Towards the rational development of molecularly imprinted polymers: 1H NMR studies on hydrophobicity and ion-pair interactions as driving forces for selectivity

The preparation of molecularly imprinted polymers (MIP) based on non-covalent interactions has become a widely used technique for creating highly specific sorbent materials predominantly used in separation chemistry. A crucial factor in a successful imprinting protocol is the optimisation of the template/functional monomer interaction in the pre-polymerisation mixture, eventually leading to a maximum of high-affinity binding sites in the resulting polymer matrix. In order to develop more efficient preparation technologies for imprinted polymers, two separate pre-polymerisation complexes were investigated by NMR spectroscopic techniques in order to identify the types of interactions occurring in the pre-polymerisation mixture, and their implications for the subsequently formed imprinted polymer. In particular, hydrophobic effects have been followed by NMR spectroscopy and their contribution to the selectivity of the resulting MIP has been investigated. The 2,4-D imprint system is used as an example to fundamentally study whether observations at the pre-polymerisation stage correlate with properties of the finally prepared MIP, and which parameters govern success of an imprinting protocol.     

Insights into the origins of binding and the recognition properties of molecularly imprinted polymers prepared using an amide as the hydrogen-bonding functional group

Molecularly imprinted polymers (MIPs) prepared using an amide hydrogen-bonding functional monomer (acrylamide) exhibited efficient enantiomeric recognition properties in both organic and aqueous media in the HPLC mode. The results indicate that the amide functional groups formed strong hydrogen-bonding interactions with the template molecule, and specific recognition sites were created within the polymer matrix during the imprinting process. When Boc-L-Trp was used as the template, an MIP prepared in a polar organic solvent (acetonitrile) using acrylamide as the functional monomer showed better enantiomeric recognition of Boc-Trp than the MIPs prepared in the same solvent using an acidic (methacrylic acid) or a basic (2-vinylpyridine) functional monomer or a combination of an acidic and a basic functional monomer (methacrylic acid + 2-vinylpyridine). Our results indicate that in organic media the degree of retention of the sample molecule on the imprinted polymer was controlled by hydrogen-bonding interactions between the sample molecule and the polymer, while in aqueous media it was determined to a considerable extent by hydrophobic interactions. In both media the shape, size and the nature of the hydrogen-bonding groups of the sample molecules were all important factors in determining the enantiomeric and substrate selectivity. In the aqueous media, however, the hydrophobicity of the sample molecules was also found to play an important role.     

Anatomy of a successful imprint: analysing the recognition mechanisms of a molecularly imprinted polymer for quercetin

This study comprises a retrospective analysis of a successful molecular imprint for quercetin with the main aim of deriving rational design strategies for more effective molecularly imprinted polymers. Hence, polymers of varying composition were synthesised and chromatographically characterised to examine the effects of monomer-template ratios. (1)H NMR analysis of the pre-polymerisation mixture yielded further information on the nature of the complexes formed prior to the polymerisation step. A direct correlation between the optimum monomer-template ratio derived from the chromatographic studies and the monomer-template ratio providing the most stable pre-polymerisation complexes observed via (1)H NMR T(1) relaxation time measurements, suggests that the formation of particularly stable pre-polymerisation complexes is responsible for an increased formation of selective binding sites during the polymerisation step. Furthermore, physical aspects of the polymerisation, such as the MIP surface area and macroscopic phase partitioning of the mixture during the polymerisation are investigated. The observed effects and their analytical assessment offer insight into the mechanisms governing MIP selectivity at a molecular level.     

Concentration dependent atrazine-atrazine complex formation promotes selectivity in atrazine imprinted polymers

An atrazine (ATR) molecularly imprinted polymer (MIP) was prepared using a non-covalent strategy. The affinity and selectivity of the polymer was initially evaluated under non-equilibrium conditions and the polymer was shown to possess good template selectivity. The selectivity of the polymer was further investigated under equilibrium conditions and over a range of concentrations using Scatchard plots and Hill plots and by assessing distribution coefficients and normalised selectivity values. It was observed that both selectivity and affinity were dependent on the concentration of the ligand and that unusually selectivity and affinity were better at higher atrazine concentrations. It was concluded that this phenomenon resulted from the formation of atrazine-atrazine complexes during the pre-polymerisation stage and during rebinding and that the polymer demonstrated improved atrazine affinity when the conditions favoured complex formation.     

Combinatorial screening of polymer precursors for preparation of benzo[α] pyrene imprinted polymer: an ab initio computational approach

A combinatorial screening procedure was used for the selection of polymer precursors in the preparation of molecularly imprinted polymer (MIP), which is useful in the detection of the air pollution marker molecule benzo[a]pyrene (BAP). Molecular imprinting is a technique for the preparation of polymer materials with specific molecular recognition receptors. The preparation of imprinted polymers requires polymer precursors such as functional monomer, cross-linking monomer, solvent, an initiator of polymerization and thermal or UV radiation. A virtual library of functional monomers was prepared based on interaction binding scores computed using HyperChem Release 8.0 software. Initially, the possible minimum energy conformation of the monomers and BAP were optimized using the semi-empirical (PM3) quantum method. The binding energy between the functional monomer and the template (BAP) was computed using the Hartree-Fock (HF) method with 6-31 G basis set, which is an ab initio approach based on Moller-Plesset second order perturbation theory (MP2). From the computations, methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) were selected for preparation of BAP imprinted polymer. The larger interaction energy (ΔE) represents possibility of more affinity binding sites formation in the polymer, which provides high binding capacity. The theoretical predictions were complimented through adsorption experiments. There is a good agreement between experimental binding results and theoretical computations, which provides further evidence of the validity of the usefulness of computational screening procedures in the selection of appropriate MIP precursors in an experiment-free way.     

Computational strategies for understanding the nature of interaction in dioxin imprinted nanoporous trappers

A new computational model capable of understanding the nature of interactions in signature complexes formed between the template (2,3,7,8‐tetrachlorodibenzo‐p dioxin (TCDD)) and the functional monomers (methacrylic acid (MAA)) using density functional theory (DFT) has been designed. The polymer precursors were optimized for geometries in polymerization media, computing the interaction energies between template molecules and functional monomers of transient pre‐polymerized complexes (PPC), and structural and vibrational properties reference to theoretical infrared spectra were computed using DFT of B3LYP/6 311+G(d,p) hybrid functional method. Atom in molecule theory was used to analyze the hydrogen‐bonding characteristics of PPC of MAA–TCDD. Considering the theoretical titrations conducted in a virtual solvent box, it was found that the 1:4 molar ratio was required to form the most stable PPC in a given solvent system. The electron density plots indicate strong hydrogen bonding as shown by the 2pz dominant highest occupied molecular orbital (HOMO) character that could be the preferable sites of binding for target molecule, TCDD. Considering HOMO approach, the active adsorption sites in molecularly imprinted polymer was modeled to get insight on molecular recognition property for targeted molecule, TCDD. The proposed computational protocol is simple, accurate, and novel to design the polymer and is useful to predict the properties of polymer systems than the conventional theoretical analysis of template–monomer interactions. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of molecularly imprinted polymers for the binding of nitrofurantoin

Novel molecularly imprinted polymers (MIPs) for the recognition of nitrofurantoin (NFT) were prepared by photoinitiated polymerisation in polar solvent using 2,6-bis(methacrylamido) pyridine (BMP) as the functional monomer and carboxyphenyl aminohydantoin (CPAH) as the analogue of the template. The binding constants of the complex between BMP and nitrofurantoin or CPAH in DMSO were determined with 1H NMR titration to be 630 ± 104 and 830 ± 146 M⁻¹, respectively. To study the influence of the functional monomer, two polymer compositions were prepared containing the template, the functional monomer and the crosslinker in the molar ratio 1:1:12 for MIP1 and 1:4:20 for MIP2, respectively. The imprinting factor at saturation concentration of nitrofurantoin, which is the ratio of the amount bound to the MIP and the non-imprinted control polymer (NIP), was determined to be 2.47 for MIP1 and 2.49 for MIP2. The cross reactivity of the imprinted polymers seems to be determined by the ability to form hydrogen bonds to the functional monomer while the shape of the molecule has no real influence.     

Composite template membranes as synthetic receptor systems 1. Preparation, specificity and template effect

Molecularly impregnated membranes for selective recognizing of adenosine 3',5'-cyclic monophosphate (cAMP) were obtained and their separation properties were studied. Composite polyvinylidenfluoride microfiltration membranes covered with a thin layer of polymer impregnated with cAMP were prepared using photoinitiated copolymerization of dimethylaminoethylmethacrylate as a functional monomer and trimethylolpropane thrimethacrylate as a cross-linker in the presence of cAMP as a template. It was concluded that the ability of MIP membranes to selective binding of cAMP is a result of both specific shape and dimension of the recognizing site as well as specific interactions between functional groups responsible for selective template binding inside the receptor site.     

Synthesis and evaluation of a selective molecularly imprinted polymer for the contraceptive drug levonorgestrel

A molecularly imprinted polymer (MIP) has been prepared using levonorgestrel (LEV) as template. The polymer was synthesised in a non-covalent approach using methacrylic acid (MAA) as functional monomer and ethylene glycol dimethacrylate (EGDMA) as cross-linking monomer via a free radical polymerization. An equivalent blank polymer was also synthesised in the absence of the template compound. Batch adsorption experiments were used to evaluate the binding affinity of the imprinted polymer. After packing MIP into a stainless steel column (150 mm x 4.6 mm i.d.), retention and elution of the template and related compounds were evaluated by high-performance liquid chromatography (HPLC). This LEV imprinted polymer was further applied for selective solid phase extraction (SPE) of LEV from human serum. It was confirmed that the binding ability of the prepared MIP for LEV was essentially sufficient in the presence of other compounds coexisting in serum sample. Therefore, as a selective and efficient solid phase material, LEV imprinted polymer has a high potential application in analysis of this steroidal hormone in clinical purposes.     

A computational approach to study functional monomer‐protein molecular interactions to optimize protein molecular imprinting

Molecular imprinting has become a promising approach for synthesis of polymeric materials having binding sites with a predetermined selectivity for a given analyte, the so‐called molecularly imprinted polymers (MIPs), which can be used as artificial receptors in various application fields. Realization of binding sites in a MIP involves the formation of prepolymerization complexes between a template molecule and monomers, their subsequent polymerization, and the removal of the template. It is believed that the strength of the monomer‐template interactions in the prepolymerization mixture influences directly on the quality of the binding sites in a MIP and consequently on its performance. In this study, a computational approach allowing the rational selection of an appropriate monomer for building a MIP capable of selectively rebinding macromolecular analytes has been developed. Molecular docking combined with quantum chemical calculations was used for modeling and comparing molecular interactions among a model macromolecular template, immunoglobulin G (IgG), and 1 of 3 electropolymerizable functional monomers: m‐phenylenediamine (mPD), dopamine, and 3,4‐ethylenedioxythiophene, as well as to predict the probable arrangement of multiple monomers around the protein. It was revealed that mPD was arranged more uniformly around IgG participating in multiple H‐bond interactions with its polar residues and, therefore, could be considered as more advantageous for synthesis of a MIP for IgG recognition (IgG‐MIP). These theoretical predictions were verified by the experimental results and found to be in good agreement showing higher binding affinity of the mPD‐based IgG‐MIP toward IgG as compared with the IgG‐MIPs generated from the other 2 monomers.     

Chiral imprinted polymers as enantiospecific coatings of stir bar sorptive extraction devices

This paper reports the design of Molecularly Imprinted Polymers (MIP) with affinity towards (S)-citalopram using computational modeling for the selection of functional monomers and monomer:template ratio. Acrylamide was selected as functional monomer and the final complex functional monomer/template resulted in a 3:1 ratio. The polymer was synthesized by radical polymerization initiated by UV onto magnetic stir-bars in order to obtain a stir bar sorptive extraction (SBSE) device capable of selective enantiomeric recognition. After successful template removal, the parameters affecting the SBSE procedure (sample volume, ionic strength, extraction time and pH) were optimized for the effective rebinding of the target analyte. The resultant chirally imprinted polymer based stir-bar was able to selectively extract (S)-citalopram from a racemic mixture in an aqueous media with high specificity (specificity factor 4) between 25 and 500 μgL(-1). The MIP coated stir-bars can have significance for enantiospecific sample pre-concentration and subsequent analysis without the need for any chiral chromatographic separation.     

Study on the recognition of templates and their analogues on molecularly imprinted polymer using computational and conformational analysis approaches

A simplified computational model was proposed to simulate the synthesis of molecularly imprinted polymers (MIP), removal of template and recognition of the template and its analogues by MIP. The MIPs with nicotinamide and iso-nicotinamide as templates were prepared using methacrylic acid as functional monomer. Based on our computational model, the interaction energies between the monomer and the template or its analogues were calculated, which were well correlated with the retention factors and imprinting factors obtained on HPLC columns packed with the corresponding MIP particles. The imprinting effects of the template and its analogues were also investigated from the viewpoint of conformational analysis. The computational data were successfully used to predict the chromatographic behaviour of some chemicals in separation on HPLC columns. We believe that the computational method will find application in designing monomers for MIP synthesis and in studying recognition of templates and their analogues on MIP.     

Surface molecular imprinting by atom transfer radical polymerization

Results are presented that demonstrate the successful preparation of ultrathin (< 10 nm), surface-confined, molecularly imprinted polymer (MIP) films on model gold substrates using atom transfer radical polymerization (ATRP). 2-Vinylpyridine (2Vpy) was investigated as the functional monomer, and ethylene glycol dimethacrylate (EGDMA) was the cross-linking monomer. Fluorescently labeled N,N'-didansyl-L-cystine and N,N'-didansyl-L-lysine were used as the template molecules to form the MIPs. Spectroscopic and ellipsometric results are presented that follow film formation and growth rates. Results are also presented from fluorescence experiments used to quantify and compare the adsorption capacities of MIP surface films and nonimprinted (NIP) control films. MIP films exhibited higher binding capacities than the control NIP films at all solution concentrations of N,N'-didansyl-L-cystine and N,N'-didansyl-L-lysine. Furthermore, template removal from these imprinted films appears to be 100% efficient. Selectivity studies showed that the MIPs display some cross-reactivity between these two molecules; nevertheless, MIPs prepared against one template showed selectivity for that template. A selectivity coefficient of 1.13 was achieved for MIP surfaces prepared against N,N'-didansyl-L-lysine; a value of 1.51 was observed for MIP surfaces prepared against N,N'-didansyl-L-cystine.     

Density functional theory characterisation of 4-hydroxyazobenzene

We report the structural properties, infrared (IR) and Raman spectra, dipole moment, polarisability, hardness and chemical potential of the trans and cis configurations of 4-hydroxyazobenzene calculated using the B3LYP functionals. All calculations were performed with the following basis sets: 6–31G, 6–31++G, 6–31G(d,p), 6–31++G(d,p), 6–31G(2d,2p), 6–31++G(2d,2p) and 6–311++G(2d,2p). We observed that 6–31++G(d,p) gives similar results to 6–311++G(2d,2p). Consequently, SVWN and PW91 methods were also used in association with 6–31++G(d,p) to test the influence of the different models of exchange and correlation functionals. A planar structure was obtained for all the optimised trans configuration structures. In both isomers, the presence of the hydroxyl group leads to an asymmetry in certain structural parameters. From these results, two IR or Raman active frequencies can be used to easily distinguish trans and cis configurations. The trans configuration was found to be more stable than the cis configuration by 67 ± 2 kJ mol⁻¹ at 0 K. The difference of the dipole moment between trans and cis for 4-hydroxyazobenzene was found to be lower than for trans and cis azobenzene.     

Effective removal of rhodamine B from contaminated water using non-covalent imprinted microspheres designed by computational approach

Molecular dynamics simulations and computational screening were used to identify functional monomers capable of interacting with rhodamine B (RhB). A library of 24 kinds of common functional monomers for preparing molecular imprinted polymer (MIP) was built and their interactions with RhB in acetonitrile were calculated using the molecular dynamics software (Gromacs 3.3). It was anticipated that the monomers giving the highest binding energy are suitable for preparing the affinity polymers. According to the theoretical calculation results, the MIP microspheres with RhB as template was prepared by reverse microemulsion polymerization method using acrylamide (AAm) as functional monomer and divinylbenzene as cross-linker in acetonitrile. Microspheres have been characterized by scanning electron microscopy (SEM). The proper adsorption and selective recognition ability of the MIP were studied by an equilibrium-adsorption method. The MIP showed outstanding affinity towards RhB in aqueous solution and the optimum pH value for binding has been found around neutral range. The molecular recognition of RhB was analyzed in detail by using molecular modeling software (Gaussian03). In addition, the MIP reusability without obviously deterioration in performance was demonstrated at least five repeated cycles.     

Cooperativity effects in linear formaldehyde oligomers using density functional theory calculations

This work reports hydrogen bonding interaction in linear formaldehyde oligomers using density functional theory method. Many-body analysis technique has been used to study the various interactions in these oligomers and to obtain % contributions from individual many-body energy terms to the binding energies of these oligomers. Co-operativity effects are studied using different indicators viz. hydrogen bond strength, inter- and intramolecular distances, dissociation energy, dipole co-operativity, energy per hydrogen bond, excess energy and non-additive energy. All these indicators show strong positive hydrogen bond co-operativity in linear formaldehyde oligomers. The dipole moment changes from 2.51 D in monomer to 20.92 D in formaldehyde heptamer.     

Theoretical study of 1-(4-hexylcyclohexyl)-4-isothiocyanatobenzene: molecular properties and spectral characteristics

The mesogenic species 4-(4-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) was studied with density functional theory and molecular mechanics in order to investigate the molecular properties, interactions between dimers and to interpret the IR spectrum. Two types of calculations were performed for model systems containing single and double molecules of 6CHBT. Calculations (involving conformation analysis) for isolated species indicated that the trans isomer, in the equatorial–equatorial conformation, is the most energetically stable. The 6CHBT molecule is polar, with a rather high (4.43 D) dipole moment with negatively charged isothiocyanato (NCS) ligand. The dimer–dimer interaction energies show that the head-to-head configuration (where van der Waals attraction forces play the major role) is the most energetically stable. Vibrational analysis provided detailed assignment of the experimental infra-red (IR) spectrum. Figure Most favorite 6CHBT head to head interaction - ESP mapped to electron density surface Dedication  This paper is dedicated to the memory of Dr. Wacław Witko, who introduced us to research on mesogenic systems.     

Validation of computational approach to study monomer selectivity toward the template Gallic acid for rational molecularly imprinted polymer design

Gallic acid (GA) is important for pharmaceutical industries as an antioxidant. It also finds use in tanning, ink dyes and manufacturing of paper. Molecularly imprinted polymers (MIP), which are tailor made materials, can play an excellent role in separation of GA from complex matrices. Molecular recognition being the most important property of MIP, the present work proposes a methodology based on density functional theory (DFT) calculations for selection of suitable functional monomer for a rational design of MIP with a high binding capacity for GA. A virtual library of 18 functional monomers was created and screened for the template GA. The prepolymerization template-monomer complexes were optimized at B3LYP/6-31G(d) model chemistry and the changes in the Gibbs free energy (ΔG) due to complex formation were determined on the optimized structures. The monomer with the highest Gibbs free energy gain forms most stable complex with the template resulting in formation of more selective binding sites in the polymeric matrix in MIPs. This can lead to high binding capacity of MIP for GA. Amongst the 18 monomers, acrylic acid (AA) and acrylamide (AAm) gave the highest value of ΔG due to complex formation with GA. 4-vinyl pyridine (4-Vp) had intermediate value of ΔG while, methyl methacrylate (MMA) gave least value of ΔG due to complex formation with GA. Based on this study, the MIPs were synthesized and rebinding performance was evaluated using Langmuir-Freundlich model. The imprinting factor for AA and AAm based MIPs were 5.28 and 4.80 respectively, 4-Vp based MIP had imprinting factor of 2.59 while MMA based MIP exhibited an imprinting factor of 1.95. The experimental results were in good agreement with the computational predictions. The experimental data validated the DFT based computational approach.     

Rational synthesis of pindolol imprinted polymer by non-covalent protocol based on computational approach

Pindolol (PDL) is a potent and specific adrenoreceptor blocking agent. It is widely used in the treatment of hypertension, cardiac arrhythmia and angina pectoris. Molecularly imprinted polymers (MIPs) are synthetic receptors having potential applications in drug delivery systems and devices such as diagnostic sensors. In the present work, ab initio quantum mechanical simulations and computational screening were used to identify functional monomer having best interactions with PDL. A virtual library of 16 functional monomers was built and the possible minimum energy conformation of the monomers and PDL were calculated using Hartree-Fock (HF) method for the synthesis of PDL imprinted polymer. The interaction energy between functional monomer and the template were corrected by means of basis set superposition error (BSSE) in all pre-polymerization complexes. The hydrogen bonding between PDL and functional monomer was evaluated by changes in bond lengths before and after complex formation. The virtual template-monomer complex with highest interaction energy is more stable during the polymerization and leads to high selectivity and specificity toward the template. The interaction energy of PDL was found to be the highest with itaconic acid followed by 4-vinyl pyridine and least with acrylonitrile. Taking a spectroscopic viewpoint, results obtained from analysis of the harmonic infrared spectrum were examined. Red and blue shifts related to the stretching frequencies of either donors or acceptors of protons were identified and compared experimentally. Stoichiometric mole ratio of template to functional monomer was optimized and confirmed by UV visible spectra titrations. The theoretical results were correlated by evaluation of binding parameters of MIPs. The experimental binding results were in good agreement with theoretical computations.     

The rational use of hydrophobic effect-based recognition in molecularly imprinted polymers

A novel molecularly imprinted polymer (MIP) system selective for D-phenylalanine is described where polymerization is performed in aqueous solution. The unique polymer system comprises a hydrophobic moiety-selective functional monomer, polymerizable beta-cyclodextrin, an electrostatic interacting functional monomer, 2-acryloylamido-2-methylpropane sulfonic acid (AMPSA), and the crosslinking agent N,N'-diacryloylpiperazine. Chromatographic evaluation of polymer-ligand recognition characteristics demonstrated ligand selectivity by the MIP and that optimal recognition was achieved through a balance of hydrophobic and electrostatic ligand-polymer interactions, indicating that recognition in these systems is regulated by enthalpy-entropy compensation. The imprinting effect was shown to be sufficient to reverse the inherent selectivity of cyclodextrin for L-phenylalanine.