Synthesis of Pyrimidines and Triazines in Ice
FULL PAPER
To test whether urea suffers degradation or biological
contamination in solution when subjected to freeze–thaw
cycles in our reactor design, a third control experiment was
performed without spark discharges under the methane/ni-
trogen/hydrogen atmosphere. After three weeks of freeze–
thaw cycles, the solution content was analyzed; no evidence
of degradation, microbial contamination, or the generation
of nitrogen heterocycles was found.
In analogy to another polar organic compound, such as pyr-
[18]
uvic acid, in frozen aqueous solutions, we can expect that
the urea remains monomeric in frozen aqueous urea solu-
tions. The extent of urea dimerization (ꢀ18% in 0.1m urea
[19]
at 258C ) is expected to increase at lower temperatures
and higher urea concentrations and to approach completion
[8,17]
a few degrees below the onset of freezing.
Thus, in our
frozen-water experiment, urea is present as hydrogen-
bonded urea dimers or higher association clusters and is ran-
domly distributed within the ice. In this environment, water
[17,18]
Discussion
can still hydrate the polar organic molecules,
urea–urea interactions will predominate.
but the
Our experiments show that the synthesis of pyrimidines
under a methane/nitrogen atmosphere is possible with high
yields if a urea source is present. In this process, the pres-
ence of frozen water or ice is a decisive factor. With water
subjected to freeze–thaw cycles, the synthesis of pyrimidines
and triazines is strongly favored and the generation of in-
soluble or polymeric organic materials or tholins is negligi-
ble. Previously reported experiments on the synthesis of cy-
tosine from cyanoacetaldehyde and guanidine or urea in ice
at À118C suggested a preference for uracil generation and
the deamination of aminopyridines or their precursors
Triazine derivatives are obtained in the laboratory and in-
[20]
dustry as products of the pyrolysis of urea.
These com-
pounds were found in the Murchinson and Orgueil meteor-
ites, and it has been argued that their origin could be the hy-
drolysis of macromolecular material present in the meteor-
[21]
[22]
ite
or the polymerization of hydrogen cyanide,
which
could form the unsubstituted s-triazine. We have shown that
the polymerization of HCN is not a necessary condition for
the generation of triazines when the urea–water system is
present at low temperatures (Figure 4). This implies that
several alternative pathways could be active, dependent on
the temperature and the presence of the reactants. As in the
case of the synthesis of polycyclic aromatic hydrocarbons
[10]
during formation.
preference sequence of pyrimidines was cytosine>uracil>
,4-diaminopyrimidine>2,4,6-trihydroxypyrimidine. Further
Under our conditions, the observed
[23]
2
(PAHs) and as stated in the previous paragraph, we could
expect that the ice medium, due to its surfaces and the
freezing process, would favor a mechanism similar to those
of the pyrolysis processes of urea, which could explain the
synthesis of triazines.
work is in progress to establish whether long-term storage in
ice under freeze–thaw cycles alters the concentrations of the
resultant pyrimidines.
The behavior of urea in an aqueous solution subjected to
a phase change could be a key factor in the results observed
with water in different phases. The urea molecules in fluid-
water solutions form pronounced hydrogen bonds with the
neighboring water molecules at both the amino and the car-
In fact, the control experiment performed under an inert
atmosphere shows the formation of triazines in the same rel-
ative abundance as the main experiment. During pyrolysis,
part of the urea undergoes isomerization to ammonium iso-
cyanate and decomposition, which releases ammonia. The
reaction of the isocyanic acid, also released, with urea gener-
ates biuret (aminocarbonyl urea). The biuret reacts with
more isocyanic acid and generates cyanuric acid and ammo-
nia, ammelide, or ammeline. The latter could react with am-
monia to form melamine. This reaction sequence, proposed
for the pyrolysis of urea, is not observed by heating below
the urea melting point and is consistent with the relative
abundance of s-triazines found with our conditions: cyanuric
[13]
bonyl groups. The number of hydration water molecules
per molecule of urea has been reported as approximately 2
[14]
at concentrations of less than 5.0m urea. Neutron diffrac-
tion measurements at 258C on aqueous 15 mol% (ꢀ10m)
urea showed that approximately 4.3 water molecules are hy-
[15]
drogen bonded to the carbonyl oxygen atom. Infrared and
dielectric spectroscopy studies showed that the two predom-
inant interactions are those of urea–urea (observed at urea
concentrations higher than 1m) and water–urea. At a con-
centration of 11m, nearly all of the urea molecules have
other urea molecules as their nearest neighbors, because few
water molecules remain to hydrate the urea molecules at
high concentrations. Concentration-dependence studies
showed that this is due to the aggregation of urea molecules
[20]
acid>ammelide>ammeline>melamine.
The photodecomposition of urea under UV radiation is a
possible source of isocyanic acid and ammonia as an alterna-
[24]
tive to thermal decomposition. Under our conditions, dis-
sipation of spark energy in the form of ultraviolet radiation
and the associated photochemical processes could be one of
the sources of isocyanic acid, together with the local effect
of the spark and the behavior of urea in frozen solutions. In
fact, the fluid microenvironments or urea oligomers generat-
ed in the ice matrix could be comparable to the molten urea
and could facilitate the condensation reactions.
[14,16]
in dimers and/or oligomers at higher concentrations.
There is a considerable amount of urea dimer or clusters
[13]
present in aqueous solutions.
Upon freezing of an aqueous solution of a polar organic
compound, the solute is segregated from the pure ice to ac-
cumulate in supercooled microfluids. The solute retains a
certain amount of water in the form of a supersaturated so-
lution. Under these conditions, the system is governed by
With regard to the relevance of our results to prebiotic
evolution, the importance of functionalized s-triazines
would not be diminished, because they could act as purines
[17]
dehydration and the association of the solute molecules.
Chem. Eur. J. 2009, 15, 4411 – 4418
ꢄ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4415