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Synthesis of C2-regioselectively substituted curdlan acetate propionate and the effect of C2 substituent on their properties Chih-Ying Chien, Yukiko Enomoto, and Tadahisa Iwata ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00415 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 7, 2019
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Synthesis
of
C2-regioselectively
substituted
2
curdlan acetate propionate and the effect of C2
3
substituent on their properties
4 5
Chih-Ying Chien,Yukiko Enomoto and Tadahisa Iwata*
6 7
Science of Polymeric Materials, Department of Biomaterial Sciences, Graduate School of
8
Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-
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8657, Japan
10 11
*Corresponding author. Tel: +81-3-5841-5266; Fax: +81-3-5841-1304.
12
E-mail:
[email protected] (T. Iwata)
13 14
Keywords: Curdlan, regioselective substitution, acetalization, structure-property relationship,
15
crystal structure
16 17
Abstract. A new C2-regioselective synthetic strategy for curdlan hetero esters was developed
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using benzaldehyde to protect the curdlan C4 and C6 hydroxyl groups as a cyclic acetal
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structure. After esterification and deprotection reactions, two C2-regioselectively substituted 1 ACS Paragon Plus Environment
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curdlan acetate propionates, 2-O-acetyl-4,6-di-O-propionyl-curdlan (CD2Ac46Pr) and 4,6-di-
21
O-acetyl-2-O-propionyl-curdlan (CD46Ac2Pr), were successfully obtained. Through the
22
differential scanning calorimetry and wide-angle x-ray diffraction analysis, the C2 ester was
23
confirmed as the decisive substituent over the melting behavior and crystal structure of curdlan
24
esters.
25 26
Introduction
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The development of alternatives to petroleum-based plastics from sustainable resources is
28
now a vital issue in polymer science. The most abundant renewable natural polymers,
29
polysaccharides, are expected as bio-based plastics and exhibit improved thermal properties
30
and solubility through chemical modification, such as esterification1-3. Curdlan is a linear
31
homopolysaccharide consisting of β-(1→3)-D-glucose units, produced by microorganisms4-9.
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Based on its high molecular weight and regular chain structure, curdlan shows great potential
33
for plastic applications. With esterification, curdlan esters exhibit diverse and interesting
34
thermal properties and crystal structures depending the chain length of introduced ester
35
groups10. Therefore, to further control the properties of curdlan esters as polymeric materials,
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the relationship between the substitution position and the properties of curdlan esters should be
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clarified.
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Multi hydroxyl groups in the monomer unit of polysaccharides are expected to have different 2 ACS Paragon Plus Environment
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contributions to the structure and properties of polysaccharides and their derivatives. Studies
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on the impact of each hydroxyl group on the polysaccharide properties have usually involved
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analyzing the properties of polysaccharide derivatives with regioselective substitution at the
42
positions of interest11-16. However, the synthesis of regioselectively substituted polysaccharide
43
derivatives remains challenging owing to the similar reactivities of hydroxyl groups among
44
three positions. Bulky protecting groups, such as trityl group which protects C6 position17-20,
45
and silyl group which protects C2 and C6 positions21,22, are routinely used to achieve
46
regioselective protection of different hydroxyl groups exploiting subtle differences in their
47
reactivities. In our previous publications, regioselective C6-hydroxyl protection of curdlan was
48
developed using trityl group. With two deprotection methods, three regioselectively substituted
49
curdlan acetate propionates, 2,6-di-O-acetyl-4-O-propionyl-curdlan (CD26Ac4Pr), 2,4-di-O-
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acetyl-6-O-propionyl-curdlan
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(CD6Ac24Pr), were successfully obtained23,24. Property analysis of these regioselectvely
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substituted curdlan esters revealed that their melting behaviors and crystal structures were
53
significantly controlled by their secondary substituents (C2 and C4 positions), or either of them.
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Therefore, curdlan acetate propionates with C2 selectivity are crucial missing structures that
55
would solve remaining questions regarding structure-property relationships.
(CD24Ac6Pr)
and
6-O-acetyl-2,4-di-O-propionyl-curdlan
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Benzaldehyde, a useful protecting group in glycochemistry that can protect 1,3-diols as a
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cyclic six-membered acetal ring25-27, is stable under alkaline conditions and can be removed 3 ACS Paragon Plus Environment
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through acid treatment28-30. In the glucose unit of curdlan, the C4 and C6 hydroxyl groups exist
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in 1,3-diol structure, that is a suitable donor for cyclic acetalization. A PEO-substituted
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benzaldehyde has been successfully reacted with curdlan through cyclic acetal formation
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between the aldehyde group and C2 and C6 hydroxyl groups to introduce a PEO chain into
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curdlan31. Therefore, benzaldehyde has potential for protecting the curdlan C4 and C6 hydroxyl
63
groups and, after esterification of the C2 position, be removed to afford C2 selectivity in
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curdlan derivatives.
65
In this study, we have used benzaldehyde as a protecting group for the C4 and C6 hydroxyl
66
groups of curdlan, with the aim to prepare CD2Ac46Pr and CD46Ac2Pr. Furthermore, the
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thermal properties and crystal structures of five regioselectively substituted curdlan acetate
68
propionates with different distributions and curdlan tri-esters were analyzed and discussed to
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better understand the structure-property relationships of curdlan esters.
70 71
Experimental section
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Materials
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Curdlan was purchased from WAKO (Tokyo, Japan) and vacuum dried before use.
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Benzaldehyde, dimethyl sulfoxide (DMSO), 10-camphorsulfonic acid (CSA), trimethyl
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orthoformate (TMOF), dimethylformamide (DMF), pyridine, acetic anhydride, propionic
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anhydride, propionyl chloride, methanol, acetone, hydrogen chloride (HCl), 30%hydrogen 4 ACS Paragon Plus Environment
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bromide (HBr) in acetic acid, and chloroform were purchased from WAKO (Tokyo, Japan).
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CD26Ac4Pr, CD24Ac6Pr and CD6Ac24Pr were synthesized according to Chien et al. (2017)
79
and Chien et al. (2018) with some modifications23,24.
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Synthesis of 4,6-O-benzylidene-curdlan (CD46BzH)
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Curdlan (500 mg) and benzaldehyde (1.0 mL) were dissolved in DMSO (16.7 mL) with
82
stirring at 80°C. CSA (72.7 mg) and TMOF (1.0 mL) were then added, and the reaction mixture
83
was heated at a predetermined temperature for 3 days. The product was regenerated by
84
precipitation in methanol (400 mL), which was filtrated, washed with methanol, and dried
85
under vacuum at 85˚C to obtain 4,6-O-benzylidene-curdlan in 87% yield (60˚C).
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Synthesis of 2-O-acetyl-4,6-O-benzylidene-curdlan (CD2Ac46BzH)
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CD46BzH (500 mg) was added to DMF (50 mL)/pyridine (25 mL) mixture under stirring at
88
80°C, and the resulting mixture was stirred for 3 h. After cooling to 60°C, acetic anhydride
89
(10.5 mL) was added to the mixture, and the reaction solution was stirred at 60°C for 3 days.
90
The product was regenerated by precipitation in methanol (1 L), which was filtered, washed
91
with ethanol, and dried under vacuum at 85˚C to obtain 2-O-acetyl-4,6-O-benzylidene-curdlan
92
in 99% yield.
93
Synthesis of 4,6-di-O-benzylidene-2-O-propionyl-curdlan (CD46BzH2Pr) (Pyridine/PrCl
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method)
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CD46BzH (200 mg) was added to pyridine (8 mL) under stirring at 50°C, and the resulting 5 ACS Paragon Plus Environment
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mixture was stirred for 1 h. Propionyl chloride (0.6 mL) was then added, and the reaction
97
mixture was stirred at 50°C for 1 day. The product was regenerated by precipitation in methanol
98
(200 mL), which was filtered, washed with methanol, and dried under vacuum at 85˚C to give
99
4,6-di-O-benzylidene-2-O-propionyl-curdlan in 95% yield.
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Synthesis of 2-O-acetyl-curdlan (CD2Ac)
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CD2Ac46BzH (300 mg) was suspended in methanol (33.0 mL) under stirring at room
102
temperature for 30 minutes. HCl (0.3 mL) was added to the solution, which then stirred for a
103
further 2 days under heterogeneous conditions. The product was collected by filtration and
104
resuspended in methanol. The filter product was dried under vacuum at 85˚C to obtain 2-O-
105
acetyl-curdlan in 95% yield.
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Synthesis of 2-O-propionyl-curdlan (CD2Pr)
107
HBr/CHCl3 method
108
CD46BzH2Pr (200 mg) was dissolved in chloroform (7 mL) under stirring at room
109
temperature. To the solution was added 30% HBr/acetic acid (0.4 mL), followed by stirring for
110
a further 3 min. The precipitate was collected by filtration and neutralized in NaHCO3 aqueous
111
solution with stirring for 1 day. The product was then wash with water, filtrated, and dried
112
under vacuum at 85˚C to obtain 2-O-propionyl-curdlan in 68% yield.
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Synthesis of 2-O-acetyl-4,6-di-O-propionyl-curdlan (CD2Ac46Pr) and 4,6-di-O-acetyl-2-
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O-propionyl-curdlan (CD46Ac2Pr) 6 ACS Paragon Plus Environment
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The esterification of curdlan derivatives was according to Chien et al (2017)23. CD2Ac46Pr
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was obtained from the propionylation of CD2Ac using propionic anhydride (yield: 97%) while
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CD46Ac2Pr was obtained from CD2Pr by treatment with acetic anhydride (yield: 55%).
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Synthesis of randomly substituted curdlan mixed acetate propionate
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The synthesis of randomly substituted curdlan mixed acetate propionate was according to
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Marubayashi et al. (2016)10. Acetic acid (10 mL) and propionic acid (10 mL) were used to
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obtain curdlan mixed acetate propionate of DSAc=1.8 and DSPr=1.2 while acetic acid (3 mL)
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and propionic acid (17 mL) were applied for curdlan mixed acetate propionate of DSAc=0.8
123
and DSPr=2.2.
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Nuclear magnetic resonance (NMR) spectroscopy
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1H
NMR, 13C NMR, COSY, HSQC and HMBC analyses were conducted on a JNM-A500
126
FT-NMR system (500 MHz, JEOL Ltd., Tokyo, Japan) at 25°C using CDCl3, DMSO-d6 or
127
trifluoroacetic acid-d (TFA-d) as solvents. The DS was calculated using equation (1)
128
DSAcyl = 7IAcyl-metheyl/3IRing proton; DSBzH = 7IBzH-CH/IRing proton
(1)
129
where I is the integral of the corresponding peak.
130
All chemical shifts of curdlan esters were assigned according to reference 23 and listed in
131
Supporting Information 1.
132
Gel permeation chromatography (GPC)
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The molecular weight was measured on a GPC system composed of a CBM-20A
134
communications bus module, DGC-20A3 degasser, LC-6AD liquid chromatograph, SIL-20A 7 ACS Paragon Plus Environment
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HT autosampler, RLD-10A refractive index detector and CTO-20A column oven (Shimadzu
136
Corp., Kyoto, Japan). Chloroform was used as the eluent at a flow rate of 0.8 mL/min under
137
40°C. Polystyrene was used as standards.
138
Infrared spectroscopy (FT-IR)
139
FTIR spectra were recorded with a Nicolet 6700 spectrometer (Thermo Scientific) in the
140
range 500-4000 cm-1 with 64 scans using the KBr technique.
141
Differential scanning calorimetry (DSC)
142
Thermal behaviors were evaluated using a DSC 8500 system (PerkinElmer Japan Co. Ltd)
143
under nitrogen gas. Samples (powder) were heated from 0°C to 300°C (1st run), cooled to 0°,
144
and then held at 0°C for 5 min before reheated to 300°C (2nd run). The heating and cooling rate
145
were fixed at 100°C /min. The melting temperature (Tm) was determined from the 1st scan of
146
DSC thermograms, while the glass transition temperature (Tg) and crystallization temperature
147
(Tc) were determined from the 2nd scan.
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Film preparation
149
Cast films were prepared by dissolving sample (300 mg) in solvent (15 mL) and pouring into
150
a Naflon® petri dish. Dichloromethane and chloroform were used as solvents for CDTAc and
151
other curdlan esters, respectively. Thermally annealing (TA) was conducted by heating solvent
152
cast films at their Tc for 24 h. For those sample with no Tc observed in DSC measurement,
153
thermal annealing was conducted under the temperature in the mean temperature of their Tg 8 ACS Paragon Plus Environment
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and Tm (CD6Ac24Pr: 175°C; CDTAc: 223°C)
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Wide angle X-ray diffraction (WAXD)
156
WAXD measurements were conducted using a RINT-2200 instrument (Rigaku Corp.) at 40
157
kV and 40mV with Cu Kα (λ=0.15418 nm) radiation in the symmetrical reflection mode,
158
equipped with a graphite monochromator (2θ=26.58˚ for Cu Kα) at the scattered X-ray position.
159
The scanned 2θ range was set to 2-40˚ with a step size of 0.1˚, where θ is the Bragg angle.
160 161
Results and discussion
162
Synthesis of CD2Ac46Pr and CD46Ac2Pr
163
Acetalization and acetylation of curdlan
164
The curdlan C4 and C6 hydroxyl groups were protected by acetalization in a homogeneous
165
curdlan/benzaldehyde/DMSO solution using pyridine as base, CSA as catalyst and TMOF as
166
dehydrating agent at 25°C31-33. Unexpected gelation occurred when TMOF was added. This
167
might be attributed to aggregation of the triple helix of 4,6-acetalized curdlan that transformed
168
from random coil of curdlan chain through formation of an apolar exterior benzylidene
169
structure at the acetalized C4 and C6 positions in DMSO31,34. After three days of acetalization,
170
a softened transparent gel was obtained. The peracetylated product (Table 1, entry 1) could
171
only be dissolved in TFA, and the DSBzH was calculated to be 0.19 from the benzaldehyde peak
172
observed in the 1H NMR spectrum (Table 1). However, the DS value obtained was considered 9 ACS Paragon Plus Environment
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inaccurate owing to the overlap of ring protons and undesirable hydroxyl groups regenerated
174
due to deacetalization in TFA solvent.
175
To increase the reaction rate, reaction temperature was increased. The reaction solution
176
remained a mixture of a white suspension and a piece of white solid at 50°C, and became a
177
pure white suspension at 60 and 70°C. For the reaction at 60°C, the peracetylated product was
178
found to have improved solubility in DMSO (entry 3). The disappeared aldehyde peak of
179
benzaldehyde at 10.0 ppm and observed acetal-CH at 5.5 ppm in the 1H NMR spectrum
180
confirmed benzylidene formation as a cyclic acetal with the C4 and C6 hydroxyl groups (Figure
181
1a)35. A DSBzH value of 0.96, very close to the target of DSBzH=1, was calculated. However,
182
overlap of the ring proton and DMSO-water peaks resulted in a small error in the DS calculation.
183
In the 13C NMR spectrum, only one acetate carbonyl peak was observed, suggesting that C4
184
and C6 positions were mostly protected in the product (Figure 1d). Therefore, the product was
185
considered to be CD2Ac46BzH. Unfortunately, no correlation between carbonyl carbon of
186
ester group and the specific ring proton was observed in the 2D NMR spectra (COSY, HSQC
187
and HMBC) to directly confirm the formation of benzylidene in this product even over 20 hours
188
of scan (Supporting Information 2), which could be due to the high molecular weight of curdlan
189
and long distance between these two atoms.
190 191
Meanwhile, the acetylated product of the reaction at 70°C (entry 4) resulted in a high DSBzH value of 1.38, and relatively small acetyl peaks in both the 1H and
13C
NMR spectra. This 10
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probably resulted from partial acetalization of the C2 hydroxyl groups to form less-stable
193
hemiacetals. However, using TFA-d as NMR solvent removed all benzylidene groups from
194
curdlan, making it difficult to examine the presence of hemiacetals.
195 196
Therefore, 60°C was considered the optimal reaction temperature for regioselective protection of curdlan C4 and C6 hydroxyl groups using benzaldehyde, to form CD46BzH.
197 198
Table 1. Results of acetalization at different reaction temperatures.
Entry
Reaction temp. (°C)
DSBzH
DSAc
1 2 3 4
25 50 60 70
0.19 0.88 0.96 1.38
2.14 0.75 1.85 0.49
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200 201
Figure 1. 1H spectra of (a) CD2Ac46BzH, (b) CD2Ac46Pr and (c) CD46Ac2Pr, and 13C NMR
202
spectra (d) CD2Ac46BzH, (e) CD2Ac46Pr and (f) CD46Ac2Pr.
203 204
Deprotection and 4,6-propionylation of CD2Ac46BzH
205
Cleavage of 4,6-O-benzylidene-glucoside is known to give different products under different
206
reaction conditions38. Treatment under mildly acidic conditions regenerates the diol, while
207
regioselective reductive ring-opening results in benzyl-glucoside with a mono-hydroxyl group.
208
With the aim to regenerate both the C4 and C6 hydroxyl groups, acid treatment was applied to
209
the deprotection of CD2Ac46BzH, using HCl as acid in methanol, followed by
210
perpropionylation of the regenerated hydroxyl groups24. The propionylated product was
211
soluble in chloroform. 1H NMR spectra showed that the benzylidene peaks had almost 12 ACS Paragon Plus Environment
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disappeared after 2-day deprotection with only trace of benzylidene peaks, indicating that most
213
benzylidene group had been removed. In addition, only the C2 acetyl peak was observed at
214
around 2 ppm, with the DSAc and DSPr calculated to be 0.84 and 2.07, respectively (Figure 1b).
215
Furthermore, the three carbonyl peaks observed in the 13C NMR spectrum were assigned to C2
216
acetate and C4 and C6 propionate, confirming this product as CD2Ac46Pr (Figure 1e).
217
Unfortunately, no correlation was observed in the HMBC spectrum for the specific position
218
determination. The molecular weight of CD2Ac46Pr was determined to be 3.1x105 by GPC
219
analysis.
220
In summary, a high degree of protection of C4 and C6 hydroxyl groups in curdlan was
221
achieved through acetalization with benzaldehyde, which can be removed by treatment with
222
HCl in methanol, to give regioselectively substituted CD2Ac46Pr after propionylation
223
(Scheme 1).
224 225
Scheme 1. Synthetic route of regioselectively substituted curdlan acetate propionate with C2 13 ACS Paragon Plus Environment
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position selectivity.
227 228
Propionylation of CD46BzH
229
To synthesize CD46Ac2Pr, the propionylation of CD46BzH was attempted using the same
230
method, but in DMF solvent using propionic anhydride as the acyl reagent (DMF/Pr2O method;
231
Supporting Information 3-1). However, the obtained product showed significant absorption for
232
the hydroxyl stretch at 3600 cm-1 in the FT-IR spectrum, suggesting an incomplete
233
propionylation at the C2 position (Supporting Information 4). The same result was also
234
obtained when changed the solvent to pyridine (Pyridine/Pr2O method; Supporting Information
235
3-2). The low propionylation efficiency observed in these two methods could be attributed to
236
the helical structure of CD46BzH, which allowed only the acid reagents small enough to enter
237
the core of the triple helix to react with the hidden C2 hydroxyl group of CD46zH. The size of
238
propionic anhydride (6.25 Å) used in these two methods, was probably larger than the diameter
239
of the helix core, leading to the low reactivity.
240
Therefore, propionyl chloride, a reactive derivative of propionic acid with smaller molecular
241
diameter (4.44 Å)39, was applied to the propionylation of CD46BzH in pyridine solvent. The
242
obtained product was found to be soluble in chloroform, and the 1H NMR spectrum showed
243
both propionyl and benzylidene peaks (Supporting Information 5). However, the DSPr value
244
was unclear owing to the overlap of ring protons and the benzylidene acetal proton. The 14 ACS Paragon Plus Environment
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complete propionylation was deduced from the absence of the hydroxyl absorption band in the
246
FT-IR spectrum. Therefore, acyl chloride was considered an appropriate acid derivative for the
247
propionylation of CD46BzH.
248 249
Deprotection and acetylation of CD46BzH2Pr
250
The deprotection of CD46BzH2Pr by HCl in methanol resulted in the dissolution of
251
CD46BzH2Pr during the reaction, and product that insoluble in most of the organic solvents
252
(Supporting Information 6). Therefore, HBr and chloroform were also applied to the
253
deprotection of CD46BzH2Pr. It’s peracetylated product was soluble only in TFA. In the 1H
254
NMR spectrum, although traces of benzaldehyde were observed (very obvious because it
255
deprotected into monomer due to the TFA NMR solvent but DSBzH < 0.1), the DSAc and DSPr
256
values were calculated to be 1.9 and 1.1, respectively (Figure 1c). Carbonyl peaks observed in
257
13C NMR spectrum were attributed to C4 and C6 acetates and C2 propionate, by the assignment
258
of CDTAc and CDTPr in TFA-d solvent, confirming the product to be CD46Ac2Pr (Figure 1d;
259
Supporting Information 7). Unfortunately, no correlation was observed in the HMBC spectrum
260
for the specific position determination.
261 262
Starting from CD46BzH, modified propionylation, deprotection, and acetylation reactions successfully afforded CD46Ac2Pr with a regioselectively substituted structure.
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Characterization of CD2Ac46Pr and CD2Pr46Ac
265
The thermal and crystalline properties of as-obtained CD2Ac46Pr and CD46Ac2Pr were
266
determined by DSC and WAXD, and compared with those of three other regioselectively
267
substituted curdlan acetate propionates (CD26Ac4Pr, CD24Ac6Pr and CD6Ac24Pr), two
268
randomly substituted curdlan mixed acetate propionates (Ac/Pr~1:2 and 2:1), and two curdlan
269
tri-esters (CDTAc and CDTPr), prepared in previous reports10,23,24, to elucidate structure-
270
property relationships in curdlan esters (Table 2; thermal stability information in Supporting
271
Information 8 ).
272 273 274 275 276 277
Table 2. Characterization of curdlan esters. DS
Selectivity
Tg
Tc
Tm
DSAc
DSPr
C2
C4
C6
(°C)
(°C)
(°C)
Preference of Tm
CDTAc CD26Ac4Pr CD24Ac6Pr CD2Ac46Pr CD6Ac24Pr CD46Ac2Pr
1.8 2.0 0.8 1.0 1.9
3.0 1.2 1.0 2.1 2.0 1.1
Ac Ac Ac Ac Pr Pr
Ac Pr Ac Pr Pr Ac
Ac Ac Pr Pr Ac Ac
168 141 147 134 123 136
177 195 171 -
278 287 287 279 227 219
Ac Ac Ac Ac Pr Pr
CDTPr
3.0
-
Pr
Pr
Pr
116
171
Tm-1: 152 Tm-2: 228
Pr 16
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278 279
Glass transition behaviors
280
The thermal transition behaviors of curdlan esters were analyzed from the DSC 2nd run
281
thermograms (Supporting Information 9). The Tg values of CD2Ac46Pr and CD46Ac2Pr were
282
134°C and 136°C, respectively, which were between the Tg values of CDTAc and CDTPr. By
283
comparison with other regioselectively substituted curdlan esters, a linear decrease was
284
observed with increasing DSPr (Figure 2a), which suggested that the Tg of curdlan acetate
285
propionates was proportional to the Ac/Pr ratio, the same tendency observed for randomly
286
substituted curdlan mixed esters, while substitution position did not influence the Tg of curdlan
287
esters.
288 289
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290 291
Figure 2. (a) Tg plotted against DSPr and (b) Tm plotted against DSPr of curdlan esters. ●:
292
Regioselectivly substituted curdlan acetate propionates; △:Randomly substituted curdlan
293
acetate propionates.
294 295
Effect of C2 substituent on the melting behavior of curdlan acetate propionates
296
The Tm of curdlan esters was determined from the DSC 1st run thermograms (Figure 3).
297
CDTAc was reported to have Tm at 287°C while two Tms have been observed in CDTPr at
298
158°C (Tm-1) and 225°C (Tm-2), which related to two distinct crystal conformations of
299
CDTPr10,40.
300
In randomly substituted curdlan mixed esters, Tm decreased linearly with the increasing 18 ACS Paragon Plus Environment
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DSPr and was dependent on the Ac/Pr ratio, similar to the tendency of Tg (Figure 2b).
302
Regioselectively substituted curdlan acetate propionates CD24Ac6Pr (Ac/Pr~2:1) and
303
CD6Ac24Pr (Ac/Pr~1:2) have been reported having Tm of 287°C and 227°C, respectively,
304
which are similar to the Tm of corresponding curdlan triesters having the same esters with their
305
secondary C2 and C4 substituents. Therefore, the Tm of curdlan acetate propionates was
306
considered to be highly related to both secondary substituents, or either of them, which are also
307
the major substituents (DS=2)24. In this study, the Tm of CD2Ac46Pr (Ac/Pr~1:2) and
308
CD46Ac2Pr (Ac/Pr~2:1) were 279°C and 219°C, respectively. The Tm of CD2Ac46Pr was the
309
same as that of CDTAc (279°C), while the Tm of CD46Ac2Pr was close to the Tm-2 of CDTPr
310
(229°C), which suggested that the melting behaviors of CD2Ac46Pr (Ac/Pr~1:2) and
311
CD46Ac2Pr (Ac/Pr~2:1) were influenced by the C2 substituent, despite the low DS of 1, rather
312
than the C4 and C6 substituents. Therefore, the melting behaviors of curdlan esters in these
313
two samples were apparently completely independent of the ester ratios, and were controlled
314
by a specific substituent, namely, the C2 substituent.
315
Comparing five regioselectively substituted curdlan acetate propionates, the Tm were
316
separated into two temperature zones close to the Tm of CDTAc and CDTPr, respectively.
317
Curdlan esters with Tm close to 280°C were classified into the CDTAc-related group, as
318
follows: CD26Ac4Pr (Tm=287°C; Ac/Pr~2:1), CD24Ac6Pr (Tm=287°C; Ac/Pr~2:1) and
319
CD2Ac46Pr (Tm=279°C; Ac/Pr~1:2). Meanwhile, those with Tm close to 228°C were classified 19 ACS Paragon Plus Environment
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320
into the CDTPr-related group, as follows: CD6Ac24Pr (Tm=227°C; Ac/Pr~1:2) and
321
CD46Ac2Pr (Tm=219°C; Ac/Pr~2:1). In the CDTAc-related group, not only CD26Ac4Pr and
322
CD24Ac6Pr with DSAc=2 exhibited identical melting behaviors to CDTAc, CD2Ac46Pr with
323
a DSAc value of only 1, also had the same Tm as CDTAc. This indicated that the melting
324
behaviors of curdlan acetate propionates in the CDTAc-related group were independent of the
325
Ac/Pr ratio and related to the character of their common substituent-C2 acetate. For the CDTPr-
326
related group, the Tm of CD6Ac24Pr has been reported to be close to that of CDTPr under the
327
influence of the major components, namely, secondary C2 and C4 propionates with DSPr=224.
328
The Tm of CD46Ac2Pr (Ac/Pr~2:1) was also close to the Tm-2 of CDTPr, despite having
329
propionyl groups as minor components, with DSPr=1. The Tm values of the curdlan acetate
330
propionates in the CDTPr-related group also indicated that their melting behaviors were
331
controlled by the C2 propionyl substituent.
332
The Tm of randomly substituted curdlan mixed acetate propionate with Ac/Pr ratio of 2:1
333
was 262°C. However, the Tm values of three regioselectively substituted curdlan acetate
334
propionates with the same Ac/Pr ratio (2:1) were found to be around 280°C (for CD24Ac6Pr
335
and CD26Ac4Pr, related to CDTAc), and 219°C (for CD46Ac2Pr, related to CDTPr). These
336
separated Tm observed for curdlan acetate propionates with the same Ac/Pr ratio indicated that
337
the effect of the C2 substituent on the melting behaviors completely outweighs that of the
338
substitution ratio in curdlan acetate propionates. Similarly, among regioselectively substituted 20 ACS Paragon Plus Environment
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curdlan acetate propionates with similar Ac/Pr ratios of 1:2, CD2Ac46Pr and CD6Ac24Pr
340
exhibited Tm value of 279°C and 227°C, respectively. These Tm values were clearly different
341
from that of the randomly substituted curdlan mixed acetate propionate with the same Ac/Pr
342
ratio of 1:2 (243°C). This suggested that the melting behaviors of regioselectively substituted
343
curdlan acetate propionates was complete independent of the substitution ratio.
344
From the results above, it was concluded that the Tm values of regioselectively substituted
345
curdlan acetate propionates were the same as those of corresponding curdlan triesters with
346
identical C2 ester substitution. This observation suggested that the C2 ester was the decisive
347
substituent among the two secondary substituents in determining the melting behaviors of
348
curdlan acetate propionates. Furthermore, the melting behavior of the curdlan acetate
349
propionates was completely independent of the substitution ratio, with C2 ester found to have
350
a significant effect even at a low DS value of 1.
351
Meanwhile, larger melting enthalpies were observed in CD26Ac4Pr (ΔHm:45 J/g) and
352
CD6Ac24Pr (ΔHm:25 J/g) compared to those of curdlan triesters with the same C2 ester
353
substitution (ΔHm of CDTAc:25 J/g; ΔHm of CDTPr:13 J/g). It could be the identical C4 and C6
354
side chain length (carbon number of 3) in these two curdlan esters that resulted in highly regular
355
molecular structure, which improved the crystallinity of these two curdlan esters.
356
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357 358
Figure 3. DSC thermograms (1st run) of curdlan esters.
359 360
Effect of C2 substituent on the crystal structure of curdlan acetate propionates
361
Curdlan triesters have been reported to have distinct crystal conformations under different
362
thermal conditions. Only one kind of crystal conformation has been observed in CDTAc, while
363
two distinct crystal conformations have been reported in CDTPr, the Form Ⅰ conformation
364
was formed by solvent annealing at room temperature, which transformed into Form Ⅱ
365
conformation after thermal annealing treatment ≥ 160°C44. In our previous study, the
366
correlation between crystal structure and secondary substituents was found to be similar to that
367
of the melting behavior through characterization of CD24Ac6Pr and CD6Ac24Pr. Therefore,
368
the crystal structures of CD2Ac46Pr and CD46Ac2Pr were also likely to provide information
369
of the structure-property relationships of curdlan esters. 22 ACS Paragon Plus Environment
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370
A solvent cast film of CD2Ac46Pr, obtained as a transparent yellow film, was prepared for
371
the crystal structure analysis (Figure 4a). The crystal structures of the curdlan esters were
372
examined by WAXD. The cast film of CD2Ac46Pr (Ac/Pr~1:2) showed a similar diffraction
373
pattern to that of CDTAc. This supported that the crystal structure of CD2Ac46Pr (Ac/Pr~1:2)
374
was predominately related to C2 acetate (Figure 4b). In contrast, the crystal structure of
375
CD46Ac2Pr remained unclear because cast film preparation was unsuccessful owing to its low
376
solubility.
377
Comparison of the X-ray diffraction patterns of curdlan esters, showed that the obtained
378
curdlan esters exhibited only two types of diffraction patterns, namely, CDTAc-related and
379
CDTPr-related diffraction patterns, the same as the types observed in Tm analysis. CD26Ac4Pr
380
(Ac/Pr~2:1), CD24Ac6Pr (Ac/Pr~2:1) and CD2Ac46Pr (Ac/Pr~1:2) showed similar diffraction
381
patterns to that of CDTAc. In contrast, both of the diffraction pattern of CD6Ac24Pr
382
(Ac/Pr~1:2) before and after thermal annealing were similar to that of CDTPr (Form Ⅱ). The
383
distinct crystal structure observed in these two groups indicated that formation of the crystal
384
structures of curdlan acetate propionates was also predominantly influenced by their C2
385
substituent.
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386 387
Figure 4. (a) The image of CD2Ac46Pr used for X-ray diffraction analysis and (b) X-ray
388
profiles of curdlan esters.
389 390
Structure-property discussion
391
Property analyses showed that both the melting behavior and crystal structure of the curdlan
392
acetate propionates were significantly dependent on only the C2 substituent. This specific C2
393
effect might be attributed to the location of the C2 position in the molecular chain. Curdlan and
394
curdlan esters are reported to have helix conformation (triple helix in curdlan and single helix
395
in CDTAc and CDTPr)40-44. In the helix structure, the C2 position is separated from the C4 and
396
C6 positions by curdlan main chain, and is located at the inner core along the helix, which
397
makes C2 position the determining position of helix formation in the glucose units of curdlan,
398
and accordingly, CDTAc and CDTPr. Therefore, helix formation of curdlan ester is directed 24 ACS Paragon Plus Environment
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399
by the substituent at C2 position, resulting in a specific crystal structure in each curdlan ester,
400
that melts at a specific temperature as a feature of helix unwinding.
401
The crystal structures of regioselectively substituted cellulose acetate propionates have been
402
reported to be affected by secondary substituents, but showed a linear decreasing Tm with the
403
decreasing Ac/Pr ratio15. In comparison, the substituent was found to have a much more
404
significant effect on the melting behavior in curdlan esters. This might be attributed to the helix
405
conformation of the curdlan chain. Unlike linear cellulose esters, CDTAc and curdlan CDTPr
406
have been reported to form 6/1 or 5/1 helices, which completely wrap the C2 ester inside the
407
helix40,44, which enhanced the impact of the helical core-facing C2 position on its properties.
408
The significant influence of the helical core-facing substituent found in curdlan esters
409
suggested that property-determining positions might be identified in other polysaccharides with
410
helix conformation.
411 412
Conclusions
413
A new protecting strategy for curdlan was developed using benzaldehyde through cyclic
414
acetalization, resulting in the protection of the C4 and C6 positions while remaining a free C2
415
hydroxyl group. We succeeded in preparing two C2-regioselectively substituted curdlan esters,
416
CD2Ac46Pr and CD46Pr2Ac, from CD46BzH. Furthermore, the thermal properties and crystal
417
structures analysis showed that the C2 ester group had a decisive effect on the crystal structure 25 ACS Paragon Plus Environment
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Page 26 of 34
418
and melting behavior of curdlan acetate propionates. These help molecular structure design to
419
achieve free and fine control of the properties of curdlan esters as polymeric materials.
420 421
Supporting Information
422
Synthetic methods, 1H, 13C, two-dimensional NMR and IR spectra of the supplemental curdlan
423
derivatives.
424 425
Acknowledgement
426
This study was supported by JST-ALCA Grand Number JPMJAL1502, Japan.
427 428
Reference
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Synopsis. This research developed a deeper understanding in the structure-property
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relationships in curdlan esters for bio-based material applications.
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