Indicator properties of oligoethylene glycol hybrids with sterically hindered phenols

Article abstract of DOI:10.1134/S1070428014030129

Aqueous solutions of hybrid compounds (conjugates) formed by polyethylene glycols modified at the terminal hydroxy groups with sterically hindered phenol lose their phase stability at a certain temperature which depends on the molecular weight of polyethylene glycol, structure of sterically hindered phenol, conjugate concentration, and composition of the medium. This property may be used to estimate the effect of structural factors on the hydrophobic-hydrophilic balance in the conjugate-water system.

Full text of DOI:10.1134/S1070428014030129

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2014, Vol. 50, No. 3, pp. 371–375. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © N.S. Domnina, O.Yu. Sergeeva, E.A. Komarova, M.E. Mikhailova, V.B. Vol’eva, I.S. Belostotskaya, N.L. Komissarova, 2014,
published in Zhurnal Organicheskoi Khimii, 2014, Vol. 50, No. 3, pp. 382–386.
Indicator Properties of Oligoethylene Glycol Hybrids
with Sterically Hindered Phenols
N. S. Domninaa, O. Yu. Sergeevaa, E. A. Komarovaa, M. E. Mikhailovaa, V. B. Vol’evab,
I. S. Belostotskayab, and N. L. Komissarovab
a St. Petersburg State University, Universitetskii pr. 26, St. Petersburg, 198504 Russia
e-mail: ninadomnina@mail.ru
b Emanuel’Institute of Biochemical Physics, ul. Kosygina 4, Moscow, 119334 Russia
Received October 3, 2013
Abstract—Aqueous solutions of hybrid compounds (conjugates) formed by polyethylene glycols modified at
the terminal hydroxy groups with sterically hindered phenol lose their phase stability at a certain temperature
which depends on the molecular weight of polyethylene glycol, structure of sterically hindered phenol,
conjugate concentration, and composition of the medium. This property may be used to estimate the effect of
structural factors on the hydrophobic–hydrophilic balance in the conjugate–water system.
DOI: 10.1134/S1070428014030129
Synthesis of hybrid compounds whose molecules
contain fragments with different functionalities con-
stitutes one of the most promising lines in the design
of modern biologically active substances. Hybrid struc-
tures are advantageous due to enhanced efficiency,
prolonged action, extended solubility range, and
reduced toxicity [1].
bone of PEG is chemically inert; on the other hand,
PEGs are characterized by high biocompatibility, the
lack of toxicity and immunogenicity, and good solu-
bility in both water and organic solvents. The presence
of terminal hydroxy groups in PEG macromolecules
ensures their covalent bonding with various biologi-
cally active compounds. This approach is widely used
Hybrid macromolecular antioxidants derived from
hydrophilic natural and synthetic polymers and steri-
cally hindered phenols (SHP) exhibit in aqueous solu-
tion antiradical activity which considerably exceeds
the activity of mixtures of the corresponding unbound
SHP and polymer [2]. Studies on such antioxidants
based on dextran showed that their enhanced anti-
radical activity is determined by hydrated structures
formed in aqueous solution, by the nature of the spacer
connecting the phenolic fragment and polymer chain,
and by the position of redox-active phenolic fragment
inside or outside the polymer hydration shell, which
depends on the spacer length [3]. The highest activity
was found for conjugates with C3 spacers, which were
prepared from 3-(3,5-di-tert-butyl-4-hydroxyphenyl)-
propanoic acid (I), 3-(5-tert-butyl-4-hydroxy-3-
methylphenyl)propanoic acid (II), and 3-(3,5-di-tert-
butyl-4-hydroxyphenyl)prop-2-enoic acid (III) [4].
Table 1. Characteristics of conjugates IVa, IVb, Va–Vc,
VIa–VIc, and VIIa–VIIc
Solubility in
Concentration
Conjugate
no.
M
water at 25°C, of SHP fragments
c = 3.0 g/dL in conjugate, wt %
IVa
Va
03400
03900
06800
21600
03400
03900
06800
21600
03900
06800
21600
–
+
+
+
+
+
+
+
+
+
+
13.3
11.8
07.1
02.4
11.4
10.0
06.5
02.0
11.5
07.1
02.4
VIa
VIIa
IVb
Vb
VIb
VIIb
Vc
Polyethylene glycols (PEGs) are of particular
interest as base polymeric structure for the synthesis of
hybrid macromolecular antioxidants. Polyether back-
VIc
VIIc
371

DOMNINA et al.
372
Scheme 1.
OH
OH
t-Bu
Bu-t
Bu-t
t-Bu
Bu-t
(1) SOCl2
(2) HO(CH2CH2O)nH (IV–VII)
OH
O
O
O
O
Bu-t
n
OH
O
I
IVa–VIIa
OH
OH
Me
Bu-t
Bu-t
Me
Bu-t
(1) SOCl2
(2) HO(CH2CH2O)nH (IV–VII)
OH
Me
O
O
O
O
n
OH
O
II
IVb–VIIb
O
Bu-t
t-Bu
O
(1) SOCl2
(2) HO(CH2CH2O)nH (IV–VII)
t-Bu
OH
OH
O
O
n
HO
HO
Bu-t
O
t-Bu
Bu-t
III
IVc–VIIc
IV, n = 77; V, n = 89; VI, n = 155; VII, n = 491.
in the design of PEG-based dosage forms of various
drugs, cosmetics, and perfumes [5].
2,2-diphenyl-1-picrylhydrazyl (DPPH) (Tables 2, 3).
The antiradical activity of conjugates based on acid II
was higher by a factor of ~1.5–2 than the activity of
those based on acid I. We failed to evaluate the rate
constants k for conjugates Vc–VIIc, for they reacted
with DPPH almost instantaneously.
Up to now, a number of conjugates have been
synthesized via condensation of acid I–III chlorides
with polyethylene glycols having molecular weights
of 3400 (IV), 3900 (V), 6800 (VI), and 21600 (VII)
[6, 7] (Scheme 1). The electronic absorption spectra of
the obtained conjugates are shown in figure, and their
other characteristics are given in Table 1. All con-
jugates displayed a high antiradical activity in aque-
ous–organic medium; their activity was assessed by
measuring the rate constants k for their reaction with
It is known that the system PEG–water is charac-
terized by a critical mixing temperature; below that
Table 3. Rate constants for the reactions of sterically hin-
dered phenol derivatives and conjugates with DPPH in
aqueous dioxane (1:1 by volume) at 20°C
Antioxidant k, l mol–1 s–1 Antioxidant k, l mol–1 s–1
Table 2. Rate constants for the reactions of sterically hin-
dered phenol derivatives and conjugates with DPPH in
aqueous ethanol (1:1 by volume) at 20°C
I
IVa
Va
VIa
VIIa
II
01.7±0.05
18.4±0.90
30.1±1.50
24.6±1.20
21.1±1.20
03.9±0.09
44.2±1.90
Vb
38.1±1.50
41.4±1.70
40.3±1.70
VIb
VIIb
III
Vc
VIc
VIIc
Antioxidant
k, l mol–1 s–1
a
–
I
03.1±0.1
45.4±2.8
87.2±4.7
a
VIIa
VIIb
VIIc
–
a
–
a
a
–
IVb
–
a
a
The rate constant was not determined because of very fast
reaction.
The rate constant was not determined because of very fast
reaction.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 3 2014

INDICATOR PROPERTIES OF OLIGOETHYLENE GLYCOL HYBRIDS
373
D
Table 4. Phase separation temperatures (TPS) of aqueous
conjugate solutions
I
0.6
VIa
Conjugate no.
Phase separation temperature, °C
0.5
0.4
0.3
0.2
0.1
0.0
IVa
Va
VIa
VIIa
IVb
Vb
VIb
VIIb
Vc
VIc
VIIc
22
38
64
>100>0
43
54
75
>100>0
49
250
260
270
280
290
λ, nm
76
>100>0
D
0.7
VIb
Table 5. Phase separation temperatures of solutions of con-
jugate Vc (c = 3.41 g/100 g of water) at different NaCl
concentrations
0.6
0.5
0.4
0.3
0.2
0.1
0.0
II
Concentration of NaCl,
g/100 g of water
Phase separation
temperature, °C
0.000
0.360
0.975
2.933
49.0
48.5
48.0
39.5
temperature no phase separation can be achieved
regardless of the polymer concentration. If the tem-
perature is higher, the system divides into layers, and
polymer separates from the solution. The critical
mixing temperature for PEG with M 5000 is above
100°C; as the molecular weight of PEG increased to
5000000, the critical mixing temperature decreased to
20°C [8]. Thus, the critical mixing temperature de-
creases as the molecular weight of PEG rises.
As expected, no phase separation was observed in
aqueous solutions of PEG used for the synthesis of
conjugates in the temperature range from 15 to 100°C.
Aqueous solutions of the conjugates became turbid
with subsequent phase separation and precipitation as
the temperature rose. Table 4 contains the phase
separation temperatures measured for a conjugate con-
centration of 3%. Increase of the phase separation tem-
perature with rise in the molecular weight of con-
jugates based on the same SHP derivative should be
noted. This is likely to be related to a shift of the
hydrophobic–hydrophilic balance of conjugate toward
higher hydrophilicity as the molecular weight in-
creases. On the other hand, variation of the phase
250
260
270
280
290 λ, nm
D
0.5
III
VIc
0.4
0.3
0.2
0.1
0.0
260
280
300
320
340
360
λ, nm
Electronic absorption spectra of sterically hindered phenols
I–III and conjugates VIa–VIc; c = 0.1 (I), 0.9 (VIa),
0.06 (II), 1.4 (VIb), 0.007 (III), 0.06 g/L ((VIc).
separation temperature for conjugates with the same
molecular weight but different SHP fragments reflects
the relative hydrophobicities of the SHP fragments,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 3 2014

DOMNINA et al.
374
Scheme 2.
O
Bu-t
Bu-t
t-Bu
O
t-Bu
O2, H2O
OH
OH
O
O
O
O
HO
HO
89
89
t-Bu
O
Bu-t
O
Bu-t
t-Bu
Va
Vc
which change in the series I > III > II. The stronger
the hydrophobic interaction, the lower the phase sepa-
ration temperature.
and photon correlation spectroscopy on a PhotoCor®
spectrometer [7]. The kinetics of the reactions of anti-
oxidants with DPPH were monitored by spectropho-
tometry (Shimadzu UV 1800) by measuring the optical
density of solutions at λ 520 nm according to the
procedure described in [3].
3-(3,5-Di-tert-butyl-4-hydroxyphenyl)propanoyl
chloride. Thionyl chloride, 0.2 mL (21 mmol), was
added to a solution of 0.5 g (18 mmol) of acid I in
5 mL of chloroform, and the mixture was heated for
3 h under reflux with protection from atmospheric
moisture. When the reaction was complete, the solvent
and excess thionyl chloride were distilled off under
reduced pressure.
Prokopov et al. [9] studied metabolism of 3-(3,5-di-
tert-butyl-4-hydroxyphenyl)propanoic acid (I) in
animals and found that this acid undergoes autooxida-
tion to 3-(3,5-di-tert-butyl-4-hydroxyphenyl)prop-2-
enoic acid (III). We also observed autooxidation of
conjugate Va (phase separation temperature TPS
=
38°C) to compound Vc (TPS = 49°C) on storage for
several months on exposure to atmospheric moisture
and oxygen. This process was accompanied by a red
shift of the absorption maximum from λ 280 nm for Va
to λ 340 nm (Vc) due to appearance of conjugated
C=C bond in the latter (Scheme 2).
Conjugates IVa–VIIa (general procedure). A solu-
tion of 0.25 g of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)
propanoyl chloride in 1 mL of chloroform and 0.2 mL
of triethylamine were added to a solution of 0.5 g of
PEG in 2 mL of chloroform. The mixture was purged
with argon over a period of 5 min, continuously stirred
for 4–5 h at 40°C, and left overnight at room tempera-
ture. The product was precipitated by pouring the mix-
ture into 60 mL of isopropyl alcohol–petroleum ether
(2:1), separated on a centrifuge, washed twice with the
precipitating mixture, filtered through a Schott filter,
washed with diethyl ether, and dried in air. The product
was additionally reprecipitated from chloroform into
diethyl ether, filtered through a Schott filter, washed
with diethyl ether, and dried under reduced pressure.
The presence of low-molecular-weight impurities was
The phase separation temperature is sensitive to the
electrolyte composition of conjugate solution, which
may be of crucial importance for the use of hybrid
antioxidants in biological systems. According to the
data of photon correlation spectroscopy, addition of
sodium chloride to a solution of Vc leads to monotonic
reduction of the phase separation temperature as the
electrolyte concentration increases (Table 5).
Thus, the results of our study have shown that poly-
ethylene glycol–sterically hindered phenol conjugates
in aqueous solution behave as temperature-sensitive
systems. The phase separation temperature of these
systems depends on the conjugate structure and elec-
trolyte composition, which may be used to estimate
hydrophobicity of SHP fragments linked to PEG. The
highest sensitivity to the SHP structure is observed for
series V conjugates.
1
checked by TLC and NMR. H NMR spectrum of Va,
δ, ppm: 1.39 s (18H, t-Bu), 2.59 br.t (2H, CH2CO),
2.83 br.t (2H, CH2C6H2), 3.61 d (178H, CH2OCH2),
4.20 br.t (2H, COOCH2), 5.05 s (1H, OH), 6.95 br.s
(2H, Harom).
Conjugates IVb–VIIb and IVc–VIIc were synthe-
sized in a similar way from polyethylene glycols IV–
VII and sterically hindered phenols II and III.
EXPERIMENTAL
The UV spectra were measured in the range from
λ 240 to 400 nm on a Shimadzu UV 1800 spectro-
photometer using quartz cells with a cell path length of
1 cm; aqueous ethanol (1:1) was used as solvent. The
1H NMR spectra were recorded from solutions in
CDCl3 on a Bruker DPX-300 instrument.
REFERENCES
Hydrodynamic properties of solutions of unmodi-
fied PEGs and conjugates were studied by viscometry
1. Shtil’man, M.I., Vysokomol. Soedin., Ser. A, 2010,
vol. 52, p. 1551.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 3 2014

INDICATOR PROPERTIES OF OLIGOETHYLENE GLYCOL HYBRIDS
375
2. Aref’ev, D.V., Belostotskaya, I.S., Vol’eva, V.B., Dom-
nina, N.S., Komissarova, N.L., Sergeeva, O.Yu., and
Khrustaleva, R.S., Izv. Akad. Nauk, Ser. Khim., 2007,
vol. 56, p. 751.
6. Domnina, N.S., Sergeeva, O.Yu., Koroleva, A.N.,
Rakitina, O.V., Dobrun, L.A., Filippov, S.K., Mikhailo-
va, M.E., and Lezov, A.V., Vysokomol. Soedin., Ser. A,
2010, vol. 52, p. 1.
7. Dobrun, L.A., Kuzyakina, E.L., Rakitina, O.V., Sergee-
va, O.Yu., Mikhailova, M.E., Domnina, N.S., and
Lezov, A.V., Zh. Strukt. Khim., 2011, vol. 52, p. 1194.
3. Aref’ev, D.V., Cand. Sci. (Chem.) Dissertation,
St. Petersburg, 1999.
4. Vol’eva, V.B., Domnina, N.S., Sergeeva, O.Yu., Komaro-
va, E.A., Belostotskaya, I.S., and Komissarova, N.L.,
Russ. J. Org. Chem., 2011, vol. 47, p. 480.
8. Tager, A.A., Vshivkov, S.A., Andreeva, V.M., and
Sekacheva, T.V., Vysokomol. Soedin., 1974, vol. 16, p. 9.
9. Prokopov, A.A., Berlyand, A.S., and Shukil’, L.V., Khim.-
5. Zalipsky, S., Bioconjugate Chem., 1995, vol. 6, p. 150.
Farm. Zh., 2006, no. 2, p. 3.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 50 No. 3 2014