Article
Crystal Growth & Design, Vol. 10, No. 2, 2010 993
Figure 9. Molecular packing of L-Glu in crystalline structure of (a) R form and (b) β form along c axis.33,34
which presumably explains why the transformation is not
observed in the solid state at room temperature. At high
temperatures g140 °C, the thermal energy of the molecules
effectively reduces the kinetic barrier to nucleation and sub-
sequent growth of the stable β polymorph.
We do not have any detailed information on the R to β
transformation mechanism. Clearly, athightemperatures, itis
not solution-mediated. It may be an entirely solid-state pro-
cess. Orpossibly, partialmeltingof R atthe R-β interface may
be involved. Further studies on this are required.
separate the R f β transformation from subsequent dehydra-
tion/melting.
This work, on the thermal transformations of L-Glu to
pyroglutamic acid and polyglutamic acid, may be of relevance
in three wider areas. First, althoughthe transformations occur
at high temperatures under our experimental conditions, the
formation of pyroglutamic acid and polyglutamic acid in-
volves dehydration, and therefore, under high vacuum condi-
tions, such as in outer space, the transformations may occur at
much lower temperatures. Biochemical processes, both natu-
ral and laboratory-based, are, of course, conducted at or
below body temperature, and are usually solution-mediated.
Our results show that temperature may be an additional
experimental variable that can be used to investigate solid-
state biochemical transformations under laboratory condi-
tions.
Second, the formation of high molecular weight polyglu-
tamic acid by thermal transformation and polymerization of
L-Glu may be regarded as a prototype amino acid polymer-
ization that gives a product of likely biochemical compat-
ibility. Similar thermally induced transformations of other
amino acids and their derivatives may be utilized for the
design and fabrication of scaffold materials for tissue engine-
ering applications.36,37
4. Conclusions
Using a combination of DSC/TG, GPC, MS and XRD, it is
found that both R and β polymorphs of L-Glu on heating in
the range 140-220 °C undergo a complex and overlapping set
of transformations involving dehydration, melting, and struc-
tural reorganization. Both XRD and MS showed the forma-
tion of pyroglutamic acid prior to formation of high
molecular weight polyglutamic acid, resolving the disagree-
ment between two groups26,27 in which the IR technique was
not adequate to differentiate pyroglutamic acid from diketo-
piperazine. Additionally, various product mixtures are possi-
ble, depending on the heating condition of temperature and
time.
Third, we have shown, for the first time, that the R to β
transformation of L-Glu is, indeed, possible in the solid state.
Polymorphism, and the stability of polymorphs with different
pharmacological activity, is important in many amino acid-
based substances. The results and methodology presented
here, with temperature (and potentially water vapor pressure)
as a key variable under dry conditions, may be applicable to a
betterunderstanding of the thermalstability ofa widerangeof
amino acid-related materials.
We observed, for the first time, a partial and irreversible
transformation of R to β polymorph attemperaturesg140 °C,
thus confirming the metastability of the R polymorph. DSC
results show a single broad endotherm peaking at ∼180 °C,
whose peak temperature increased somewhat with heating
rate. At least three processes may occur simultaneously and
contribute to this endotherm: R f β transformation, melting,
and decomposition/dehydration/transformation. The peak
temperature was observed to depend on heating rate; this is
probably because a thermally activated and complex sequence
of overlapping reactions and transformations occur and are
represented by this endotherm. The isothermal studies of
L-Glu at different temperatures reveal the sequence of the
reaction to be R f β transformation, followed by internal
cyclization to give pyroglutamic acid and subsequent poly-
merization to give polyglutamic acid.
The isothermal heat treatments as a function of time have
shown that it is possible to at least partially separate the
different steps in the transformation sequence, allowing iso-
lation of pure pyroglutamic acid as an intermediate. These
studies also showed that, with sufficient time, the various
transformationsoccur attemperaturesaslow as140°C, which
is well below the endotherm seen during the dynamic DSC
experiments. Further studies are required to find conditions to
Acknowledgment. We thank Rob Hanson for DSC analy-
sis and Dr. John Haycock for helpful discussions.
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