Strong Binding of Hydrogen-Bonded Poly(carboxylate) Ligands on
Calcium Carbonate Crystal
Norikazu Ueyama, Kazuyuki Takahashi, Taka-aki Okamura, and Hitoshi Yamamoto
(Department of Macromolecular Science, Graduate School of Science, Osaka University)
Abstarct
13
C CP/MAS NMR and FE/TEM measurements of
aragonite brick in Pinctada fucata nacreous layer
indicate that the brick consists of numbers of
biopolymer domains and highly oriented aragonite
nanocrystals. On the surface of CaCO3 crystal, Ca
cation is presumably surrounded with 1- charge of
CO32- and 1- charge of oxyanion of poly(carboxylate)
and four water molecules, totally giving a neutral
Ca(II) complex. Synthetic poly-carboxylate having
intramolecular NH---O hydrogen bond forms
nanosized conglomerate of metastable vaterite crystals
with a strong binding between poly(carboxylate) and
CaCO3 crystal under neutral conditions. The strong
binding in the hydrogen-bonded poly(carboxylate) is
due to increase of the Ca-O formation constant by the
pKa shift of polycarboxylic acid and due to covalent
Ca-O bond character enhanced by the NH···O
hydrogen bond in the neutral Ca(II) site of the crystal
surface.
strongly bind to the CaCO3 crystal and prevent the
Ca–O (carboxylate) bond from dissociation due to the
NH···O hydrogen bond.
EXPERIMENTAL
Synthesis of Polymer Ligands and Their CaCO 3
Composites
Poly(allylaminocarboxylate)s were synthesized from
poly(allylamine hydrochloride and maleic anhydride
in ethyleneglycol was added triethyl- amine. The
elemental analysis indicates the 63 % transduction of
carboxylic
acid
group.
Poly[3(methacrylolamino)alkyrate]s were synthesized by
homopolymerization of the corresponding monomer
using two stereospecific polymerization catalysts.
Ammonium carbonate aqueous solutions were
dropped into an ethanol solution of polymer ligand
and calcium chloride dihydrate at 303 K (pH 3.8 ~
4.0). The pH value of the solutions was tuned to 7.2 ~
7.4 by addition of ammonium carbonate aqueous
solution. The final CaCO3/polymer ligand ratio
became 100/1. The obtained CaCO3 crystals were
washed with methanol and distilled water to remove
unbound polymer ligands, and dried over P 2O5 under
reduced pressure.
Introduction
Biomineralization processes are often mediated and
regulated by a small amount of biopolymer. Most of
the unusually acidic proteins found in aragonite- or
calcite-containing mineralized tissues of invertebrates
are rich in acidic residues, which are typically Asx
(i.e., Asp or Asn) or Glx (i.e., Glu or Gln). Many
artificial carboxylates were synthesized for
poly(carboxylate)-CaCO3 composites as a mimick of
biominarals.
However, a strong binding of biopolymer ligands
to CaCO3 crystal is one of the puzzles on mimicking
biominerals. We focuses that an NH∙∙∙O hydrogen
bond to Ca(II)-coordinated oxygen effectively affects
to formation of a Ca complex. Two types of Ca(II)
complexes of Ca(II) and hydrogen-bonded benzoate,
2,6-di-acetylamino- benzoate, were synthesized as
(1:2) and (1:4) Ca/carboxylate complexes in neutral
and 2- anion states, respectively. The (1:2) complex
exhibits a short Ca-O bond distance (2.3 Å) due to
d-p interaction enhanced by the intramolecular
NH---O hydrogen bond from amide NH against
carboxylate oxyanin, whereas the (1:4) complex
shows a long Ca-O bond distance (2.5 Å) in common
ionic mode. In the both cases, the NH∙∙∙O hydrogen
bond prevents the metal–oxygen bond from
dissociating, 1-4 due to the pKa shift,5 that contributes
to a higher formation constant to Ca ion.
Previously, we have studied on CaCO3 composites with poly(carboxylate) ligands, e.g. poly(partially amidated acrylate) (1), that have a 6- or
8-membered ring with an intramolecular NH···O
RESULT AND DISCUSSION
Poly(carboxylate)/CaCO3 Composites and Pearl
Novel polymer ligands with NH∙∙∙O hydrogen bond in
the same side-chain were synthesized, such as
poly(2-allylcarbamoylacetate)
(2),
poly(3-allylcarbamoylpropionate)
(3),
and
poly(4-allylcarbamoylbutyrate) (4), to form 6-, 7-, or 8membered-ring intra-side-chain NH∙∙∙O hydrogen
bonds, respectively.8 CaCO3 composite with these
polymer ligands shows that the formations of strong
NH∙∙∙O hydrogen bond stabilizes Ca–O bond on the
surface of CaCO3 crystal. Furthermore, these polymers
control CaCO3 crystal growth to yield one of the
metastable morphology, vaterite.
7-Membered-ring
intra-side-chain
hydrogenbonded
polymer,
poly[(Z)-3-allyl-carbamoyl-2propenate] (5) is easily dislodged from CaCO3 crystals
by washing, although this ligand forms a similar
7-membered-ring intra-side-chain NH∙∙∙O hydrogen
bond. 8 The polymer ligand forms a dynamically weak
NH∙∙∙O hydrogen bond, whereas 3 has the strong
NH∙∙∙O hydrogen bond. Formation of the weak NH∙∙∙O
hydrogen bond is due to a conformational equilibrium
between two conformers with NH∙∙∙O hydrogen bond
and without one. The Ca-O bond of the
none-hydrogen bonded conformer in 5 is readily
hydrolyzed by
hydrogen bond between the carboxylate group and
neighboring amide NH.6, 7 These polymer ligands
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O
O
t-Bu
N
H O
O
O
O
O
O
H
N
N
O
O
O
n
1
O
H
H
N
n
2
n
3
n
4
O
O
O
O
O
H
H
N
N
O
Me
n
5
O
O
H
Me
n
6
N
7
O
O
O H
N
Me
n
O
n
8
Scheme 1
water because the conformer possesses a higher pKa
due to the closely located carbonyl negative charge.
Then, the polymer ligand becomes carboxylic acid to
be dislodged from the surface of CaCO3 crystal. The
hydrolysis is promoted with the long life of the
none-hydrogen bonded conformer. Thus, the strong
NH∙∙∙O hydrogen-bonded polymer ligand strongly
binds to CaCO3 crystal and controls CaCO3 crystal
growth to yield vaterite crystals, whereas the weak
NH∙∙∙O hydrogen-bonded polymer ligand is easily
dislodged from CaCO3 crystal by washing.
In order to investigate the effect of stereoregularity of each carboxylate in the polymer main
chain, three types of polymer ligands, poly[3methyl-2-(methacryloylamino)butyrate] (6), poly[3-(methacryloylamino)propionate] (7), and poly[4-(methacryloylamino)butyrate] (8), were synthesized. 9 The three polymer ligands have hydrogenbonded structures in their side-chains. Isotactic-rich
and synsiotactic-rich polymers for 7 were examined in
the terms of the surface coordination. The strength of
the NH∙∙∙O hydrogen bond in the polymer ligands, 6, 7
and 8, reflects the morphologies of CaCO3 crystals.
The formation of a weak hydrogen bond in the
polymer ligand does not affect the CaCO3 crystal
growth to form the most stable morphology, calcite,
without restrictions. However, a strong one restricts
crystal growth to yield vaterite crystals. Five- or
syndiotactic
6-membered-ring
intra-side-chain
NH···O hydrogen-bonded polymer ligand-CaCO3
composites form calcite crystals due to the formation
of the weak hydrogen bond, while isotactic-rich 6- or
7-membered-ring
intra-sidechain
NH···O
hydrogen-bonded polymer ligand- /CaCO3 composites
form vaterite crystals because of the formation of the
strong hydrogen bond.
Furthermore, we have studied on the aragonite
brick of the nacreous layer in mollusk shell using 13C
CP/MAS, FE/TEM and EELS. Although the brick has
been thought to be a single crystal, these analytical
results indicate that the aragonite brick is composed by
highly controlled aragonite nano- crystals with small
biopolymers. 10 These nano- crystals in the brick
resemble to vaterite nano- crystals supported by the
synthetic hydrogen- bonded polymer ligand. It is
likely that biopolymer ligand in aragonite of pearl
possesses a similar NH∙∙∙O hydrogen bond presumably
derived from Asp and Asn residues.
This stabilization is mainly related to two factors.
One is that the NH∙∙∙O hydrogen bond lowers the pKa
value of a carboxylic acid. The low pKa value
increases the complex formation constant in Ca
complex. Another is that neutral surface around Ca ion
forms covalent Ca-O bond to contribute to the strong
Ca-O bond. Figure 1 shows the proposed surface
structure of poly(carboxylate)- /CaCO3.
0
O
N
C
O
H 2O
CaII
O
O
C
H
R
OH2
O
C
2- O
Fig. 1. Proposed surface structure of
poly(carboxylate)/CaCO3.
REFERENCES AND NOTES
1. Onoda, A.; Yamada, Y.; Okamura, T.; Doi, M.;
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