A1260887413fYHTc

1
Identification of the Mechanism for Chiral
Modification of Calcite Morphology by Aspartic
Acid Enantiomers Rachel L. Reese and Sausan Jaber
(faculty advisor Dr. Ryan E. Sours) Department
of Chemistry, Towson University, Towson, MD
21252, USA
Results
Discussion
Calcite, one of three polymorphs of crystalline
calcium carbonate, is a common biomineral found
in many structures including the shells of
mollusks and a part of the human inner ear.
However, simple mechanistic models of crystal
growth do not explain these relatively complex
biological crystal formations. This signifies
that impurities, such as soluble amino acids and
proteins, could be responsible for the varied
crystal shapes found in these biomineral
structures. Orme et al tested the effects of
adding D- and L- aspartic acid (Asp) to the
solution during calcite crystal growth and found
that the presence of D-Asp resulted in crystals
which were non-superimposable mirror images of
crystals formed in the presence of L-Asp.1 The
mechanism for this induced chirality is not well
understood. Gaining a mechanistic understanding
of how selective binding of aspartic acid induces
chirality in calcite crystals has broader
applications in understanding and possibly
controlling crystal growth. This has implications
in preventing scale formation in heat exchangers,
developing lightweight materials, and even
gaining insight into liver and kidney stone
formation.2 Also, a knowledge of the mechanism
controlling this induced asymmetry is another
step towards understanding the origin of
homochirality and the links between life and
enantio-selectivity and -specificity.3
The observation of fluorescence, due to the
presence of dansylated L-aspartic acid, indicates
that L-Asp readily adsorbed onto the calcite
crystal surface and was included into the bulk of
the crystal during growth. An impurity inclusion
mechanism for calcite growth modification is
contrary to the previously reported mechanism, in
which aspartic acid inclusion was not necessary.1
The presence of dansylated L-Asp resulted in
asymmetric growth and therefore macroscopic
chirality of the calcite morphology, as
previously observed for unlabeled L-Asp.1 In
addition, the trend in the normalized
fluorescence intensity indicates preferential
inclusion of the impurity. This implies a
stereospecific preference for L-aspartic acid to
bind to the left side of the calcite crystal face
(as oriented in Figure 2) despite the presence of
the relatively large dansyl group.
aspartic acid
dansyl chloride
dansylated aspartic acid
Figure 1. Derivatization of aspartic acid with
dansyl chloride.

• Figure 2. Photomicrographs of a calcite crystal
  grown in the presence of 0.01M dansylated L-Asp
  
• bright-field image
• fluorescence image

Future Research
In the future, a parallel study using dansylated
D-aspartic acid will be performed. Also, to
ensure that the aspartic acid and not the dansyl
label is driving the observed fluorescence
asymmetry, a control experiment will be performed
using dansylated glycine. Glycine is an achiral
amino acid and should therefore show a
symmetrical fluorescence pattern. With the
additional data, the mechanism controlling the
amino acid-induced chirality of calcite
morphology should be better understood.
L-Asp Derivitization (see Figure 1) A 10mM
carbonate buffer solution (pH 8.5) containing
10mM L-Asp was mixed with a 1mM solution of
dansyl chloride in acetone and the resulting
crystals of pure dansylated L-Asp were collected
by filtration. SAM Preparation A clean
gold-coated mica substrate was placed in a 1mM
methanolic solution of 11-mercapto-1-undecanol.
After 24 hours, the substrate was removed from
solution and rinsed with methanol. Calcite
crystal growth SAM substrates were immersed
in a 5mM solution of calcium chloride containing
10mM L-Asp or dansylated L-Asp and placed inside
a chamber with solid ammonium carbonate. The
resulting crystals were viewed under a Nikon
Eclipse LV100POL optical microscope.
References

1. C.A. Orme, et al, Nature, 2001, 411, 775-779.
2. D A Walters, et al, Biophys. J. 1997, 72,
   1425-1433.
3. R. M. Hazen, T. R. Filley, G. A. Goodfriend,
   Proc. Natl. Acad. Sci. U S A. 2001, 98,
   54875490.

Figure 3. Comparison of fluorescence intensity
trends for 16 crystals grown in the presence of
0.01M dansylated L-Asp. Measurements were made
from the left to the right side of the calcite
crystal face in three different regions. a)
Model of modified calcite crystal morphology with
regions color coded to correspond to graphs b-d.
b) Normalized fluorescence intensity for all
crystals. c) Normalized fluorescence intensity
for selected crystals d) Normalized fluorescence
intensity for crystals not included in the c
subset.
We would like to thank Dr. Jennifer Swift at
Georgetown University for generously providing
the gold-coated mica substrates.