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Food and Beverage Chemistry/Biochemistry
Phenylboronic Acid Functionalized Adsorbents for Selective and Reversible Adsorption of Lactulose from Syrup Mixtures Mingming Wang, Fayin Ye, He Wang, Habtamu Admassu, Yinghui Feng, Xiao Hua, and Ruijin Yang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b02152 • Publication Date (Web): 15 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018
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Journal of Agricultural and Food Chemistry
Phenylboronic Acid Functionalized Adsorbents for Selective and Reversible Adsorption of Lactulose from Syrup Mixtures
Mingming Wang1, 2, Fayin Ye3, He Wang4, Habtamu Admassu1, 2, Yinghui Feng1,2, Xiao Hua1, 2*, Ruijin Yang1, 2*
1. State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, China 2. School of Food Science and Technology, Jiangnan University, 214122 Wuxi, China 3. College of Food Science, Southwest University, 400715 Chongqing, China 4. Jiyang College, Zhejiang Agriculture and Forestry University, Zhuji, Zhejiang 311800, China
*Correspond to: Dr. Xiao Hua and Dr. Ruijin Yang Xiao Hua:
[email protected] (X. Hua) Ruijin Yang:
[email protected] (R. Yang)
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ABSTRACT: :Boronate affinity materials have been widely used for enrichment of
2
cis-diol molecules. In this work, phenylboronic acid functionalized adsorbents were
3
prepared via a simple and efficient procedure by grafting phenylboronic acid groups
4
onto amino macroporous resins. Elemental analysis has confirmed the successful
5
functionalization of AR-1M and AR-2M with approximately 2.17% and 0.73% weight
6
percentage of boron. Comparatively, AR-1M possessed higher lactulose adsorption
7
capacity ( , 84.78±0.95 mg/g dry resin) under neutral condition (pH=7), while
8
the introduced glutaraldehyde spacer arms on AR-2M resulted in excellent adsorption
9
selectivity (α~23), high adsorption efficiency (π~22%), and fast adsorption/desorption
10
rate. The purity of lactulose ( ) through pH-driven adsorption (pH 7-8) and
11
desorption (pH 1.5) can be effectively improved depending on the ratio of lactulose to
12
lactose. When lactulose: lactose ≥1:1, ~95% was achieved. No significant drop
13
in (>90%) was observed after ten-consecutive repeats. Results demonstrated
14
that the newly developed method may achieve satisfactory performance in lactulose
15
purification.
16 17
KEYWORDS: Lactulose, phenylboronic acid functionalized adsorbent, adsorption
18
selectivity, pH-responsive, recyclability
19 20 21 22 23 24 25 2
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INTRODUCTION
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As a valuable lactose-originated non-digestive disaccharide, lactulose has been
28
commonly used in pharmaceuticals, nutraceuticals and food industries due to its
29
well-documented prebiotic properties.1-3 Currently, lactulose can be produced by
30
chemical-based isomerization of lactose (industrial scale) or through enzymatic-based
31
synthesis (lab-scale) using β-galactosidase, Caldicellulosiruptor saccharolyticus
32
cellobiose 2-epimerase (CsCE), etc.4-8 However, undesirable by-products of
33
monosaccharides and residual lactose remained in the isomerization syrup mixture
34
will most often lead to rising the costs associated with downstream lactulose
35
purification, since high-purity lactulose is mandatory for its medical uses and food
36
applications, especially for those who are lactose intolerance.1, 4, 9
37
Purification processing is a key operation for high-purity lactulose production.
38
Although monosaccharide can be efficiently removed from raw lactulose syrup by
39
using membrane separation, the fractionation of lactulose and lactose from
40
isomerization mixture is a challenging task due to the complexity of the syrup and the
41
highly structural similarity among them. Recently, several separation technologies,
42
such as methanolic crystallisation procedure10,
43
(SC-CO2) with alcohol-type cosolvent technique12, pressurized liquid extraction
44
(PLE)13 and room temperature ionic liquids (ILs)14, 15 approach have been applied to
45
obtain high-purity lactulose. However, those methods, efficient as they are, may not
46
appropriate for industrial process due to their sophisticated and tedious procedures,
47
considerable energy and organic solvent consumption. Very recently, a new innovative
48
strategy for lactulose and oligosaccharide purification has been developed through
49
selective fermentation of monosaccharides and lactose with Saccharomyces cerevisiae
50
and Kluyveromyces marxianus.9, 16 However, the final product was still a mixture of
11
, supercritical carbon dioxide
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lactulose and oligosaccharides.
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Boronate affinity based technique has been extensively used in the selective capture
53
and enrichment of cis-diol molecules (carbohydrates, nucleosides, glycoconjugates
54
and etc) from complex samples.17-23 The high selectivity inherits from the unique
55
chemical properties of boronic acids, which can reversibly form complexation with
56
cis-1,2- or cis-1,3-diols to generate 5- (stable) or 6-membered (less stable) cyclic
57
esters.17, 24 Furthermore, lactulose with typical cis-1,2-diol units shows higher binding
58
affinity than lactose with boronic acid groups (Supplementary Figure 1), which would
59
facilitate the separation of lactulose and lactose.25 To promote solid phase separation
60
and enhance the reusability of boronic acid groups, it generally requires immobilizing
61
this functional group on insoluble solid supports.26 Macroporous adsorption resins
62
(MARs) with high specific surface areas, large pore volumes (adsorption capacity),
63
regular and tunable pore sizes, and low cost, have been studied extensively and used
64
widely as excellent supports for industrial adsorption.26-28 Moreover, MARs generally
65
possess stable and interconnected frameworks with active pore surfaces, which make
66
them possible for further modification or functionalization with functional groups.26
67
The grafting of functional boronic acids onto MARs may somewhat favors the
68
selective capture of target lactulose from isomerization mixtures.
69
In the present work, a simple and effective modification procedure for the synthesis of
70
phenylboronic acid functionalized adsorbents (AR-1M and AR-2M) was established
71
by grafting phenylboronic acid groups onto amino macroporous resins (AR-0).
72
Physical characterization and adsorption/desorption properties (adsorption/desorption
73
kinetics and adsorption selectivity) of phenylboronic acid functionalized adsorbents
74
towards lactulose from isomerisates were investigated. The feasibility of adsorbent
75
recyclability and stability in ten consecutive capture/release cycles was also explored. 4
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MATERIALS AND METHODS
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Materials. Aminated poly (styrene-co-divinylbenzene) resin (AR-0, ~6.4 mmol/g dry
78
resin of -NH2 functional groups) was kindly donated by Tianjin Nankai Hecheng S&T
79
Co., Ltd. (Tianjin, China). 3-Aminophenylboronic acid (APBA, purity 98%),
80
p-formylphenylboronic acid (p-FPBA, purity 95%), sodium cyanoborohydride
81
(NaBH3CN, purity 95%), N,N-dimethylformamide (DMF, purity 99.8%) and HPLC
82
grade (purity>98%) of lactulose, lactose and epilactose were purchased from
83
Sigma-Aldrich (Shanghai, China). All other chemicals were of analytical grade and
84
obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).
85
Preparation of Phenylboronic Acid Functionalized Adsorbents. Two synthetic
86
protocols were used to functionalize poly(styrene-co-divinylbenzene) resins with
87
phenylboronic acid groups as depicted in Fig.1. Before functionalization, AR-0 was
88
first swelled thoroughly with DMF for 24 h. Then for route one, p-FPBA was
89
covalently bound to AR-0 via the formation of Schiff’s base between the formyl
90
groups of p-FPBA and the amino groups on the surface of AR-0.22, 23, 26 As to route
91
two, AR-0 was activated first by grafting of glutaraldehyde, and then the activated
92
AR-0 was chemically modified with APBA. Finally, sodium cyanoborohydride power
93
was added portionwise to reduce the Schiff’s base and the phenyboronic acid
94
functionalized adsorbents AR-1M (route 1) and AR-2M (route 2) were obtained after
95
washed with DMF, ethanol and deionized water and air-dried.
96
Characterization of Phenylboronic Acid Functionalized Adsorbents. Fourier
97
transform infrared spectroscopy (FT-IR) measurements of AR-0, AR-1M and AR-2M
98
were carried out on a Thermo Nicolet IS10 spectrometer (Nicolet, USA). X-ray
99
photoelectron spectroscopy (XPS) measurements for elemental analysis (C, N and B)
100
were performed using an AXIS Ultra DLD spectrometer (Shimadzu/Kratos, Japan). 5
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The surface morphology of AR-0, AR-1M and AR-2M was observed with scanning
102
electron microscopy (SEM, 5.0 kV, SU1510; Hitachi Co., Japan).
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Selective Adsorption of Lactulose and Lactose. The densities of phenylboronic acid
104
groups (µmol/g dry resin) on AR-1M and AR-2M was determined by calculating the
105
content of p-FPBA (λmax 256 nm) and APBA (λmax 293 nm) on a UV-3600
106
spectrometer (Techcomp, Shanghai, China) before and after the formation of boronic
107
ester27, 28, respectively. Unless otherwise stated, the adsorption studies were conducted
108
as follows: 1.0 g (dry mass) of the phenyboronic acid functionalized adsorbents was
109
added to 25 mL of lactulose and lactose binary solution (Lu:La=1:1, ×5 mg/mL) in a
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100 mL Erlenmeyer flask. The initial pH values of the syrup ranged from 6.0 to 10.0
111
were adjusted by 0.1 M HCl/NaOH solution. The mixture was then shaken (150 r/min)
112
at 10-45oC for 12 h and samples were withdrawn at varying predetermined times
113
intervals for further analysis.
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The equilibrium adsorption capacity in adsorption experiment, (mg/ g dry resin),
115
was calculated according to Eq. (1): = − ×
1
116
where and are the initial and equilibrium substrate concentrations (mg/mL),
117
respectively. refers the volume of syrup solution (mL), and is the dry mass (mg)
118
of the adsorbents.
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The adsorption capacity at contact time (t) in adsorption experiment, (mg/ g dry
120
resin), was calculated as: = − ×
2
121
where represents the substrate concentrations (mg/mL) at contact time (min/h).
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Adsorption selectivity (α) and adsorption efficiency (π, %) of adsorbents towards 6
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lactulose from Lu-La binary solution were quantitatively defined as Eqs. (3) and (4): α=
⁄ 3 ⁄
π % = 100 ×
1000 × 4 × 342.30
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where and (mg/g dry resin) denote respectively the equilibrium adsorption
125
capacities of lactulose and lactose. and are the equilibrium concentrations
126
(mg/mL) of lactulose and lactose, respectively. represents the content of
127
phenylboronic acid groups (µmol/g dry resin) on adsorbents.
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pH-driven Desorption of Lactulose and Lactose from Adsorbents. After the
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adsorption equilibrium was reached, pH-driven desorption of lactulose and lactose
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from adsorbents was carried out as follows: the adsorbate-loaded AR-1M/AR-2M was
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filtered and then desorbed with 25 mL pre-treated deionized water (pH 1-5, adjusted
132
by 1M HCl/NaOH) in 100 mL Erlenmeyer flask. Subsequently, the flask was
133
incubated at 25oC on a rotary shaker (150 r/min) for 2 h. Samples were withdrawn at
134
certain time intervals for further HPLC analysis.
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The desorption capacity of lactulose and lactose ( , mg/g dry resin) and desorption
136
ratio (D, %) were defined to assess the desorption efficiency: = ×
5
% = ×
× 100% 6
−
137
where is the concentrations (mg/mL) of lactulose and lactose in the desorption
138
solution. is the volume (mL) of the desorption solution.
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Selective Capture of Sugars by Phenylboronic Acid Functionalized Adsorbents
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from Syrup Mixtures. AR-1M and AR-2M were tested for selective adsorption of
141
lactulose, lactose, fructose, glucose and epilactose from four syrup solutions: (1) 7
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lactulose-lactose binary mixture (Lu:La=1:5, 1:2.5, 1:1, 2:1, 3:1, 4:1 and 5:1, ×5
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mg/mL); (2) fructose-glucose binary mixture (Fru:Glu=1:1, ×5 mg/mL); (3)
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fructose-glucose-lactulose-lactose mixture (Fru:Glu:Lu:La=1:1:1:1, ×5 mg/mL); and
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(4) enzymatic reaction mixture (Lu:Epi:La=6.35:1.58:4.0, mg/mL) by using CsCE.6, 25
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The pH values of these four different syrup solutions were adjusted to 8.0 by adding
147
0.1 M NaOH/HCl. Adsorption and desorption experiments were performed in an
148
identical manner as described above. The aliquots were sampled at different time
149
intervals and sugar concentrations were determined with HPLC analysis.
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Initial purity ( " , %) and desorption purity (" , %) were defined as Eqs. (7) and (8)
151
in order to quantitatively evaluate the adsorption and desorption selectivity of
152
AR-1M/AR-2M towards different sugars from syrup solutions. " % =
" × × 100% 7
" % =
" × × 100% 8
153
" and " refer the initial and desorption concentration (mg/mL) of certain sugar,
154
respectively. and are, respectively, the total amount (mg) of sugars in the
155
initial and desorption solutions.
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Recyclability of Phenylboronic Acid Functionalized Adsorbents towards
157
Lactulose. The reusability of AR-1M/AR-2M in sugar capture was investigated in
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ten-consecutive adsorption/desorption cycles. Briefly, 1.0 g of AR-1M/AR-2M (dry
159
mass) was added to 25 mL of Lu-La binary solution (Lu:La=1:1, ×5 mg/mL, pH 7.0
160
for AR-1M and pH 8.0 for AR-2M) in a 100 mL Erlenmeyer flask. The mixture was
161
shaken (150 r/min) for 2 h at 25oC, and then the adsorbents were filtered. The
162
supernatant was collected for further HPLC analysis. Next, desorption process was
163
performed as described above (pH 1.5, 25oC, for 2 h). The separated adsorbent was 8
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washed three times by deionized water. After air-dried overnight, the adsorbent was
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used for sugar adsorption once again. The above adsorption/desorption cycles were
166
repeated for ten consecutively times.
167
HPLC Method for Sugar Analysis. Quantification of sugars concentration was
168
performed on a Hitachi L-2000 HPLC system (Hitachi Co., Japan) equipped with an
169
RI detector, using Asahipak NH2P-50 4E chromatographic column (4.6 mm×250 mm;
170
Showa Denko K.K, Japan).29 CsCE reaction mixture (lactulose, epilactose and lactose)
171
was analyzed using a Waters Alliance e2695 HPLC system (Waters Co., USA)
172
equipped with an RI detector (Waters 2414; Waters Co., USA) and Shodex VG-50 4E
173
chromatographic column (4.6 mm×250 mm; Shodex, Tokyo, Japan).27, 28
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Statistical Analysis. All data expressed in this study were reported as mean±SD
175
(standard deviation). Each value represents the mean for three independent
176
experiments performed in duplicate with average standard deviations not exceeding
177
5%. SPSS statistical software 22.0 (SPSS Inc., Chicago, IL, USA) was used to
178
perform the statistical analysis.
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RESULTS AND DISCUSSION
180
Characterization of Phenylboronic Acid Functionalized Adsorbents. FT-IR
181
spectra of AR-0, AR-1M, p-FPBA (Fig.2a) and AR-2M, APBA (Fig.2b) were used to
182
verify the successful grafting of phenylboronic groups. N-H stretching vibration at
183
3440 cm-1, 1501 cm-1, 903 cm-1 and C-N stretching vibration at 1108 cm-1 in both
184
AR-0, AR-1M and AR-2M indicated the presence of amino groups.24, 25 In Fig.2a and
185
b, it was found that both p-FPBA and APBA showed a characteristic stretching
186
vibration at 1342 cm-1, which is corresponded to B-O stretching of the phenylboronic
187
groups.18, 30, 31 When compared with AR-0, the adsorption band around 1342 cm-1 was
188
emerged for both AR-1M and AR-2M. The obtained results could indirectly prove that 9
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the phenylboronic groups have been successfully introduced to AR-1M and AR-2M
190
through the amidation reaction.21-23
191
Chemical bonding of phenylboronic groups onto AR-1M and AR-2M surface was
192
further confirmed by XPS analysis. As shown in Fig.3, N 1s signal appeared at ~400
193
eV was observed for both AR-0 (Fig.3a), AR-1M (Fig.3c) and AR-2M (Fig.3e), but
194
only AR-1M (Fig.3d) and AR-2M (Fig.3f) exhibited the typical B 1s peak observed at
195
184-194 eV. Specifically, the peak emerged at 190.04 eV was corresponded to the
196
typical trigonal sp2-hybridized neutral B-species of the phenylboronic acid.19, 31 These
197
data indeed confirmed the success coupling of phenylboronic groups onto adsorbents
198
through boronic ester formation method.30, 31
199
The morphologies of the adsorbents were characterized by SEM. It was observed that
200
AR-0 showed a compact structure with a smooth surface (Fig.4a), whereas the pure
201
resin sample exhibited a rough surface, which occurs due to solvent swelling effect
202
(Fig.4b). The rougher surface also stated that the inner amino groups were much more
203
exposed, thus enabling the subsequent amidation reactions. In addition to AR-0,
204
AR-1M (Fig.4c) and AR-2M (Fig.4d) also had a rough surface accompanying by
205
pores with well distributed tiny holes, while all the skeletons retained their porous
206
structure and showed nearly uniform particle size (~300 µm). The changes on the
207
surface morphology would be beneficial for grafting more phenylboronic acid groups
208
which could obviously promote sugars capture.
209
Adsorption Properties of Phenylboronic Acid Functionalized Adsorbents. The
210
adsorption capacities of AR-1M (Fig.5a) and AR-2M (Fig.5b) on lactulose ( )
211
and lactose ( ) were gradually increased with the increasing of phenylboronic
212
acid groups due to the reversible covalent interaction between phenylboronic acid
213
groups and cis-diol compounds. More importantly, as depicted in Fig.S1, the binding 10
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affinity of phenylboronic acid for lactulose (4-O-β-D-galactosyl-D-fructose) and
215
fructose was much higher than that of lactose (4-O-β-D-galactopyranosyl-D-glucose)
216
and glucose because of the typical cis-1,2-diol unit supplied by lactulose and
217
fructose,25 which can be easily linked with boronic acid group and to form a stable
218
5-membered boronic ester ring.17, 24 This is evidenced in Fig.5 that the increasing in
219
was much higher compared with that of . The discordance in adsorption
220
performance for lactulose and lactose could obviously increase the adsorption
221
selectivity (α) according to Eq.(3) and can be a principle of separation lactulose from
222
complex syrup. Furthermore, as depicted in Fig.1, the residual amino groups (-NH2,
223
~3,800 µmol/g dry resin) of AR-1M could cause nonspecific adsorption21, besides, the
224
residual amino groups might also participate in the adsorption of cis-dilos (lactulose
225
and lactose) through the B-N coordination.21, 32, 33 Those obviously exert influence on
226
and α. Whereas for AR-2M, the adsorption performance was well maintained
227
because of the presence of the interaction between amino groups and glutaraldehyde.
228
Moreover, the introduced flexible chains of glutaraldehyde (spacer arms) on AR-2M
229
were obviously favorable for boronate esterification with cis-diol compounds.
230
Therefore, it can be seen from Fig.5 that AR-2M not only has a superior adsorption
231
selectivity but also displays a higher adsorption efficiency (α=7.64, π~15.4% at
232
phenylboronic acid content of ~750 µmol/g dry resin) than AR-1M (α=2.27, π~11.2%
233
at phenylboronic acid content of ~2,600 µmol/g dry resin).
234
Accordingly, the weight percentages of the major elements in the phenylboronic acid
235
functionalized adsorbents were listed in Table 1. The mass concentrations of boron in
236
AR-1M (~2600 µmol of phenylboronic acid groups) and AR-2M (~750 µmol of
237
phenylboronic acid groups) were ~2.17% and ~0.73%, respectively. Comparatively,
238
the weight percentage of boron in AR-1M was almost 3 times higher than that of 11
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AR-2M. Such a drastic decline in the content of boron could be reasonably attributed
240
to the successful grafting of bifunctional glutaraldehyde (spacer arms) on AR-2M
241
(Fig.1, route 2), which may exist in two types: resin-NH2-glutaraldehyde-NH2-resin
242
and resin-NH2-glutaraldehyde-phenylboronic acid group. These observations and
243
explanations provide insight that phenylboronic acid was presumably the sole
244
functional group of AR-2M, which might result in high lactulose selectivity, but failed
245
to increase the lactulose adsorption capacity.
246
Adsorption pH is a critical factor for boronate affinity. As stated in previous reports,
247
the binding of boronic acid groups (usually phenylboronic acid groups) to cis-1,2-diol
248
compounds occurred in two steps: phenylboronic acid group was first transformed to
249
tetrahedral anionic form [-B(OH)4-] (sp3) at alkaline conditions, and [-B(OH)4-] (sp3)
250
subsequently bonded with cis-1,2-diol units to form a stable 5-membered cyclic ester
251
(Fig.1).17, 30, 34 Given that most boronic acids were generally weak acids having a pKa
252
value of 8-9 (pKa of 8.7-8.9 for phenylboronic acid), it means that at pH≤ the pKa
253
value, most of the boronic acids still existed in the form of trigonal [-B(OH)2] (sp2),
254
which cannot react with cis-diol groups.21, 24 Based on this principle, phenylboronic
255
acid functionalized adsorbents required a basic pH (usually the pH should be> the pKa
256
value of the phenylboronic acid ligand) for binding.35 This is in good agreement with
257
the results obtained from AR-2M. As described above, phenylboronic acid was the
258
sole functional group on AR-2M, thus of AR-2M was definitely pH-dependent
259
and dramatically increased from ~5 to ~85 (mg/g dry resin) when the pH increased
260
from 6.0 to 8.0 and then kept almost unchanged (Fig.6b). Interesting, of
261
AR-2M was almost constant regardless the pH applied, this further confirmed that the
262
phenylboronic acid was the sole functional group of AR-2M. However, for AR-1M
263
(Fig.6a), was significantly decrease from approximately 60 mg/g dry resin at 12
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pH 6.0 to about 30-40 mg/g dry resin at pH 7.0-10.0. The reason for this decline may
265
due to the influence of the pH affecting residual amino groups (-NH2), which is
266
stronger for lactose (through nonspecific adsorption) than for lactulose (through
267
reversible covalent binding with phenylboronic acid groups). Moreover, no obviously
268
change on was observed as the pH increased from 6.0 to 10.0, this might be
269
reasonably ascribed to the influence of residual amino groups, which could form an
270
alkaline microenvironment around the surface of AR-1M. Besides, as illustrated in
271
Fig.1, the residual amino groups of AR-1M may also react with the trigonal [-B(OH)2]
272
(sp2) and to form a B-N coordination complex (regard as Wulff-type boronic acids).32
273
B-N coordination complex with a low pKa value was obviously favors the adsorption
274
of cis-diols under neutral pH conditions.21, 32, 33 This interesting phenomenon is a great
275
advantage compared with traditional boronate affinity materials (such as AR-2M),
276
because it allows the selective adsorption of lactulose under neutral condition, which
277
is more favorable for industrial applications. Further studies are needed to understand
278
the positive adsorption of cis-diols through the B-N coordination.
279
Fig.6c and 6d provides the effect of adsorption temperature on sugars capture. It was
280
shown that of AR-1M was significantly (p96%, respectively. The same trend was also observed in AR-2M. Such a rapid 16
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reduction in could be largely attributed to the inhibitory effect of the load of
365
more lactulose molecules at higher ratio of lactulose to lactose, since the binding
366
affinity of phenylboronic acid for lactulose (cis-1,2-diol unit) is much higher than that
367
of lactose.25 However, Fig.S2 shows that both and increased with the
368
concentration of lactulose and lactose at fixed Lu-La ratio of 1:1, which further
369
confirming the decisive effect of the ratio of lactulose to lactose on lactulose selective
370
capture. Moreover, it can be seen from Table 2A that AR-2M exhibited a lower
371
but a much higher at fixed Lu-La ratio compared with AR-1M, demonstrating
372
again that the introduced glutaraldehyde spacer arms on AR-2M can enhance the
373
adsorption selectivity towards lactulose but failed to improve the adsorption capacity
374
due to the reduction in phenylboronic acid groups.
375
Structure difference between lactulose (4-O-β-D-galactosyl-D-fructose) and lactose
376
(4-O-β-D-galactopyranosyl-D-glucose), namely the fructose moieties of lactulose
377
with typical cis-1,2-diol unit is responsible for their different binding affinity with
378
boronic groups.17, 22, 24 This explanation can be further supported by using fructose
379
and glucose as the model substrates. Table 2B clearly shows that a higher binding
380
affinity towards fructose was observed as expected at 1:1 ratio of fructose to glucose,
381
which ultimately resulted in a much higher &' for AR-1M (79.17±3.25%) and
382
AR-2M (90.45±5.47%).
383
Considering the fact that commercial lactulose syrup produced from chemical based
384
isomerization is a mixture of monosaccharides (i.e. fructose and glucose) and
385
disaccharides (e.g. lactulose and lactose).1,
386
mixtures of fructose-glucose-lactulose-lactose at a ratio of 1:1:1:1 was formulated to
387
investigate the applicability of AR-1M and AR-2M to selective adsorption of target
388
lactulose from the chemical isomerisates. Apparently, selective enrichment of
4
Herein,a model syrup consisting in
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389
lactulose and fructose from isomerisates by using AR-1M and AR-2M are presented
390
in Table 2C. As a result, &' and were remarkably increased from 25% to
391
43.47±1.25%, 38.83±2.09% for AR-1M and 45.35±2.97%, 45.44±2.02% for AR-2M,
392
respectively. More importantly, small molecules such as monosaccharides can be
393
efficiently separated from lactulose syrup by using nanofiltration (NF) due to different
394
molecular weight cut-offs,37, 38 thus the affect of fructose on the selective adsorption
395
of AR-1M and AR-2M towards lactulose can be remarkably diminished through prior
396
NF process, implying that both AR-1M and AR-2M are still satisfactory in purifying
397
real isomerisates.
398
Table 2D claims the selective enrichment effect of AR-1M and AR-2M towards
399
lactulose from real CsCE-catalyzed reaction mixture, which contains lactulose,
400
epilactose and residual lactose at the ratio of 6.35:1.58:4.00 (×g/L). was
401
considerably increased from ~53% to 75.75±1.43% for AR-1M and 85.95±0.59% for
402
AR-2M, respectively. More importantly, both of the loaded lactose and epilactose
403
expect for lactulose could be released from adsorbents by using deionized water
404
(pH~5.8) because of their weak binding strength with boronic groups and/or the
405
nonspecific adsorption with amino groups (-NH2).21 Therefore, a higher * could
406
be obtained for both AR-1M (91.58±1.50%) and AR-2M (>98%) after first desorbing
407
with deionized water. The above results exhibited a good practicability of AR-1M and
408
AR-2M for real lactulose-enriched syrup.
409
Recyclability of Phenylboronic Acid Functionalized Adsorbents towards
410
Lactulose. From the viewpoint of practical applications, regeneration and
411
recyclability of phenylboronic acid adsorbents are other important features. Thus the
412
reutilization potential of AR-1M and AR-2M was evaluated in ten-consecutive
413
pH-driven adsorption/desorption cycles. As illustrated in Fig.8, no dramatic loss of 18
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414
lactulose adsorption capacity was observed and the recovery was still up to ~90%
415
after 10-repeats. The slight reduction in adsorption capacity was probably attributed to
416
the cleavage of some unreduced Schiff’s base (-CH=N-) (Fig.1) under desorption
417
acidic condition (pH96
>96
50.80±0.76
53.18±1.77
80.68±3.10
97.33±2.07
102.33±2.86
116.21±2.23
1.36±0.23
0.61±0.41
ND
ND
ND
ND
92.75±3.70
95.67±1.92
>98
>98
>98
>98
(B) fructose-glucose mixture (Fru:Glu=1:1, ×5 mg/mL). Fru:Glu &' (%) AR-1M &' (mg/g) ,- (mg/g) ()./) (%) * AR-2M &' (mg/g) ,- (mg/g) ()./) (%) *
1:1 50 68.50±2.49 56.49±3.54 79.17±3.25 37.25±3.01 8.91±0.76 90.45±5.47
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(C) fructose-glucose-lactulose-lactose mixtures (Fru:Glu:Lu:La=1:1:1:1, ×5 mg/mL). Fru:Glu:Lu:La
1:1:1:1 25 46.79±1.95 34.18±2.27 37.15±2.57 4.42±0.96 43.47±1.25 9.19± ±1.76 38.83± ±2.09 8.51± ±0.87 27.30±3.40 7.01±0.59 32.84±2.47 0.67±0.28 45.35±2.97 5.18± ±0.39 45.44± ±2.02 4.04± ±0.24
&' , ,- , , AR-1M
AR-2M
(%) &' (mg/g) ,- (mg/g) (mg/g) (mg/g) ()./) ) * (%) 12) ()* (%) ) ()+) ) * (%) ()+3 ) * (%) &' (mg/g) ,- (mg/g) 45 (mg/g) 46 (mg/g)
()./) ) * (%) 12) ()* (%) ) ()+) ) * (%) ()+3 ) * (%)
(D) CsCE reaction mixture (Lu:Epi:La=6.35:1.58:4.00, ×mg/mL). Lu: Epi: La 6.35:1.58:4.00 (g/L) 53.23 (%) 97.74±3.95 AR-1M (mg/g) 14.78±1.13 789 (mg/g) (mg/g) 40.20±2.07 75.75± ±1.43 ()+) ( %) ) * * 91.58±1.50 ()+) ( %) ) * 98.68±2.88 AR-2M (mg/g) 10.17±1.34 789 (mg/g) 15.14±3.25 46 (mg/g) +) 85.95± ±0.59 ()* (%) ) * >98 ()+) ( %) ) * *: desorption purity of lactulose from AR-1M and AR-2M after first washed with deionized water (pH 5.8~6.0).
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Figures Fig. 1.
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Fig.2.
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Fig.3.
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Fig. 4. (a)
(b)
(c)
(d)
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Fig. 5.
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Fig.6.
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Fig.7.
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Fig.8.
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Graphic for table of contents
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