¼ k1 þ k2) of Thr were plotted in
the Arrhenius diagram (Figure 2). These values are in good
agreement with the Thr decrease rates reported previously.5,10
The slope of the regression line in Figure 2 gives the apparent
activation energy of 147 kJ/mol, in agreement with the values
in literature data.5,10
Now, by using k, [Thr] and [Gly] values at each tempera-
tures to the Eq 3, we can obtain k1 values for each data points:
the difference in values for the same temperature might be origi-
nated from the experimental errors in temperature and concen-
trations of Thr and Gly. Therefore the average k1 values were de-
termined at each temperature. The obtained average values of k1
4
5
6
7
8
9
10 J. Li and T. B. Brill, J. Phys. Chem. A, 107, 5987 (2003).
Published on the web (Advance View) December 25, 2004; DOI 10.1246/cl.2005.158