Spectroscopy of hydrothermal reactions, 19: pH and salt dependence of decarboxylation of α-alanine at 280-330°C in an FT-IR spectroscopy flow reactor

Article abstract of DOI:10.1002/kin.10045

The spontaneous decarboxylation of 0.5m aqueous α-alanine solutions as a function of pH (1-9 at 320°C, where neutrality is approximately 6) was determined with a flow reactor at 280-330°C and 275 bar by FT-IR spectroscopy. The kinetics for the cationic and anionic forms have not been previously reported. The rate constants for the cationic form [CH₃(NH₃⁺)CHCO₂H], the anionic form [CH₃(NH₂)CHCO₂^-], and the zwitterion form [CH₃(NH₃⁺)CHCO₂^-] were obtained and followed the first-order rate law. The rate of decarboxylation of the zwitterion is three times greater than that of the cationic and anionic forms in the temperature and pH ranges of study. The corresponding Arrhenius parameters were determined and compared with previously reported data. The addition of KCl (1 and 2 m) at the natural pH of α-alanine resulted in a reduction of the decarboxylation rate, suggesting that the transition state is less polar than the zwitterion and/or that the activity of the zwitterion has been reduced. The α-alanine solution is therefore somewhat more robust in solutions of high ionic strength, such as seawater, than it is in pure water.

Full text of DOI:10.1002/kin.10045

Spectroscopy of
Hydrothermal Reactions, 19:
pH and Salt Dependence
of Decarboxylation
of -Alanine at 280–330 C
in an FT-IR Spectroscopy
Flow Reactor
JUN LI, XIAOGONG WANG, MICHAEL T. KLEIN, THOMAS B. BRILL
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
Received 7 June 2001; accepted 27 November 2001
DOI 10.1002/kin.10045
ABSTRACT: The spontaneous decarboxylation of 0.5 m aqueous -alanine solutions as a
function of pH (1–9 at 320 C, where neutrality is approximately 6) was determined with a
flow reactor at 280–330 C and 275 bar by FT-IR spectroscopy. The kinetics for the cationic
and anionic forms have not been previously reported. The rate constants for the cationic
form [CH3(NH3+)CHCO2H], the anionic form [CH3(NH2)CHCO2 ], and the zwitterion form
[CH3(NH3+)CHCO2 ] were obtained and followed the first-order rate law. The rate of decar-
boxylation of the zwitterion is three times greater than that of the cationic and anionic forms
in the temperature and pH ranges of study. The corresponding Arrhenius parameters were de-
termined and compared with previously reported data. The addition of KCl (1 and 2 m) at the
natural pH of -alanine resulted in a reduction of the decarboxylation rate, suggesting that the
transition state is less polar than the zwitterion and/or that the activity of the zwitterion has
been reduced. The -alanine solution is therefore somewhat more robust in solutions of high
C
ionic strength, such as seawater, than it is in pure water. 2002 Wiley Periodicals, Inc. Int J
Chem Kinet 34: 271–277, 2002
a lively debate with strong arguments both for [1–9]
and against [10–17] the role of these vents in the ori-
gin of life on the planet. It has been determined that
amino acids are irreversibly destroyed in hydrother-
mal solutions when the hydrogen fugacity and redox
state of the environment are not controlled [13,16–
23]. The decarboxylation rate of amino acids is, how-
ever, sharply reduced in the simulated hydrothermal
INTRODUCTION
The synthesis and stability of amino acids at the condi-
tions of ocean-floor hydrothermal vents have produced
Correspondence to: Thomas B. Brill; e-mail: brill@udel.edu.
Contract grant sponsor: National Science Foundation.
Contract grant number: CHE-9807370.
c
2002 Wiley Periodicals, Inc.

272
LI ET AL.
vents, with high hydrogen fugacity [7] or with the re-
dox state maintained by adding the pyrite–pyrrohotite–
magnetite mineral assembly [9]. The ionic strength of
the ocean may also be a substantial factor in that it was
recently mentioned that the addition of KCl reduced
the decarboxylation rate of amino acids [21].
The pH of the solution determines the form of
the amino acid as the zwitterion, the anion, or the
cation. These three forms are expected to have dif-
ferent decarboxylation rates. Indeed, the decarboxyla-
tion of anionic and cationic forms of glycine proceeds
about 10 times more slowly than the zwitterion in the
170–260 C range [21].
Two issues motivated this work. First, the hydro-
thermal stability of amino acids has been previously
investigated only in the batch mode. In order to ob-
tain rate constants and Arrhenius parameters of an
amino acid with real time measurements at hydrother-
mal conditions, we performed a kinetic study of the
spontaneous decarboxylation of ␣-alanine as a func-
tion of the pH by using FT-IR spectroscopy with a
flow reactor. The results are compared to values re-
ported mostly at lower temperatures and at pH values
where the zwitterion dominates. Second, we sought a
more detailed description of the effect of added salts
on the decarboxylation rate than has been reported. At
the end, the decarboxylation rates of solutions of ␣-
alanine were compared with various added amounts
of KCl.
A Nicolet 560 Magna FT-IR spectrometer with an
MCT-A detector was used for the transmission IR
spectroscopy experiments. Kinetic measurements were
1
recorded at 4 cm on 32 summed spectra. The total
collection time at each temperature was approximately
10 s. Each spectrum was normalized against the back-
ground spectrum of pure water recorded at the same
conditions. Only the asymmetric stretch of aqueous
1
CO2 centered at 2343 cm was observed in the band
pass of sapphire windows. The band area of CO2 was
converted to concentration by using the Beer–Lambert
law and the previously determined molar absorptivity
of aqueous CO2 [28]. The weighted least-squares re-
gression [29] was performed in which the statistical
weight was set to be 1/ 2, where is the standard de-
viation of the variables.
RESULTS AND DISCUSSION
Simple amino acids decarboxylate to produce CO2
and the alkylamine [14]. ␣-Alanine was chosen here
for study because the decomposed products remained
water-soluble, in contrast to glycine and most other
amino acids, which produced some insoluble residue.
For ␣-alanine, the products are CO2 and ethylamine by
reaction 1.
CH3(NH2)CHCO2H → CH3CH2NH2 + CO2 (1)
Figure 1 shows the spectrum of CO2 recorded at a
residence time of about 36 s during the decarboxylation
EXPERIMENTAL
The DL ␣-alanine (99%) was obtained from Aldrich
Chemical Company and used without further treat-
ment. Milli-Q deionized water was sparged with com-
pressed argon before use so as to expel the atmospheric
gases. The pH values of aqueous alanine solutions were
adjusted by adding aqueous HCl or KOH, and were in-
dicated by the use of an Orion model 330 pH meter.
The molality of the alanine solution was maintained at
0.5 m in all cases. To test the effect of increasing the
ionic strength on the decarboxylation rate, KCl was
added at concentrations of 1 and 2 m at the natural pH
of ␣-alanine.
The flow reactor constructed from titanium with
sapphire windows has been described in detail else-
where [24–27]. Because of the limitations of the seals,
the decarboxylation kinetics were determined only up
to 330 C and 275 bar. This temperature is below that
which is required to obtain a wide range of rate con-
stants at the highest and lowest pH values where the
rates are the lowest. Corrosion of the cell by strong
acid prevented determination of the rate constant for
decarboxylation below pH 1.
Figure 1 FT-IR spectra of aqueous CO2 from decarboxy-
lation of 0.5 m ␣-alanine solution at different temperatures
under 275 bar and at a residence time of approximately 36 s.

SPECTROSCOPY OF HYDROTHERMAL REACTIONS
273
escape.
Ka1
CO2 + H2O
HCO3
HCO3 + H+
(5)
(6)
(7)
Ka2
Kb
CO3 + H+
2
CH3CH2NH2 + H+
CH3CH2NH3
+
The equilibrium constants for reactions 5–7 were writ-
ten in iso-Coulombic form and extrapolated from the
room temperature to high temperature [31]. This ex-
trapolation employs Kw [32] and the specific volume
of water [33], apparent dissociation constants K1 and
K2, and equilibrium constants Kb [34], Ka1, and Ka2
[35]. The extrapolated K1, K2, Kb, Ka1, and Ka2, as
well as calculated solution pH values at high temper-
atures are given in Table I. This method does not take
into account the effects of ion activity or complexation
on the fluid speciation.
Scheme 1
of aqueous ␣-alanine solution under 275 bar at differ-
ent temperatures. Similar data were recorded at each
value of the pH. The conversion of the resulting band
areas (concentration of CO2 [CO2]obs) into useful ki-
netics for ␣-alanine however requires consideration of
the equilibria in the solution.
According to Eq. (1), Eq. (8) applies at each tem-
perature and residence time,
Four forms of an amino acid are possible in
aqueous solution: the cationic form [CH3(NH3+)-
CHCO2H] at lowest pH, the anionic form [CH3(NH2)-
CHCO2 ] at highest pH, the zwitterion form
[CH3(NH3+)CHCO2 ], and the uncharged form
[CH3(NH2)CHCO2H] at intermediate pH values. The
equilibrium among these species is shown in Scheme 1.
The concentration of the zwitterion of an amino acid
is much larger than that of uncharged form. For ex-
ample, KZ = 105.41 for ␣-alanine in neutral solution at
room temperature [30]. Therefore, the uncharged form
was not considered further in the present study. K1 and
K2 are experimentally determined apparent dissocia-
tion constants [30].
To obtain the rate constants of decarboxylation of
␣-alanine from the CO2 concentration, the ioniza-
tion of the CO2 [Eqs. (5) and (6)] and protonation
of the ethylamine [Eq. (7)] products must be taken
into account because the decarboxylation reaction oc-
curred in a closed system where the product does not
[CH3(NH2)CHCO2H]T,t
= [CH3(NH2)CHCO2H]0 [CO2]T,t (8)
where [CH3(NH2)CHCO2H]0 is the initial concen-
tration, [CH3(NH2)CHCO2H]T,t is total concentra-
tion of ␣-alanine at any time Eq. (9), and [CO2]T,t
is the total running concentration of CO2 from
Eq. (10).
[CH3(NH2)CHCO2H]T,t
=[CH3(NH3+)CHCO2H] + [CH3(NH3+)CHCO2
]
+ [CH3(NH2)CHCO2
]
(9)
2
[CO2]T,t = [CO2]obs + [HCO3 ] + CO3
Ka1
[H+]
Ka1 Ka2
[H+]2
= [CO2]obs 1 +
+
(10)
Table I Apparent Dissociation Constants for Alanine (K1 and K2), Ionization Constants of Ethylamine (Kb) and CO2
(Ka1 and Ka2), and the Calculated pH at Different Temperatures for 0.5 m Aqueous Alanine Solutionsa
pH25, Exp.
1
2.3 3.32 4.02 4.95
pH, Calc.
5.22 5.6 5.66 5.72 6.39
6
7.2
8.5
9.5 11
Temperature ( C) pK1 pK2 pKb pKa1 pKa2
280
290
300
310
320
330
3.64 7.70 10.63 7.70 11.18
3.76 7.75 10.62 7.82 11.29
3.89 7.81 10.62 7.95 11.41
4.03 7.89 10.62 8.10 11.54
4.19 7.98 10.62 8.25 11.69 1.2
4.36 8.10 10.62 8.43 11.86 1.15 3.8
4.5
3.52 4.57 5.3
5.66 5.72 5.76 6.41 7.07
4.65 5.35 5.7 5.76 5.81 6.42 7.09
3.6
3.7
4.74 5.44 5.75 5.81 5.85 6.44 7.1
3.75 4.82 5.5
5.8
5.86 5.9
6.46 7.12 8.9
7.15
4.9 5.55 5.85 5.9
5.93 6.5
9
aAdjusted by adding HCl or NaOH.

274
LI ET AL.
Figure 2 The first-order rate plot of 0.5 m ␣-alanine solu-
tion at 275 bar and the natural pH (6.0).
The solution pH must be known at each tempera-
ture and residence time. The charge and mass balance
equations make this calculation possible. At neutral
conditions, Eq. (11) follows:
[H+] + [CH3(NH3+)CHCO2H] + [CH3CH2NH3
]
+
= [OH ] + [CH3(NH2)CHCO2
+ [HCO3 ] + 2 CO3
]
2
(11)
At acidic and basic conditions Eqs. (12) and (13), re-
spectively, follow:
[H+] + [CH3(NH3+)CHCO2H] + [CH3CH2NH3
]
+
= [OH ] + [CH3(NH2)CHCO2
+ [HCO3 ] + [Cl ]
]
(12)
[H+] + [CH3(NH3+)CHCO2H] + [CH3CH2NH3
]
+
+ [K+] = [OH ] + [CH3(NH2)CHCO2
]
2
+ [HCO3 ] + 2 CO3
(13)
Kb[H+][CH3CH2NH2]T,t
[CH3CH2NH3+] =
1 + Kb[H+]
Kb[H+][CO2]T,t
=
(14)
1 + Kb[H+]
The concentration of protonated ethylamine was cal-
culated from Eq. (14).

SPECTROSCOPY OF HYDROTHERMAL REACTIONS
275
Table III The Rate Constants and Arrhenius Parameters of Cationic Form k1, Zwitterion k2, and Anionic form k3
at 275 Bar
Temperature ( C)
k1 103
k2 103
k3 103
310
320
330
Ea (kJ mol
ln A (s
2.92 2.2
5.63 2.82
14.04 4.37
241.02 23.63
43.80 4.75
9.64 0.74
18.53 1.21
33.92 2.1
183.57 2.42
33.24 0.49
1.76 3.08
5.33 3.35
16.51 3.9
332.84 4.54
62.30 0.91
1
)
1
)
The rate expression for the decarboxylation of
␣-alanine at 275 bar is given by Eq. (15), where
[CH3(NH2)CHCO2H]T,t is obtained from Eq. (8).
Fitting the nonlinear Eq. (16) with the observed rate
constants at different values of the pH yielded k1, k2,
and k3. The fitted k1, k2, and k3 values are listed in
Table III. Figure 3 shows an example of the compar-
ison of the observed and calculated rate constants at
320 C. The change in the rates roughly followed the
distribution of the ␣-alanine species as a function of
the solution pH at 320 C. The rate constant of anionic
form is similar to that of cationic form. In the temper-
ature range of study, the rate constant of zwitterion is
however approximately three times greater than that
of cationic and anionic forms. This difference extrapo-
lates reasonably well from the region of measurement
to the factor of 10 difference in these rates for glycine
at about 210 C reported by Snider and Wolfenden [21]
for the cation and zwitterion.
d[CH3(NH2)CHCO2H]T,t /dt
= kobs[CH3(NH2)CHCO2H]T,t
(15)
Figure 2 shows the first-order rate plot of decarboxy-
lation of neutral ␣-alanine solution. The observed rate
constantsatdifferentvaluesofthesolutionpHarelisted
in Table II. In acidic, neutral, and basic solutions, the
predominating species are the cationic, zwitterionic,
and neutral forms, respectively. Each has different first-
order rate constants. If k1 represents the first-order rate
constant for the cationic form, k2 for the zwitterion,
and k3 for anionic form, then Eq. (16) applies.
Figure 4 is an Arrhenius plot for decarboxylation
of the cationic, anionic, and zwitterion forms. The
[H+]2k1 + K1[H+]k2 + K1K2k3
kobs
=
(16)
[H+]2 + K1[H+] + K1K2
Figure 4 Arrhenius plot of the cationic form
[CH3(NH3+)CHCO2H] (k1), the zwitterion form
Figure 3 Comparison of observed and fitted rate constants
and the distribution of ␣-alanine species in solution at 320 C
and 275 bar.
[CH3(NH3 )CHCO2
[CH3(NH2)CHCO2 ] (k3) of ␣-alanine, including previ-
ously reported data.
]
(k2), and the anionic form

276
LI ET AL.
Table IV Observed First-Order Rate Constants (k 103) and Arrhenius Parameters for Decarboxylation of 0.5 m
-Alanine (pH25 6) at Different Concentrations of Added KCl
Temperature ( C)
0 m KCl
1 m KCl
2 m KCl
300
310
320
330
1
4.72 0.1
9.09 0.24
18.03 0.48
37.0 1.0
0.79 0.03
2.43 0.08
5.93 0.14
12.62 0.20
0.54 0.06
1.30 0.06
4.00 0.14
10.44 0.21
Ea (kJ mol
)
251
6
6
259 13
46.7 2.7
296
8
1
ln A (s
)
45.7
54.5 1.67
Arrhenius parameters are given in Table III. For com-
parison, the previously reported values for k2 are dis-
played in Fig. 4. These latter data were obtained at
mostly lower temperatures and in the batch mode. In
all cases the pH values used were those at room tem-
perature loading conditions rather that at the reaction
conditions. In addition, the pH values previously clus-
ter around neutral (see Fig. 4) and therefore apply only
to the zwitterion. The present study applies to both the
zwitterion and the cationic form of ␣-alanine. Most of
the data in Fig. 4 are consistent within the experimen-
tal error; however the trend in the data of Qian et al.
[16] and of Conway and Libby [20] suggests slower
reaction rates at higher temperatures than do the data
in other reports.
show the rate constants in the 300–330 C range when
0.00, 1.00, and 2.00 m KCl is present. The Arrhenius
plot is shown in Fig. 5. The decarboxylation rate is
clearly retarded by an increase in the ionic strength.
These data have several possible explanations. One in-
terpretation is that they show qualitatively that the tran-
sition state for the decarboxylation process is less polar
than the zwitterion reactant, perhaps because of ion and
solvent association. A second interpretation is that the
K+ and Cl ions form complexes with the ionic sites
of the ␣-alanine zwitterion, which reduces the activity
of the free alanine.
The present study was successful in experimentally
determining the decarboxylation kinetics of all forms
of ␣-alanine for the first time. Although the pH range
was limited to some extent by the reality of the mea-
surement methods, the pH values used are correct for
the actual reaction conditions as opposed to previous
workinwhichthepHatonlytheloadingconditionswas
used. The study addresses the issue of stability of these
species in hydrothermal vents peripherally in the sense
that an increase in the ionic strength tends to retard
the decarboxylation rate, although the ionic strength
values used in this work exceed those of seawater.
The effect of ionic strength on the decarboxylation
rate was tested by adding KCl to 0.5 m ␣-alanine at its
natural pH of 6.0–6.2 (at 25 C). The data in Table IV
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