Functionalised mesoporous silica: A good opportunity for controlled peptide oligomerisation

Article abstract of DOI:10.1039/c0jm04492j

In this paper, mesoporous organosilicas functionalised with aminopropyl groups have been successfully used for peptide oligomerisation. For this purpose, three mesoporous silica SBA-15 type containing different amounts of aminopropyl groups were prepared

Full text of DOI:10.1039/c0jm04492j

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PAPER
Functionalised mesoporous silica: a good opportunity for controlled peptide
oligomerisation†
a
b
a
a
a
b
Gilles Subra,* Ahmad Mehdi,* Christine Enjalbal, Muriel Amblard, Luc Brunel, Robert Corriu
a
and Jean Martinez
Received 22nd December 2010, Accepted 14th February 2011
DOI: 10.1039/c0jm04492j
In this paper, mesoporous organosilicas functionalised with aminopropyl groups have been
successfully used for peptide oligomerisation. For this purpose, three mesoporous silica SBA-15 type
containing different amounts of aminopropyl groups were prepared by direct synthesis and using 3-
tert-butyloxycarbonylamino propyltriethoxysilane. Thanks to amino groups and under well selected
experimental conditions, amino acids N-carboxyanhydride-polymerisation has been achieved within
the pores with control of the physical properties of peptide functionalised hybrid materials. NCA of
alanine, side chain protected glutamic acid and methionine were used for this study. For the first time,
direct LDI-MS analysis was successfully performed on the resulting covalently bound supported
oligomers. To demonstrate the potential application of this class of hybrid bio-organic–inorganic
material as supported catalysts, one of the methionine-functionalized OMSs was used to promote
disulfide bond formation in a model peptide.
Introduction
of the residual silanol (SiOH) groups at the surface as well as
7
their accessibility, are possible. A one step alternative approach,
Organic–inorganic hybrid materials based on silica and obtained
by sol–gel process have attracted considerable attention during
the last decades, as they constitute a fascinating class of materials
combining the properties of organic moieties and inorganic
overcoming the main restrictions of the post-synthesis method,
8,9
has been developed. It consists of the copolymerisation of
tetraethoxysilane (TEOS) and an organotrialkoxysilane RSi
0
(
OR )
3
in the presence of a structure-directing agent. In this case,
1–3
matrix.
the functional groups of the resulting materials were regularly
10
distributed on the pores’ surface. Among organic–inorganic
4
Since the discovery of ordered mesoporous silicas, many
investigations have focused on their functionalization to render
them suitable for applications such as in catalysis, separation,
hybrid materials, those prepared from ordered mesoporous
material (OMS) are of special interest. Indeed, silica can easily be
functionalised through trialkoxysilane chemistry, providing
functional groups used as anchoring points for covalent immo-
bilization of organic moieties. Moreover the synthesis of OMS
which proceeds in the presence of surfactants, enables the
tunable control of this structure at the nanometric scale yielding
pore diameters from 2 to 30 nm. Compared to other materials of
undefined porosity such as silica gels, the use of OMS greatly
rationalizes the inclusion of organic compounds in a non-cova-
lent or covalent way. Large proteins such as enzymes and heme
5
chemical sensors, etc. Two main approaches have been used to
anchor organic groups onto the inner pore surface of meso-
porous silicas: grafting method (post-synthesis) and co-conden-
sation method (direct synthesis). Post-synthesis grafting of an
0
organotrialkoxysilane RSi(OR )
3
onto the pore surface of the
mesoporous silica was the first method established for func-
6
tionalization. This method is generic and allows the incorpo-
ration of many R groups including bulky ones. However, neither
the loading control nor the distributions of the functional
groups, which depend on several parameters such as the number
11
proteins were recently covalently immobilized on OMS. More
recently, nanocomposite materials were prepared from an OMS
obtained by post-synthetic grafting of 3-(trimethoxysilyl)propan-
a
Institut des Biomol eꢀ cules Max Mousseron (IBMM), UMR5247 CNRS,
Universit eꢀ s Montpellier 1 et 2, 15 avenue Charles Flahault, 34000
1
-amine and N-carboxyanhydride (NCA) of alanine and side
Montpellier, France. E-mail: gilles.subra@univ-montp1.fr; Fax: +33 4 67
54 86 54
12
chain protected lysine.
b
Institut Charles Gerhardt, Chimie Mol eꢀ culaire et Organisation du Solide,
Universit eꢀ Montpellier 2, cc 1701, Place E. Bataillon, 34095 Montpellier
Cedex 5, France. E-mail: ahmad.mehdi@univ-montp2.fr; Fax: +33 4
In this paper, we report the preparation of hybrid oligopep-
tide–silica material starting from well defined aminopropyl
functionalised silica of the SantaBarbara Amorphous type
6
†
1
714 38 52; Tel: +33 4 67 14 38 32
1
3
(
SBA) and NCA of alanine, protected glutamic acid and
Electronic supplementary information (ESI) available: See DOI:
0.1039/c0jm04492j
methionine. The pore size and aminopropyl group loading
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ꢀ
1
(
mmol g ) of each OMS sample were carefully controlled thanks
OMS was washed 3 times with 10 ml of DMF and 3 times with
the same volume of DCM.
to the direct synthesis approach consisting of the hydrolysis and
co-condensation of variable amounts of TEOS and 3-tert-buty-
loxycarbonylaminopropyltriethoxysilane in the presence of
surfactant self-assemblies under acidic conditions.
Solids characterization
13
The solid-state CP-MAS C NMR spectra were recorded on
a BRUKER FTAM 300 by using the TOSS technique. The
repetition time was 5 seconds with contact times of 3 milli-
Experimental
Chemicals
1
seconds. The duration of the H pulse was 4.2 microseconds and
the MAS rate was 10 kHz. Chemical shifts (d, ppm) were refer-
Pluronic P123 triblock copolymer (PEO20PPO70PEO20 with
4
enced to Me Si. The nitrogen adsorption isotherms were
PEO ¼ poly(ethylene oxide), PPO ¼ poly(propylene oxide) and
measured at liquid temperature (77 K) using a Micromeritics
Tristar 3000 analyser. Before the measurements, the samples
M
av ¼ 5800), 3-aminopropyltriethoxysilane (APTES), tetra-
ethoxysilane and di-tert-butyl dicarbonate were purchased from
ꢁ
were out gassed under vacuum for 12 h at 100 C. The specific
Aldrich and used as supplied.
surface areas were calculated by the Brunauer–Emmett–Teller
(
BET) method (using 74 points and starting from 0.01 as the
Synthesis
value of the relative pressure) and the pore size distributions were
determined by the BJH method applied to the adsorption
branch. Elemental analyses of Si, N, S, and P were performed by
the Service Central d’Analyse (CNRS, Vernaison, France).
3
-tert-Butyloxycarbonylaminopropyltriethoxysilane 1. Product
was prepared by mixing 11.30 g (51.1 mmol) of 3-amino-
propyltriethoxysilane and 12.50 g of di-tert-butyl dicarbonate
57.3 mmol) in 50 ml of ethanol. The resulting mixture was
1
(
stirred overnight at room temperature. The solvent was removed
Mass spectrometry analyses
under vacuum and the residual liquid was distilled to afford
ꢁ
1
2.10 g of 1 (37.3 mmol, 73%) as a colourless liquid (bp 95 C at
A MALDI-ToF/ToF apparatus (Ultraflex III mass spectrom-
eter, Bruker Daltonics, Germany) was used for all MS analyses
(three sets of experiments according to the sample deposit
protocol on the MALDI target). The source was operated in the
positive mode. An acceleration voltage of 25.0 kV (IS1) was
applied for a final acceleration of 21.95 kV (IS2). The ToF mass
analyzer was set in the reflectron mode (voltages of 26.3 kV and
13.8 kV). A pulsed Nd : YAG laser at a wavelength of 355 nm
was operated at a frequency of 100 Hz with a delayed extraction
time of 50 ns. Data were acquired with the Flex Control software
and processed with the Flex Analysis software. External cali-
bration was performed with a commercial peptide mixture
(calibration peptide standard 2, Bruker Daltonics, Wissembourg,
France). Mass spectra were acquired from 250 laser shots, the
laser fluence being adjusted for each studied sample. Ions were
detected over a mass range from m/z 200 to 5000. No deflection
was applied in heterogeneous deposit conditions whereas
a deflection up to m/z 500 was used for homogeneous deposit to
remove matrix ions.
1
0
.05 Torr). H NMR (d ppm, 200 MHz, CDCl
3
): 0.60 (m, 2H,
), 1.41 (s, 9H, C
), 3.09 (m, 2H, CH N), 3.79
), 4.76 (s, 1H, NH). Si NMR (d
): ꢀ45.40 MHz.
3
CH
2
Si), 1.20 (t, 9H, JHH ¼ 6.90 Hz, OCH
), 1.54 (m, 2H, CH CH CH
q, 6H, JHH ¼ 7.00 Hz, OCH
2 3
CH
(
(
CH
3
)
3
2
2
2
2
3
29
2
ppm, 40 MHz, CDCl
3
ꢀ1
Aminopropyl functionalised mesoporous silica at 0.8 mmol g .
4
.0 g of triblock copolymer [EO PO EO with PEO ¼ poly
2
0
70
20
(
ethylene oxide) and PPO ¼ poly(propylene oxide)] Pluronic
P123 as surfactant were dissolved in an aqueous HCl solution
160 ml, pH z 1.5). This solution was poured into a mixture of
(
TEOS (8.86 g, 42.6 mmol) and 0.72 g (2.2 mmol) of 3-tert-
butyloxycarbonylaminopropyltriethoxysilane 1, at ambient
temperature. The mixture was stirred for 2 h giving rise to
a microemulsion. After heating this perfectly transparent solu-
ꢁ
tion at 60 C, a small amount of NaF (75.4 mg) was added under
stirring to induce the polycondensation. The mixture was left at
ꢁ
6
0 C under stirring for 48 h. The resulting solid was filtered and
washed with ethanol and ether. The surfactant was removed by
hot ethanol extraction in a Soxhlet apparatus for 24 hours. After
Sample deposit protocols on the MALDI target. Every sample
ꢁ
filtration and drying at 60 C under vaccum, 3.10 g (95%) of
was analysed with all 3 deposit protocols.
a white solid were obtained.
The obtained solid was introduced into a one-neck round
ꢁ
1 Direct deposit on MALDI steel plate without matrix. The
bottom flask. The flask was heated at 160 C under vacuum for
silica (1 mg) was suspended in a solution (1 ml) of water–
acetonitrile–trifluoroacetic acid (70/30/0.1 v/v/v) and thoroughly
sonicated. 0.5 ml of such a suspension was then rapidly aliquoted
and directly deposited on the steel plate. The deposit was allowed
to air-dry.
1
2 hours. The resulting solid was washed with ethanol and ether.
ꢁ
After filtration and drying at 60 C under vacuum, 2.4 g of
aminopropyl functionalised mesoporous silica were obtained as
a white solid.
ꢀ1
NCA oligomerisation (exp. 1). 200 mg of OMS (0.4 mmol g of
free amino groups) were placed in 12 ml plastic syringe equipped
with frit. 8 ml of freshly prepared 0.1 M Ala NCA (92 mg,
2
Direct deposit on MALDI steel plate with matrix. HCCA
(a-cyano-4-hydroxycinnamic acid) was chosen as the matrix at
ꢀ1
a concentration of 20 mg ml in water–acetonitrile–trifluoro-
acetic acid (70/30/0.1 v/v/v). 0.5 ml of the matrix was deposited
first on the steel plate. The same protocol was then applied for
the silica except that the aliquoted suspension (0.5 ml) was
0
.8 mmol) solution in dry DMF were added to the syringe. The
syringe was placed for 3 hours on an orbital shaker under gentle
agitation. The syringe was percolated and the functionalised
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322 | J. Mater. Chem., 2011, 21, 6321–6326
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directly mixed with 0.5 ml of the matrix solution and then allowed
to air-dry.
3
Silica solubilization and sample deposit on MALDI steel
12
plate with matrix. Silica solubilization was performed in
a mixture (1 : 1 v : v) of aqueous fluorhydric acid (48% w) and
hexafluoroisopropyl alcohol. Trihydroxyacetophenone (THAP)
and HCCA were chosen as the matrix for two sets of experi-
ments. A solution in water–acetonitrile (70 : 30 v : v) was mixed
with the silica solution (1 : 1 v : v). 0.5 ml of such a mixture was
deposited on the MALDI steel plate and air-dried.
Results and discussion
Fig. 1
N
2
adsorption–desorption isotherms of OMS containing 0.8% of
Three amino functionalised mesoporous silicas with different
aminopropyl groups before and after peptide oligomerisation.
ꢀ1
loadings (0.4, 0.8 and 1.6 mmol g ) were prepared by direct
14
synthesis following the method already described. For this
study, SBA-15 mesoporous silica containing variable amounts of
tert-butyloxycarbonylamino (NHBoc) groups were prepared
2
The N adsorption–desorption measurements at 77 K for all
aminopropyl groups functionalised OMS showed type IV
isotherms (see Fig. 1 as example) with H1-type hysteresis loop at
relatively high pressure characteristic of mesoporous materials
with high surface areas. The BET surface areas were in the range
(
Scheme 1). The synthesis of these materials was achieved by co-
polymerization of 1 and different amounts of tetraethoxysilane
TEOS) in the presence of P123 as structure directing agent (see
(
2
ꢀ1
of 400 to 700 m g , the total pore volumes are from 0.60 to
Experimental).
3
.30 cm g with regular pore size distributions.
ꢀ1
1
The surfactant was removed by washing to give the tert-
butyloxycarbonylaminopropyl functionalised material in high
yield. It is worth noting that no solid was obtained under the
same conditions by using APTES instead of 1. This result
demonstrates the importance of the approach with protected
amino groups.
In this way, the influence of the physical properties of the
starting aminopropyl functionalised OMS as well as the
concentration and the equivalents of NCA used for oligomeri-
zation was examined. Hybrid materials were characterized by
several spectroscopic and spectrometric measurements. Notably,
we first demonstrated that direct laser desorption ionization mass
spectrometry (LDI-MS) analysis of these organic–inorganic
hybrid materials is feasible without prior dissolution of such
silica-based samples in HF aqueous solution. In contrast to
Thermal treatment of the material containing NHBoc at
ꢁ
1
60 C under vacuum gave rise quantitatively to mesoporous
silica containing aminopropyl groups.
13
C CP-MAS NMR technique was used to demonstrate the
efficiency of the non-ionic template extraction procedure, as well
as to control the organic incorporation within the materials.
The spectrum of OMS containing 0.8% of aminopropyl groups
Table 1 Functionalised OMS prepared with NCA
OMS loading
mmol of
Exp. average
oligomer
Eq. of NCA [NCA] length
(
see ESI 1†) indicates the complete removal of the block copol-
(
Exp free NH
ymer surfactant. The resonances at 9.3, 24.5 and 42.7 ppm refer
ꢀ1
2
g
)
NCA
respectively to the carbons in a, b and g positions of the silicon
atom, settling the incorporation of –(CH
2
)
3
NH
2
groups in the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0.4
0.4
0.4
0.8
0.8
0.8
1.6
1.6
1.6
0.4
0.4
0.4
0.8
0.8
0.8
1.6
1.6
1.6
0.8
0.8
0.8
0
Ala
Ala
Ala
Ala
Ala
Ala
Ala
Ala
5
10
20
5
10
20
5
10
20
5
10
20
5
10
20
5
10
20
5
0.1
0.2
0.4
0.1
0.2
0.4
0.1
0.2
0.4
0.1
0.2
0.4
0.1
0.2
0.4
0.1
0.2
0.4
0.1
0.2
0.4
0.1
0.2
0.4
4.0
6.3
10.9
2.9
4.0
9.7
1.2
1.7
4.8
4.5
4.2
5.3
2.4
2.2
2.2
0.1
0.1
0.1
0.7
0.4
0.4
–
material. It is also worth noting that the observed signals at 58
and 17 ppm were assigned to residual –SiOCH CH groups.
2
3
Ala
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
Met
Met
Met
Met
Met
Met
Met
Met
Met
Glu(OBzl)
Glu(OBzl) 10
Glu(OBzl) 20
Ala
Glu(OBzl) 20
Met 20
20
0
0
–
–
Scheme 1 General way for the preparation of controlled oligopeptide
modified SBA.
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ꢀ
1
literature data based on the latter tedious and potentially
vibration bands at 1658 and 1541 cm witnessing the formation
12,15
dangerous analytical protocol,
each of the prepared func-
of peptide bonds on the silica. The strong silyl ether band was
ꢀ1
tionalised mesoporous silica was suspended in water, sonicated
and an aliquot was deposited on the MALDI steel plate either
directly as such (LDI analysis) or mixed with an organic MALDI
matrix (MALDI analysis).
observed at 1061 cm . Due to the direct synthesis of amino
ꢀ1
OMS, no silanol band at 3450 cm was observed (see ESI 2†).
Secondly, the average polymer weight was determined by
weighing freeze-dried materials after polymerization. At a fixed
concentration and fixed number of equivalents of NCA, the
length of the polypeptide chain decreased when the loading of the
OMS increases (i.e. comparing exp. 1, 4, 7 and exp. 10, 13, 16).
This can be interpreted as an effect of steric hindrance generated
by the proximity between two close amino groups of the OMS
that did not allow the peptidyl chain to grow longer. Moreover,
the pore size (initially ꢂ5 nm) changed during oligomerization
process and the NCA monomers probably react more easily with
the N-terminus of the growing peptide chains in the less func-
tionalised OMS material. When comparing oligo-OMS 4 (Ala),
13 (Met) and 19 (Glu), one can note that polymerization was
highly influenced by the nature of the amino acid side chain. The
more bulky the side chain was (i.e. benzyl ester protected gluta-
mic acid) the less polymerization occurred. Moreover, when non-
hindered Ala NCA was used, for a given OMS, increasing the
number of equivalents of NCA resulted in an increase of the
polymer chain. As an example, when comparing oligo-OMS 1, 2
Although peptide analyses by LDI-MS from various inert
supports, including silica (of non-controlled porosity), has been
16,17
previously investigated in the laboratory,
the peptides were
not covalently attached to the material, but only adsorbed onto
the surface. Suppression of the solubilization step allowed
gaining in analysis safety, robustness and speed. For all the
studied organic–inorganic hybrid materials, covalent bond
cleavage was achieved upon laser irradiation in the MALDI mass
spectrometer ion source, liberating in the gas phase positively
charged oligopeptide ions.
Each amino functionalised OMS reacted three hours with
different concentrations of NCA solution (0.1, 0.2 and 0.4 M) in
dry dimethylformamide yielding an excess of 5, 10 and 20-fold
respectively (Table 1). Silica was filtrated and washed thoroughly
with DCM and DMF. Aliquots were dried, weighed and
analyzed by various methods. Firstly, transmission FTIR was
used to check the appearance of the two characteristic carbonyl
Fig. 2 LDI MS spectra of oligoAla-OMS material (exp. 4) acquired under three different conditions: 1, direct deposit on the MALDI steel plate
heterogeneous deposit as shown in the inset, no matrix); 2, direct deposit on the MALDI steel plate with a prespotted HCCA matrix solution; 3, OMS
(
12
solubilization and sample deposit on MALDI steel plate with THAP matrix. Ions were produced as cationized species (sodiated ions indicated by a star
and potassium adducts representing the most intense polymeric distribution).
6
324 | J. Mater. Chem., 2011, 21, 6321–6326
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ꢀ1
and 3 obtained from 0.4 mmol g OMS, the average number of
Ala residues increased from 4.0 to 10.9 while the equivalents of
NCA increase from 5 to 20. On the contrary, whatever excess
of bulkier Met NCA was used, it has almost no influence on
polymer length. Thirdly, three sets of mass spectrometry analyses
were carried out from solid samples (heterogeneous sample
deposit conditions with and without MALDI matrix) and from
HF solubilized silicas serving as analytical controls (homoge-
neous sample deposit conditions with a MALDI matrix).
In the case of alanine (exp. 1–9), the expected polymeric
distributions were observed with an increased intensity and wider
mass range detection in the presence of an organic matrix mixed
with the silica particles. The same results were obtained in
standard homogeneous conditions from solubilized materials
indicating that the described analytical method is not only effi-
cient but above all, simple, safe, rapid and ready to use in
laboratories equipped with a conventional MALDI mass spec-
trometer. As an illustration, the mass spectra recorded for exp. 4
are displayed in Fig. 2. The LDI MS data of exp. 6 corresponding
to the same polymerization with 4 times more equivalents of Ala
NCA can be found in ESI 3†. Compared to alanine, glutamic
acid derivatives provided shorter peptides that failed to be
identified after solubilization in aqueous HF according to the
Scheme 2 Preparation of oligoMet[O]-OMS and its use for disulfide
bond formation in crustacean cardioactive peptide (CCAP).
In addition, transmission electron microscopy (TEM) micro-
graph (see inset of Fig. 3) confirms that the material exhibits local
hexagonal symmetry.
It is worth noting that non-functionalised OMS (exp. 22–24)
neither displayed any characteristic peptide bands in trans-
mission FTIR, nor ions in the MALDI mass spectra, nor gained
significant weight after NCA reaction.
12
published protocol. Only direct MS characterization upon
heterogeneous LDI conditions allowed the detection of few ions
separated by the mass of Glu(OBzl) i.e. 221 Da (exp. 19–21) (see
ESI 4†). Methionine used in exp. 10–18 provided MS spectra that
were more difficult to interpret, no obvious polymeric distribu-
tion being clearly detected whatever be the used method. Thus,
for this residue transmission FTIR, weighing freeze-dried mate-
rials and elemental analysis of sulfur content were preferred.
The nitrogen adsorption–desorption isotherm of functional-
ised OMS (exp. 4) material after peptide oligomerisation (Fig. 1)
shows a similar type of isotherm to that of the starting material
As a first application, we proposed to prepare an OMS sup-
ported reagent. Supported oligomers of methionine sulfoxide
were efficient in promoting intramolecular disulfide bond
18
formation in peptides containing two or more cysteine residues.
To that purpose oligoMet-OMS 14 was treated with hydrogen
0
peroxide to yield oligoMet[O]-OMS 14 (Scheme 2). The presence
of sulfoxide was controlled by FTIR showing the appearance of
ꢀ1
a broad band at 3350 cm .
OligoMet[O]-OMS 14 was mixed in water with 5 mM solution
of purified (free sulfhydryl) form of crustacean cardioactive
peptide (CCAP) obtained by classical microwave assisted Fmoc/
2
ꢀ1
3
ꢀ1
with 375 m g of surface area, 0.8 cm g of pore volume. This
2
diminution of surface area (from 700 to 375 m g ) and pore
ꢀ1
3
ꢀ1
19
volume (from 1.3 to 0.8 cm g ) demonstrated that the oligomers
developed within the pores of mesoporous materials.
tBu SPPS. After 30 minutes stirring, the oligoMet[O]-OMS 14
was filtered and the solution was analysed showing <90%
disulfide bond formation. After 3 hours, only the oxidized form
of CCAP was detected (see ESI 5†).
Fig. 3 shows the powder X-ray diffraction pattern of OMS
(
exp. 4) which exhibits an intense diffraction Bragg peak corre-
sponding to d100 spacing with 11.2 nm as lattice parameter a
o
.
Conclusions
In this work, we described the synthesis and characterization of
new organic–inorganic hybrid materials functionalised with
peptide units. We demonstrated that physical properties of
peptide–OMS hybrid materials can be tuned by experimental
conditions used for NCA-polymerisation within the pores.
Direct LDI-MS analysis can be performed on the resulting
covalently bound supported oligomers. Gathering the structural
features of OMS and peptide properties, this class of hybrid bio-
organic–inorganic material could find other applications in
separation and molecular recognition.
References
Fig. 3 XRD pattern of OMS containing 0.8% of aminopropyl groups
after peptide oligomerisation. The inset shows TEM image for the same
material. Scale bar ¼ 100 nm.
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