Exploring the meaning of sugar configuration in a supramolecular environment: Comparison of six octyl glycoside micelles by ITC and NMR spectroscopy

Article abstract of DOI:10.1039/c4md00122b

A series of octyl α- and β-glycosides of the manno- galacto- and gluco-series were synthesized and employed in formation of homo- and hetero-micelles in water. Critical micelle concentrations (cmc), thermodynamic quantities of demicellation and, to some extent, the hydrodynamic radii of glycomicelles were determined by isothermal titration calorimetry (ITC) and diffusion NMR studies. The goal of this work was to determine the significance of anomeric configuration as well as of epimerisation at the sugar ring for supramolecular features of the respective glycoside. A new projection of glycoside structures is proposed to facilitate interpretation of structure-property relationships in this regard. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4md00122b

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Exploring the meaning of sugar configuration in a
supramolecular environment: comparison of six
octyl glycoside micelles by ITC and NMR
spectroscopy†
Cite this: Med. Chem. Commun., 2014,
, 1218
5
Jorn Schmidt-Lassen and Thisbe K. Lindhorst*
¨
A series of octyl a- and b-glycosides of the manno- galacto- and gluco-series were synthesized and
employed in formation of homo- and hetero-micelles in water. Critical micelle concentrations (cmc),
thermodynamic quantities of demicellation and, to some extent, the hydrodynamic radii of glycomicelles
were determined by isothermal titration calorimetry (ITC) and diffusion NMR studies. The goal of this
work was to determine the significance of anomeric configuration as well as of epimerisation at the
sugar ring for supramolecular features of the respective glycoside. A new projection of glycoside
structures is proposed to facilitate interpretation of structure–property relationships in this regard.
Received 16th March 2014
Accepted 29th May 2014
DOI: 10.1039/c4md00122b
www.rsc.org/medchemcomm
such as for the study of self-assembly properties of glycolipids,
Introduction
7–10
for example.
They are amenable to different biophysical
Carbohydrates are constituents of every cell surface (glycocalyx) techniques, e.g. surface tension measurements, uorescence
8
where they play crucial roles in cellular communication, both in spectroscopy, or various scattering methods (DLS, SANS, SAXS).
health and disease states of an organism. A lot is known about In addition, methods that allow to measure kinetic and
1
the biological importance of the glycosylated cell surface; but
little has been disclosed about the meaning of specic sugar
congurations within this supramolecular environment.
However, for the advancement of the medicinal chemistry of
carbohydrates, it is important to improve our knowledge about
the impact of sugar conguration on the cell surface biology. To
date, it is not obvious how the subtle differences between bio-
2
logically important carbohydrate structures, for example D-
mannose, D-galactose or D-glucose, contribute to specicity and
regulation in cell biology, while being embedded into a highly
dynamic molecular context that expands over 100 nm width or
more. Moreover, there are no direct experimental means to
investigate and compare the structural effects of individual
monosaccharides in vivo. Therefore, we have used glycomicelles
as a highly simplied but well approved multivalent and supra-
3–5
molecular construct, to probe how the conguration of sugars
inuences their interactions in water. Eventually, determination
of structure–property relationships in micelle formation will
allow us to draw conclusions about the impact of congurational
differences in a supramolecular carbohydrate environment.
Glycomicelles are robust and well investigated supramolec-
6
ular systems that have been used earlier in biological chemistry
Otto Diels Institute of Organic Chemistry, Christiana Albertina University of Kiel,
Otto-Hahn-Platz 3-4, D-24098 Kiel, Germany. E-mail: tklind@oc.uni-kiel.de
Fig. 1 Structures of the investigated amphiphilic octyl glycosides. Two
series of sugars differing in their configurational characteristics were
employed for glycomicelle formation: a- and b-glycosides 1, 3, and 5
and 2, 4, and 6, respectively.
†
Electronic supplementary information (ESI) is available: Containing details of
synthesis, ITC and DOSY NMR measurements. See DOI: 10.1039/c4md00122b
1218 | Med. Chem. Commun., 2014, 5, 1218–1226
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thermodynamic parameters of micelles are of relevance in a ITC measurements of homo-glycomicelles
biological context.
At rst, isothermal titration calorimetry (ITC) was used to study
homo-micelle formation of the octyl a-glycosides 1, 3, 5 and of
the corresponding octyl b-glycosides 2, 4 and 6. ITC is a stan-
Here, a collection of six different octyl glycosides were
synthesized and employed (Fig. 1) in isothermal calorimetry
titrations and NMR-spectroscopic studies, including well-
known octyl b-glucoside (6), which is commercially available
and regularly used for the formation of glycomicelles. Two
series of octyl glycosides were tested, a-D-mannoside (1), a-D-
galactoside (3), and a-D-glucoside (5), and the respective b-
glycosides 2, 4, and 6. Hence, the inuence of the anomeric
conguration on micelle formation can be compared, as well as
the meaning of inversion of conguration at C2 and C4 of the
sugar ring, respectively. In addition, mixed, so-called hetero-
glycomicelles were formed and compared with the respective
homo-glycomicelles.
dard technique to determine thermodynamic parameters of
11–15
molecular interactions in solution
including micelle
formation and determination of critical micelle concentrations
16,17
(cmc).
Cmc values are frequently obtained in demicellation
studies employing concentrated solutions of each sample to be
titrated into water. In our study, as expected, exothermic dem-
icellation processes were observed in all cases as reected by the
obtained ITC diagrams (Fig. 2); cmc values were deduced from
their inection points. The obtained data are collected in the
table insert of Fig. 2. Owing to its low water solubility, the cmc of
aOctylGlc (5) could not be determined by ITC. The cmc of 5,
however, has been reported in the literature based on tensio-
18,19
metric studies (Fig. 2, table insert).
Results
Synthesis
Thus, the effect of anomeric conguration on micelle
formation could be studied in the manno- as well as the galacto-
The employed amphiphilic octyl glycosides 1–6 are literature- series (Fig. 2a and b). No signicant difference between the a-
known and were synthesized according to described procedures and b-glycomicelles was detected in the galacto-series, but the
(cf. Experimental and ESI†).
manno-series, on the other hand, exhibits a distinctive effect
Fig. 2 Integrated ITC titration curves of homo-glycomicelles obtained at 298 K. Effect of anomeric configuration: (a) a- and b-octyl mannoside
micelles formed of 1 and 2; (b) a- and b-octyl galactoside micelles formed of 3 and 4. Effect of configuration of the sugar ring: (c) a-anomeric
micelles formed of 1 and 3, respectively (5 was not sufficiently soluble in water); (d) b-anomeric micelles formed of 2, 4, and 6, respectively.
R
Representative curves are shown. D H: reaction enthalpy per injection. ITC starting concentrations c and obtained mean cmc values are listed in
19
19
18
19
19
16
20
the table insert; SD: standard deviation. (a) 6.0; (b) 16; (c) 6.3, 12; (d) 20, 23, 25.8.
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with a much lower cmc for the a-micelle (10.9 mM) than for the
respective b-micelle (22.9 mM). Accordingly, formation of a-
mannomicelles starts at ꢀhalf the concentration which is
DG ¼ DH ꢂ TDS
(2)
ꢁ
The enthalpy of demicellation (DHdemic) can be calculated
required for formation of the corresponding b-micelle. In the according to eqn (3).
gluco-series, we determined a cmc of 28.3 mM for the b-gluco-
csyringe
ꢁ
ꢁ
DH
¼
D
R
H
(3)
demic
demic
side. Literature reports on cmc values of octyl glucosides diverge
cmic
to some extent, which can be due to different determination
18
19
methods employed. Focher and Matsumura and co-workers
used a surface tension method for cmc determination, where
effects of dilution and all intermolecular interactions are not
taken into account. Consequently lower cmc values were found.
Focher et al. determined the cmc value for the b-glucoside 6 as
For this calculation, csyringe was considered as equal to
cmono + cmic. Here the approximation is used that above the cmc,
the concentration of the monomers remains constant. Thus, the
thermodynamic parameters of demicellation could be deduced
from measured cmc values and are collected in Table 1.
18
1
9 mM and for the a-glucoside 5 as 6.3 mM. Paula et al., on the
other hand, used titration calorimetry and obtained a cmc of
3.0 mM (ref. 16) for the b-glucoside 6. Regardless of these
The thermodynamic data summarized in Table 1 show that
ꢁ
micelle formation is an exergonic process with negative DGmic
2
ꢁ
values. Enthalpy changes of micelle formation, DHmic, are
differences, the gluco-series shows, in analogy to the manno-
case, that a-glucomicelles are formed at lower concentrations
than b-glucomicelles. This effect is less pronounced in case of
the galactoside micelles.
23
positive though, but enthalpy–entropy compensation results
ꢁ
in an overall exergonic process. Positive DS
values indicate
mic
that micelle formation is entropically favoured, an effect which
has been explained by desolvation of monomers during micelle
Next, the inuence of congurational characteristics of the
sugar ring on micelle formation was studied (Fig. 2c and d). The
two a-glycosides 1 and 3 show a signicant difference in gly-
comicelle formation, with the cmc of mannoside 1 (10.9 mM)
being much smaller than the cmc of galactoside 3 (30.2 mM). In
the series of the three b-glycosides 2, 4, and 6, again the cmc of
the galactoside (4, 31.7 mM) was found to be the highest.
Overall, within both series of a- and b-octyl glycosides a
carbohydrate-specic dependency of cmc values was observed,
namely cmc(aMan) z cmc(aGlc) < cmc(aGal), and cmc(bMan) <
cmc(bGlc) < cmc(bGal).
24
formation. This behavior is observed for all investigated
glycomicelles.
ITC measurements of hetero-glycomicelles
In the next step, we extended our project to the investigation of
mixed glycomicelles, called hetero-micelles. Such systems are of
particular interest as they reect more of the carbohydrate
diversity of the natural glycocalyx than homo-glycomicelles.
Equally, hetero-glycoclusters have become important in the
glycosciences, showing unexpected, so-called hetero-glyco-
Next, thermodynamic data of micelle formation were
deduced from the measured ITC curves, using the so-called
25
cluster effects. To date, self-aggregation of mixed micellar
16
systems formed of octyl glycosides and ionic surfactants have
phase-separation model. In this model, the glycomicelle is
considered as a separate microphase. At high aggregation
numbers of the investigated micellar systems (assumed
26,27
been described.
Furthermore, the aggregation behavior of
mixed micelles formed of alkyl maltosides with varying alkyl
28
16,20–22
chain length was investigated. Also bolaamphiphiles were
according to the literature
) the Gibbs free energy of dem-
29
studied as mixed micelles with octyl glycoside doping.
icellation is dened by the chemical potential of the amphiphile
in water and the amphiphile in micellar phase (eqn (1)). To
obtain the Gibbs free energy DG from measured cmc values,
Here, we have employed 1 : 1-mixtures of two amphiphilic
glycosides of the same anomeric conguration in a rst series of
experiments. The standard demicellation protocol was followed
to obtain ITC curves of the investigated six mixed micelles, three
(1) a-hetero-micelles (Fig. 3a) and three b-hetero-micelles (Fig. 3b).
Strikingly, in all cases the titration curves show exactly one
point of inection (reecting one cmc value) indicating the
0
molar fractions cmc are pluged in.
ꢁ
¼ mw ꢂ m ¼ ꢂRT ln cmc0
0
0
mic
DG
demic
ꢁ
Demicellation enthalpy, DHdemic, and demicellation entropy,
ꢁ
ꢁ
DSdemic, are obtained from DGdemic according to the Gibbs– formation of just one type of mixed micelle. Hence, it can be
Helmholtz relation (2).
concluded that the employed 1 : 1-mixtures form homogeneous
ꢁ
ꢁ
ꢁ
ꢁ
Table 1 Thermodynamic parameters of demicellation (DHdemic, DGdemic, TDSdemic and DSdemic) of homo-glycomicelles on the basis of the
16
0
phase-separation model according to eqn (1) and Gibbs–Helmholtz relation (2); cmc ¼ critical micelle concentration in mole fractions; T ¼
98 K; ꢃstandard deviation
2
aOctylMan 1
bOctylMan 2
aOctylGal 3
bOctylGal 4
bOctylGlc 6
0
ꢂ4
cmc (ꢄ10
)
1.95 ꢃ 0.13
ꢂ8.55 ꢃ 0.07
21.2 ꢃ 0.2
4.11 ꢃ 0.06
ꢂ7.80 ꢃ 0.03
19.3 ꢃ 0.1
5.42 ꢃ 0.07
ꢂ7.52 ꢃ 0.04
18.6 ꢃ 0.1
5.68 ꢃ 0.06
ꢂ7.47 ꢃ 0.03
18.5 ꢃ 0.2
5.07 ꢃ 0.09
ꢂ7.59 ꢃ 0.05
18.8 ꢃ 0.2
0
ln cmc
DGdemic [kJ mol ] ¼ ꢂDGmꢁ ic
ꢁ
ꢁ
ꢂ1
ꢂ1
ꢂ1
DHdemic [kJ mol
]
ꢂ10.3 ꢃ 1.7
ꢂ31.5 ꢃ 1.9
ꢂ105.6 ꢃ 6.5
ꢂ9.7 ꢃ 0.9
ꢂ9.2 ꢃ 1.0
ꢂ8.4 ꢃ 0.7
ꢂ6.4 ꢃ 1.2
ꢁ
TDSdemic [kJ mol
]
ꢂ29.1 ꢃ 0.8
ꢂ97.5 ꢃ 2.8
ꢂ27.9 ꢃ 1.1
ꢂ93.5 ꢃ 3.3
ꢂ26.9 ꢃ 1.2
ꢂ90.2 ꢃ 3.1
ꢂ25.2 ꢃ 1.4
ꢂ84.6 ꢃ 4.6
ꢁ
ꢂ1
ꢂ1
DSdemic [J K mol
]
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Fig. 3 Integrated ITC titration curves of hetero-glycomicelles obtained at 298 K: (a) micelles formed of two different a-glycosides (aOctylMan 1
aOctylGlc 5, aOctylMan 1 + aOctylGal 3, aOctylGal 3 + aOctylGlc 5); (b) micelles formed of two different b-glycosides (bOctylMan 2 +
+
bOctylGlc 6, bOctylMan 2 + bOctylGal 4, bOctylGal 4 + bOctylGlc 6). ITC starting concentrations c (1 : 1 ratio of two glycosides) and deduced
average cmc values are listed in the table insert; SD: standard deviation.
hetero-glycomicelles and that phase separation or homo- supramolecular properties of mixed micelles can differ signi-
micelle formation is not effectively competing in this self- cantly from pure homo-micelles. Similar effects have found
33
aggregation process.
technical applications in other binary systems.
In Table 2 the thermodynamic parameters of the investi-
Also in hetero-micelle formation, the investigated manno-
sides exerted a special effect. All mannoside-containing hetero- gated hetero-micelles are summarized which were calculated in
micelles had lower cmc values than mannoside-free hetero- analogy to what has been described for the homo-glycomicelles.
micelles. In the a-series, the mixed micelles containing octyl Again, formation of hetero-glycomicelles is entropically driven
mannoside had cmc values of 13.0 ꢃ 0.7 mM and 15.4 ꢃ 0.3 leading to overall exergonic processes.
mM, whereas the mannose-free hetero-micelle had cmc ¼ 25.4
ꢃ 0.7 (Fig. 3, table insert). In the b-series, the analogous
DOSY NMR experiments
behaviour was observed. It should be noted, that calculation of
cmc values of mixed micelles has been reported in the litera- While ITC experiments are suited to describe the equilibrium
30,31
ture.
In these cases, cmc values were either obtained state of a micellar system, diffusion ordered NMR spectroscopy
according to a model for ideal mixing behaviour, or the (DOSY) can be used to measure the diffusion rate of molecules
employed relation was adapted for non-ideal mixing of the or aggregates, respectively. Thus, information about the
components using regular solution theory by Rubingh and dynamics of self-assembly and the size of formed micelles can
32
34,35
colleagues.
be gained.
NMR diffusion measurements involve the so-
In our experiments it was striking, that it was possible to called pulsed eld gradient spin echo NMR experiment (PFGSE)
employ the weakly water-soluble octyl a-D-glucoside 5 in a which has found numerous applications in supramolecular
binary system and to determine the cmc values of the respective chemistry. It is based on a pulse sequence, in which the
hetero-glycomicelles.
This
demonstrates
that
the intensity of an echo signal depends on molecular motion in a
ꢁ
ꢁ
ꢁ
ꢁ
Table 2 Calculated thermodynamic parameters of demicellation (DHdemic, DGdemic, TDSdemic and DSdemic) of mixed glycomicelles on the basis of
16
0
the phase-separation model according to eqn (1) and Gibbs–Helmholtz-relation (2); cmc ¼ critical micelle concentration in mole fractions; T
298 K; ꢃstandard deviation
¼
aOctylMan 1,
aOctylGlc 5
aOctylMan 1,
aOctylGal 3
aOctylGal 3,
aOctylGlc 5
bOctylMan 2,
bOctylGlc 6
bOctylMan 2,
bOctylGal 4
bOctylGal 4,
bOctylGlc 6
0
ꢂ4
cmc (ꢄ10
)
2.34 ꢃ 0.14
ꢂ8.36 ꢃ 0.06
20.7 ꢃ 0.2
2.76 ꢃ 0.06
ꢂ8.20 ꢃ 0.03
20.3 ꢃ 0.1
4.48 ꢃ 0.16
ꢂ7.71 ꢃ 0.07
19.1 ꢃ 0.2
4.18 ꢃ 0.04
ꢂ7.78 ꢃ 0.01
19.3 ꢃ 0.1
4.39 ꢃ 0.07
ꢂ7.73 ꢃ 0.04
19.2 ꢃ 0.2
5.71 ꢃ 0.06
ꢂ7.47 ꢃ 0.03
18.5 ꢃ 0.1
0
ln cmc
DGdemic [kJ mol ] ¼ ꢂDGmꢁ ic
ꢁ
ꢁ
ꢂ1
ꢂ1
ꢂ1
DHdemic [kJ mol
]
ꢂ12.3 ꢃ 0.1
ꢂ33.1 ꢃ 0.1
ꢂ110.9 ꢃ 0.4
5.2 ꢃ 0.3
ꢂ10.6 ꢃ 0.1
ꢂ29.7 ꢃ 0.1
ꢂ99.8 ꢃ 0.3
ꢂ8.6 ꢃ 0.4
ꢂ8.5 ꢃ 0.2
ꢂ7.2 ꢃ 0.9
ꢁ
TDSdemic [kJ mol
]
ꢂ25.2 ꢃ 0.3
ꢂ85.7 ꢃ 1.3
ꢂ27.9 ꢃ 0.4
ꢂ93.4 ꢃ 1.3
ꢂ27.7 ꢃ 0.3
ꢂ92.8 ꢃ 0.9
ꢂ25.7 ꢃ 0.9
ꢂ86.1 ꢃ 3.1
ꢁ
ꢂ1
ꢂ1
DSdemic [J K $mol
]
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pulsed eld gradient. Faster moving molecules lead to echo
In (5), k is an interaction constant, representing dynamic
signals of lower intensity. From the signal decrease the diffu- interactions of hard spheres. According to the literature, k was
36
38
sion coefficient can be calculated.
chosen as 1.73.
f
represents the volume fraction of the
mic
Here, the two anomeric glycosides, a- and b-octyl mannoside micelles. It was obtained using eqn (6).
and 2, were selected for diffusion NMR experiments, as they
1
M
fmic ¼ ðctotal ꢂ cmcÞ d
(6)
showed the most pronounced effect of anomeric conguration
on micelle formation. For determination of the diffusion coef-
1

cients Dobs, four different regions in the H NMR spectra of the
In eqn (6), M is the molecular weight of the octyl mannoside
1 or 2, respectively) and d its density (cf. ESI).
By insertion of eqn (5) and (6) into (4), expression (7) is
obtained that was used for tting of the data points of the PGSE
NMR experiment (cf. Fig. 5).
investigated compounds were used (d ¼ 4.00–3.30, 1.65–1 : 37,
(
1
.36–1.00, and 0.90–0.60 ppm). Samples were measured at
concentrations between 2.5 and 250 mM for 1, and 10 and 250
mM for 2, employing the PFGSE experiment and measured Dobs
values plotted against sample concentration (Fig. 4). As expec-
!
34
ted,
D
obs values decrease rapidly with concentrations
2
cmc
ðctotal ꢂ cmcÞ sk
D
obs ¼ c
D
mono
þ
ꢄ Dmic;0
(7)
increasing beyond the cmc, resulting from the limited molec-
ular motion within the micellar assembly.
total
c
total
M1;2
d1;2
All diffusion NMR experiments showed just one diffusing with s ¼
¼ 0:248
species. Thus, it can be concluded that the equilibrium between
Thus, self-diffusion coefficients Dmic,0 for the micellar state
of octyl mannosides could be determined and the obtained data
are summarized in Fig. 5.
Both experimental methods, ITC and DOSY NMR spectros-
copy, led to very similar cmc values for 1 and 2. However, the
cmc values determined by diffusion NMR spectroscopy for
mannoside 1 (9.1 mM) and 2, (18.8 mM) were found somewhat
lower than the respective values determined by ITC (10.9 mM
for 1 and 22.9 mM for 2). This nding has in principal been
monomeric and micellar molecules is fast on the time scale of
the NMR experiment. Consequently, the observed diffusion
coefficient Dobs is a weighted average from two states of the
investigated octyl mannoside (1 or 2, respectively), monomeric
34,37
and micellar. Hence, Dobs can be expressed by eqn (4).
cmc
c
total ꢂ cmc D
D
obs ¼ c
D
mono
þ
mic
(4)
total
c
total
39
Eqn (4) is valid for concentrations >cmc, and thus only described before for association constants of calixpyrrols and
respective data points were used. Dmono was inserted into the can be explained by additional intermolecular interactions
equation as a xed parameter that was obtained by NMR during demicellation occurring in the ITC experiment.
3
4
measurements at concentrations below the cmc. ctotal was
inserted as an independent variable.
The diffusion coefficients Dmono determined for both man-
nosides, 1 and 2, were found equal within error limits (4.35 ꢄ
ꢂ10
2
ꢂ1
ꢂ10
2
ꢂ1
However, in eqn (4) obstruction effects that occur at nite 10
aggregation concentrations, are not taken into account. There- published data for bOctylGlc (6). On the contrary, the diffusion
fore, according to Nilsson and S ¨o derman, eqn (4) was cor- coefficients Dmic,0 of the respective micelles were found to differ
m s for 1 and 4.21 ꢄ 10
m s for 2) and similar to
40
34
ꢂ11
2
ꢂ1
rected using relation (5).
signicantly with Dmic,0 (1) of 3.27 ꢄ 10
m s and Dmic,0 (2)
ꢂ11
2
ꢂ1
of 4.42 ꢄ 10
m s . Hence, the a-mannosidic micelle
Dmic ¼ Dmic,0 (1 ꢂ kfmic
)
(5) diffuses more slowly than the b-mannosidic micelle. This
corresponds to larger aggregates in the a-case than in the b-
case. Thus, again a signicant inuence of the anomeric
conguration of the mannosidic amphiphile on self-aggrega-
tion was revealed.
To obtain an estimate of the size of the investigated micelles
the Stokes–Einstein eqn (8) was employed.
B
k T
R
h
¼ 6
(8)
phDmic;0
In (8) R
h
is the hydrodynamic radius of the micelle, h the
dynamic viscosity of the used solvent, T the temperature and k
B
the Boltzmann constant. With the dynamic viscosity of D
2
O ¼
ꢂ2 41
h
1.10 mNs m , R of the a-mannosidic micelle was calculated
˚
as 60.7 A and for the corresponding b-mannosidic micelle as
˚
4
4.9 A. Thus, the a-mannosidic micelle is signicantly bigger
Fig. 4 Concentration-dependent diffusion coefficients of mannosidic
micelles in D O formed of octyl a-D-mannopyranoside 1 and octyl b-
D-mannopyranoside 2, respectively, as determined by diffusion NMR
spectroscopy at 298 K. In the inset the transition from monomeric to
micellar state of the dissolved glycoside is enlarged.
than its b-analogue. It has to be kept in mind, however, that the
Stokes–Einstein equation assumes a spherical shape for
micelles, while the investigated systems might have a different
form and thus other sizes than the calculated ones (vide infra).
2
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Fig. 5 Determination of cmc values and diffusion coefficients Dmono and Dmic,0 for aggregation systems formed with a- and b-mannosides 1 and
2, respectively, at 298 K. For fitting, eqn (7) and data points of ctotal > cmc were used; (a) Dmono at ctotal ¼ 5 and 7.5 mM; (b) Dmono at ctotal ¼ 15 mM.
As occurring obstruction effects were taken into account in our
Discussion and conclusion

h
tting procedure, the reason for the large R values may be
Glycomicelles have been employed in various research to date. found by re-considering the shape of the investigated micelles.
This account is dedicated to glycomicelles in order to study the In the employed Stokes–Einstein equation a spherical shape of
signicance of congurational changes of glycosides in a micelles is assumed, however, the herein investigated micelles
supramolecular context. Thus, micelle formation has been might adopt a different form, probably rodlike or ellipsoidal.
investigated with the complete set of amphiphilic octyl glyco- We will perform additional studies to specify the shape of octyl
sides of the manno-, gluco- and galacto-series of both anomeric mannoside micelles and understand differences caused by the
congurations using ITC and DOSY NMR spectroscopy. With anomeric conguration (a or b).
this selection, we were able to investigate the inuence of
It has become obvious in our study, that subtle structural
anomeric conguration as well as of the conguration at C-2 changes such as epimerisation at C-2 of octyl glycosides lead to
and C-4 of the sugar ring on glycomicelle formation. Our data, signicant effects in a supramolecular context, including
in agreement with the available published work, have clearly hetero-micelles However, how structural details determine the
revealed that the tendency to form micelles in water decreases self-aggregation process, remains a secret until this day.
from mannose over glucose to galactose (cmc(aMan) z
cmc(aGlc) cmc(aGal), and cmc(bMan) cmc(bGlc) <
It has been reasoned earlier that conguration and different
tilt angles of a- and b-glycosides inuence the geometry as well
<
<
cmc(bGal)). Moreover, it was shown that all investigated a- as the hydrophilicity of glycosides, factors that can determine
18,19,45
congured glycomicelles form more easily than their b- glycomicelle formation.
In addition, theoretical studies
analogues. Overall, octyl a-mannoside requires the lowest have highlighted the importance of variable hydrogen bonding
46
concentrations for micelle formation in water among the six in glycomicelle formation of glucosides. MD simulations with
investigated octyl glycosides (Fig. 6).
octyl b-glucoside and octyl b-galactoside suggested that the b-
The employed mannosides were also unique when employed galactoside has a slightly higher tendency to form inter- and
in the formation of mixed micelles. Thus, the cmc value that is intramolecular hydrogen bonds than the corresponding b-
47
obtained for aOctylMan/aOctylGal hetero-micelles is signi- glucoside. How this results in lower cmc values for the
cantly affected by the a-mannosidic component (Fig. 3). glucoside is not completely clear given that micelle formation is
Equally, the cmc values obtained for bOctylMan/bOctylGlc as an entropically driven process.
well as for bOctylMan/bOctylGal hetero-micelles are particularly
affected by the b-mannoside component bOctylMan, hence tionships, we suggest an alternative structure view on glycosides
increasing the tendency to micelle formation.
to assess the congurational characteristics of the investigated
The micellar hydrodynamic radii R
obtained for 1 and 2 a- and b-octyl glycosides in new light (Fig. 6). In the proposed
To facilitate the interpretation of structure–property rela-
h
˚
˚
projections, the relative positions of the hydroxy groups at C-2
and C-4 are depicted at respective edges of two fused triangles,
using a PFGSE NMR experiment, namely 60.7 A and 44.9 A, are
rather large compared to reported micelle sizes (ꢀ23–27 A˚ ).
42–44
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Fig. 6 A new view on structural characteristics of the investigated a- and b-octyl glycosides is suggested and related to classical chair
conformations and Newman projections, respectively. This structure representation highlights the variable hydroxyl groups at C-2 and C-4 of the
sugar ring at respective edges of two fused front face triangles, in relation to another two triangles representing the far end of the glycoside. The
relative positioning of the octyl aglycone to the front face triangles depends on whether the depicted glycoside is a- or b-configured. Increasing
ease of micelle formation is indicated by arrows with aOctylMan requiring the least concentrated solution for micelle formation in water.
forming the ‘front face’ of the glycoside, in relation to the octyl
aglycone moiety in the rear of the molecule. Interestingly, this
structural representation suggests that the glucosides can be
Experimental
Synthesis of octyl glycosides
19,48
19,49
considered as ‘transition’ between mannoside (low cmc) and The 1,2-trans glycosides 1,
4,
and 6 (ref. 50) were
galactoside (high cmc) features. The two front triangles of the synthesized according to the literature. The 1,2-cis glycosides
5
0,51
mannosides on the one hand and the glucosides on the other, 2,
3 and 5 (ref. 19) were prepared using Fischer glycosyla-
52
53
are mirror images of one another, whereas the relative position tion according to modied literature procedures. Separation
of the aglycone moiety at the far end of the molecule remains of anomeric mixtures was accomplished by ash column
constant. The different positioning of the 2,4-dihydroxy group chromatography. Analytical data of the synthetic glycosides are
pair relative to the octyl aglycone in the a- and b-case, respec- reported in the ESI.†
tively, also becomes very obvious using the suggested projec-
tion. The depicted projection could even be further expanded by ITC demicellation protocol16,54
including the direction of the O–H bonds. This would demon-
ITC measurements were performed on a MicroCal VP-ITC
strate how the hydroxyl hydrogen atoms are exposed to the
(
MicroCal, Inc., Northhampton, MA, now GE Healthcare).
surrounding of the glycoside, featuring a particular hydrogen
bonding behavior. The proposed projection could equally
support the interpretation of self-aggregation of mixed micelles,
which were shown to display different properties than the
homo-micelles. In our further work, we will expand the present
study in both an experimental and a theoretical context and we
will make an attempt to draw inspiration from the proposed
Sample preparation
For ITC, highly concentrated solutions (10-fold concentration of
cmc in double distilled water) were prepared in a 1 or 2 mL
volumetric ask and used aer ultrasonic degassing for 5 min.
‘
trinity projection’ of glycosides. In particular, we have Titration procedure
commenced MD simulations to understand the notable effects
of octyl a- and b-mannosides on micelle formation.
The cells (reference cell and sample cell) were lled with double
distilled water and the amphiphilic solution was added using
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the injection syringe. The number of aliquots was 70 up to 93. At
higher aliquot numbers an injection volume of 3 mL was used, at
lower aliquot numbers 4 mL were titrated into the sample cell.
An interval of 540 s was used between titration steps. The
sample was stirred by the syringe during titration with stirring
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Cleaning of the injection syringe and the cells
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Aer draining off residual solution, the syringe was washed with
100 mL of double distilled water and 50 mL methanol (HPLC
grade). Using the ThermoVac the syringe was dried for 10 min
under vacuum and maintained at reduced pressure for least 1 h
until the next measurement was performed. The sample cell
was rst washed with Contrad 70 liquid detergent (10% in
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2
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4
91–494.
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Acknowledgements
We thank Prof. Dr. F. D. S ¨o nnichsen and Dipl.-Chem. H. Kobarg
for their support in NMR diffusion experiments. Valuable
discussions with V. Baronin Sta ¨e l von Holstein, Dipl.-Ing. are
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