Switchable polymer-grafted mesoporous silica's: From polyesters to polyamides biosilica hybrid materials

Article abstract of DOI:10.1016/j.tet.2013.05.090

We report in this piece of work the synthesis of biosilica hybrid materials able to develop from hydrophobic polyesters to intermediate hydrophilic polyesters and then to polyamides through an O-N acyl transfer shift. After demonstration of the O-N acyl shift on a model depsipeptide anchored in the pores of ordered mesoporous silica (OMS), the preparation of three bio oligomer-silica hybrid materials is described. The oligomerisation of protected serine lactones was initiated in the pores of carboxylate functionalized OMS. Hybrid protected serine polyester-OMS were obtained and could be converted (i) into cationic polyesters, which could be switched (ii) into neutral polyamide-OMS in basic conditions through a multi O-N acyl shift.

Full text of DOI:10.1016/j.tet.2013.05.090

Tetrahedron 69 (2013) 7670e7674
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Switchable polymer-grafted mesoporous silica’s: from polyesters
to polyamides biosilica hybrid materials
,
,
b,
*
Said Jebors a b, Christine Enjalbal a, Muriel Amblard a, Gilles Subra a , Ahmad Mehdi
,
*
Jean Martinez a
a
ꢀ
ꢀ
Institut des Biomolecules Max Mousseron (IBMM), UMR5247 CNRS, Universites Montpellier 1 et 2, 15 avenue Charles Flahault, 34093 Montpellier,
France
b
ꢀ
ꢁ
Institut Charles Gerhardt (ICG), UMR5253 CNRS, CMOS, Universite Montpellier 2, Place Eugene Bataillon, 34095 Montpellier, France
a r t i c l e i n f o
a b s t r a c t
Article history:
We report in this piece of work the synthesis of biosilica hybrid materials able to develop from hydro-
phobic polyesters to intermediate hydrophilic polyesters and then to polyamides through an OeN acyl
transfer shift. After demonstration of the OeN acyl shift on a model depsipeptide anchored in the pores
of ordered mesoporous silica (OMS), the preparation of three bio oligomeresilica hybrid materials is
described. The oligomerisation of protected serine lactones was initiated in the pores of carboxylate
functionalized OMS. Hybrid protected serine polyestereOMS were obtained and could be converted (i)
into cationic polyesters, which could be switched (ii) into neutral polyamideeOMS in basic conditions
through a multi OeN acyl shift.
Received 13 March 2013
Received in revised form 18 May 2013
Accepted 20 May 2013
Available online 24 May 2013
Keywords:
OeN acyl shift
Hybrid materials
OMS
Ó 2013 Elsevier Ltd. All rights reserved.
Lactone polymerization
Serine polyester
1. Introduction
precursor) with an organotrialkoxysilane RSi(OR0)3 in the presence of
a surfactant as a structure-directing agent. After elimination of the
Silica hybrid based materials obtained by the solegel process have
attracted considerable attention since a quarter of a century. They
constitute a fascinating class of materials combining the properties of
organic moieties and inorganic matrices.1e4 Since the discovery of
orderedmesoporoussilicas,5 theirsurfacemodificationofinnerpores
with organic functional groups has been extensively investigated to
render them suitable for a wide number of applications,6 such as
catalysis,7 separation,8 chemical sensors,9,10 etc. Incorporation of
several types of functional organic units into the channels of meso-
porous silica has attracted many efforts for the design and organi-
zation of multi-functionalized mesoporous materials.11
surfactant, ordered mesoporous silica (OMS) with high porosity,
surface area, and tuneable pores of 2e30 nm diameter can be
obtained.
Compared to other materials of undefined porosity, such as
silica gels, the use of OMS greatly rationalizes the inclusion of or-
ganic compounds in a non-covalent or covalent way. Large protein,
such as enzymes15,16 and heme proteins17 were covalently immo-
bilized on hexagonal18 and cubic19 phases of OMS. Recently, hybrid
peptide oligomeresilica OMS were prepared by amino acid N-car-
boxyanhydride (NCA)20 polymerization, initiated by amino groups
present in the pores of OMS.21,22
Generally, organic functionalities can be incorporated into mes-
oporous materials, either by post synthesis12 (grafting) or by direct
synthesis (one-pot) method.13 It was demonstrated that the direct
synthesis method produces hybrid material with high content of
organic groups (up to 25% molar) and more homogeneous surface
coverage of the silica inner pores.14 This approach consists in the co-
hydrolysis and polymerization of tetraethylorthosilicate (TEOS: silica
On the other hand, serine lactone has already been used to
obtain polyesters.23,24 In a general manner, esters formed on the
side chain of serine, are known to easily undergo an OeN acyl
shift.25,26 This reaction is extensively used in the field of peptide
science for the synthesis of difficult sequences,27 cyclic peptides,28
C-terminal peptide alcohols.29 More recently, we take profit of this
acyl transfer reaction to the synthesis of serine polyester oligomers
in solution able to shift to serine polyamide oligomers.30
In this paper, we applied the same methodology to the prepa-
ration of bioorganic hybrid OMS whose pores contain switchable
serine oligomers. Three different hybrid materials can be obtained
sequentially (i.e., protected serine polyester, cationic polyester, and
* Corresponding authors. Tel.: þ33 4 1175 9606; fax: þ33 4 1175 9641 (G.S.); tel.:
þ33 4 6714 3832; fax: þ33 4 6714 3809 (A.M.); e-mail addresses: gilles.subra@
univ-montp1.fr (G. Subra), ahmad.mehdi@univ-montp2.fr (A. Mehdi), martinez@
univ-montp1.fr (J. Martinez).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.05.090

S. Jebors et al. / Tetrahedron 69 (2013) 7670e7674
7671
neutral polyamide silica hybrids) using a multi OeN acyl transfer
that takes place within the pores of OMS. Such materials could find
potential applications as controllable nanocontainers.
OMS 3 was determined by elemental analysis (0.11 mmol gꢁ1) (Table
1). Boc protecting groups were removed by neat TFA treatment
yielding cationic material 4. Boc removal and NeO acyl shift were
monitored bythe Kaiser test. Ithas to beunderlined that theOeNacyl
transfer could not take place till the amino group was free. This was
achieved by treatment of hybrid silica 3 with 10% Et3N in DCM. After
12 h, the hybrid silica 5 was obtained.
2. Results and discussion
2.1. Model depsipeptide grafted in OMS pores
Table 1
Properties of OMS determined by N2 adsorption and elemental analysis
As a proof of concept, we first studied the OeN acyl shift on
a model depsipeptide Boc-Ser(Fmoc-Phe)-Lys-NH2 1 grafted on
nonfunctionalized OMS. For that purpose, we used the methodology
we recently set up31 consisting in the synthesis of hybrid trialkox-
ysilyl protected peptides thatweredirectly involvedinhydrolysis and
condensation on the surface of the OMS pores (Scheme 1, step f). The
OMS used here was SBA-15 type, which was obtained by hydrolysis
and polycondensation of TEOS in the presence of Pluronic P123 as
surfactant.32 The depsipeptide 1 reacted with trialkoxysilyl propyl
isocyanate to yield the hybrid pseudo peptide 2, which was isolated
and characterized (Supplementary data S5eS6). Compound 2 was
grafted into the pores of OMS by reaction between alkoxy groups and
silanol (SiOH) of the surface. It is worth noting this new type of bio-
molecule grafting did not require any ligation chemistry between
inorganic and bioorganic moieties. Briefly, SBA-15 suspended in
a DMF solution containing the hybrid peptide blocks was stirred for
1 h at rt, then 24 h at 80 ꢀC. This two-step procedure allowed the
hybrid peptide block to penetrate into the pores. Hybrid material 3
was characterized by X-ray diffraction, elemental analysis, BET, and
FTIR. N2 adsorptionedesorption measurements at 77 K showed type
IV isotherms with a clear H1-type hysteresis loop at high relative
pressure (Supplementary data S1), characteristic of mesoporous
Area surfacea (m2 gꢁ1
)
Pore volumea (cm3 gꢁ1
)
%Nb
SBA
674
1.18
d
3
337
0.63
0.85
/
/
1.02
d
4
520
0.91
5
457
0.86
6
643
1.22
7
540
0.83
9aeb
10aeb
11aeb
271e133
389e323
281e157
0.81e0.35
0.89e0.65
0.74e0.44
3.0e4.7
/
/
a
Calculated by BET.
Calculated by elemental analyses.
b
2.2. Hybrid biopolymeresilica material
The next step of the study was the use of the OeN acyl shift for
the synthesis of a hybrid biopolymeresilica material. To obtain the
starting polyesteresilica hybrid material, we set up the polymeri-
zation of protected serine lactone. This ring opening polymerization
(ROP)33 could be initiated by the carboxylate salt prepared from
cyano OMS 6. Thus, a mesoporous material functionalized with CN
groups with controlled loading (0.7 mmol gꢁ1) was prepared in one-
pot by hydrolysis and co-condensation of tetraethylorthosilicate
(TEOS) with the desired amount of cyanopropyltriethoxysilane in
the presence of P123 as surfactant under acidic conditions. The
obtained OMS 6 was well organized in a 2D-hexagonal structure as
shown by the XRD pattern diffraction (Supplementary data S3).
Carboxylic functionalized mesoporous silicas (0.7 mmol gꢁ1
loading) were prepared by oxidation of the nitrile 6 OMS following
materials. XRD patterns exhibited three low-angle reflections, d100
,
d110, and d200, characteristic of well-ordered SBA-15 type materials
(Supplementary data S2). Transmission Electron Microscopy (TEM)
provided further evidence of an ordered hexagonal structure
(Supplementary data S2). Taken together, these results indicated that
properties of OMS were conserved after chemical modification with
depsipeptide units. The efficiency of depsipeptide loading of hybrid
O
b
c
a
d
e
NH
NH
NH2
NH
(EtO)3Si
N
H
NH
O
Cl
O
O
O
H
N
H
N
H
N
H
N
NH2
NH2
NH2
NH2
NH2
Boc
N
Boc
N
Boc
N
H
Fmoc
N
Boc
N
H
H
H
O
O
H
O
O
O
HO
O
O
H
N
O
H
N
H
N
Fmoc
O
Fmoc
O
Fmoc
O
2
1
f
O
O
O
O
O
O
O
Si
O
N
NH
O
O
O
Si
O
N
NH
H
Si
O
N
NH
O
O
Si
O
N
NH
O
O
H
H
H
g
h
O
Fmoc
O
H
N
HN
NH2
TFA
H
H3N
O
NH2
Boc
N
H
H2N
O
NH2
N
NH2
N
H
N
N
O
H
H
O
O
O
O
O
H
N
HO
H
N
H
N
Fmoc
O
Fmoc
O
Fmoc
O
5
4
3
O-N acyl shift
Scheme 1. Synthesis of a hybrid trialkoxysylyl depsipeptide and preparation of switchable hybrid peptide-OMS. (a) Fmoc-Lys-NH2, DIEA; (b) Fmoc deprotection: DMF/pip; cou-
pling: Boc-Ser-OH, BOP, DIEA; (c) esterification: Fmoc-Phe-OH, DIC/DMAP; (d) TFE/DCM 3/7 v/v; (e) Et3OSi(CH2)3NCO, DMF; (f) grafting on SBA-15 OMS in DMF; (g) Boc removal,
TFA; (h) 10% Et3N in DCM for 24 h, OeN acyl shift.

7672
S. Jebors et al. / Tetrahedron 69 (2013) 7670e7674
the method already described.34 Carboxylate potassium salt was
prepared by treatment of the carboxylic acid OMS 7 with potassium
pentadionate in anhydrous ethanol. OMS 7 was characterized by
13C CP-MAS NMR, porosimetry measurements, and elemental
analysis. The spectrum revealed three signal resonances at 13.5,
18.6, and 37.0 ppm attributed to the propyl chain and an additional
signal at 180.4 ppm for the carbonyl (Supplementary data S4).
N2 adsorptionedesorption measurements of OMS
7
(Supplementary data S1) showed an isotherm type IV with H1-type
hysteresis loop at relative high pressure, characteristic of a meso-
porous material with large pores and regular pore size distribution
with high surface area (540 m2 gꢁ1) and pore volume (0.83 cc gꢁ1).
Carboxylate functionalized OMS reacted overnight with different
concentrations of Boc serine lactone (0.1 and 0.2 M, corresponding to
5 or 10 equiv) in dry dichloromethane solution (Table 2). Resulting
samples of hybrid mesoporous silicas 9 were filtered and thoroughly
washed with DCM and DMF. Aliquots were dried, weighted, and an-
alyzed by various methods. Firstly, transmission FTIR was used to
check the appearance of the characteristic carbonyl vibration band at
1720 cmꢁ1. As described before, Boc protecting groups were removed
from hybrid material 9 by neat TFA treatment to yield cationic poly-
ester 10. After treatment of polyester 10 by DCM/Et3N (9/1 v/v), two
characteristic carbonyl vibration bands at 1658 and 1549 cmꢁ1
appeared indicating formation of peptide bonds on the silica due to
the multi OeN acyl transfer, yielding polyamide 11 (Scheme 2). As
expected, in hybrid materials 6e11 a strong silyl ether band at
1061 cmꢁ1 was observed. It is worth noting that Kaiser test did not
reveal the presence of free amino groups (Fig. 1).
Fig. 1. (A) FT-IR, (B) Kaiser test for OMS, before and after OeN acyl shift.
The nitrogen adsorptionedesorption isotherm of functionalized
material 9 after lactone oligomerisation is shown in the Supple-
mentary data section (SD Fig. S1). It showed a very similar isotherm
to that of the starting material with 271 and 133 m2 gꢁ1 of surface
area, 0.81 and 0.35 cc gꢁ1 of pore volume, respectively for OMS 9a
(n¼4) and 9b (n¼8). This surface area and pore volume diminution
demonstrated that oligomers developed within the pores of mes-
oporous materials. In addition, elemental analysis of nitrogen
contents (Table 1) for OMS 9 clearly showed an increase of oligomer
size from 4 (9a) to 8 units (9b). After TFA treatment, an increase of
both the surface area and the pore volume, 389 and 323 m2 gꢁ1, and
Table 2
Hybrid polyesters OMS 9 prepared from Boc-serine lactone
Exp.
OMS loading
(mmol of free COOK/g)
Equiv of
lactone
[Lactone]
Exp. average
oligomer lengtha
a
b
0.7
0.7
5
10
0.1
0.2
w4.1
w8.1
a
Determined by weight increase/elemental analysis.
O
O
O
O
O
O
O
O
O
O
O
O
N
H
O
O
O
O
O
a
O
O
n
O
b
Si
CN
Si
Si
Si
OH
O
O
O
OH
c
ROP
base H
NH
6
7
8
9
O
polyester
d
ght contraction
HO
O
O
O
O
O
n
O
O
O
O
O
O
O
O
O
n
O
e
Si
Si
Si
O
OH
O
OH
N
H
OH
NH3
NH2
n
TFA
polyamide (polyserine)
multi O-N acyl shift
11
10
Scheme 2. Synthesis of switchable hybrid polymer-OMS. (a) Hydrolysis of CN into COOH by H2SO4 (50%), 100 ꢀC; (b) Potassium pentadionate; (c) Polymerization initiated by
complexed carboxylate salts; (d) Boc removal by TFA; (e) 10% Et3N in DCM, 24 h, OeN acyl shift.

S. Jebors et al. / Tetrahedron 69 (2013) 7670e7674
7673
0.89 and 0.65 cc gꢁ1 was observed, respectively for OMS 10a and
10b, indicating the removal of Boc protecting groups, confirmed by
a positive Kaiser test.35 After treatment of cationic polyester 10 by
DCM/Et3N (9/1 v/v), a decrease of specific area and pore volume
(100 MHz, DMSO-d6): 173.7, 171.9, 168.9, 158.5, 156.4, 155.8, 144.1,
141.1, 138.0, 129.5, 128.7, 128.1, 127.5, 126.9, 125.7, 120.6, 79.2, 66.2,
64.9, 58.1, 55.9, 53.9, 52.7, 46.9, 42.5, 36.8, 32.5, 30.3, 28.6, 24.0,
22.9, 18.7, 7.7. 29Si NMR (75 MHz, DMSO-d6): ꢁ45.1. LC/MS m/z
949.4 [MþH]þ; HPLC tr, 2.04 min; m/z 847.3 Siliconium ions [dep-
sipeptide-Si(OH)2]þ; HPLC rt, 1.64 min. HRMS (ESI) m/z calcd for
C42H55O11N6Si [M]þ 847.3698, found 847.3704.
(281 and 157 m2 ꢁ1; 0.74 and 0.44 cc gꢁ1) was observed despite
g
chain length contraction, due to the increase of volume oligomer
and decrease of hydrophilicity, a negative Kaiser test confirming
the disappearance of free amino groups.
In conclusion, we showed that OMS hybrid materials of SBA-15
type, functionalized with COOH groups could be used as nano-
reactors for controlled Ring Opening oligopolymerization of lactone
units. The obtained switchable oligomers confined in the pores
were fully accessible and the polyester to polyamide trans-
formation was quantitative. These tailored oligomers could be used
as switchable nanovalves in multifunctionalized nanobiomaterials
for drug delivery control for instance.
3.3. Preparation of hybrid depsipeptideeOMS by grafting 3
To a solution of SBA-15 (500 mg) in 20 mL of dry DMF was added
hybriddepsipeptideblock2(50mg). Thereactionmixturewasstirred
for1 hatroom temperatureand 24 hat80ꢀC. ThefunctionalizedSBA-
15 (SBA-Pep) wasfiltered off andwashed with DMF (3ꢂ10 mL),1DCM
(3ꢂ10 mL), diethylether (3ꢂ10 mL), and dried in vacuo.
The deprotection step was realized by treatment of the hybrid
silica with 5 mL TFA/DCM (2/1 v/v) for 1 h. The functionalized
deprotected SBA-15 (SBA-Pep) was filtered off and washed with
DMF (3ꢂ10 mL), DCM (3ꢂ10 mL), diethylether (3ꢂ10 mL), and dried
in vacuo.
3. Experimental section
3.1. Synthesis of depsipeptide 1
Fmoc-Lys-NH2 (4 equiv) was anchored on a trityl chloride resin
using DIEA (8 equiv) and anhydrous DMF. The reaction mixture was
stirred at room temperature for 2 h, and then washed with DMF/
MeOH/DIEA (17/2/1) (3ꢂ), DCM (3ꢂ), MeOH (1ꢂ), DCM (1ꢂ).
3.4. Synthesis of carboxylic functionalized mesoporous silica
7 at 0.7 mmol gL1
Quantitative Fmoc tests were performed as spot checks. The N
a
-
4.0 g of triblock copolymer [EO20PO70EO20 with PEO [poly(-
ethylene oxide)] and PPO [poly(propylene oxide)], and Pluronic
P123 as surfactant were dissolved in an aqueous HCl solution
(160 mL, pH¼1.5). This solution was poured into a mixture of TEOS
(8.86 g, 42.6 mmol) and 0.5 g (2.2 mmol) of cyanopropyltriethox-
ysilane at room temperature. The mixture was stirred for 2 h giving
rise to a microemulsion. After heating this perfectly transparent
solution at 60 ꢀC, a small amount of NaF (80 mg) was added under
stirring to induce polycondensation. The mixture was left at 60 ꢀ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 h. After filtration and
drying at 60 ꢀC in vacuo, 3.10 g (95%) of the nitrile functionalized
OMS 6 were obtained as a white solid. 500 mg of nitrile function-
alized OMS was converted into carboxylic acid OMS 7 by treatment
with 5 mL of H2O/H2SO4 solution (1/1 v/v), stirred at 150 ꢀC for 5 h,
filtered, and washed with water and acetone, and dried in vacuo.
Fmoc deprotection step was performed by treatment with a solu-
tion of piperidine (20% v/v in DMF) for 20 min.
Boc-Ser-OH elongation was performed using the Fmoc/tBu
strategy using BOP as coupling reagent. The coupling step was
carried out on the resin using Boc-Ser-OH (3 equiv) in DMF in the
presence of BOP (3 equiv) and DIEA (6 equiv) for 2 h at room
temperature. After the coupling step, the resin was washed with
DMF (3ꢂ), DCM (3ꢂ), MeOH (1ꢂ), and DCM (1ꢂ).
DIC (3 equiv) and DMAP (0.3 equiv) were added to a solution of
Fmoc-Phe-OH (6 equiv relative to the resin substitution) in a mix-
ture of anhydrous DCM and DMF (50/50) at 0 ꢀC (3 mL mmolꢁ1).
The resulting mixture was stirred with the previously functional-
ized resin for 6 h at room temperature. This procedure was re-
peated twice, and resin was washed with DMF (3ꢂ), DCM (3ꢂ),
MeOH (1ꢂ), and DCM (1ꢂ).
Solid-supported depsipeptides were cleaved by stirring the
resin with a TFE/AcOH/DCM (2/1/7 v/v/v) solution for 1 h. After
filtration, the resin was submitted to a second cleavage step for an
additional 1 h and filtered. The combined filtrates were concen-
trated under reduced pressure. The depsipeptide 1 was precipitated
with diethylether, filtered, and dried under high vacuum. Crude
compounds were analyzed by analytical HPLC and LC/MS and used
without further purification. LC/MS m/z 702.3 [MþH]þ; HPLC tr,
1.55 min. HRMS (ESI) m/z calcd for C38H48O8N5 [MþH]þ 702.3503,
found 702.3497.
3.5. Preparation of serine lactone
To a stirred solution of dried Ph3P (4.9 mmol) in 100 mL of an-
hydrous THF at ꢁ80 ꢀC was added DEAD (4.9 mmol) dropwise over
10 min. After 15 min, a solution of Boc-L-Serine (4.88 mmol) in THF
was added dropwise over 15 min to the stirred solution at ꢁ80 ꢀC.
The solution was stirred at ꢁ80 ꢀC for 20 min and overnight at room
temperature. Solvent was removed in vacuo, and the resulting
product was purified by chromatography on silica gel using EtOAc/
Hexane (30/70 v/v) as eluent.
3.2. Synthesis of hybrid of triethoxysilyl depsipeptide 2
To a solution of depsipeptide (1) (0.1 mmol) in 100 mL of DMF
was added DIEA (2.1 equiv) and 3-isocyanatopropyltriethoxysilane
(1.2 equiv). The reaction mixture was stirred for 2 h at room tem-
perature. Reaction was monitored by HPLC. Ether (30 mL) was
poured into the reaction mixture. The precipitate was suspended in
ether and collected by filtration. This procedure was repeated three
times to remove TICPS and DIEA. All crude compounds were ana-
lyzed by analytical HPLC, LC/MS, and NMR and used without further
purification. 1H NMR (400 MHz, DMSO-d6): 0.47 (m, 2H), 1.14 (t, 9H,
J¼7.2 Hz), 1.23 (m, 2H), 1.37 (m, 6H), 2.88e2.97 (m, 6H), 3.69e3.76
(q, 6H, J¼7.2 Hz, J¼14), 4.06e4.40 (m, 8H), 5.68e5.78 (m, 2H), 7.06
(s, 2H), 7.10 (s, 2H), 7.15e7.64 (m, 13H), 7.88 (br s, 3H). 13C NMR
3.6. Lactone oligomerisation (exp 1, 2)
500 mg of carboxylic acid mesoporous silica 7 was poured into
a solution of potassium pentadionate (1.2 equiv) in 5 mL of anhy-
drous ethanol. The suspension was refluxed for 24 h, filtered,
washed with ethanol, and dried to give the respective potassium
carboxylate salt. 200 mg of this material were poured into a solu-
tion of dibenzo-18-crown-6 ether in dichloromethane placed in
a 12 mL plastic syringe equipped with frit. A solution of serine
lactone was added to the syringe, placed overnight under gentle
stirring. Syringe was percolated and functionalized OMS was

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S. Jebors et al. / Tetrahedron 69 (2013) 7670e7674
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washed with DCM (3ꢂ10 mL), DMF (3ꢂ10 mL), diethylether
(3ꢂ10 mL), and dried in vacuo.
Acknowledgements
The University Montpellier 1 Post-Doc Program supported this
study.
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Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2013.05.090.
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