ASTROBIOLOGY
Volume 7, Number 3, 2007
© Mary Ann Liebert, Inc.
DOI: 10.1089/ast.2006.0083
Research Paper
Sugar Synthesis from a Gas-Phase Formose Reaction
ABRAHAM F. JALBOUT,1 LEIF ABRELL,2,3 LUDWIK ADAMOWICZ,2,3
ROBIN POLT,2,3 A.J. APPONI,2,3,4,5 and L.M. ZIURYS2,3,4,5
ABSTRACT
Prebiotic possibilities for the synthesis of interstellar ribose through a protic variant of the
formose reaction under gas-phase conditions were studied in the absence of any known catalyst. The ion-molecule reaction products, diose and triose, were sought by mass spectrometry, and relevant masses were observed. Ab initio calculations were used to evaluate protic
formose mechanism possibilities. A bilateral theoretical and experimental effort yielded a
physical model for glycoaldehyde generation whereby a hydronium cation can mediate
formaldehyde dimerization followed by covalent bond formation leading to diose and water. These results advance the possibility that ion-molecule reactions between formaldehyde
(CH2O) and H3O lead to formose reaction products and inform us about potential sugar formation processes in interstellar space. Key Words: Astrobiology—Molecular processes—Laboratory methods—Interstellar medium, molecules—Interstellar medium, clouds. Astrobiology
7(3), 433–442.
INTRODUCTION
O
more than 75%
of known interstellar chemical species
(Muller et al., 2005). Gas-phase interstellar reactions dominated by two-body, exothermic, ionmolecule reactions permit theoretically feasible
mechanistic pathways that lead to interstellar organics, including the more complex species [up
to 12 atoms (Herbst and Klemperer, 1976; Lee et
al., 1996)]. Molecular hydrogen (H2), H3, and
H3O are all present in the interstellar medium
(H2 is the most abundant interstellar chemical
species), which suggests that protonation may be
RGANIC MOLECULES MAKE UP
an important ion-molecule reaction. Formaldehyde (CH2 : O) is significantly abundant among
the 140 known interstellar molecules, and protonated formaldehyde (CH2 : OH) and glycoaldehyde (HOCH2CH:O) have been detected
by radio telescope (millimeter) astronomy (Ohishi
et al., 1996; Hollis et al., 2000; Halfen et al., 2006).
Could these organic species present in interstellar molecular clouds be the chemical building
blocks of life?
In 1861, the Russian chemist Alexander
Michailowitsch Boutlerow noted that formaldehyde reacts under basic conditions [Ca(OH)2 solution] to form a mixture of larger sugars, in-
1Instituto
de Quimica, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico D.F.
Astrobiology Institute, Departments of 3Chemistry and 4Astronomy, and 5Steward Observatory, The University of Arizona, Tucson, Arizona.
2NASA
433
434
cluding ribose (Boutlerow, 1861). This sugar formation, the “formose reaction,” where n 1–5
for nCH2O (CH2O)n, was explained by Ronald
Breslow in 1959 as a series of aldol condensation
mechanisms, though he acknowledged that the
first “induction” step from pure formaldehyde
(CH2O) to glycoaldehyde (CH2O)2 is puzzling
and not an aldol condensation (Breslow, 1959). A
Nazarov-like mechanism was proposed to help
explain this difficult first step, but the leading induction step question remains unanswered
(Halfen et al., 2006). Nevertheless, the formose reaction persists in the literature as an admired
route to prebiotic ribose (CH2O)5, a compulsory
feature of the envisaged “RNA World” (Gilbert,
1986).
Further work on the base-catalyzed formose reaction has revealed a variety of stabilizers and catalysts for effective transformation of formaldehyde
to ribose (Zubay, 1999). Under terrestrial conditions, the formose reaction product ribose is stabilized by borate [e.g., colemanite Ca2B6O11 5H2O
(Benner et al., 2004; Ricardo et al., 2004)]. Many hydroxy ketones can catalyze the reaction, and,
based on the same mechanism, one molecule of glycoaldehyde will theoretically start the formose reaction cycle, effectively bridging the first induction
step. Productive basic conditions can also be created by Mg(OH)2 or plumbous salts or minerals,
e.g., PbS, galena, and others (Zubay, 1999). Our interest in prebiotic possibilities for interstellar ribose
requires appreciation of the formose reaction under low-density, low-temperature conditions. This
problem was approached through low-pressure
experimentation and theoretical calculations in the
absence of any known catalyst.
In consideration of the extreme conditions in
the interstellar medium (the presence of H3 and
H3O donors, low temperatures, and low pressures), we sought possible methods to study the
formose reaction under similar conditions in the
laboratory. Proton transfer reaction-mass spectrometry inherently operates under comparable
conditions and was, therefore, chosen for performing these experiments.
A gas-phase protic variant of the formose reaction was explored at 2 mbar using hydronium
ion (H3O) as a proton-donating source to
formaldehyde. Reaction products were observed
and measured by single quadrupole mass spectrometry, with a special interest in higher mass
sugars, where n 2–5 for (CH2O)n. Although
structural information to distinguish between iso-
JALBOUT ET AL.
baric products of (CH2O)n is still needed to identify possible formose reaction species, the observation of masses that correspond to n 2 advance the possibility that ion-molecule reactions
between formaldehyde (CH2O) and a proton
source (i.e., H3 or H3O) can lead to sugars (such
as diose). These low-resolution results may serve
to inform us about potential sugar formation
processes in interstellar space as a possible source
of prebiotic material.
Without any positive identifications of our reaction products, an assessment of theoretical evidence for proposed reaction mechanisms was undertaken. Results revealed incompatibility of an
isolated proton (H) donor due to high-energy interactions associated with this ion, so an alternate
source was considered. Other proton sources that
were tested produced either unbearably high barriers or a lack of stable transition states. This system was quite sensitive to the computational
methods employed, and only through careful calculations was the mechanism adequately modeled.
As a result of the bilateral theoretical and experimental effort, a physical model was generated
by which a hydronium cation (H3O) mediated
formaldehyde dimerization in a -stacking
arrangement, with two formaldehyde molecules
arranged in a head-to-tail van der Waals intermediate. This complex then underwent covalent bond
formation and eventually separated into diose and
water. To our knowledge, there is no physical
model like this used to explore and explain gasphase formose chemistry.
Herein, we report results from experimental
gas-phase proton transfer reactions that support
theoretical calculations to provide detailed information about possible formose mechanisms. Although mass spectrometry experiments were not
able to distinguish between five possible isobaric
structures with identical mass, theoretical calculations clearly established a possible protic, gasphase formose mechanism that leads to glycoaldehyde (a structure of significant interest to
astrobiologists). In the future, we intend to explore the gas-phase formose mechanism by additional experimental methods.
MATERIALS AND METHODS
Gas-phase proton transfer reactions
A protic variant of the formose reaction was
studied in the gas phase under conditions de-
FIG. 1.
Structures and geometrical parameters for all species studied in this work. Bond lengths in angstroms (Å) and bond angles in degrees (°) are also shown.
436
signed for protonated formaldehyde to undergo
induction to glycoaldehyde followed by aldol
condensations. Hydronium ion was used to protonate formaldehyde under reduced pressure
wherein further ion-molecule reactions could
proceed. Resulting single unit ion masses were
observed by quadrupole mass spectrometry.
Formaldehyde gas was generated by heating
several grams of solid paraformaldehyde in a dry
environment (Pengelly et al., 1996). A 120-ml Erlenmeyer flask containing paraformaldehyde was
partially immersed in a 80°C water bath while argon gas (10 psi, 0.145 mbar) was passed through
the closed flask. The formaldehyde–argon gas
mixture was delivered through a 12-m Teflon®
(DuPont, Wilmington, DE) tubing (0.125 in, 0.32
cm inner diameter) to the sample gas inlet of a
proton transfer reaction mass spectrometer.
Proton transfer reaction-mass spectrometry has
become a unique tool for analysis of trace volatile
organic compounds. The proton transfer reaction-mass spectrometer is based on a flow drift
tube design originally used for investigating ion
molecule reactions (Ferguson, 1992). For a further
description of the proton transfer reaction-mass
spectrometer see reviews by Lindinger et al.
(1998, 2001).
Hydronium ions (H3O) were used as a proton donor reagent gas in the proton transfer reaction mass spectrometer in a procedure similar
to atmospheric pressure chemical ionization. Primary H3O ions produced by a hollow-cathodedischarge ion source (water flow rate was 7.5 cm3
min1) traveled into a flow drift tube (reaction
chamber) where the gaseous CH2O-Ar sample
was added. The proton transfer reaction (H3O M MH H2O) proceeded in the reaction region of a 393-ml flow drift tube. This reaction is
energetically favorable for formaldehyde because
its proton affinity (170.4 kcal mol1) exceeds that
of water (165.2 kcal mol1) (Hansel et al., 1997;
Lindinger et al., 2001). Products from subsequent
ion collisions of the type (Ma MH MbH)
in the same reaction chamber were also observed
by quadrupole mass spectrometry. The drift tube
was maintained at 2.0 mbar, 410 V, and 343 K.
Structural information to distinguish between
isobaric ion collision products (MbH) was
sought from mass spectral fragmentation patterns using atmospheric pressure chemical
ionization-tandem mass spectrometry [liquid
chromatography-mass spectrometry-mass spec-
JALBOUT ET AL.
trometry on a ThermoFinnigan (Waltham, MA)
TSQ-7000 in the University of Arizona Proteomics Facility Core].
Theoretical calculations
The calculations presented in this work were
performed using the ab initio methods implemented in the GAUSSIAN03 package of computer codes (Frisch et al., 2003). The MP2/6311G** method was used for geometry
optimizations and frequency calculations. Stationary points and transition states were verified
by frequency calculations; if the structures had
one imaginary frequency, then the species was
identified as a transition state, if not a stationary
point. This was followed by single point calculations performed at the CCSD (T)/6-311G** level
of theory. More information about these methods
is available elsewhere (Foresman and Frisch,
1996; Jalbout et al., 2004). These combinations of
methods have been shown to produce reasonably
good agreement with the experiment and are generally quite accurate (Drougas et al., 2003, 2004;
Jalbout et al., 2006; Zhou et al., 2006). These calculations were done in the gas phase, and assume
that the molecules do not interact with the solvent.
The geometrical structures for all complexes
and transition states considered in this work are
shown in Fig. 1, and energies (total and relative)
are presented in Table 1. In Fig. 1, all bond lengths
shown are in angstroms (Å), and the bond angles
are in degrees (°). Stretching modes for S2 and
S6 were pinpointed by extracting the vibrational
modes that corresponded to C—H-O and C-H—
O stretching for both molecules.
RESULTS
Proton transfer reaction products
from formaldehyde
Proton transfer reaction products produced in
the flow drift tube of a proton transfer reactionmass spectrometer were observed by single
quadrupole mass spectrometry. Protonated
formaldehyde (MH, m/z 31) was the predominant ion measured. A less predominant ion
was observed at m/z 49 for the water cluster of
protonated formaldehyde (CH2OH H2O)
(Hansel et al., 1997). Figure 2 shows a clear in-
437
SUGAR SYNTHESIS IN GAS-PHASE FORMOSE REACTION
TABLE 1.
TOTAL ENERGIES
Species
H2O
H3O
H2CO
H2COH
H2COH H2O
H2COH H2CO
H2CO H2CO
H2CO H3O
S1
S1 H3O
S2
S3
S3 H2CO
S4a
S4b
S5
S6
S7
S7 H2O
TS1
TS2
TS3
TS4
E1
E2
E3a
E3b
E4a
HF4a
HR4a
E4b
E5
E6
HF6
HR6
E7
HF7
HR7
E8
E9
HF9
HR9
IN
HARTREES/PARTICLE, WHERE 0 K
IS THE
SUM
OF
ELECTRONIC
AND
ZERO-POINT ENERGIES
MP2/6311G**
MP2/6-311G**
(0 K)
CCSD (T)/6311G**a
CCSD (T)/6311G** (0 K)b
76.2747200
76.5497659
114.2416092
114.5218141
190.7965341
228.7634233
228.4832184
190.7913751
228.4895165
305.0392824
305.1222137
190.8482129
305.0898221
305.1171534
228.8137943
305.0897237
305.1336350
228.8146643
305.0893843
305.0685992
305.0614640
305.0085879
228.7041734
3.952
35.667
17.150
32.428
17.212
30.468
13.256
31.608
52.039
31.652
13.920
17.733
27.554
50.913
78.467
27.767
0.546
68.787
69.333
76.2530410
76.5147510
114.2146620
114.4804710
190.7335120
228.6951330
228.4293240
190.7294130
228.4332950
304.9480460
305.0275370
190.7832120
304.9978740
305.0174240
228.7453980
304.9886990
305.0347280
228.7400563
304.9930973
304.9684912
304.9599020
304.9116770
228.6329940
2.492
33.759
12.268
31.187
18.025
30.705
12.680
31.541
49.881
25.510
7.440
18.070
28.883
48.331
77.215
26.123
3.352
70.534
67.182
76.2863956
76.5636519
114.2666841
114.5513799
190.8377755
228.8180640
228.5333682
190.8303360
228.5398259
305.1034778
305.1861803
190.8878177
305.1545018
305.1820177
228.8673453
305.1531991
305.1965425
228.8671493
305.1535449
305.1347436
305.1248612
305.0802238
228.7644417
4.052
36.070
17.266
31.401
18.084
29.664
11.581
30.924
51.900
31.200
13.418
17.782
27.198
45.792
72.990
26.981
0.123
64.572
64.449
76.2647166
76.5286370
114.2397371
114.5100369
190.7747534
228.7497737
228.4794742
190.7683741
228.4836049
305.0122419
305.0915033
190.8228168
305.0625537
305.0822887
228.7989493
305.0521744
305.0976355
228.7925413
305.0572579
305.0346356
305.0232992
304.9833129
228.6931487
2.592
34.163
12.384
30.160
18.897
29.902
11.006
30.858
49.737
25.058
6.938
18.119
28.527
43.211
71.737
25.337
4.021
66.390
62.369
E are the energies of reaction (in kcal mol1), which are as follows: E1, H2CO H2CO → S1; E2, H2CO H3O
→ S3; E3a, S3 H2CO → S4; E3b, S3 → H2COH H2O; E4a, S4a → S5; E4b, H2COH H2CO →
S4b; E5, S1 H3O → S2; E6, S1 H3O → S5; E7, S5 → S6; E8, S6 → S7 H2O; E8, S4b → S7,
where HF4,7–9 are the forward barrier heights (and HR4,78 are the reverse barriers) for S4a → TS1, S1 H3O →
TS2, S5 → TS3, S4b → TS4.
aMP2/6-311 G** geometries were used.
bMP2/6-311 G** geometries and zero-point energies were used.
crease in ion counts for these two species during
the sample addition period beginning at 344 s and
ending at 605 s. The ion m/z 61 also increased
during the sample addition period, which
indicates a product with molecular formula
C2H4O2H also formed.
H2CO dimerization
All steps in the reaction mechanisms discussed
below are displayed in Fig. 1 and analyzed numerically in Table 1. To discover a method and
mechanism by which sugars can form from sin-
438
JALBOUT ET AL.
FIG. 2. Selected ion counts versus time for m/z 31, 49, and 61 (solid circles, hollow circles, and solid triangles,
respectively). Each mass had a dwell time of 1 s, and the cycle time was 8 s. The formaldehyde–argon sample was
applied to the reaction chamber/drift tube between 340 and 605 s. The background level of formaldehyde was high
before sample addition because of residual sample in the Teflon tube from previous experiments.
gle formaldehyde (CH2O) starting components,
the CH2O self-reaction that leads to a van der
Waals complex as the reaction initiation step was
considered first. This complex corresponds to
structure S1 in Fig. 1, and the energy of reaction
(E1) for this process is around 2.5 kcal/mol (at
the MP2 level with zero-point effects factored in)
and 2.6 kcal/mol at the CCSD (T)//MP2 level
of theory. A protonated formaldehyde product
(S3) channel was also considered since this
species was clearly observed by proton transfer
reaction-mass spectrometry. As shown in Fig. 1,
CH2O and H3O can interact to form S3, a hydrogen-bonded water complex. The energy of reaction (E2) for this process is 33.7 kcal/mol
and 34.2 kcal/mol at the MP2 and CCSD (T)
levels of theory, respectively. This process is also
barrier-less, and has no activation energy. Subsequently, S3 can form a van der Waals complex
with another equivalent of CH2O to yield the
S4a molecular species. The energy of reaction
(E3a) for this process is 12.3 kcal/mol and
12.4 kcal/mol at the MP2 and CCSD (T) levels
of theory, respectively. The dissociation of S3
into CH2OH and water (E3b) is also a possibility, though it should not be dominant. The MP2
value is 31.2, compared with 30.2 at the CCSD (T)
level of theory.
H3O addition to dimer intermediate
Hydronium cation (H3O) catalysis of a ringclosing step in the mechanism is described below.
Figure 1 shows that the product from the previous dimerization step (S1) can interact with H3O
to yield a separated product (S2) or a covalently
bound product (S5). The former product is barrier-less (no transition state located) and is
around 49.9 kcal/mol and 49.7 kcal/mol more
stable than the reactants (S1H3O) at the MP2
and CCSD (T)//MP2 levels of theory, respectively. This energy difference is denoted as E5
in Table 1. The van der Waals complex formed
from the interaction of CH2O with its protonated
counterpart (S4) can also interact via TS1 to
form S5. The energy of reaction for this process
(E4a) is 18.03 kcal/mol and 18.9 kcal/mol at the
MP2 and CCSD (T) levels of theory, respectively.
The barrier height for this reaction (HF4a) is 30.7
kcal/mol and 29.9 kcal/mol at the MP2 and
CCSD (T) levels of theory, respectively. Energies
(E4b) for the complexation of a H bridged
439
SUGAR SYNTHESIS IN GAS-PHASE FORMOSE REACTION
dimer (S4b) originating from CH2OH were additionally computed because this species is a possible isobaric species observed by proton transfer
reaction-mass spectrometry at m/z 61. The energies of reaction for this are about 31.5
kcal/mol and about 30.9 kcal/mol at the MP2
and CCSD (T) levels of theory, respectively.
Diose formation and water removal
A cyclic transition state (TS3) that involves
proton migration from carbon in S5 to oxygen
in S6 and formation of a new C-C bond is proposed in the final step of this mechanism for the
formation of diose (S7 in Fig. 1). This process
has a barrier height (HF3) of 48.3 kcal/mol and
43.2 kcal/mol at the MP2 and CCSD (T)//MP2
levels of theory, respectively. The energy of reaction for this process, relative to S6 (denoted
as E7 in Table 1) is around 28 kcal/mol at both
levels of theory considered.
As shown for the CH2OH pathway, S4b can
also lead to diose (S7) through TS4. The energy
of reaction (E9) for this process is 3.4 kcal/mol
and 4.0 kcal/mol at the MP2 and CCSD(T) levels
of theory, respectively. Also, the barrier height for
this process (HF9) is about 70.5 kcal/mol and
66.4 kcal/mol at the MP2 and CCSD (T) levels of
theory, respectively. This is low enough for it to
be a plausible pathway.
DISCUSSION
Proton transfer reaction products
from formaldehyde
Based on calculations explained above, the
mass 61 ion product could well be glycoaldehyde
(see S7 in Fig. 1). However, four other isobaric
structures with molecular formulae C2H4O2H
are theoretically possible: methyl formate
(CH3OCOH2), acetic acid (CH3COOH2), hydroxymethyl methyleneoxonium (H2COCH2
OH; CAS# 137516-32-6), and a bridged formaldehyde dimer (H2CO—H—OCH2; S4b, see results above). To distinguish between these
structures, experiments with formaldehyde, 1,4dioxane-2,5-diol, and paraformaldehyde samples
were explored by atmospheric pressure chemical
ionization-mass spectrometry/mass spectrometry to gain fragmentation information, but sample ionization was very poor, and no spectral information was obtained.
H3O addition of dimer to intermediate
While S2 is stable, calculations could not conclusively link this product to a reaction cascade
leading to a diose. The values for formation of
S5 through TS1 were high, but should also be
considered as possible mechanisms. Hydronium
ion may have dual catalytic properties in these
mechanisms [indicated by our recent work on the
Nazarov reaction (Jalbout et al., 2007)]; therefore
consideration was given for its involvement in
the ring-closing mechanism of S1 to yield cyclic
product S5. This occurs through TS2, which has
a forward barrier (HF7) of 7.4 kcal/mol (MP2)
and 6.9 kcal/mol (CCSD (T)//MP2) and an energy of reaction of around 25 kcal/mol at both
the MP2 and CCSD (T)//MP2 levels of theory.
Although the barrier is negative, the mechanism
should occur very fast, and if an excess of CH2O
and H3O is present in the reaction they should
eventually complex together.
Diose formation and water removal
Final product (S6) is stable relative to cyclic
product S5 from the previous step. The charge on
this molecule is stabilized by water, shown by the
amount of energy required to remove it (E8) to
yield the final diose product (S7). The removal of
water requires around 26.1 kcal/mol and 25.3
kcal/mol at the MP2 and CCSD (T)//MP2 levels
of theory, respectively. A transition state structure
for this process was not located, probably because
of the sensitive nature of the final charged species,
or to the barrier-less nature of this portion of the
potential energy surface.
Many potential states for the diose cation have
been found, and there should also be a variety of
excited states that may potentially exist. This area
is now under investigation in our group.
CONCLUSIONS
Experimental results from ion-molecule reactions with formaldehyde and hydronium ion
clearly demonstrate the formation of a species
with molecular formula C2H4O2H, which had to
arise from two molecules of formaldehyde. Although this species remains structurally uncharacterized, evidence for the potential formation of
diose from simple formaldehyde precursors has
been presented through ab initio calculations. Figure 3A shows relative energies of all structures
440
JALBOUT ET AL.
FIG. 3. Relative energy diagram (relative to the lowest point on the singlet cation potential energy surface, which
is S6), in kcal/mol, which was computed using the CCSD (T)/6-311G**//MP2/6-311G** level of theory for the
(A) H3O-mediated pathway and (B) non–H3O-mediated pathway.
441
SUGAR SYNTHESIS IN GAS-PHASE FORMOSE REACTION
(S6 taken as the 0.0 reference value) for reactions involving H3O mediation leading to diose
(shown in Fig. 1). Figure 3B shows the non–water-mediated case (through S4b). Evidence for
interaction between CH2O and the protonated
form of CH2O (S3) to yield a H bridged complex was also presented. Through TS4 the protonated formaldehyde product (CH2OH) can react with formaldehyde (CH2O) to yield the diose
product with a low barrier height. However, the
dominance of H3O in the interstellar medium
and in the proton transfer reaction-mass spectrometer experiment compels a thorough consideration of it also. Both pathways are potential avenues for a mechanism to glycoaldehyde (diose)
and must be considered in conjunction with experimental data. Calculations in this study revealed that the hydronium-mediated channel
lowers the barrier substantially (about 20 kcal/
mol).
Also, calculations in this study suggested that
stretching modes distinct for S2 should be seen
at 273 cm1 and at 1600–1800 cm1, whereas
stretching frequencies should be observed for
S6 at 1300–1400 cm1 and 2200–2250 cm1.
However, if the protonated formaldehyde complex (S3) is also observed, then peaks at 1471
cm1 and 1762 cm1 should be observed that correspond to the stretching vibrational modes along
the C-H—O axis. If, on the other hand, S4b
(CH2OH complex with CH2O) is formed, vibrational frequencies at 838 cm1, 1680 cm1, and
1739 cm1 are expected. These values could be
insightful clues to determine the nature of products obtained from this formose reaction.
These results may reveal a hitherto poorly considered process for the prebiotic synthesis of sugars—precursors for life—in the interstellar and atmospheric medium. We have presented new
experimental and theoretical results for which
formaldehyde (CH2O) in the gas phase can be
used as a starting component for larger sugar synthesis. While such synthetic procedures have
been previously proposed, the exact mechanism
of this reaction, up to now, has not been explained.
ACKNOWLEDGMENTS
This work was supported by NASA through
the NASA Astrobiology Institute under Cooperative Agreement Number CAN-02-OSS-02 issued
through the Office of Space Science. L.A. was partially supported by grant CHE 0216226 from the
National Science Foundation. In addition, we
thank Mark Smith (University of Arizona Department of Chemistry) for valuable discussions,
Varada Datar for proton transfer reaction-mass
spectrometry assistance, the University of Arizona for valuable computational resources, and
grant 0216226 from the National Science Foundation for proton transfer reaction-mass spectrometer acquisition.
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Address reprint requests to:
Abraham F. Jalbout
Department of Physical Chemistry
Institute of Chemistry
National Autonomous University of Mexico
Col. Coyoacan, Del. Copilco
04510 Mexico City, Mexico
E-mail: [email protected]