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Ion-fuelled L -Phenylalanine controlled pumping by hybrid membrane materials
Reactive & Functional Polymers 86 (2015) 259–263
Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Ion-fuelled L-Phenylalanine controlled pumping by hybrid membrane materials Pascal Blondeau 1, Mihail Barboiu ⇑, Eddy Petit Adaptative Supramolecular Nanosystems Group, Institut Européen des Membranes – ENSCM/UM2/CNRS 5635, IEM/UM2, CC 047, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 14 June 2014 Received in revised form 11 August 2014 Accepted 15 August 2014 Available online 27 August 2014 Keywords: Hybrid materials Supramolecular membranes Self-organization Dialysis
a b s t r a c t We report organic–inorganic membrane materials for controlled translocation of ion-pairs along directional conduction pathways. The supramolecular/sol–gel combined procedure has been realised in two steps. First, the self-organization properties of 3-(ureidoarene)propyltriethoxysilane compound, 1 in aprotic solvents were determined, revealing the formation of supramolecular oligomers, in the selfassembly step. They led in a second sol–gel transcription step to hybrid membrane materials. Then, the transport of organic molecules of variable charge, like L-Phenylalanine can be directionally tuned, via ion-fuelled membrane polarization phenomena. Using this strategy, the transport of the cationic, zwitterionic and anionic forms of the L-Phenylalanine can be controlled and directionally oriented from the feed to strip phase and vice versa. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Most of the vital functions depend on selective transport of metabolites through the membrane cell [1]. The transport of essential metabolites depends on unique properties of hydrophilic inner-domains of the protein channels [2–4]. Artificial supramolecular ion-channels, extensively studied with the hope to mimic the natural ionic conduction [4–11], give the possibility to extend and to engineer the dimensionality of active transporting membranes materials across the extended scales [12]. Hybrid organic–inorganic materials produced by sol–gel process offer the opportunity to achieve versatile nanostructured materials [13–17]. Of special interest is structure-directed function of such hybrid materials and to control their build-up by self-organization. Recently, we have proved the potentiality of hybrid fixedsite complexant membranes. Such thin-layer solid membranes may present functional properties such as: (a) multiple molecular recognition [18–25] and (b) directional conduction transport via supramolecular self-assembled pathways [26–30]. The first generation of thin-layer hybrid membranes contains the polytopic macrocyclic receptors, randomly dispersed in a hybrid silica matrix
⇑ Corresponding author. Tel.: +33 467 14 91 95; fax: +33 467 14 91 19. E-mail address: [email protected] (M. Barboiu). Present address: Department of Analytical and Organic Chemistry, Universitat Rovira i Virgili, C/. Marcellí Domingo, s/n, 43007 Tarragona, Spain. Tel.: +34 977 55 81 55; fax: +34 977 55 84 46. 1
http://dx.doi.org/10.1016/j.reactfunctpolym.2014.08.002 1381-5148/Ó 2014 Elsevier B.V. All rights reserved.
[18–25]. The transport mechanism is based on ‘‘fixed-site jumping’’ diffusion [31–35] of solutes, controlled by the specific interactions with the fixed receptors. Then, our next objective was to minimize the distance between the selective receptor sites, with the hope to reduce the non-selective solute-diffusion between the active sites and to generate directional diffusion patterns into the solid dense material. Toward this objective, we have been previously developed self-organized hybrid membranes containing macrocyclic [26], aromatic [27–29] and sulfonic [30] groups, that form by collective self-assembly, directional transporting pathways for synergetic translocation of ion-pairs. Interesting pumping transport mechanisms have been described by using these membranes: the fast transport of small cationic species (i.e. H+, Na+) from the feed to the strip solutions, may induce an opposite (negative) membrane potential (e), which slow the diffusion of cations and accelerate the diffusion of anions (Fig. 1) [26]. Herein we describe novel findings related to the previously observed pumping/membrane polarization effects. The novel hybrid organic–inorganic membrane materials described here, have been prepared in two steps. First, the selfassembling properties of 3-(4-pyridylureido) propyltriethoxysilane compound, 1 in aprotic solvents were determined, revealing the formation of supramolecular oligomers. They led in a second sol– gel transcription step to thin-layer hybrid membrane materials organized at the nanoscopic scale (Scheme 1). These membranes have been tested in competitive dialysis transport experiments. The initial influx of lithium cations envisioned to diffuse along
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Teflon mould and then thermically polymerized at 80 °C, to afford a white brittle self-supported film which have been used further for characterization. The thin layer hybrid supported membranes were obtained by coating of the sol onto commercial ultrafiltration planar Polyacrylonitrile (PAN) supports (ORELIS – IRIS membranes) using a tape-casting method (see Ref. [19] for details). Scanning electron microscopy (SEM, Hitachi S-4500 apparatus, under a tension of 0.5–30 kV) allowed the thickness and the quality of the active deposited layer to be determined (Scheme 1). 2.3. Dialysis transport procedure
Fig. 1. Proposed cation-powered pumping mechanism of the anions against their concentration gradient (see text for details).
pyridine aromatic conduction pathways, can be used to directionally pump cationic and anionic forms but not zwitterionic form of LPhenylalanine across the membrane. 2. Experimental 2.1. Materials and methods The 3-(4-pyridylureido)propyltriethoxysilane compound, 1, was obtained from 3-isocyanato-propyltriethoxysilane and the corresponding 4-amino-pridine (benzene, 80 °C, 48 h) to afford after precipitation the compound 1, as a white powder (yield 85%). NMR spectroscopic analyses of precursors 1 are in accord with the proposed formula. 3-(4-Pyridylureido)-propyltriethoxysilane, (1) 4-aminopyridine (0.5 g; 5.312 mmol) and 3-isocyanatopropyl-triéthoxysilane (3.96 mL; 7.918 mmol) were dissolved in benzene and reaction mixture was refluxed under Ar for 48 h. After removal of the solvent, the residue was washed with hexane to afford 1, as a pure compound. 1H RMN (CDCl3): d = 0.68 (t, J = 7.8, 2H), 1.25 (t, J = 3.1, 3H), 1.65 (qt, J = 6.89, 2H), 3.28 (m, 2H), 3.79 (m, 2H), 5.56 (s, 1H), 7.35 (m, 2H), 7.6 (s, 1H), 8.39 (d, J = 6.19, 2H); 13C RMN (CDCl3) d 8.0, 18.6, 23.8, 42.8, 58.8, 113.1, 148.1, 150.2, 155.5.
Membrane transport experiments were performed with a bicompartmental device, magnetically stirred at room temperature (see Ref. [40] for details). It consists of two Teflon cell device separated by the solid membrane oriented with the active dense film to the feed phase. Nitrogen permeation measurements were performed to ensure that they were dense and defect free. The feed and the strip phases were both, aqueous solutions of the 10 3 M L-Phenylalanine aqueous solutions at different pH values: (a) pH = 1, (b) pH = pKa = 6.1 and (c) pH = 10. In order to generate the ion-concentration driven polarization effects, a 10 1 M LiCl has been added in the feed phase. The membrane consisted of supported dense membrane (S = 5.32 cm2). The Li+ concentrations were monitored at different time intervals in both feed and strip phases using the Atomic Absorption Spectrophotometer – AAS Varian 220 FS (k = 670.8, 1 nm c = 0.02–5 lg/ml). The amounts of LPhenylalanine in the both phases were determined by an HPLC– MS system (MICROMASS Platform II; Waters TM 600S Controller equipped with a Waters 515 HPLC pump (flow = 0.05 mL/min, T = 90 °C and voltage cone, VC = 25V). The acidic and amphoteric forms have been determined by using ESI + Mass spectrometry (H2O/CH3CN/CF3COOH:80/20/0.1) and the basic form has been determined by using ESI-mass spectrometry (H2O/CH3CN:80/20). This method allowed the concentration of the L-Phenylalanine to be determined with an error of 4%. The flux values, representing concentration of ions transported per hour through feed/membrane or membrane/receiving interfaces, have been calculated using the solution–diffusion model, from the slope of the initial linear part of concentration versus time profiles (time frame 0–10 h) [19].
2.2. Preparation and characterization of membranes 3. Results and discussions The compound 1 gelate the benzene solutions (C > 0.8 M). In order to obtain hybrid membrane materials we have first conducted the sol–gel experiments in benzene saturated with water. A slow hydrolysis process is occurring even for the long periods of time (t = 100 h) After solvent evaporation we obtain a low condensed transparent organic sol of 1, which has been casted in a
3.1. Precursor 1 and membrane material characterizations The generation of self-organized ribbon-type superstructures of 1 in solution, sol/gel and in the solid state is based on three encoded features (Scheme 1): (1) they contain pyridine and urea
Scheme 1. Structure of ligand 1 and its self-organization in ribbon-type architectures in solution, followed by the sol–gel transcription of encoded transporting pathways for cations and anions into a hybrid dense membrane material. Cross section micrograph of the thin-layer dense hybrid membrane deposited onto PAN polymeric support.
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compound 1 [37] (Fig. 3a). It is proving the importance of additional p–p stacking interactions in the self-organization process [38,39]. By contrast the self-association of 1, highly increase in the non-polar solvent like C6D6 when compared with the low-polar solvent CDCl3, while the cooperativity is remaining constant (Fig. 3b). Evidence for H-bond formation was obtained from the FTIR spectra: in both organogels and in the solid hybrid, the urea vibration shifts were detected: mNAH = 3320 (Dm = 30), mC@O(amideI) = 1635 (Dm = 15) and mCAN(amide II) = 1565 (Dm = 25) cm 1. After hydrolysiscondensation reactions FTIR spectra of hybrid material showed the appearance of the vibrations of the mSiAOASi = 1000–1200 cm 1 instead the vibrations mSiAOEt = 1075, 1100 and 1170 cm 1, initially observed for the molecular precursor 1. The 29Si MAS NMR spectroscopic experiments agree with a progressive condensation of alkoxysilane moieties: (a) in the first step by stirring at room temperature, a very weak connected hybrid sol was observed, mostly composed of T0 ( 46.2 ppm) corresponding to RSi(OEt)3 and with the residual T1 and T2 species; (b) more advanced condensation of about 81% with a higher proportion of T1 ( 53.1 ppm), T2 ( 61.4 ppm) and T3 ( 67.4 ppm) species were obtained by heating up to 80 °C for the cross-linked hybrid membrane material (Vitreous transition temperature: Tg = 88.8 °C), which have been found to be thermally stable up to 200 °C. 3.2. Dialysis membrane transport experiments
Fig. 2. Chemical shifts of a) the Alkyl–N–H (red squares) and Aryl-N–H (black squares) protons plotted against total concentration of 1 in CDCl3 at 25 °C and of the Aryl-N–H protons plotted against total concentration of 1 in CDCl3 (red squares) and in C6D6 (black squares) at 25 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
moieties known to bind metal cations (i.e. Li+) [36] and the anions, respectively [37,38]; (2) the supramolecular guiding interaction is the urea head-to-tail H-bond association, [35,38] reminiscent with the amide moiety’s H-bonding in proteins, (3) the ureidopyridyl residues are covalently bonded to triethoxysilyl groups allowing to transcribe the self-organized superstructures into the hybrid materials by sol–gel process [13,14]. 1 H NMR dilution experiments showed the strong downfield shifts of NHalif (Dd = 0.82 ppm) or NHarom (Dd = 2.2 ppm) protons upon increasing concentration of 1, which is indicative of self-association through the intermolecular hydrogen bonding in CDCl3 (Fig. 2a). Equilibration between H-bonded and non-H bonded states in CDCl3 for a given NAH proton is almost always fast on the NMR time scale and observed proton chemical shifts are weighted averages of the chemical shifts of contributing states. The downfield shifts of NHarom (Dd = 4.0 ppm) proton of 1 increase significantly in C6D6 (Fig. 2b) indicating stronger association a less polar solvent as expected. The association of 1 could be described with a cooperative association model [11,12]. This method assumes that all polymerization constants, but not the first equilibrium constant K2 describing dimerization, are the same Kp (Fig. 3). High values of Kp and K2 indicate strong association via H-bonding, while high values of the ratio Kp/K2 are related to the cooperativity of the self-association process. Calculation of the association constants revealed that the strength of the self-association of compound 1 and the process cooperativity are both decreased, when compared with the values determined for the bis-aromatic
The membrane transport performances designed to transport ion-pairs, depend on encoded molecular features of heteroditopic precursor 1. In order to demonstrate the formation of cation–anion translocation pathways into synthesized membranes we carried out dialysis transport experiments. The L-Phenylalanine transport across the membranes according to the directional-diffusion transport mechanism and against its thermodynamic gradient was
Fig. 3. Dimerization (K2) and polymerization (KP) association constants for numerical fits to a monomer/dimer/defined aggregate model of (a) pyridyl-propyl-type compound 1 and pyridyl-Aryl compound 1 and (b) pyridyl-propyl-type compound 1 in CDCl3 and C6D6 solvents.
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Fig. 4. Membrane transport of L-Phenylalanine and Li+ through the membranes (a) concentration versus time transport profiles of the (a) acidic cationic ammonium, pH = 1 (b) neutral, zwitterionic, pH = pKa = 6.1 and (c) basic anionic carboxylate, pH = 10 L-Phenylalanine. Proposed Li+-powered pumping mechanism of the L-Phenylalanine against its concentration gradient. The flux of Li+ ions is represented by the black arrow, the induced membrane polarization potential by the light-blue arrow and the flux of the LPhenylalanine in (a) red (pH = 1), (b) violet, pH = 6.1 and (c) blue (pH = 10) arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
evaluated using ion-driven pumping conditions: experiments have been conducted with equal concentrations of the L-Phenylalanine of 10 3 M initial in the feed and in the strip solutions, while the LiCl is present only in the feed phase, CLi+ = 10 1 M. Fig. 4 shows the concentration versus time transport profiles through the hybrid membrane. The concentration profiles versus time showed a non-linear dependence for the transport of cationic and anionic amino acid forms through hybrid membranes. This is reminiscent with a dual-mode profile diffusion assisted by complexation–
decomplexation at the level of fixed-site components, directionally self-organized within the membrane [19–23]. We first noted a fast transport of the Li+ ions from the feed to the strip solutions with the pH-dependent fluxes: JLi+ = 1.7–3.4 10 4 mol/L h. This induce an opposite membrane polarization potential (e) which intend to slow the diffusion of Li+ ions. This potential activates the transport of acidic L-Phenylalanine ammonium cation (JPhe+ = 1.5 10 5 mol/L) from the strip to the feed phase (Fig. 4a) and of the basic L-Phenylalanine negative carboxylate anion
P. Blondeau et al. / Reactive & Functional Polymers 86 (2015) 259–263
(JPhe = 2.8 10 5 mol/L) (Fig. 4c) oppositely from the feed to the strip phase against their concentration potential (i.e. note that the concentrations of L-Phenylalanine are the same, 10 3 M, in the feed and in the strip phases at the beginning of the experiment). As expected, the neutral zwitterionic L-Phenylalanine is not transported through the membrane under the same conditions (Fig. 4b). This unusual but previously observed [26] behaviour, can be rationalized by considering the charged L-Phenylalanine ammonium cation and carboxylate anion as ‘‘active spectators’’ of the diffusion events of more concentrated and fast diffusive Li+ and Cl partners. 4. Conclusion Controlled formation of the high-performance directional transporting pathways in silica, make hybrid materials presented here of interest for the development of a supramolecular approach toward functional membrane materials through self-organization. Membrane systems of increasing behavioral and functional addressability (directional transport, selectivity, ionic pumps, etc.) can be obtained by combining molecular/supramolecular and sol–gel chemistry approaches [12]. Within this context hybrid membrane systems may be employed to successfully transport charged molecules of biological interest across lipophilic membranes, by using ion-powered pumping mechanisms [26]. Herein we have proven that the transport of charged organic aminoacids can be directionally controlled via ionfuelled membrane polarization. The transport L-Phenylalanine, presenting cationic, zwitterionic and anionic forms, can be oriented and controlled via the pH from the feed to strip phase and vice versa. Our results imply that the control of molecular interactions and the polarization of the membrane materials can modulate and control the transport of charged organic molecules across the membrane. This is reminiscent with the organization of binding sites in channel-type proteins collectively contributing to the selective translocation of solutes along the hydrophilic ways [1–4]. Acknowledgements This work was supported by funds from ITN DYNANO, PITN-GA2011-289033 (www.dynano.eu) and ANR 2011 BS08 00604-MULTISELF Project. References [1] F. Hucho, C. Weise, Angew. Chem. Int. Ed. 40 (2001) 3100–3116. [2] S. Cukierman, Biophys. J. 78 (2000) 1825–1834.
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Contents lists available at ScienceDirect
Reactive & Functional Polymers journal homepage: www.elsevier.com/locate/react
Ion-fuelled L-Phenylalanine controlled pumping by hybrid membrane materials Pascal Blondeau 1, Mihail Barboiu ⇑, Eddy Petit Adaptative Supramolecular Nanosystems Group, Institut Européen des Membranes – ENSCM/UM2/CNRS 5635, IEM/UM2, CC 047, Place Eugène Bataillon, F-34095 Montpellier Cedex 5, France
a r t i c l e
i n f o
Article history: Received 14 June 2014 Received in revised form 11 August 2014 Accepted 15 August 2014 Available online 27 August 2014 Keywords: Hybrid materials Supramolecular membranes Self-organization Dialysis
a b s t r a c t We report organic–inorganic membrane materials for controlled translocation of ion-pairs along directional conduction pathways. The supramolecular/sol–gel combined procedure has been realised in two steps. First, the self-organization properties of 3-(ureidoarene)propyltriethoxysilane compound, 1 in aprotic solvents were determined, revealing the formation of supramolecular oligomers, in the selfassembly step. They led in a second sol–gel transcription step to hybrid membrane materials. Then, the transport of organic molecules of variable charge, like L-Phenylalanine can be directionally tuned, via ion-fuelled membrane polarization phenomena. Using this strategy, the transport of the cationic, zwitterionic and anionic forms of the L-Phenylalanine can be controlled and directionally oriented from the feed to strip phase and vice versa. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Most of the vital functions depend on selective transport of metabolites through the membrane cell [1]. The transport of essential metabolites depends on unique properties of hydrophilic inner-domains of the protein channels [2–4]. Artificial supramolecular ion-channels, extensively studied with the hope to mimic the natural ionic conduction [4–11], give the possibility to extend and to engineer the dimensionality of active transporting membranes materials across the extended scales [12]. Hybrid organic–inorganic materials produced by sol–gel process offer the opportunity to achieve versatile nanostructured materials [13–17]. Of special interest is structure-directed function of such hybrid materials and to control their build-up by self-organization. Recently, we have proved the potentiality of hybrid fixedsite complexant membranes. Such thin-layer solid membranes may present functional properties such as: (a) multiple molecular recognition [18–25] and (b) directional conduction transport via supramolecular self-assembled pathways [26–30]. The first generation of thin-layer hybrid membranes contains the polytopic macrocyclic receptors, randomly dispersed in a hybrid silica matrix
⇑ Corresponding author. Tel.: +33 467 14 91 95; fax: +33 467 14 91 19. E-mail address: [email protected] (M. Barboiu). Present address: Department of Analytical and Organic Chemistry, Universitat Rovira i Virgili, C/. Marcellí Domingo, s/n, 43007 Tarragona, Spain. Tel.: +34 977 55 81 55; fax: +34 977 55 84 46. 1
http://dx.doi.org/10.1016/j.reactfunctpolym.2014.08.002 1381-5148/Ó 2014 Elsevier B.V. All rights reserved.
[18–25]. The transport mechanism is based on ‘‘fixed-site jumping’’ diffusion [31–35] of solutes, controlled by the specific interactions with the fixed receptors. Then, our next objective was to minimize the distance between the selective receptor sites, with the hope to reduce the non-selective solute-diffusion between the active sites and to generate directional diffusion patterns into the solid dense material. Toward this objective, we have been previously developed self-organized hybrid membranes containing macrocyclic [26], aromatic [27–29] and sulfonic [30] groups, that form by collective self-assembly, directional transporting pathways for synergetic translocation of ion-pairs. Interesting pumping transport mechanisms have been described by using these membranes: the fast transport of small cationic species (i.e. H+, Na+) from the feed to the strip solutions, may induce an opposite (negative) membrane potential (e), which slow the diffusion of cations and accelerate the diffusion of anions (Fig. 1) [26]. Herein we describe novel findings related to the previously observed pumping/membrane polarization effects. The novel hybrid organic–inorganic membrane materials described here, have been prepared in two steps. First, the selfassembling properties of 3-(4-pyridylureido) propyltriethoxysilane compound, 1 in aprotic solvents were determined, revealing the formation of supramolecular oligomers. They led in a second sol– gel transcription step to thin-layer hybrid membrane materials organized at the nanoscopic scale (Scheme 1). These membranes have been tested in competitive dialysis transport experiments. The initial influx of lithium cations envisioned to diffuse along
260
P. Blondeau et al. / Reactive & Functional Polymers 86 (2015) 259–263
Teflon mould and then thermically polymerized at 80 °C, to afford a white brittle self-supported film which have been used further for characterization. The thin layer hybrid supported membranes were obtained by coating of the sol onto commercial ultrafiltration planar Polyacrylonitrile (PAN) supports (ORELIS – IRIS membranes) using a tape-casting method (see Ref. [19] for details). Scanning electron microscopy (SEM, Hitachi S-4500 apparatus, under a tension of 0.5–30 kV) allowed the thickness and the quality of the active deposited layer to be determined (Scheme 1). 2.3. Dialysis transport procedure
Fig. 1. Proposed cation-powered pumping mechanism of the anions against their concentration gradient (see text for details).
pyridine aromatic conduction pathways, can be used to directionally pump cationic and anionic forms but not zwitterionic form of LPhenylalanine across the membrane. 2. Experimental 2.1. Materials and methods The 3-(4-pyridylureido)propyltriethoxysilane compound, 1, was obtained from 3-isocyanato-propyltriethoxysilane and the corresponding 4-amino-pridine (benzene, 80 °C, 48 h) to afford after precipitation the compound 1, as a white powder (yield 85%). NMR spectroscopic analyses of precursors 1 are in accord with the proposed formula. 3-(4-Pyridylureido)-propyltriethoxysilane, (1) 4-aminopyridine (0.5 g; 5.312 mmol) and 3-isocyanatopropyl-triéthoxysilane (3.96 mL; 7.918 mmol) were dissolved in benzene and reaction mixture was refluxed under Ar for 48 h. After removal of the solvent, the residue was washed with hexane to afford 1, as a pure compound. 1H RMN (CDCl3): d = 0.68 (t, J = 7.8, 2H), 1.25 (t, J = 3.1, 3H), 1.65 (qt, J = 6.89, 2H), 3.28 (m, 2H), 3.79 (m, 2H), 5.56 (s, 1H), 7.35 (m, 2H), 7.6 (s, 1H), 8.39 (d, J = 6.19, 2H); 13C RMN (CDCl3) d 8.0, 18.6, 23.8, 42.8, 58.8, 113.1, 148.1, 150.2, 155.5.
Membrane transport experiments were performed with a bicompartmental device, magnetically stirred at room temperature (see Ref. [40] for details). It consists of two Teflon cell device separated by the solid membrane oriented with the active dense film to the feed phase. Nitrogen permeation measurements were performed to ensure that they were dense and defect free. The feed and the strip phases were both, aqueous solutions of the 10 3 M L-Phenylalanine aqueous solutions at different pH values: (a) pH = 1, (b) pH = pKa = 6.1 and (c) pH = 10. In order to generate the ion-concentration driven polarization effects, a 10 1 M LiCl has been added in the feed phase. The membrane consisted of supported dense membrane (S = 5.32 cm2). The Li+ concentrations were monitored at different time intervals in both feed and strip phases using the Atomic Absorption Spectrophotometer – AAS Varian 220 FS (k = 670.8, 1 nm c = 0.02–5 lg/ml). The amounts of LPhenylalanine in the both phases were determined by an HPLC– MS system (MICROMASS Platform II; Waters TM 600S Controller equipped with a Waters 515 HPLC pump (flow = 0.05 mL/min, T = 90 °C and voltage cone, VC = 25V). The acidic and amphoteric forms have been determined by using ESI + Mass spectrometry (H2O/CH3CN/CF3COOH:80/20/0.1) and the basic form has been determined by using ESI-mass spectrometry (H2O/CH3CN:80/20). This method allowed the concentration of the L-Phenylalanine to be determined with an error of 4%. The flux values, representing concentration of ions transported per hour through feed/membrane or membrane/receiving interfaces, have been calculated using the solution–diffusion model, from the slope of the initial linear part of concentration versus time profiles (time frame 0–10 h) [19].
2.2. Preparation and characterization of membranes 3. Results and discussions The compound 1 gelate the benzene solutions (C > 0.8 M). In order to obtain hybrid membrane materials we have first conducted the sol–gel experiments in benzene saturated with water. A slow hydrolysis process is occurring even for the long periods of time (t = 100 h) After solvent evaporation we obtain a low condensed transparent organic sol of 1, which has been casted in a
3.1. Precursor 1 and membrane material characterizations The generation of self-organized ribbon-type superstructures of 1 in solution, sol/gel and in the solid state is based on three encoded features (Scheme 1): (1) they contain pyridine and urea
Scheme 1. Structure of ligand 1 and its self-organization in ribbon-type architectures in solution, followed by the sol–gel transcription of encoded transporting pathways for cations and anions into a hybrid dense membrane material. Cross section micrograph of the thin-layer dense hybrid membrane deposited onto PAN polymeric support.
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compound 1 [37] (Fig. 3a). It is proving the importance of additional p–p stacking interactions in the self-organization process [38,39]. By contrast the self-association of 1, highly increase in the non-polar solvent like C6D6 when compared with the low-polar solvent CDCl3, while the cooperativity is remaining constant (Fig. 3b). Evidence for H-bond formation was obtained from the FTIR spectra: in both organogels and in the solid hybrid, the urea vibration shifts were detected: mNAH = 3320 (Dm = 30), mC@O(amideI) = 1635 (Dm = 15) and mCAN(amide II) = 1565 (Dm = 25) cm 1. After hydrolysiscondensation reactions FTIR spectra of hybrid material showed the appearance of the vibrations of the mSiAOASi = 1000–1200 cm 1 instead the vibrations mSiAOEt = 1075, 1100 and 1170 cm 1, initially observed for the molecular precursor 1. The 29Si MAS NMR spectroscopic experiments agree with a progressive condensation of alkoxysilane moieties: (a) in the first step by stirring at room temperature, a very weak connected hybrid sol was observed, mostly composed of T0 ( 46.2 ppm) corresponding to RSi(OEt)3 and with the residual T1 and T2 species; (b) more advanced condensation of about 81% with a higher proportion of T1 ( 53.1 ppm), T2 ( 61.4 ppm) and T3 ( 67.4 ppm) species were obtained by heating up to 80 °C for the cross-linked hybrid membrane material (Vitreous transition temperature: Tg = 88.8 °C), which have been found to be thermally stable up to 200 °C. 3.2. Dialysis membrane transport experiments
Fig. 2. Chemical shifts of a) the Alkyl–N–H (red squares) and Aryl-N–H (black squares) protons plotted against total concentration of 1 in CDCl3 at 25 °C and of the Aryl-N–H protons plotted against total concentration of 1 in CDCl3 (red squares) and in C6D6 (black squares) at 25 °C. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
moieties known to bind metal cations (i.e. Li+) [36] and the anions, respectively [37,38]; (2) the supramolecular guiding interaction is the urea head-to-tail H-bond association, [35,38] reminiscent with the amide moiety’s H-bonding in proteins, (3) the ureidopyridyl residues are covalently bonded to triethoxysilyl groups allowing to transcribe the self-organized superstructures into the hybrid materials by sol–gel process [13,14]. 1 H NMR dilution experiments showed the strong downfield shifts of NHalif (Dd = 0.82 ppm) or NHarom (Dd = 2.2 ppm) protons upon increasing concentration of 1, which is indicative of self-association through the intermolecular hydrogen bonding in CDCl3 (Fig. 2a). Equilibration between H-bonded and non-H bonded states in CDCl3 for a given NAH proton is almost always fast on the NMR time scale and observed proton chemical shifts are weighted averages of the chemical shifts of contributing states. The downfield shifts of NHarom (Dd = 4.0 ppm) proton of 1 increase significantly in C6D6 (Fig. 2b) indicating stronger association a less polar solvent as expected. The association of 1 could be described with a cooperative association model [11,12]. This method assumes that all polymerization constants, but not the first equilibrium constant K2 describing dimerization, are the same Kp (Fig. 3). High values of Kp and K2 indicate strong association via H-bonding, while high values of the ratio Kp/K2 are related to the cooperativity of the self-association process. Calculation of the association constants revealed that the strength of the self-association of compound 1 and the process cooperativity are both decreased, when compared with the values determined for the bis-aromatic
The membrane transport performances designed to transport ion-pairs, depend on encoded molecular features of heteroditopic precursor 1. In order to demonstrate the formation of cation–anion translocation pathways into synthesized membranes we carried out dialysis transport experiments. The L-Phenylalanine transport across the membranes according to the directional-diffusion transport mechanism and against its thermodynamic gradient was
Fig. 3. Dimerization (K2) and polymerization (KP) association constants for numerical fits to a monomer/dimer/defined aggregate model of (a) pyridyl-propyl-type compound 1 and pyridyl-Aryl compound 1 and (b) pyridyl-propyl-type compound 1 in CDCl3 and C6D6 solvents.
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Fig. 4. Membrane transport of L-Phenylalanine and Li+ through the membranes (a) concentration versus time transport profiles of the (a) acidic cationic ammonium, pH = 1 (b) neutral, zwitterionic, pH = pKa = 6.1 and (c) basic anionic carboxylate, pH = 10 L-Phenylalanine. Proposed Li+-powered pumping mechanism of the L-Phenylalanine against its concentration gradient. The flux of Li+ ions is represented by the black arrow, the induced membrane polarization potential by the light-blue arrow and the flux of the LPhenylalanine in (a) red (pH = 1), (b) violet, pH = 6.1 and (c) blue (pH = 10) arrows. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
evaluated using ion-driven pumping conditions: experiments have been conducted with equal concentrations of the L-Phenylalanine of 10 3 M initial in the feed and in the strip solutions, while the LiCl is present only in the feed phase, CLi+ = 10 1 M. Fig. 4 shows the concentration versus time transport profiles through the hybrid membrane. The concentration profiles versus time showed a non-linear dependence for the transport of cationic and anionic amino acid forms through hybrid membranes. This is reminiscent with a dual-mode profile diffusion assisted by complexation–
decomplexation at the level of fixed-site components, directionally self-organized within the membrane [19–23]. We first noted a fast transport of the Li+ ions from the feed to the strip solutions with the pH-dependent fluxes: JLi+ = 1.7–3.4 10 4 mol/L h. This induce an opposite membrane polarization potential (e) which intend to slow the diffusion of Li+ ions. This potential activates the transport of acidic L-Phenylalanine ammonium cation (JPhe+ = 1.5 10 5 mol/L) from the strip to the feed phase (Fig. 4a) and of the basic L-Phenylalanine negative carboxylate anion
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(JPhe = 2.8 10 5 mol/L) (Fig. 4c) oppositely from the feed to the strip phase against their concentration potential (i.e. note that the concentrations of L-Phenylalanine are the same, 10 3 M, in the feed and in the strip phases at the beginning of the experiment). As expected, the neutral zwitterionic L-Phenylalanine is not transported through the membrane under the same conditions (Fig. 4b). This unusual but previously observed [26] behaviour, can be rationalized by considering the charged L-Phenylalanine ammonium cation and carboxylate anion as ‘‘active spectators’’ of the diffusion events of more concentrated and fast diffusive Li+ and Cl partners. 4. Conclusion Controlled formation of the high-performance directional transporting pathways in silica, make hybrid materials presented here of interest for the development of a supramolecular approach toward functional membrane materials through self-organization. Membrane systems of increasing behavioral and functional addressability (directional transport, selectivity, ionic pumps, etc.) can be obtained by combining molecular/supramolecular and sol–gel chemistry approaches [12]. Within this context hybrid membrane systems may be employed to successfully transport charged molecules of biological interest across lipophilic membranes, by using ion-powered pumping mechanisms [26]. Herein we have proven that the transport of charged organic aminoacids can be directionally controlled via ionfuelled membrane polarization. The transport L-Phenylalanine, presenting cationic, zwitterionic and anionic forms, can be oriented and controlled via the pH from the feed to strip phase and vice versa. Our results imply that the control of molecular interactions and the polarization of the membrane materials can modulate and control the transport of charged organic molecules across the membrane. This is reminiscent with the organization of binding sites in channel-type proteins collectively contributing to the selective translocation of solutes along the hydrophilic ways [1–4]. Acknowledgements This work was supported by funds from ITN DYNANO, PITN-GA2011-289033 (www.dynano.eu) and ANR 2011 BS08 00604-MULTISELF Project. References [1] F. Hucho, C. Weise, Angew. Chem. Int. Ed. 40 (2001) 3100–3116. [2] S. Cukierman, Biophys. J. 78 (2000) 1825–1834.
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