Tetrazoles: Calcium oxalate crystal growth modifiers

Article abstract of DOI:10.1039/c4ce01717j

Molecules containing tetrazole substituents have become of interest due to their being bioisosteres of carboxylic acids and like their carboxylate counterparts, tetrazolate anions have been able to affect the crystal growth of barium sulphate and calcium carbonate. In this proof of principle study, we show that this behaviour also extends to calcium oxalate and therefore opens the possibility of using tetrazole-based additives for investigating mineralization processes of human pathological relevance. This journal is

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Tetrazoles: calcium oxalate crystal growth
modifiers†
Cite this: DOI: 10.1039/c4ce01717j
Calum J. McMulkin, Massimiliano Massi and Franca Jones*
Received 20th August 2014,
Accepted 20th February 2015
Molecules containing tetrazole substituents have become of interest due to their being bioisosteres of
carboxylic acids and like their carboxylate counterparts, tetrazolate anions have been able to affect the
crystal growth of barium sulphate and calcium carbonate. In this proof of principle study, we show that this
behaviour also extends to calcium oxalate and therefore opens the possibility of using tetrazole-based
additives for investigating mineralization processes of human pathological relevance.
DOI: 10.1039/c4ce01717j
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hypotension (low blood-pressure) and hypokalemia (potas-
Introduction
3
,4
sium depletion).
Not surprisingly, major effort has been
Biomineralization encompasses many biological crystalliza-
tion events including urolithiasis (the formation of stones in
the urinary tract). Thus, the formation of kidney stones is an
devoted to controlling the size and shape of the calcium oxa-
late with the aim of finding better strategies to deal with this
condition.
1
undesired biomineralization event within human bodies. Cal-
cium oxalate, calcium phosphate and uric acid constitute
Crystal growth modifiers are additives that can be used to
gain control over crystallization processes, either in the form
of promotion or inhibition. To date, a variety of additive
8
9% of renal calculus, commonly known as kidney stones.
5
,6
7
Calcium oxalate dominates the constituents of kidney stones
types including; calixarenes, glycosaminoglycans, double-
2
8
9
at 70%. Calcium oxalate has three hydrate forms, with cal-
hydrophilic block copolymers, phospholipid monolayers,
amino acids, proteins and carboxylic acids have been shown
cium oxalate monohydrate (COM, CaC O ĴH O) being the
2
4
2
1
0
thermodynamically stable phase. Calcium oxalate dihydrate
COD, CaC Ĵ2H O) and calcium oxalate trihydrate (COT,
CaC O Ĵ3H O) are the meta-stable phases. All three crystal
to impact the crystallization of calcium oxalate. Our previ-
ous work investigated the use of tetrazoles as crystal growth
modifiers for inorganic solids. We showed that both the crys-
tallization of barium sulphate and calcium carbonate are
impacted on by tetrazoles in a unique way compared to their
(
O
2 4
2
2
4
2
phases can be found within kidney stones.
The formation of kidney stones causes extreme pain in
patients and currently the lifetime risk of developing a kidney
stone is 10–15% in the developed world and up to 20–25% in
1
1
parent carboxylic acids. In this manuscript, we report a
proof of principle study to determine whether tetrazoles can
impact on the crystallization of calcium oxalate and if they
do so at comparable levels to carboxylic acids. Thus, this is a
first step in determining whether tetrazoles might hold any
potential for crystal growth modification of biologically
important minerals.
3
the Middle East. The current best-practises in dealing with
kidney stones are medical expulsive therapy (MET) and
shock-wave lithotripsy (SWL) both of which are not methods
of prevention but rather are aimed at spontaneous passage of
formed ureteral calculi, and ultrasound degradation of the
particles too big for passage. The best pharmaceutical treat-
ments include the use of hydrochlorothiazide, chlorthalidone
or indapamide which act as thiazide or thiazide-like diuretics
and inhibit the kidneys ability to retain water; these drugs
however have potential drawbacks associated with the risk of
The methods of synthesising tetrazoles has been exten-
1
2–19
sively researched
due to the multitude of applications of
tetrazole-containing compounds, including the use of tetra-
2
0,21
zoles in materials science and medicinal chemistry.
Tetrazoles have been increasingly used in medicine due to
2
2
their bioisostere nature with carboxylic acids. Carboxylic
acids and tetrazoles are isosteres due to their similar pKa
Curtin University, Department of Chemistry, GPO Box U1987, Perth, WA,
Australia 6845. E-mail: F.Jones@curtin.edu.au; Fax: +61 618 9266 4699;
Tel: +61 618 9266 7677
a
values resulting from their dissociation (details on the pK 's
calculated or known for the molecules investigated in this
work can be found in Table S1 in the ESI†). Thus it has
become possible to replace the carboxylic acid functional
group with that of the tetrazole ring for medicinal applica-
†
Electronic supplementary information (ESI) available: SEM images, turbidity
spectra, and Raman spectra of calcium oxalate particles in the presence of
tetrazole and carboxylate additives. Tables of quantitative data extracted from
SEM images, dissolution experiment concentration information and additive
2
3,24
pK
a
values. See DOI: 10.1039/c4ce01717j
tions such as anti-asthmatic and anti-cancer drugs.
An
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important aspect of this replacement is due to the tetrazole
being a more metabolically stable substituent thus making it
less susceptible to biodegradation.
to begin the reaction. The vials were left in the 30 °C water
bath for 3 days before the glass disc from each sample vial
was removed and the excess fluid adsorbed using tissue
paper. The glass vial was then placed in a new, dry sample
vial for analysis. All samples had a constant final volume of
2
5
Experimental
Tetrazole synthesis
2
0 mL and pH ~7. The final concentrations of calcium oxalate
−
4
−4
in the vials ranged between 1.0 × 10 M and 5.0 × 10 M,
but the results for a final concentration of 0.30 mM are
presented here. All the experiments and the number of
repeats are given in the ESI† (Table S2).
Fig. 1 shows the structures of the tetrazole molecules and car-
boxylic acid equivalents investigated in this work. All of the
tetrazoles were prepared following a previously reported pro-
1
9
cedure, involving the 1,3-dipolar cycloaddition of the corre-
sponding nitrile with sodium azide (NaN ) in the presence of
triethylammonium hydrochloride. The tetrazole-containing
3
Scanning electron (SEM)
SEM was performed at the Curtin University Electron
Microscope Facility using a Zeiss Evo 40XVP SEM using either
secondary electron or back-scatter detectors. Energy-
dispersive X-ray spectroscopy (EDX) was also employed in
determining the qualitative composition of our samples.
Each sample analysed by SEM was coated with either gold
or platinum.
2
carboxylic acid (TzBaH ) was synthesised by selective hydroly-
sis in aqueous sodium hydroxide at room temperature from
the corresponding methyl ester.
Crystallization of calcium oxalate methodology
Stock solutions of the starting two reagents (calcium chloride
and sodium oxalate) were prepared. To a flask (250 mL), cal-
cium chloride (0.444 g, 0.004 mol) was added into ultrapure
water (200 mL) to make the stock calcium chloride solution
Complexation experiments
−
1
(
0.02 mol L ). Next, sodium oxalate (0.536 g, 0.004 mol) was
Commercially obtained COM (AR grade) was added in excess
(0.25 g, 1.71 mmol) to 50 mL vials with varying concentra-
added to ultrapure water (200 mL) in a 250 mL flask to make
−
1
−1
the stock sodium oxalate solution (0.02 mol L ), with the use
of a sonicator when necessary. Stock solutions of the modify-
ing species (tetrazoles or carboxylic acids) were made up to
tions of organic additives (0–5 g L ), filled with ultrapure
water and left for 1 week at room temperature.
After 1 week, the filtrate was collected by vacuum
through 0.2 μm filter membranes for analysis. Inductively
coupled plasma-atomic emission spectroscopy (ICP-AES)
analysis was recorded on a Varian Vista Axial CCD ICP-AES
at Murdoch University's Centre for Marine and Freshwater
Research Laboratory. The final measured calcium ion con-
centrations for these experiments can be found in Table S3
in the ESI.†
−
1
1
0 g L concentrations, at pH 7 (adjusted to ~pH 7 by 1.0 M
sodium hydroxide solution), ready for addition. To obtain the
calcium oxalate crystals 300–500 μL of the calcium chloride
solution was added to 20 mL glass vials as required. Cleaned
glass discs with a radius of 0.5 cm were placed at the bottom
of each vial to provide a removable crystal growth site. Next
the required amount of modifier was added to the flask
followed by the addition of ultrapure water, almost up to a
final volume (20 mL). The solution was equilibrated in a
water bath to the reaction temperature, 30 °C, before the
addition of sodium oxalate solution (equimolar, 300–500 μL)
Measurement of size from SEM images
Each SEM image used in either the manuscript or supple-
mentary information was run through the free software,
ImageJ, to determine the number of particles, their size and
standard deviation of particles from the mean. The measur-
ing tool and scale bar calibration was used to measure the
length (c-axis length) and width of each particle in the SEM
image. Table S4 in the ESI† was generated from the collection
of this data.
Confocal raman spectroscopy
While XRD would have been the characterization method of
choice to determine the different hydrates of calcium oxalate
formed, the small amount of solids present meant this tech-
nique was not appropriate. Thus, Raman, which has also
been shown to be specific to mineral form and chemistry,
was used instead.
The Confocal Raman Spectroscopy (CRM) and Scanning
Near-Field optical Microscopy (SNOM) measurements were
Fig. 1 The tetrazole-substituted additives and carboxylic acids investi-
gated in this work: 5-phenyl-2H-tetrazole (TzPhH), 4-IJ2H-tetrazol-5-
yl)benzaldehyde (TzPhaH), 4-IJ2H-tetrazol-5-yl)benzonitrile (TzPhCNH),
26–29
1
,4-bis-IJ2H-tetrazol-5-yl)benzene (TzTzH
2
), benzoic acid (BaPhH),
4-formylbenzoic acid (BaPhaH), 4-cyanobenzoic acid (BaPhCNH),
terephthalic acid (BaBaH
2
), 4-IJ2H-tetrazol-5-yl)benzoic acid (BaTzH
2
).
obtained using the WITec alpha 300SAR utilising a
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frequency-doubled NdYAG laser of wavelength 532 nm and of Results and discussion
5
0 mW power. Each Raman spectrum collected consists of 10
Control calcium oxalate monohydrate
accumulations at an integration time of 0.08371 seconds and
a grating of 600 g mm . Analysis was performed using the
WITec Project FOUR software.
−
1
In order to determine the impacts of both the tetrazole
containing and the carboxylate containing molecules many
control samples were imaged and analysed. Fig. 2 shows an
SEM of the typical control crystals with COM morphology. As
previously mentioned, COM is the thermodynamically stable
phase for calcium oxalate and thus would be the expected
product after 72 hours. These particles were confirmed as
COM by confocal Raman spectroscopy (see ESI,† Fig. S1 and
S2). The mean length of the control particles was found to be
approximately 8 μm in length. No fluorescence was observed
in the presence of COM for all laser strengths applied. ESI†
Fig. S3 illustrates the crystal face indexing for COM used
Timed experiments
All conditions were the same as mentioned in the crystalliza-
tion of calcium oxalate except that the samples were removed
at different time periods up to a time of 1 month. The addi-
tives investigated were TzPh (6.84 mM) and BaPh (8.18 mM)
as further investigation into the dissolution mechanism
occurring in the presence of the phenyl-tetrazole species
−
−
−
TzPh was required.
30
throughout this manuscript.
Impact of carboxylic acid derivatives as compared to tetrazole
derivatives on calcium oxalate crystallization
Turbidimetric methodology
Ultrapure water (3.48 mL) was added to a ~5 mL cuvette and
a background reading was recorded before the addition of
organic additive (0.4 mL), calcium chloride and sodium oxa-
late solutions (60 μL each) before the turbidity was moni-
tored for thirty minutes. Turbidity was measured by using a
UV-vis instrument (GBC Double beam UV/VIS Spectropho-
tometer 916) at 900 nm at an integration time of 0.5 seconds
and slit width of 2.0 nm for all samples. All samples were fil-
tered through a 0.1 μm membrane filter to remove any parti-
cles in the solution that would provide a seed for nucleation.
The final concentration of calcium oxalate in the cuvettes
Fig. 3 presents the comparison between some of the carbox-
ylic acid and tetrazole additives on calcium oxalate crystalli-
zation in this study. The impact of tetrazole additives are all
shown on the left side of Fig. 3, while their carboxylic acid
equivalent groups are shown on the right hand side. Starting
from the top, both additives with an aldehyde functional
−
−
group; Fig. 3a (TzPha ) and b (BaPha ) formed calcium oxa-
late crystals with shortened crystal lengths in the <001>
direction (c-axis, see Fig. S3† for a schematic of the crystal
directions) compared to the control samples previously seen.
Upon close inspection, some crystals (Fig. 3a) have the {010}
face missing and/or twinning is reduced resulting in some
cube shaped crystals (see Fig. S4† for a magnified view of
Fig. 3a). While both additives have modified the crystalliza-
−
4
was 3.0 × 10 M. Turbidity experiments were conducted mul-
tiple times and averaged to give the final curve for induction
time determination.
−
tion of calcium oxalate, the TzPha additive was found to
have altered the crystal shape and increased the average par-
−
ticle length to ~16.5 μm (see Table S4 in ESI†). BaPha , on
the other hand, decreased the average particle length com-
pared to the control.
2
−
The addition of TzTz resulted in many crystals with
shortened a-axis lengths (Fig. 3c), meaning the crystal thick-
2
−
ness was reduced, whilst the presence of BaBa (Fig. 3d)
resulted in crystals more than twice the size of the control
samples (see Table S4†). The final images show calcium
−
−
oxalate crystals in the presence of TzPh and BaPh
Fig. 3e and f, respectively). The tetrazole species has
(
impacted the COM crystals by showing some re-dissolution
of the crystal centre on the {100} face through the entire crys-
tal width. Investigating the possible means by which the
tetrazole does this is discussed later. The carboxylic acid
additive on the other hand, showed no signs of this re-
dissolution and did not show significant signs of morpholog-
ical impact or alteration of the aggregation state of the crys-
−
−
tals. Both species TzPh and BaPh had similar gross mor-
phology and mean particle sizes (Table S4†) indicating that
preferential adsorption onto specific crystal faces does not
appear to be occurring.
Fig. 2 SEM image of calcium oxalate monohydrate control particles
(
0.30 mM).
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−
1
−
−1
−
−1
Fig. 3 SEM images of calcium oxalate (0.30 mM) formed in the presence of (a) 1.0 g L (5.74 mM) TzPha , (b) 1.0 g L (6.66 mM) BaPha , (c) 1.0 g L
(
2
−
−1
2−
−1
−
−1
−
4.66 mM) TzTz , (d) 1.0 g L (6.02 mM) BaBa , (e) 1.0 g L (6.84 mM) TzPh and (f) 1.0 g L (8.18 mM) BaPh .
2
−
The species, TzBa showed only weak signs of inhibiting
morphology of those particles that were not broken do not
appear to be significantly different to the control particles,
and the average length of the particles in the presence of this
additive was found to be ~6 μm compared to the control
calcium oxalate crystallization (see Fig. 4 and S5†) and these
were determined to be COM by Raman, unlike some carbox-
ylic acids that also change the hydrate formed.
3
1,32
The
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their surface free energy (provided supersaturation is not sig-
3
6
nificantly altered). Due to the small volumes and low
concentrations used in this study, there was scatter in the
turbidimetric data, thus 4–6 data sets were averaged to mini-
mise these errors. The complete set of turbidity curves can be
found in the supplementary information along with the
induction time shown by an arrow (Fig. S8–S16†). The most
important information, however, was how the presence of the
additives altered the nucleation rate compared to the control,
thus, trends being more important than actual values. The
control sample (Fig. S14†) showed an induction time of
between 25–35 seconds based on the time required for the
turbidity to rise above the background. When the additive
2
−
Fig. 4 SEM image of calcium oxalate (0.30 mM) particles formed in
the presence of 5 g L TzBa (0.0301 M).
TzTz (Fig. S15†) was present during crystallization the
−
1
2−
induction time was found to be ~10 seconds. In comparison
2
−
the presence of BaBa (Fig. S16†) was found to have an even
shorter induction time, almost occurring instantaneously, at
~5 seconds.
length of ~7 μm, which is within the standard deviation of
the measurements.
The presence of TzPhCN (Fig. 5a) showed a similar action
to that when BaPhCN (Fig. 5b) additive was present, the
None of the tetrazole additives showed a significant
lengthening of the induction time. In fact for most of the
tetrazole additives the opposite was found. Two tetrazoles,
−
−
−
−
additives appearing to have a significant impact on growth in
the <100> (the thickness of the crystals). In the presence of
both these additives the calcium oxalate crystals formed are
significantly thinner than the control crystals. The presence
TzPh and TzPha , showed similar induction times to the
control. In addition, this result suggests that despite the fact
that the doubly charged additives complex more with calcium
in solution (see section below on solution behaviour) they do
not lower the nucleation rate of calcium oxalate mono-
−
−
of TzPhCN appears to have a greater impact than BaPhCN
on the size as confirmed by the quantitative data (see Table
−
−
hydrate. For the carboxylate molecules, BaPh , BaPhCN and
−
1
−
S4†). At the lower concentration of 1 g L of additive, the
COM particles are almost indistinguishable from the control
particles (see Fig. S6 and S7†).
BaPha , each was found to promote nucleation somewhat
with induction times of 15–25 seconds, 5–15 and 10–15 sec-
onds, respectively. Similar to the tetrazoles then, the carbox-
ylic acid derivatives appear to promote nucleation slightly.
The ability of the tetrazoles to complex calcium was inves-
tigated to determine whether the modifiers altered supersatu-
ration significantly and whether complexation could lead to
dissolution of the formed COM particles.
Turbidity
To better determine the impact of these small molecules on
the nucleation behaviour of calcium oxalate, we undertook
turbidimetric studies to assess their impact on homogenous
nucleation. Turbidity was used to monitor the onset of nucle-
ation and the induction time from these experiments deter-
mined. The important parameter in these studies is not the
turbidity (which is impacted by many factors such as size,
Complexation behaviour
The ability of the various molecules to complex calcium was
undertaken by measuring the dissolution of commercially
3
3,34
shape and number of particles)
which is the time the turbidity increases above baseline.
but the induction time,
3
5
Induction time is inversely related to the homogenous nucle-
ation rate. Increasing induction time suggests that the organic
molecules are adsorbing onto the nuclei and changing
2
+
−1
Fig. 5 SEM image of calcium oxalate (0.30 mM) particles formed in
the presence of 5 g L a) TzPhCN (0.029 M) and b) BaPhCN (0.034 M).
Fig. 6 Dissolution of Ca (mg L ) from COM in the presence of
−1
−
−
additive species in solution measured by ICP-AES.
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−
1
−1
obtained COM with the six inhibitors highlighted in Fig. 6, in
ultrapure water. The calcium ion content after a week was
analysed by ICP-AES, which determined that increased solu-
bility by complexation did not significantly occur for many of
main identified peaks at ~1350 cm and ~1622 cm are
believed to be the tetrazole peaks (Fig. S21b†) but modified
on adsorption (for example, due to de-protonation).
The unadsorbed tetrazole has these peaks at 1411 cm and
1602 cm . In any event, these bands are different to the con-
trol COM crystal's Raman spectrum (Fig. S21c†), showing the
expected doublet for the monohydrate at 1460, 1480 cm
and which does not emit such broad fluorescence as seen in
Fig. S21a.†
This implies that the tetrazole species is present in the
crystal prior to re-dissolution. The data thus far suggests it is
the incorporation of the tetrazole that causes this re-
dissolution by weakening the crystal structure at defects or
areas of strain.
3
7
−
1
−
−1
the molecules (Fig. 6), in particular in the presence of TzPh .
2
−
However, it was shown from this data set that the TzTz ,
2
−
2−
−1
BaBa and TzBa additives showed significant increases in
the dissolution of calcium oxalate monohydrate compared to
the other additives analysed (see Table S3† for concentration
information). The molecules containing more than one func-
tional group, therefore, would change the supersaturation
solutions and may account for the observed morphological
impacts in the presence of these additives.
−
In addition, since the tetrazole, TzPh , does not signifi-
cantly complex calcium the re-dissolution does not appear to
be due to subsequent complexation and dissolution.
Conclusions
From this study we have shown that tetrazoles can impact on
COM morphology and size significantly. Of the molecules
Timed experiments
−
2−
−
Prolonged timed experiments were conducted to determine
if,
i) in the presence of the additive TzPh , the crystals grew
in the manner they are observed and
ii) after the COM dissolved whether some other solid
phase re-precipitated.
investigated the TzPhCN , TzTz and TzPha , were found to
be the most potent. The growth directions most impacted by
the tetrazoles are the <100> and <001> of COM. In all cases
we found COM was formed in the presence of these tetra-
zoles, not COD, and this is thought to be due to the pH of
the experiments falling between 6.5 and 7.5, while COD has
−
3
8
Initially, the majority of particles formed were the thermo-
dynamically stable COM (see Fig. S17a†) having a morphol-
ogy that is also equivalent to the control particles. Particles
larger in mean length than an average control sample
were obtained according to the quantitative data; standard
COM control crystals usually averaged ~7 μm in length while in
the presence of the tetrazole average lengths from 9–22 μm were
observed in different repeats. After >3 days dissolution of the
interior of the particles can be observed (as shown in Fig. S17b†)
and is a reproducible affect seen in Fig. 3e, S18 and S19.†
No evidence of calcium oxalate re-precipitation is observed
of any form. After 19 days few particles with COM shape
remained, and there is microbial action evident as the labora-
tory conditions were not sterile. This issue persisted even
when we attempted to sterilise by filtration (filtering all solu-
tions using a 0.2 μm pore membrane). Experiments with
azide are currently underway. The timed experiments did
show that the crystals did not grow in a hollowed manner
from the beginning but were in fact similar to control crys-
tals, however we were not able to determine any recrystalliza-
tion due to the bacterial growth. This bacterial growth did
not occur for the control experiment, nor in the presence of
the carboxylate containing molecule even after 19 days (see
Fig. S20a, b and c†).
been shown to form favourably in basic conditions. The
tetrazole molecules were generally found to promote homoge-
nous nucleation.
The tetrazole additives throughout this study have been
shown to impact more than their carboxylic acid equivalent
groups (as lower concentrations are required to achieve
similar or more significant morphological impacts) across a
variety of functional groups. The increase in the number of
functional groups from one to two tetrazole or carboxylic
acid groups is seen to increase their impact on calcium
oxalate crystallization as has been observed for other
1
1
systems.
−
The re-dissolution observed in the presence of TzPh was
investigated in more detail. It was found that this tetrazole
does not complex the calcium ion to any significant extent
but is present with the COM prior to the dissolution occur-
ring. The timed experiments showed that the COM doesn't
grow in this fashion and that therefore the most likely mech-
anism is that the tetrazole is incorporated in the crystal and
that the dissolution is due to this incorporation weakening
the crystal structure. This is further being investigated
due to microbial action interfering with the longer-term
experiments.
In conclusion, it can be stated that tetrazoles can be used
as crystal growth modifiers of calcium oxalate monohydrate.
Furthermore, we plan to continue research in this area to
explore a variety of organic backbones, different functional
groups and their stereochemistry. It will be interesting to
investigate the differences in stereoisomers in the ortho and
meta substituted positions in addition to further investiga-
tions into functional groups that are electronegative and
Using Confocal Raman spectroscopy, the crystals taken
after 3 days in the presence of the tetrazole; still in typical
COM shape with no visible signs of hollowing were analysed.
These particles showed a Raman spectrum (Fig. S21a†) with a
significant degree of fluorescence in the background. Further
investigation found that this fluorescence only occurred in
the presence of the tetrazole molecule. In addition, the two
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electropositive; this is due to the most significant modifiers
being identified, in this study, as those that are electron
withdrawing. Experiments closer to physiological conditions
and biologically relevant environments are also envisaged to
ascertain whether the ability of the tetrazole to impact crys-
tallization is retained.
15 A. N. Chermahini, A. Teimouri and A. Moaddeli, Heteroat.
Chem., 2011, 22, 168–173.
16 A. Teimouri and A. Najafi Chermahini, Polyhedron, 2011, 30,
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17 R. Huisgen and H. Markgraf, Chem. Ber., 1960, 93, 2106–2124.
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1
9 K. Koguro, T. Oga, S. Mitsui and R. Orita, Synthesis, 1998,
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9
Acknowledgements
2
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and Commonwealth Governments.
2
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