Synthesis of C2-regioselectively substituted curdlan acetate

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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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ACS Sustainable Chemistry & Engineering

1

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-

9

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

18

using benzaldehyde to protect the curdlan C4 and C6 hydroxyl groups as a cyclic acetal

19

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.

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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.

32

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

37

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-

50

acetyl-6-O-propionyl-curdlan

51

(CD6Ac24Pr), were successfully obtained23,24. Property analysis of these regioselectvely

52

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.

54

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

57

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

60

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.

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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

69

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).

78

CD26Ac4Pr, CD24Ac6Pr and CD6Ac24Pr were synthesized according to Chien et al. (2017)

79

and Chien et al. (2018) with some modifications23,24.

80

Synthesis of 4,6-O-benzylidene-curdlan (CD46BzH)

81

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).

86

Synthesis of 2-O-acetyl-4,6-O-benzylidene-curdlan (CD2Ac46BzH)

87

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

94

method)

95

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.

100

Synthesis of 2-O-acetyl-curdlan (CD2Ac)

101

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.

106

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.

113

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

116

was obtained from the propionylation of CD2Ac using propionic anhydride (yield: 97%) while

117

CD46Ac2Pr was obtained from CD2Pr by treatment with acetic anhydride (yield: 55%).

118

Synthesis of randomly substituted curdlan mixed acetate propionate

119

The synthesis of randomly substituted curdlan mixed acetate propionate was according to

120

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)

122

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

125

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.

148

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)

155

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.

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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

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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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192

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

199

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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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212

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


226

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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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245

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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301

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


Page 20 of 34

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

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


339

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



Page 22 of 34

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

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.



Page 24 of 34

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


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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551



552

Page 34 of 34

Table of Contents

553 554 555

Synopsis. This research developed a deeper understanding in the structure-property

556

relationships in curdlan esters for bio-based material applications.


Synthesis of C2-regioselectively substituted curdlan acetate

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