Modification of cellulose acetates for preparing chiral sorbents

Article abstract of DOI:10.1134/S1070427214090225

Modification of cellulose acetates via sorption-desorption of vapors of mesogenic solvents in which the polymer forms a lyotropic liquid crystal phase and of mixtures of these solvents with water leads to the formation of a new chiral structure of the polymeric sample. This is manifested in a significant change in the value and even sign of the specific optical rotation of the polysaccharide system. The sorbents based on cellulose acetates that have been modified by such treatment exhibit specific affinity for definite optical antipodes. When a racemic mixture of L- and D-isomers of amino acids is passed through this sorbent, it acts as a chiral filter owing to "steric recognition" of one of the enantiomers, so that the filtrate contains an optically pure product (isomer). The revealed effects served as a basis for the development of a new procedure for preparation of optically pure stereoisomers of chiral products.

Full text of DOI:10.1134/S1070427214090225

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 9, pp. 1326−1333. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © A.B. Shipovskaya, N.O. Gegel’, S.Yu. Shchegolev, 2014, published in Zhurnal Prikladnoi Khimii, 2014, Vol. 87, No. 9, pp. 1336−1344.
MACROMOLECULAR COMPOUNDS
AND POLYMERIC MATERIALS
Modification of Cellulose Acetates
for Preparing Chiral Sorbents
A. B. Shipovskayaa, N. O. Gegel’a, and S. Yu. Shchegolevb
a Chernyshevsky State University, ul. Astrakhanskaya 83, Saratov, 410102 Russia
b Institute of Biochemistry and Physiology of Plants and Microorganisms,
Russian Academy of Sciences, pr. Entuziastov 49, Saratov, 410049 Russia
e-mail: ShipovskayaAB@rambler.ru
Received October 8, 2014
Abstract—Modification of cellulose acetates via sorption–desorption of vapors of mesogenic solvents in which
the polymer forms a lyotropic liquid crystal phase and of mixtures of these solvents with water leads to the
formation of a new chiral structure of the polymeric sample. This is manifested in a significant change in the
value and even sign of the specific optical rotation of the polysaccharide system. The sorbents based on cellulose
acetates that have been modified by such treatment exhibit specific affinity for definite optical antipodes. When a
racemic mixture of L- and D-isomers of amino acids is passed through this sorbent, it acts as a chiral filter owing
to “steric recognition” of one of the enantiomers, so that the filtrate contains an optically pure product (isomer).
The revealed effects served as a basis for the development of a new procedure for preparation of optically pure
stereoisomers of chiral products.
DOI: 10.1134/S1070427214090225
The development of highly selective chiral sorbents
and of simple procedures for separation of optical
isomers of bioproducts and drugs is a topical problem
for chemical, pharmaceutical, and biochemical industry
and medicine [1]. The technology for the production of
the majority of drugs is limited to preparation of the final
product as a racemic mixture of chiral molecules. The
therapeutic effect of such racemates is due, as a rule, to the
action of only one of enantiomers. The other enantiomer
is less active or inactive at all; furthermore, it can even
exert negative pharmacological effects [2, 3].
Among chromatographic methods, high-performance
liquid chromatography and affinity (or biospecific)
liquid chromatography with optically active sorbents
(chiral stationary phases, chiral “hosts”) are the most
widely used. The selectivity of the interaction of such
sorbents with heterochiral substrates in the course of
separation is determined by the principles of molecular
recognition and steric correspondence [11]. Such
sorbents are prepared by grafting of various chiral groups
(fragments of optically active amines, amino acids,
antibiotics, etc. [12, 13], or special “hooks” created by
methods of genetic engineering [14–16]) or synthesized
on the basis of crown compounds, porphyrins, or
cyclodextrins as three-dimensional network structures
with “chiral voids” allowing selective penetration of
only one of the enantiomers [17–21]. Chiral-selective
composite membranes based on cellulose [22], cellulose
acetobutyrate [23], etc., are also used as stationary phases
for separating optically active substances.
Equimolar mixtures of levo- and dextrorotating
enantiomers are separated into optical antipodes using,
as a rule, chromatographic [4–7] and electrokinetic
[8–10] methods. The latter methods are based on
principles of capillary electrophoresis, which, in turn,
is based on hydrophobic, dipole–dipole, and other
interactions of stereoisomers with a chiral selector.
Various cyclodextrins, acyclic oligosaccharides, anionic
or cationic polysaccharide derivatives, etc., are used as
such selectors.
It should be noted that chiral selectors, sorption media,
and stereoselective membranes in such approaches are
1326

MODIFICATION OF CELLULOSE ACETATES
Table 1. Characteristics of cellulose acetate samples
1327
Amount of bound
CH3COOH γ, %
Viscosity-average molecular mass
Bulk density ρ,
Polymer
CDA-1
Producer
Mη × 104, kDa
g cm–3
OAO Khimvolokno, Vladimir
OAO Khimvolokno, Engels
54.9
7.7
1.32
CDA-2
CTA
55.1
62.2
8.2
7.0
1.32
1.28
prepared by multistep directional synthesis. Furthermore,
despite high efficiency, both chromatography and
electrophoresis are semipreparative methods requiring
choice of additional methods and conditions (eluent, pH,
ionic strength, etc.) for extraction the homochiral substance
from the support. All these facts make the development
of simpler methods for preparing stereospecific sorbents
and of feasible methods for racemate separation a topical
problem. In this connection, we believe that attention
should be paid to the possibility of solving these problems
via physicochemical modification of optically active
cellulose acetates in the active vapor phase specifically
interacting with the polymeric molecules.
preparation of polymeric materials with new specific
functional possibilities that are not exhibited by the
initial polymer. For example, cellulose acetate filters and
membranes capable of selective separation of optically
active components of blood plasma were prepared by this
procedure. These materials efficiently retain cholesterol
or bilirubin, but do not retain proteins and electrolytes
[30–32].
It was interesting to determine whether the steric
organization of the macrochains is preserved in other
procedures for further processing of the polymeric
substances modified in vapors of mesogenic solvents, in
particular, after desorption of the sorbate vapor taken up
by the polymeric matrix directly in the powdered samples,
without dissolving the polymer with the sorbed vapor in
appropriate solvent (the latter procedure is used, as a rule,
for the subsequent formation of the condensed polymeric
material). This study was also aimed at evaluating
the stereospecificity of the induced chiral structure of
the obtained cellulose acetate samples for separating
racemates of optically active compounds, with racemic
mixtures of α-amino acids as example.
Previous studies of the nature and formation conditions
of a lyotropic liquid crystal (LC) phase of cellulose esters
showed that these esters are capable to change their
asymmetric structure and form stereoisomers differing
in the optical activity [24–29]. For example, when
cellulose acetate powders take up vapors of mesogenic
solvents in which they form an LC phase or of their
mixtures with water, the supramolecular and stereomeric
structure of the polymer undergoes rearrangement. This
is manifested in changes in the value and sign (in most
cases) of the specific optical rotation of solutions of
modified samples, reflecting changes in the conformation
(steric arrangement) of the chiral macromolecules. The
action of vapors of such solvents on the structure and
optical activity of cellulose acetate is described by the
dose–effect relationship. The most significant effect is
exerted by small doses (no more than 10–12 wt %) of
the absorbed sorbate vapor.
EXPERIMENTAL
We chose for our study commercial samples of
cellulose acetates: diacetate (CDA) and triacetate (CTA),
used for the production of acetate yarns for textiles. The
sample characteristics are given in Table 1.
As sorbates we used nitromethane, dimethyl sulfoxide
(DMSO), dimethylformamide (DMF), acetic acid (AA),
and their binary mixtures with distilled water (component
ratio from 1 : 99 to 20 : 80). The choice of these liquids
was governed by the fact that they belong to the class of
mesogenic solvents in which cellulose and its derivatives
form a lyotropic LC phase. In addition, as shown in
The discovered fact that the new sterically modified
metastable structure of the cellulose ester sample is
preserved upon dissolution of the polymer powder
(with the sorbed vapor) in the process solvent and
upon formation of the ready product opens the way to
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1328
SHIPOVSKAYA et al.
solution and solvent, respectively (deg), l is the optical
path length (dm), and с is the solution concentration
(g dL–1).
[24–32], vapors of these sorbates influence not only the
supramolecular structure of cellulose acetates, but also
their stereomeric structure (at its fixation with the sorbed
vapors in the condensed material).
To evaluate structural changes that occurred in the
polymeric matrix upon sorption and sorption–desorption
of the sorbate, we performed comparative analysis
of cellulose acetate solutions prepared by different
procedures. In the first procedure, the modified samples
after the dissolution were left in air for the desorption of
the absorbed vapor; in the second procedure, as in [30–
32], the samples with the sorbed vapors were immediately
placed into the solvent.
The CDA and CTA modification was performed as
follows.Apolymer powder sample was kept in the sorbate
vapor at 20°С in a hermetically closed desiccator filled to
1/25 with the liquid medium. The sample was arranged
in sieves with the pore diameter of 0.25 mm at a distance
of approximately 5 cm from the liquid surface, and the
sorption was performed. The desorption of the absorbed
vapor was performed in air at 20 ± 2°C. The amounts of
the sorbed, сs (wt %), and desorbed, сs–1 (wt %), vapor
were determined by the formulas
The selectivity of the separation of racemic mixtures
of optically active substances was evaluated by
chromatography on a special separation cell at T = 20 ±
2°C. The cell was a glass tube 17 cm long, 1 cm in
diameter, with two compartments isolated from each other.
The first (upper) compartment was a hollow tube 13.5 cm
high, and the second (lower) compartment was a glass
filter with a pore diameter of approximately 0.1 cm and
a height of 3.0 cm. The sample of the powdered polymer
was placed into the upper hollow tube of the column to a
bed height of 3 cm and compacted, after which 20 mL of
a solution of an optically active substance was passed. We
used aqueous solutions (с = 0.1–0.2 g dL–1) of racemic
mixtures of L- and D-amino acids (1 : 1): tryptophan,
serine, aspartic acid, and valine (chemically pure grade).
Data on the specific optical rotation of the individual
enantiomers of amino acids, [α]e, and of their racemic
mixtures, [α]r, are given in Table 2.
m1 – m0
сs = ––––––– ×100,
(1)
(2)
m0
m1 – m2
сs–1 = ––––––– ×100,
m1
where m0 is the initial sample weight (g), and m1 and m2
are the sample weights after the sorption and desorption,
respectively (g).
Because of strong influence of small amounts of
the absorbed vapors on the structure of CDA and CTA
samples, the amount of the vapor sorbed by the polymer
did not exceed 10–12 wt %.
The IR spectra of the CDA and CTA powders were
recorded with an FSM 1201 Fourier IR spectrometer
from thin layers of mulls in mineral oil between KBr
windows. The 13C NMR spectra of CDA and CTA
solutions in DMSO-d6 were taken with a Varian-400
NMR spectrometer (USA).
The efficiency of separating the enantiomers was
monitored by variation of the specific optical rotation
of the amino acid solution before ([α]r) and after ([αi])
passing through the sorbent, taking into account [α]e.
The separation selectivity S (%) was determined by the
formula
The optical activity of the control and modified samples
of CDA and CTA was measured with an SM-2 circular
polarimeter at the wavelength λNa = 589 nm, T = 20°С. We
used solutions of concentration с = 0.5 g dL–1 in common
process solvents: acetone–water mixture (95 : 5) for CDA
and methylene chloride–ethanol mixture (90 : 10) for
CTA. All the solvents were of analytically pure grade.
The specific optical rotation α (deg mL dm–1 g–1) of the
solutions was determined by the formula
[α]i
S = ––––––– × 100,
[α]0
(4)
where [α]i is the specific optical rotation of the permeate
and [α]0 is that of either L- or D-enantiomer.
As shown in [24–26], the sorption by CDA and CTA
(powder, film, fiber) from the vapor phase formed by
nitromethane, DMSO, DMF, AA, and their mixtures
with water (Н2О content no more than 20–25%) is
not described by Fick’s law and is characterized by
(α –α0) ×100
[α] = ––––––––––––,
(3)
lc
where α and α0 are the measured rotation angles for the
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 9 2014

MODIFICATION OF CELLULOSE ACETATES
1329
anomalous swelling curves. Under the same conditions,
acetate yarns and films undergo spontaneous elongation
and the inverse effect of spontaneous “shrinkage” of the
specimens that underwent elongation on exposure to
the vapor. Relaxation of internal stresses in the polymer
system occurs; it is accompanied by a decrease in the
misorientation angle and by an increase in the mechanical
characteristics of the materials. Stable optical anisotropy
in film specimens is realized [27–29]. Sorption of vapors
of active mesogenic solvents also affects the optical
activity of the polysaccharides under consideration,
varying it within wide limits [26–32]. This fact suggests
changes in the overall chiral structure of the polymer
system and enhancement of its optical heterogeneity. It
should be emphasized once again that all these effects
are observed with cellulose acetates that have taken up
vapor of a specific sorbate.
Table 2. Characteristics of aqueous solutions of amino acids
Specific optical rotation (λ = 589 nm),
deg mL dm–1 g–1
Amino acid
[α]e
[α]r L : D
a,b
(1 : 1)
L-Enantiomera D-Enantiomera
Tryptophan
Valine
Serine
Aspartic acid
–32.1
–6.06
–14.32
–25.5
+32.9
+6.42
+14.45
+25.5
0
0
0
0
a Reference data.
b Experimental data.
[α] = f(сs), obtained in [29] for the system of CDA-2 with
nitromethane vapor, is also shown for comparison.
As we found, the samples modified by sorption–
desorption of vapor of specific active media undergo not
only changes in the absolute values of the specific optical
rotation, but also the inversion of the sign of [α].
The experimental data shown in Fig. 1 indicate that
the control samples of CDApowders in the acetone–water
solvent (95 : 5) have positive specific optical rotation,
[α] = +25 ± 1 deg mLdm–1 g–1 for CDA-1 and [α] = +30 ±
2 deg mL dm–1 g–1 for CDA-2. For CDA-1 modified by
the first and second procedures and for CDA-2 modified
by the second procedure, the optical activity sharply
changes already after the absorption of the first, relatively
small (up to ~2.0 wt %) amounts of the sorbate vapor, with
the change in the sign of [α] (Fig. 1a, curves 1–3). As the
amount of the vapor taken up by the polymer is increased
to 4–5 (CDA-1) and 7–8 wt % (CDA-2), [α] remains
negative and further increases in the absolute value. The
Figure 1 shows as example the dependence of the
specific optical rotation [α] of solutions of CDA-1 and
CTA modified by the first and second procedures on the
amount of vapors of nitromethane and DMSO–water
mixture (10 : 90), taken up by the polymer. For the
samples modified by the first procedure, i.e., by sorption
and subsequent desorption of the sorbate vapor, the
quantity сs (as in the second procedure) corresponds to the
degree of vapor sorption by the polymer. The dependence
20°C
(b)
20°C
589 nm
(a)
589 nm
8 cs, wt %
16 cs, wt %
20°C
589 nm
Fig. 1. Specific optical rotation α
of solutions (с = 0.5 g dL–1) of the initial CDA powders (points on the ordinate), CDA powders
modified in 95 : 5 acetone–water mixture, and CTA powders modified in 90 : 10 methylene chloride–ethanol mixture as a function of the
amount сs of vapor of (a) nitromethane and (b) 10 : 90 DMSO–H2O mixture. (1, 2) CDA-1 modified by the first and second procedures,
respectively; (3) CDA-2 modified by the second procedure [29]; and (4) CTA modified by the first procedure.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 9 2014

1330
SHIPOVSKAYA et al.
cycle are similar: The specific optical rotation varies in a
Enantiomers
(L, D)
wide range, and for CDA powder [α] also changes sign.
The polymer modified by both the first (desorption of the
sorbed vapor before dissolution) and second (dissolution
of the sample with the sorbed vapor in the process solvent)
procedures is characterized by the altered chiral structure.
Cellulose acetate sorbent
with induced chiral
structure
There are good grounds to state that the steric structure
of the polymer, modified as a result of sorption of vapors
of specific media, is preserved after desorption of the
sorbate vapor. The Fourier IR data for powders of CDA
and CTA modified by the first procedure and the 13C
NMR data for solutions of these substances show that the
substances do not contain mesogene molecules, i.e., that
the sorbate molecules are virtually completely removed
from the polymeric matrix. Thus, the desolvation of the
swollen polymer powder, apparently, leads to fixation of
the supramolecular structures and of the conformations
that the macromolecules acquired upon interaction with
the specific active medium.
Fig. 2. Scheme of chiral filtration of racemic mixtures of
α-amino acids on cellulose acetate sorbents with induced chiral
structure.
Taking into account the fact that the altered steric
organization of the modified polymeric sample strongly
affects the properties of the materials obtained [30–32],
we estimated the stereospecificity of the induced chiral
structure of CDAand CTAmodified by the first procedure.
For this purpose, we performed chromatographic separa-
tion of racemic mixtures of L- and D-isomers of amino
acids, chosen as example, with the modified cellulose
derivatives. The results of chromatographic separation of
aqueous solutions of racemic α-amino acids (tryptophan,
valine, serine, aspartic acid) on the initial powdered cellu-
lose acetate sorbents and on the samples with the induced
chiral structure are given in Table 3.
uptake of still larger amounts of nitromethane by CDA-1
and CDA-2 (more than 5 and 8 wt %, respectively) results
in that [α] becomes positive again. This trend is clearly
manifested for CDA-1 modified by the first procedure
(curve 1) and for CDA-2 modified by the second proce-
dure (curve 3). Probably, similar shape of the [α] = f(сs)
dependence will also be observed for CDA-1 modified
by the second procedure at сs > 13–14 wt % (curve 2).
The same is true for other systems based on CDA-1 with
the vapor phase formed by the mixtures DMSO : Н2О =
1 : 99–10 : 90 (Fig. 1b), DMF : water = 10 : 90–20 : 80,
and AA : water = 5 : 95 as sorbate.
The initial samples of CTA powder in methylene
chloride with ethanol (90 : 10) are characterized
by negative specific optical rotation, [α] = –20 ±
2 deg mLdm–1 g–1. The nitromethane sorption–desorption
cycle is not accompanied by the inversion of the sign
of [α] of CTA solutions. However, both CDA and CTA
modified by the first procedure undergo changes in the
value of the specific optical rotation (Fig. 1a, curve 4). It is
appropriate to note that similar dependence [α] = f(сs) was
also obtained for the CTApowder modified by the second
procedure in vapors of nitromethane, AA, trifluoroacetic
acid, and formic acid [29]. The latter two acids, like the
liquids used in this study, are also mesogenic solvents
for cellulose acetates.
Passing of aqueous solutions of amino acid racemates
through a column packed with the initial polymer does not
result in separation of the enantiomers. However, when
the modified CDA or CTA powder, characterized by the
induced negative [α] of polymer solutions in standard
solvents, is used as stationary phase, the D-isomer is
retained in the column, whereas the L-isomer passes
through the sorbent (Table 3). On the contrary, when a
solution of the racemic mixture is passed from the CDA
sorbent with positive [α], the D-isomer passes through
the column.
It engages attention that the most efficient influence
on the steric structure of the polymer is exerted by small
amounts of the absorbed vapor (сs < 5–7 wt %) of the
active medium. For example, the CDA sorbent after
the uptake of 3 wt % vapor of the 1 : 99 DMSO : water
Our results show that the dependences [α] = f(сs) both
for the CDA and CTA samples that sorbed the sorbate
vapor and for those subjected to the sorption–desorption
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MODIFICATION OF CELLULOSE ACETATES
1331
Table 3. Results of chromatographic separation of racemic mixtures of amino acid L-, D-enantiomers on cellulose acetate sorbents
with induced chiral structure
Sorbent characteristics
“Adsorbate” characteristics
S, %
cs,
[α]589 nm
,
[α]i 589 nm,
polymer
CDA-1
active medium
amino acid
wt %
deg mL dm–1 g–1
deg mL dm–1 g–1
–
–
–
–
–
–
3.0
5.0
+25
+25
+25
0
Tryptophan
Serine
Aspartic acid
Tryptophan
''
0
0
0
0
0
0
AA : water (5 : 95)
0
0
''
–14
–32.5
100
DMF : water:
10 : 90
10 : 90
20 : 80
3.0
5.0
5.0
–13
–8
–15
''
''
''
–30.1
–32.1
–23.9
94
100
75
DMSO : water:
1 : 99
5 : 95
10 : 90
10 : 90
3.0
5.0
3.0
4.0
1.5
1.5
3.5
7.0
11.0
–10
–2
–15
–5
–19.5
–19.5
–22
0
''
''
–32.7
–32.5
–32.3
–22.9
–5.4
–30.7
–32.8
0
100
100
100
71
21
96
100
0
51
Serine
Tryptophan
Aspartic acid
Tryptophan
Nitromethane
''
''
''
''
''
''
''
+25
+16.6
CTA
–
–
–
–
–20
–20
–21
–44
–44
–29
–21
''
Valine
Tryptophan
Valine
Tryptophan
''
0
0
0
0
78
45
82
Nitromethane
0.6
4.5
4.5
7.0
9.0
–25.1
–2.7
–26.6
–32.3
–33.0
"
''
''
''
100
100
''
to the L- or, more seldom, D-isomer to the filtrate in the
optically pure form (Fig. 2). This phenomenon can be
of great practical importance, because it is well known
that many organic compounds (including drugs) exhibit
the maximal biological and pharmacological activity
specifically in the form of their L-isomers.
mixture, followed by its removal, separates racemic
tryptophan with 100% yield of the L-isomer, and after the
uptake and subsequent desorption of the same amount of
the vapor of the 10 : 90 DMSO : water it separates racemic
serine with 100% yield of the L-isomer (Table 3). The
data obtained allow modified CDA and CTA powders
to be considered as promising sorbents for separation
of racemates of α-amino acids and, probably, of other
optically active substances and of their heterochiral
mixtures.
The revealed effects served as the basis for developing
a new procedure for preparing optically pure stereoisomers
of chiral products. The procedure has been patented in
the Russian Federation [33].
Thus, the cellulose acetate sample modified by
appropriate treatment and characterized by the altered
chiral structure exhibits specific affinity (possibly, steric
complementarity) for definite optical antipodes. When
passing a racemic mixture of L-, D-isomers, the sorbent
actually acts as a chiral filter, which leads to “steric
recognition” of one of the enantiomers and to the passing
CONCLUSIONS
(1) Modification of cellulose acetate powder of dif-
ferent degrees of acetylation by sorption–desorption of
vapors of individual mesogenic solvents and of their
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 9 2014

1332
SHIPOVSKAYA et al.
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Mosk. Univ., Ser. 2: Khim., 2012, vol. 53, no. 3, pp. 147–
156.
mixtures with water involves transformations altering the
overall chiral structure of the polysaccharide system. The
stereomeric organization of the polymeric matrix, induced
by the vapors, is preserved after the desorption of the ab-
sorbed sorbate, which is manifested in changes in the value
and sign of the specific optical rotation of the solutions
of the modified samples. The most significant influence
on the optical activity of the polysaccharide is exerted by
relatively small doses of the vapor used as sorbate.
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Methods and Applications, Chichester: Horwood, 1988.
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dose–effect relationship and can be completed in any step
of the formation of the chiral structure of the polymer
sample. The conversion can be monitored by measuring
the specific optical activity of the samples.
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(3) Sorbents with the altered chiral structure,
optimized for preparing optically pure isomers of
α-amino acids from the corresponding racemates, were
prepared from cellulose diacetate and triacetate powders
modified by appropriate treatment. The sorbents show
high performance in “chiral recognition” of definite
optical antipodes (D- or L-isomers) in the course of
chromatographic separation on the powdered cellulose
acetate material.
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sorbents prepared by this procedure ensure selective
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ACKNOWLEDGMENTS
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