Molecular properties of 2-pyrone-4,6-dicarboxylic acid (PDC) as a stable metabolic intermediate of lignin isolated by fractional precipitation with na+ Ion

Article abstract of DOI:10.1246/bcsj.80.2436

A chemically stable metabolic intermediate of lignin, 2-pyrone-4,6- dicarboxylic acid (PDC), was isolated, and the molecular properties were comprehensively investigated by using thermal analysis, optical spectroscopy, potentiometric titration, and X-ray crystallography. UV absorption spectra of PDC did not show any solvent effects, whereas the fluorescence spectra displayed distinct solvatochromism. Despite the fairly good planarity of the PDC geometry, dual fluorescence was observed in some small donor number solvents. The emission behaviors are thought to be due to the significant twist between the pyrone ring and carboxylate substituents, found in the X-ray crystal structures of the Na⁺ complex. The crystal structure also showed the intermolecular, centrosymmetric short hydrogen bonding of the carboxylate moieties with an O...H...O distance of 0.2452 nm, which made it possible to assign the pK_a values to the molecular structure. PDC was thermally stable and could be sublimed even at atmospheric pressure, offering the preparation possibility of ultra-thin films by vapor-deposition techniques. The fact that the chemically and thermally stable compound possessing many attractive functions, such as unique emission, facile sublimation, and strong acidity, resulting in exceptionally short intermolecular contacts, can be obtained from biomass on a large quantity will lead to establishment of carbon neutral system in future industry.

Full text of DOI:10.1246/bcsj.80.2436

2436 Bull. Chem. Soc. Jpn. Vol. 80, No. 12, 2436–2442 (2007)
Ó 2007 The Chemical Society of Japan
Molecular Properties of 2-Pyrone-4,6-dicarboxylic Acid (PDC)
as a Stable Metabolic Intermediate of Lignin Isolated
by Fractional Precipitation with Naþ Ion
Tsuyoshi Michinobu,1;2 Masami Bito,2 Yoshiko Yamada,2 Yoshihiro Katayama,3 Keiichi Noguchi,4
ꢀ1;2
Eiji Masai,5 Masaya Nakamura,6 Seiji Ohara,7 and Kiyotaka Shigehara
1Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology,
Koganei, Tokyo 184-8588
2Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
3Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
Koganei, Tokyo 184-8588
4Instrumentation Analysis Center, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
5Department of Bioengineering, Nagaoka University of Technology, Nagaoka 940-2188
6Department of Applied Microbiology, Forestry and Forest Products Research Institute, Tsukuba 305-8687
7Department of Biomass Chemistry, Forestry and Forest Products Research Institute, Tsukuba 305-8687
Received July 30, 2007; E-mail: jun@cc.tuat.ac.jp
A chemically stable metabolic intermediate of lignin, 2-pyrone-4,6-dicarboxylic acid (PDC), was isolated, and the
molecular properties were comprehensively investigated by using thermal analysis, optical spectroscopy, potentiometric
titration, and X-ray crystallography. UV absorption spectra of PDC did not show any solvent effects, whereas the fluo-
rescence spectra displayed distinct solvatochromism. Despite the fairly good planarity of the PDC geometry, dual fluo-
rescence was observed in some small donor number solvents. The emission behaviors are thought to be due to the sig-
nificant twist between the pyrone ring and carboxylate substituents, found in the X-ray crystal structures of the Naþ com-
plex. The crystal structure also showed the intermolecular, centrosymmetric short hydrogen bonding of the carboxylate
moieties with an O H O distance of 0.2452 nm, which made it possible to assign the pK values to the molecular struc-
a
ꢁꢁꢁ ꢁꢁꢁ
ture. PDC was thermally stable and could be sublimed even at atmospheric pressure, offering the preparation possibility
of ultra-thin films by vapor-deposition techniques. The fact that the chemically and thermally stable compound possess-
ing many attractive functions, such as unique emission, facile sublimation, and strong acidity, resulting in exceptionally
short intermolecular contacts, can be obtained from biomass on a large quantity will lead to establishment of carbon
neutral system in future industry.
Utilization of biomass in valuable fuels and industrial prod-
ucts has attracted much attention, because society requires a
change from nonrenewable carbon resources to renewable bio-
resources.1 Lignin is one of the most abundant natural carbon
resources, existing in trees at 15–36% by weight. However, an
advanced system of utilizing it as biomass has not been estab-
lished, because the highly networked three-dimensional struc-
tures make it difficult to transform into well-defined versatile
substances by conventional synthetic methods. Accordingly,
the metabolic conversion of lignin into stable and functional
intermediates has become the focus in this research field,2,3
and we have recently succeeded in producing pure 2-pyrone-
4,6-dicarboxylic acid (PDC) on a large scale from lignin via
protocatechuate by using transformed bacterium.4 PDC is
chemically stable and expected to be a versatile functional
substance, because the pyrone ring has some useful functions,
such as an acceptor property in donor–acceptor type chromo-
phores5,6 as well as the biodegradability induced by PDC hy-
drolase.7 Additionally, two carboxylic acids attached to the
pseudo-aromatic ring can be chemically modified when larger
molecular systems are constructed. A typical example is the
use as a bifunctional monomer for polycondensation with
another bi- or tri-functional monomer, furnishing linear and
networked polymers, respectively. Furthermore, PDC cannot
be synthesized using petrochemistry, only from plant compo-
nents at the moment in spite of much effort to synthesize natu-
rally occurring compounds.3,4,8 This fact would lead to com-
pletely novel functional materials. Thus, PDC is highly prom-
ising as a renewable biosource for establishing carbon neutral
system in industrial materials science, and to this end, a
comprehensive understanding of PDC properties is needed.
Many reports on PDC, including the first attempt of the iso-
lation and tentative characterization by Maruyama,9 suggest
that it is difficult to purify, probably due to the inherent sensi-
tivity of the 2-pyrone ring towards bases.10 Therefore, there
has been no systematic study on the detailed properties of
PDC. In the course of development of the preparation protocol
on a large scale, we have found that PDC can be selectively
Published on the web December 13, 2007; doi:10.1246/bcsj.80.2436

T. Michinobu et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007) 2437
8000
COOH
6000
4000
2000
0
O
O
COOH
Chart 1. Chemical structure of 2-pyrone-4,6-dicarboxylic
acid (PDC).
recovered as the Naþ salt when NaCl is added to the centri-
fuged aqueous solution. Subsequent acidification of the Naþ
salt-dispersed solution, followed by the solvent extraction,
readily gives the pure PDC. Here, we report detailed properties
of PDC, determined by using thermal analysis, optical spec-
troscopy, potentiometric titration, and X-ray crystallography.
250
300
350
400
450
Wavelength / nm
The PDC Naþ complex was also investigated by using X-ray
ꢁ
Fig. 1. UV absorption spectrum of PDC in water at 20 ꢃC.
crystallography, by which a remarkable, centrosymmetric
short hydrogen bonding as a result of partial deprotonation
of the carboxylic acids was observed (Chart 1).
Table 1. Donor Number of the Employed Solvents and
Optical Absorption and Fluorescent Spectral Data for
PDC in Various Solvents
Results and Discussion
Characterization of PDC. Crude PDC produced by the
metabolic pathway was isolated by addition of NaCl, followed
by the acidification of the solution. Since purification difficulty
generally arises in many biomass production processes owing
to the co-existence of bacteria bodies and metabolic wastes,
such simple recovery of the product is remarkable. Pure PDC
Solvent
Donor numbera) ꢀmax ꢀem, max Stokes shift
/nm
/nm
/cmꢂ1
Acetic anhydride
Acetonitrile
1,4-Dioxane
Acetone
Water
Methanol
Ethanol
Diethyl ether
Dimethylformamide
10.5
14.1
14.8
17.0
18.0
19.0
19.2
19.2
26.6
316 451, 468
10278
8254
8102
6426
7318
8281
7507
8227
4977
316
320
327
313
316
317
319
319
428
432
414
406b)
428
416
433
379
1
was unambiguously characterized by using H and 13C NMR,
FT-IR, and MALDI-TOF mass spectrometry. The compound
was very soluble in polar solvents, such as water, acetone,
methanol, THF, and acetonitrile, but hardly soluble in apolar
solvents, such as benzene, hexane, and heptane. Complexation
with Naþ ion decreased the solubility even in polar solvents.
For example, the solubility in water decreased from 182 mM
a) Taken from the Ref. 12a. b) The quantum yield measured in
water at 20 ꢃC against 2-aminopyridine is 7.9%.
(PDC) to 9.5 mM (1 M = 1 mol dmꢂ3) (PDC Naþ complex)
ꢁ
upon complexation. Although PDC tends to interact strongly
with Naþ ion, the complexed cationic peak could not be de-
tected by the MALDI-TOF-MS measurements in the positive
ionization mode, but the molecular ion peak was observed
in the negative ionization mode. This result is consistent
with the fact that the pyrone ring has electron-accepting
features especially when electron-withdrawing substituents
are attached.5,6
find utility in the vast and fascinating fields of industry, it is
significant to investigate the detailed optical properties of
PDC. PDC showed a longest-wavelength absorption peak
(ꢀmax) at 313 nm with a molar extinction coefficient (") of
6200 Mꢂ1 cmꢂ1 in water at 20 ꢃC (Fig. 1). The ꢀmax value
was hypsochromically shifted, and the " value is much smaller
as compared to the reported 2-pyrone compounds with extend-
ed ꢁ-conjugations. This result is also explained by the elec-
tronic effects of the substituents. Introduction of donor groups
at the 4- and 6-positions of 2-pyrone ring is reported to induce
intramolecular donor–acceptor interactions, leading to batho-
chromic shift of ꢀmax. In this case, carboxylic acid groups at
the 4,6-positions of PDC did not generate such interactions
in the ꢁ-conjugated chromophore. UV absorption spectra,
measured in different solvents with various polarities or other
interactions, such as hydrogen bonding, did not show signifi-
cant solvent effects except for shifted ꢀmax value in acetone
(Table 1). However, considerable solvatochromism was
observed in the fluorescence spectra. Emission maxima
(ꢀem,max) of PDC range from 379 nm in N,N-dimethylform-
amide to 468 nm in acetic anhydride (Table 1 and Fig. 2a).
The fluorescence spectra did not display any relationship with
solvent polarities, whereas a linear trend was seen in relation to
the hydrogen-bonding ability of the solvents. In this case, we
used solvent donor number (quantitative measurement of
The thermal stability of PDC was investigated by using
thermogravimetric analysis (TGA). When measurements were
performed at a heating rate of 10 ꢃC minꢂ1 under N2, no
weight loss was observed up to at least ca. 200 ꢃC, and the de-
composition temperature was determined to be 253 ꢃC (see
Supporting Information). However, the melting point measure-
ment indicated that slow sublimation occurs at ca. 190 ꢃC. For
example, leaving PDC at 190 ꢃC for 1 day provided very thin
white films deposited on a glass plate, of which the purity was
confirmed by using NMR measurements. This result offers the
possibility of preparing functional ultra-thin films, which
possesses attractive optical properties of PDC (vide infra),
by using vapor-deposition techniques.11
UV Absorption and Fluorescence Spectroscopy. It is re-
ported that 2-pyrone ring possesses efficient electron-accepting
properties as well as strong fluorescence when conjugation is
extended.5,6 Since biomass-based functional materials must

2438 Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007)
2-Pyrone-4,6-dicarboxylic Acid
1.2
(a)
(a)
O3
C6
C3
DMF
Water
Acetone
Ethanol
O4
C2
1
0.8
0.6
0.4
0.2
0
θ
= -2.59°
Methanol
C4
1,4-Dioxane
O7
Acetonitrile
Ether
C5
C1
O2
O6
O8
C7
O5
O1
= 4.07°
Acetic anhydride
θ
(b)
400
450
500
550
600
Wavelength / nm
30
(b) 30
20
20
10
10
20000
4000
28000
8000
12000
24000
em,max / cm-1
-1
Stokes shift / cm
λ
Fig. 2. (a) Fluorescence spectra of PDC in various solvents
at 20 ꢃC. (b) Plots of donor number of the employed sol-
vents versus emission peak maxima or Stokes shift values
for PDC.
Fig. 3. (a) ORTEP drawing of PDC 2H2O with vibrational
ꢁ
ellipsoids shown at the 50% probability level and
(b) arrangement of neighboring molecules via hydrogen
bonding with water molecules in the crystal packing of
PDC 2H2O.
ꢁ
Lewis basicity) as a measure of hydrogen-bonding strength
with a solute.12 Plots of the ꢀem,max values against the donor
numbers showed a linear correlation (R ¼ 0:88) between the
two quantities (Fig. 2b). Since the absorption ꢀmax values do
not vary remarkably in the employed solvents, the fluorescence
can also be characterized by a linear increase in the Stokes
shift with decreasing solvent donor number (R ¼ 0:85)
(Fig. 2b). It should be noted here that no clear relationship
between the absorption ꢀmax values and solvent features, in-
cluding polarity and hydrogen bonding, was found for PDC.
These results suggest that the emitting excited state is more
strongly associated with solute–solvent hydrogen-bonding
interactions than the ground state.
For some small donor number solvents, two emission peaks
or one emission peak with an additional shoulder were ob-
served. The most noticeable example is the emission in acetic
anhydride, in which two peaks were observed at 451 and
468 nm (Fig. 2a). This dual fluorescence behavior has pre-
viously been reported for organic donor–acceptor type com-
pounds as a result of a twisted intramolecular charge-transfer
state (TICT).13 If this is also true for PDC, the emission might
originate from both the initial excited state and a lower-energy
TICT state, in which the carboxylic acid groups take an or-
thogonal orientation with respect to the planar 2-pyrone ring.
the Naþ complex crystallize in the monoclinic space group
C2=c (Z ¼ 8) and P21=c (Z ¼ 4), respectively. ORTEP draw-
ings of the asymmetric unit are depicted in Figs. 3a and 5a,
respectively, and selected bond lengths and angles are listed
in Table 2. In the crystals of PDC 2H2O, the 2-pyrone ring
ꢁ
and the two carboxylic acid moieties are in a relatively planar
orientation. The interplanar angles between the least-square
plane of the 2-pyrone ring and the plane of the carboxylic
acids at the 4- and 6-positions are 2.59 and 4.07ꢃ, respectively.
Each PDC molecule is further linked to the neighboring
molecules via hydrogen bonding with a water molecule in a
planar fashion, leading to a two-dimensional sheet-like struc-
ture (Fig. 3b). The interplanar distance between the sheets is
0.301 nm. This unusual short packing is probably caused by
the strong antiparallel dipolar interactions of the carbonyl
moieties of the pyrone ring as well as of the carboxylic acids
(Fig. 4).14
The highly polarizable electronic state within the small
molecular size of PDC results in facile release of protons. Pre-
viously, the pKa values of two carboxylic acids of PDC have
been estimated to be <2:1, but the presumably low purity
has prevented the complete determination of the values.9 Since
we obtained the pure compound, the pKa values were carefully
determined by potentiometric titration in an aqueous solution
of PDC at 30 ꢃC. Addition of aq NaOH solution to the PDC
solution showed a distinct pH jump exactly at a [NaOH]/
X-ray Crystal Structures.
the molecular geometry, the crystal structures of PDC 2H2O
To obtain information on
ꢁ
and the Naþ complex were determined. Single crystals of
PDC 2H2O and the Naþ complex suitable for X-ray crystallo-
ꢁ
graphic analysis were grown in water at 5 ꢃC. PDC 2H2O and
[COOH] ratio of 1 (see Supporting Information). The pKa
ꢁ
1

T. Michinobu et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007) 2439
Table 2. Summary of Selected Bond Lengths and Angles of
the Crystals of PDC 2H2O and PDC Naþ Complex
(a)
ꢁ
ꢁ
O8
PDC 2H2O
ꢁ
PDC Naþ complex
ꢁ
O3
O4
Bond length/nm
C4
O(1)–C(1)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(3)–C(6)
C(4)–C(5)
O(1)–C(5)
C(5)–C(7)
O(2)–C(1)
O(3)–C(6)
O(4)–C(6)
O(5)–C(7)
O(6)–C(7)
0.13759(16) O(1)–C(1)
0.14484(16) C(1)–C(2)
0.13481(18) C(2)–C(3)
0.14339(19) C(3)–C(5)
0.15006(16) C(3)–C(4)
0.13345(16) C(5)–C(6)
0.13650(15) O(1)–C(6)
0.14902(19) C(6)–C(7)
0.12115(15) O(2)–C(1)
0.12137(16) O(3)–C(4)
0.13091(16) O(4)–C(4)
0.12212(15) O(5)–C(7)
0.12997(16) O(6)–C(7)
0.13786(13)
0.14534(16)
0.13464(14)
0.14365(15)
0.15128(16)
0.13495(16)
0.13660(13)
0.15064(15)
0.12138(13)
0.12077(13)
0.13170(13)
0.12213(13)
0.12914(11)
θ
= -4.78°
C2
C5
C3
O1
θ
= -11.29°
C1
C6
O6
O5
C7
O2
O7
Na1
(b)
Bond angle/ꢃ
C(1)–O(1)–C(5) 121.40(9)
O(1)–C(1)–C(2) 117.05(11)
C(1)–C(2)–C(3) 120.41(12)
C(2)–C(3)–C(4) 119.99(11)
C(3)–C(4)–C(5) 118.71(11)
O(1)–C(5)–C(4) 122.44(12)
C(1)–O(1)–C(6)
O(1)–C(1)–C(2)
C(1)–C(2)–C(3)
C(2)–C(3)–C(5)
C(3)–C(5)–C(6)
O(1)–C(6)–C(5)
122.38(8)
116.58(8)
120.17(10)
120.93(10)
118.11(9)
121.78(9)
1.0788 nm
0.2452 nm
Fig. 5. ORTEP drawings of (a) the repeat unit and (b) zig-
zag chain structure of the PDC Naþ complex with vibra-
ꢁ
tional ellipsoids at the 50% probability level. The O H O
ꢁꢁꢁ ꢁꢁꢁ
distance of the short hydrogen bonding and the intrachain
Na Na distance through the PDC moiety are 0.2452 and
1.0788 nm, respectively.
ꢁꢁꢁ
short hydrogen bonding between carboxylic acids has some-
times been seen for the complexes of electron deficient carbox-
ylic acids and alkali or alkaline earth metals.15 Some neutron
diffraction studies have shown that the hydrogen atom shared
between two carboxylate groups in a manner of COO–H
ꢁꢁꢁ
ꢂOOC $ COOꢂ H–OOC is located exactly on a center to
ꢁꢁꢁ
symmetry between the two oxygen atoms, which also coin-
cides with the inversion center of the crystals.16 A database
search of Cambridge Structural Database indicated the average
Fig. 4. Crystal packing of PDC 2H2O, showing the anti-
ꢁ
parallel stacking of carbonyl moieties with an unusual
short distance of 0.29–0.32 nm.
O H O distance of such a short hydrogen bonding in the 76
examples found in the database is 0.2443 nm, which is quite
short as compared to the usual dimers of carboxylic acids
ꢁꢁꢁ ꢁꢁꢁ
and pKa values, calculated from the dibasic acid model, in
(ca. 0.27 nm).17 The observed O H O distance for the PDC
ꢁꢁꢁ ꢁꢁꢁ
ꢁ
2
which the dissociation of two protons is under equilibrium,
were 1.13 and 2.52 (R = 0:99), respectively. This strong acidic
nature of PDC is supported by the finding during the develop-
ment of the purification procedure that addition of NaCl to an
aqueous PDC solution afforded a white precipitate in an almost
Naþ complex is 0.2452 nm, which is fairly in good agreement
with the average value of the previously reported examples.
This remarkable short hydrogen bonding as well as the
Naþ O=C coordination, as a result, cause the formation of
an alternated zigzag chain structure (Fig. 5b).18 The intrachain
ꢁꢁꢁ
quantitative yield. The crystal structure of the PDC Naþ com-
Na Na distance through the PDC moiety is 1.0788 nm. This
ꢁꢁꢁ
ꢁ
plex showed that the carbonyl group of the 2-pyrone ring, in-
stead of the carboxylate moieties at the 4,6-positions, coordi-
nates to the Naþ ion (Fig. 5a). Remarkably, short centrosym-
metric intermolecular hydrogen bonding occurs between the
6-carboxylic acids of the 2-pyrone rings as a result of partial
deprotonation of the carboxylic acids (Fig. 5b). This type of
one-dimensional coordination chain further interacted with
each other via hydrogen bonding and coordination with water
molecules to provide the two- and three-dimensional net-
worked structures, respectively (see Supporting Information).
The interchain distance along the a axis is 0.358 nm, which
is greater than that for the PDC 2H2O crystals.
ꢁ

2440 Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007)
2-Pyrone-4,6-dicarboxylic Acid
crystals. Two broad peaks appeared at 3488 and 3338 cmꢂ1
Comparison of the crystal structures between PDC 2H2O
,
ꢁ
and the Naþ complex gave some more information on the
molecular properties of PDC. As shown by the X-ray crystal
structure of the Naþ complex (Fig. 5a), each of two pKa values
indicating that hydrogen bonding, especially associated with
water molecules, is much weaker (Fig. 6b). Consequently, a
one-dimensional chain structure composed of the Naþ coordi-
nation and short hydrogen bonding of the carboxylic acids is
suggested for this complex (vide supra). Upon complexation
with Naþ ion, Fermi resonance of the pyrone ring disappeared,
and a strong single peak ascribed to the coordinated C=O
vibration appeared at 1719 cmꢂ1. Furthermore, a noticeable
peak was assigned to the vibration derived from the intermo-
lecular short hydrogen bonding of the carboxylate moieties.19
A broad, but relatively strong, peak was observed at 772
could be assigned to the molecular structure. The pKa corre-
1
sponds to the carboxylic acid at the 6-position of the pyrone
ring, associated with the short hydrogen bonding based on a
proton release. Accordingly, pKa originates from the other
2
carboxylic acid at the 4-position. The bond lengths and angles
of the PDC framework are almost the same between the two
crystals, except for the torsion angles of the carboxylate moie-
ties at the 4- and 6-positions (Table 2). In comparison to the
PDC 2H2O crystals, in the Naþ complex, the carboxylate
cmꢂ1 for the PDC Naþ complex, which does not exist in the
ꢁ
ꢁ
moieties were distorted farther from the pyrone ring plane.
For example, the interplanar angles between the least-square
plane of the 2-pyrone ring and the plane of the carboxylate
groups at the 4- and 6-positions are 4.78 and 11.29ꢃ, respec-
tively (Fig. 5a). The latter significant deviation from the co-
planarity might represent the possible molecular geometry of
PDC in the excited state responsible for the observed dual
fluorescence in small donor number solvents. Small donor
number solvents can less efficiently interact with PDC mole-
cules than large donor number solvents. Thus, the intermolec-
ular short and strong hydrogen bonding of the two carboxylic
acids should predominantly occur in these kinds of solvents,
leading to the deviation of the substituents from the pyrone
ring planarity and, consequently, a dual fluorescence.
spectrum of the PDC 2H2O crystals. Judging from previous
reports, this peak could be ascribed to the asymmetric stretch-
ꢁ
ing vibration of the O H O bond in the short hydrogen bond-
ꢁꢁꢁ ꢁꢁꢁ
ing. Thus, the IR spectra of both crystals reasonably explain
the X-ray crystallographic analysis, which is also helpful in
understanding the PDC properties.
Conclusion
The molecular properties of 2-pyrone-4,6-dicarboxylic acid
(PDC), a lignin-derived stable metabolic intermediate, were
comprehensively investigated by using thermal analysis, opti-
cal spectroscopy, potentiometric titration, and X-ray crystal-
lography. It was revealed that PDC shows solvent-dependent
fluorescence and, in particular, dual fluorescence in small
donor number solvents. The unique optical properties suggest
that the emitting excited state is strongly associated with
solute–solvent hydrogen-bonding interactions. Further detailed
investigations are expected to determine its opto-electronic
applications. The fluorescence behavior was also elucidated
by using potentiometric titration as well as the X-ray crystal
IR Spectra. There was also a significant difference in the
ꢁ
IR spectra of the crystals of PDC 2H2O and the Naþ complex.
The frequency of the OH stretching vibrations is a good guide
to the hydrogen-bonding strength. The ꢂOH of the PDC 2H2O
ꢁ
crystals appeared at a very low frequency (3102 cmꢂ1) as a
single broad peak, which completely overlapped the sharp ꢂCH
peak of the pyrone ring (Fig. 6a). This low frequency indicates
very strong hydrogen bonding between water and PDC mole-
cules, resulting in the two-dimensional sheet structure (vide
supra). The spectrum also included multiple C=O stretching
vibrations ascribed to the carboxylic acids and Fermi reso-
structures of PDC 2H2O and the Naþ complex. A more pro-
ꢁ
nounced deviation of the carboxylic acid groups from the py-
rone ring plane was found in the crystal of the Naþ complex,
which forms a one-dimensional polymeric structure via Naþ
ion coordination to the carbonyl moiety of the pyrone ring
and short intermolecular hydrogen bonding of the carboxylate
nance of the 2-pyrone ring in the region of 1757–1726 cmꢂ1
.
On the other hand, the ꢂOH of the PDC Naþ complex was
ꢁ
moieties with an O H O distance of 0.2452 nm. This rare,
ꢁꢁꢁ ꢁꢁꢁ
shifted to a higher frequency relative to that of the PDC 2H2O
ꢁ
very short hydrogen bonding has sometimes been seen in
the crystal structures of electron-deficient carboxylic acids,20
but to the best of our knowledge, this is the first report for
biomass-derived products.
(a)
For material applications of PDC, we mainly envision
two approaches. One is the preparation of ultra-thin films by
vapor-deposition techniques, which are often employed for de-
vice fabrications of functional small molecules. The other is
the chemical conversion into covalently linked polymeric
structures by the use of PDC as a bifunctional monomer for
polycondensation with another bi- or tri-functional monomer.
As an advantage, a wide variety of monomers with desired
functionalities can be employed as a co-monomer, thus leading
to biomass-derived functional polymers. Furthermore, poly-
meric materials can be readily applied for device fabrications
by using simple and low-cost methods, such as spin-coating.
The recent trend to substitute petrochemicals with biomass-
based products will enhance the future development of PDC-
containing functional materials and devices.
3102
(b)
1757-1726
3338
3488
772
1719
2000
Wavenumber / cm-1
4000 3000
1000
500
Fig. 6. IR spectra of the crystals of (a) PDC 2H2O and
ꢁ
(b) PDC Naþ complex in KBr at 20 ꢃC.
ꢁ

T. Michinobu et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 12 (2007) 2441
93.1 K for (C7H4O6) 2(H2O) [Mr ¼ 220:14] monoclinic, space
ꢁ
Experimental
group C2=c, Dcalcd ¼ 1:589 g cmꢂ3
,
Z ¼ 8, a ¼ 1:50912ð6Þ,
General Apparatus. Thermogravimetric analysis (TGA) was
carried out on a Rigaku Thermo plus TG8120 under nitrogen flow,
at a heating rate of 10 ꢃC minꢂ1 from 20 to 450 ꢃC. H NMR and
b ¼ 0:512354ð17Þ, c ¼ 2:38057ð9Þ nm, ꢅ ¼ 91:481ð3Þꢃ, V ¼
1:84005ð12Þ nm3. Rigaku RAXIS-RAPID, Cu Kꢆ radiation
(ꢀ ¼ 0:154187 nm), ꢇ ¼ 1:3386 mmꢂ1. Numbers of measured
1
13C NMR spectra were measured on a JEOL AL400 MHz spec-
trometer at 20 ꢃC. Chemical shifts are reported in ppm downfield
from SiMe4, using the solvent’s residual signal as an internal ref-
erence. Infrared (IR) spectra were recorded on a JASCO FT/IR-
4100 spectrometer. UV/Vis spectra were recorded on a JASCO
V-550 spectrophotometer. Fluorescence spectra were measured
on a JASCO FP6500 spectrophotometer. The spectra were mea-
sured for the dehydrated PDC in a quartz cuvette of 1 cm. The
quantum yield was determined against 2-aminopyridine in 0.1 M
H2SO4 (ꢃF ¼ 0:60).21 MALDI-TOF-MS spectrum was measured
on an Applied Biosystems model Voyager-DE STR instrument
with 2-(4-hydroxyphenylazo)benzoic acid as a matrix. The hydro-
gen ion activities were determined with a Mettler Toledo Portable
Ion Analyzer MA130. Sodium nitrate (0.1 M) was added as the
supporting electrolyte. The initial PDC concentration was 5 mM.
Chemicals were purchased from Kanto, Wako, and Tokyo Kasei,
and used as received.
and unique reflections are 9720 and 1678, respectively (Rint
¼
0:031). Final RðFÞ ¼ 0:0309, wRðF2Þ ¼ 0:0869 for 151 parame-
ters and 1402 reflections with I > 2ꢈðIÞ and 3:7ꢃ < ꢉ < 68:2ꢃ.
X-ray Crystal Structure of PDC Naþ Complex: Crystal
ꢀ
data at 103.1 K for (C7H3:5O6) 2(H2O) Na0:5 [Mr ¼ 231:13]
ꢁ
ꢁ
monoclinic, space group P21=c, Dcalcd ¼ 1:749 g cmꢂ3, Z ¼ 4,
a ¼ 0:35841ð12Þ, b ¼ 1:0175ð5Þ, c ¼ 2:4081ð9Þ nm, ꢅ ¼
91:329ð14Þꢃ, V ¼ 0:8779ð6Þ nm3. Rigaku RAXIS-RAPID, Mo Kꢆ
radiation (ꢀ ¼ 0:071075 nm), ꢇ ¼ 0:1832 mmꢂ1. Numbers of
measured and unique reflections are 8679 and 2000, respectively
(Rint ¼ 0:019). Final RðFÞ ¼ 0:0280, wRðF2Þ ¼ 0:0770 for 150
parameters and 1640 reflections with I > 2ꢈðIÞ and 3:2ꢃ <
ꢉ < 27:4ꢃ.
Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre: Deposition numbers CCDC-
654109 and CCDC-654110 for PDC 2H2O and PDC Naþ com-
ꢁ
ꢁ
plex, respectively. Copies of the data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge, CB2 1EZ, UK; Fax: +44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk).
Preparation.
2-Pyrone-4,6-dicarboxylic acid (PDC) was
prepared from protocatechuate via the metabolic pathway of
Sphingomonas paucimobilis SYK-6.4 After the removal of bacte-
ria by centrifugation, the crude PDC was first precipitated as a
brown solid by addition of NaCl. This salt (24 g) was dissolved
in hot water (1500 mL), and acetone (1500 mL) and concn. HCl
aqueous solution (45 mL) were added. Activated carbon (20 g)
was added to this solution, and the mixture stirred for 15 min.
After filtration, the filtrate was evaporated to ꢄ1500 mL, which
was again acidified with concn. HCl aqueous solution (18 mL).
The solution was extracted with ethyl acetate (500 mL ꢅ 5), and
the organic phase was dried over MgSO4. After filtration, the
solvent was evaporated to afford PDC as a white solid (11 g).
1H NMR (400 MHz, DMSO-d6, 20 ꢃC): ꢄ 6.93 (d, J ¼ 3 Hz, 1H),
7.24 (d, J ¼ 3 Hz, 1H), 13.13 (brs, 2H); 13C NMR (100 MHz,
DMSO-d6, 20 ꢃC): ꢄ 108.77, 121.93, 145.10, 150.62, 160.78,
160.88, 164.59 ppm; IR (KBr): ꢂ 3102, 1757, 1743, 1726, 1669,
1603, 1559, 1456, 1425, 1239, 1211, 1186, 1135, 1086, 912,
891, 818, 782, 765, 719, 685, 669, 645, 571, 460 cmꢂ1; MALDI-
TOF-MS (negative mode, matrix: 2-(4-hydroxyphenylazo)benzoic
acid): m=z: calcd for C7H4O6ꢂ: 184.00; found: 184.44 [M]ꢂ; ele-
mental analysis calcd (%) for C7H4O6 (184.10): C, 45.67; H,
2.19%. Found: C, 45.48; H, 2.06%.
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (#18208028) and the 34th
Award for Encouragement by the Agricultural Chemical
Research Foundation, Japan (T. M.). We thank Dr. Y. Otsuka
(Forestry and Forest Products Research Institute) and Prof.
S. Kajita (Tokyo University of Agriculture and Technology)
for useful discussion.
Supporting Information
Thermogravimetric analysis and potentiometric titration of
PDC and the crystal packing of PDC Naþ complex. This material
ꢁ
is available free of charge on the Web at: http://www.csj.jp/
journals/bcsj/.
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