Remarkable enantioselectivity of molecularly imprinted TiO2 nano-thin films

TiO(2) nano-thin films with imprinted (R)- and (S)-enantiomers of propranolol, 1,1'-bi-naphthol, and 2-(4-isobutylphenyl)-propionic acid were fabricated on quartz plates by spin-coating their solutions with Ti(O-(n)Bu)(4) in a toluene-ethanol mixture (1:1, v/v). After template removal, the imprinted films showed better binding for original templates than to the corresponding enantiomers. The assessment of template incorporation, template removal, and re-binding was conducted through UV-vis measurements. Significant enhancement of enantioselectivity was achieved by optimization of the film thickness and by heat-treatment of the imprinted films. After subtraction of non-specific binding, the optimized films provided chiral recognition with the enantioselectivity of almost 100% for (R)-propranolol and 95% for (S)-propranolol.     

Molecularly imprinted microparticles for capillary electrochromatographic enantiomer separation of propranolol

Molecularly imprinted microparticles imprinted against (S)-propranolol were synthesised and studied for use in capillary electrochromatographic separation of propranolol enantiomers. The imprinted microparticles were in the size range of 0.2-0.5 micron as determined by scanning electron microscopy. The microparticles were suspended, in high concentration, in the electrolyte and used to perform enantiomer separation by a partial filling technique.     

RP-HPLC拆分普萘洛尔对映体的洗脱顺序及其荧光响应研究

应用反相高效液相色谱法,以（Ｒ,Ｒ）－Ｏ,Ｏ－二乙酰基酒石酸酐（ＤＡＴＡＡＮ）为手性衍生化试剂,拆分了普萘洛尔（ｐｒｏｐｒａｎｏｌｏｌ）对映体。实验结果显示：普萘洛尔对映体经衍生化而生成的两个非对映异构体分子的空间结构有着明显的差异,即（Ｒ,Ｒ,Ｒ）－异构体中存在着分子内氢键,而（Ｒ,Ｒ,Ｓ）－异构体中却没有,这种结构上的区别不仅成为控制洗脱顺序的主要因素,而且对其荧光响应也产生重要的影响。     

二乙酰酒石酸酐柱前手性衍生化反相高效液相色谱法拆分心得安对映体

以（Ｒ，Ｒ）－Ｏ，Ｏ－二乙酰基酒石酸酐作柱前手性衍生化试剂，用ＯＤＳ柱拆分了心得安对映体，探讨了衍生化反应条件，研究了ＰＨ及流动相组成对色谱保留行为的影响，发现心得安的两种非对映异构体衍生物的荧光响应有显著差别，对心得安对映体的半制备色谱拆分规律作了新的探讨，确定用Ｓｈｉｍ－ｐａｃｋＣＬＣ－ＯＤＳ柱进行半制备色谱拆分的最佳条件，得到的光学异构体纯度在９９％以上。     

Noncovalent imprinting in the shell of core-shell nanoparticles

Propranolol was imprinted using noncovalent interactions in the shell of core-shell nanoparticles prepared by aqueous emulsion polymerization in the presence and absence of toluene. The imprinted particles were characterized, and their capacity to rebind propranolol from both organic and aqueous media was analyzed. Results showed that the amount of template incorporated into the polymer and the presence of toluene as a "porogenic" agent influenced the ability of the nanoparticles obtained to rebind propranolol. The presence of toluene during imprinting increased rebinding by about 2-fold in buffer and by 3-fold in toluene, compared with similar materials made in the absence of toluene during imprinting. It also influenced the final surface area of the particles. Binding site affinity, assessed by radioligand displacement, was measured as IC50 values of about 1-10 microM. This compares with about 3 microM for bulk polymer made with a similar composition. Finally, to demonstrate the advantages of structured particles for analytical applications a new property, fluorescence, was incorporated into the core of the particles without interfering with the imprinted shell and its ability to rebind propranolol.     

Enantioselective drug monitoring of (R)- and (S)- propranolol in human plasma via derivatization with optically active (R,R)-O,O-diacetyl tartaric acid anhydride

A sensitive high-performance liquid chromatographic method was developed for the stereoselective assay of (R)- and (S)-propranolol in human plasma. The method involves diethyl ether extraction of the drugs and a racemic internal standard, N-tert.-butylpropranolol, followed by derivatization of the compounds with the chiral reagent (R,R)-O,O-diacetyl tartaric acid anhydride. The resulting diastereomeric derivatives were separated isocratically on a reversed-phase column. Quantitation was achieved by the peak-height ratio method with reference to the internal standard. The assay was accurate and reproducible in the concentration range 1-100 ng of (R)- and (S)-propranolol per ml plasma, using fluorescence detection at lambda ex 290 nm and lambda em 335 nm. The applicability of this method was demonstrated for the determination of concentration-time profiles of propranolol enantiomers in the course of comparative pharmacokinetic studies.     

Uniform molecularly imprinted microspheres and nanoparticles prepared by precipitation polymerization: the control of particle size suitable for different analytical applications

Molecularly imprinted polymers (MIPs) are being increasingly used as selective adsorbents in different analytical applications. To satisfy the different application purposes, MIPs with well controlled physical forms in different size ranges are highly desirable. For examples, MIP nanoparticles are very suitable to be used to develop binding assays and for microfluidic separations, whereas MIP beads with diameter of 1.5-3 μm can be more appropriate to use in new analytical liquid chromatography systems. Previous studies have demonstrated that imprinted microspheres and nanoparticles can be synthesized using a simple precipitation polymerization method. Despite that the synthetic method is straightforward, the final particle size obtained has been difficult to adjust for a given template. In this work, we initiated to study new synthetic conditions to obtain MIP beads with controllable size in the nano- to micro-meter range, using racemic propranolol as a model template. Varying the composition of the cross-linking monomer allowed the particle size of the MIP beads to be altered in the range of 130 nm to 2.4 μm, whereas the favorable binding property of the imprinted beads remained intact. The chiral recognition sites were further characterized with equilibrium binding analysis using tritium-labeled (S)-propranolol as a tracer. In general, the imprinted sites displayed a high chiral selectivity: the apparent affinity of the (S)-imprinted sites for (S)-propranolol was 20 times that of for (R)-propranolol. Compared to previously reported irregular particles, the chiral selectivity of competitive radioligand binding assays developed from the present imprinted beads has been increased by six to seven folds in an optimized aqueous solvent.     

Time-resolved fluorescence reveals two binding sites of 1,8-ANS in intact human oxyhemoglobin

Time-resolved fluorescence of 1,8-anilinonaphthalene sulfonate (1,8-ANS) fluorescent probe bound to intact human oxyhemoglobin (HbO2) is investigated. Fluorescence emission spectra of 1,8-ANS in a potassium buffer solution (pH 7.4) of HbO2 undergo a substantial blue shift during first 6 ns after pulsed optical excitation at 337.1 nm. Nonexponential fluorescence kinetics of 1,8-ANS in the HbO2 solution are studied by the decay time distribution and conventional multiexponential analyses for a set of emission wavelength range of lambdaem = 455-600 nm. These fluorescence decays contain components with mean decay times of <0.5 ns, 3.1-5.5 ns, and 12.4-15.1 ns with spectrally-dependent relative contributions. The shortest decay component is assigned to free 1,8-ANS molecules in the bulk buffer environment, whereas the two longer decay components are assigned to two types of binding sites of 1,8-ANS in the HbO2 molecule presumably differing by polarity and accessibility to water molecules. The results represent the first experimental evidence of heterogeneous binding of 1,8-ANS to intact human oxyhemoglobin.     

Encapsulation of fluorescent molecules by functionalized polymeric nanocontainers: investigation by confocal fluorescence imaging and fluorescence correlation spectroscopy

Nanocontainers (NCs) were prepared from amphiphilic triblock copolymers, having an average molecular weight of around 8000 g/mol, by using previously published preparation methods consisting of dispersing the polymer in an aqueous buffer solution containing molecules for encapsulation. A small molecular weight fluorophore, sulforhodamine B, as well as the fluorescent protein avidin labeled with Alexa 488 were encapsulated, and the resulting nanocontainers were characterized using fluorescence correlation spectroscopy (FCS) and fluorescence cross-correlation spectroscopy (FCCS). Nanocontainer size determination by FCS is very robust and compares well with results obtained from photon correlation spectroscopy: the measured diameters of the polymeric nanocontainers vary between 140 and 172 nm. Encapsulation of fluorescent molecules was determined by evaluating the molecular brightness of nanocontainers with an encapsulated fluorescently labeled protein (avidin-Alexa 488). Results indicate that the number of encapsulated avidin-Alexa 488 molecules corresponds well with the initial concentration of the fluorescently labeled protein and the encapsulated volume. A nanocontainer binding assay was developed using biotinylated fluorescently labeled nanocontainers. Binding of biotinylated nanocontainers to fluorescently labeled streptavidin was followed by fluorescence cross-correlation spectroscopy. The intrinsic dissociation constant, K(d), of labeled streptavidin to the ligand-modified nanocontainers is 1.7 +/- 0.4 x 10(-8) M, and about 1921 +/- 357 molecules of labeled streptavidin are bound to each nanocontainer.     

Molecular recognition in imprinted polymers: thermodynamic investigation of analyte binding using microcalorimetry

This study aimed at elucidating the interaction mechanism between an imprinted polymer and its template in aqueous environment with thermodynamic aspects. The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) was chosen as a model template to imprint a co-polymer of 4-vinylpyridine (4-VP) and ethyleneglycol dimethacrylate. Equilibrium binding isotherm analysis and isothermal titration microcalorimetry were used to quantify the contribution of enthalpy and entropy to the binding process, identify the nature of the interactions involved and confirm the existence of binding pockets with shape-complementarity to the template. For the binding process of 2,4-D to the imprinted polymer, we postulate three subprocesses: (1) dehydration of the binding pocket and of the 2,4-D, (2) adsorption of 2,4-D, and (3) rearrangement of the water molecules from the dehydration process. We found that binding in aqueous environment was due to the cumulative effect of pi-stacking and electrostatic interactions between the template and the functional monomers. At pH<6, entropy is the dominating driving force, while at pH>6 where the highest difference in binding between the imprinted and a non-imprinted reference polymer was observed, the enthalpy change accounts for most of the binding free energy. The developed microcalorimetric method sheds light on the binding mechanism of analyte molecules with imprinted polymers, in particular if the polymers are used in aqueous solvents.     

A fluorescence polarisation molecular imprint sorbent assay for 2,4-D: a non-separation pseudo-immunoassay

The first pseudo-immunoassay which employs a molecularly imprinted receptor and a fluorescent probe, and quantifies the bound analyte directly using the fluorescence anisotropy of the polymer-probe-analyte suspension, is described.     

Influence of salt ions on binding to molecularly imprinted polymers

Salt ions were found to have an influence on template binding to two model molecularly imprinted polymers (MIPs), targeted to penicillin G and propranolol, respectively, in water–acetonitrile mixtures. Water was detrimental to rebinding of penicillin G whereas propranolol bound in the entire water–acetonitrile range tested. In 100% aqueous solution, 3-M salt solutions augmented the binding of both templates. The effects followed the Hofmeister series with kosmotropic ions promoting the largest increase. Binding was mainly of a non-specific nature under these conditions. In acetonitrile containing low amounts of water, the specific binding to the MIPs increased with the addition of salts. Binding of penicillin G followed the Hofmeister series while an ion-exchange mechanism was observed for propranolol. The results suggest that hydration of kosmotropic ions reduces the water activity in water-poor media providing a stabilizing effect on water-sensitive MIP–template interactions. The effects were utilized to develop a procedure for molecularly imprinted solid-phase extraction (MISPE) of penicillin G from milk with a recovery of 87%.     

Direct determination of S-(-)- and R-(+)-propranolol glucuronide in rat hepatic microsomes by RP-HPLC

Propranolol, available commercially as a racemic mixture, is a non-selective beta-adrenergic blocking agent used in the treatment of hypertension, angina pectoris and cardiac arrhythmias. We have developed and validated an RP-HPLC assay method for direct determination of R-(+)- and S-(-)-propranolol glucuronide in rat hepatic microsomes to investigate the enantioselectivity of propranolol glucuronidation metabolism. A baseline separation of propranolol glucuronide enantiomers was achieved on a 5 microm reversed-phase ODS column, with a mixture of phosphate buffer (pH 3.5, 0.067 mol/L) and methanol (55:45, v/v) as mobile phase. Ultraviolet detection was set at 220 nm, and p-nitrobenzoic acid was used as internal standard. The standard curve of assay for R-(+)- and S-(-)-propranolol glucuronide in spiked microsomal incubate showed good linearity throughout the concentration range from 0.50 to 20.0 micromol/L. The analytical method affords average recovery of 99.8 and 100.1% for R-(+)- and S-(-)-propranolol glucuronide, respectively. The method provides a high sensitivity and good precision for R-(+)- and S-(-)-propranolol glucuronide (RSD < 10%). The LOD was 0.15 micromol/L and the LOQ was 0.5 micromol/L (RSD < 8%, n = 5) for both R-(+)- and S-(-)-propranolol glucuronide. The method is simple, precise and accurate, and is suitable for quantifying the propranolol glucuronides enantiomers in rat hepatic microsomes.     

Novel probes showing specific fluorescence enhancement on binding to a hexahistidine tag

The introduction of hexahistidine (His tag) is widely used as a tool for affinity purification of recombinant proteins, since the His tag binds selectively to nickel-nitrilotriacetic acid (Ni(2+)-NTA) complex. To develop efficient "turn-on" fluorescent probes for His-tagged proteins, we adopted a fluorophore displacement strategy, that is, we designed probes in which a hydroxycoumarin fluorophore is joined via a linker to a metal-NTA moiety, with which it forms a weak intramolecular complex, thereby quenching the fluorescence. In the presence of a His tag, with which the metal-NTA moiety binds strongly, the fluorophore is displaced, which results in a dramatic enhancement of fluorescence. We synthesized a series of hydroxycoumarins that were modified by various linkers with NTA (NTAC ligands), and investigated the chemical and photophysical properties of the free ligands and their metal complexes. From the viewpoint of fluorescence quenching, Ni(2+) and Co(2+) were the best metals. Fluorescence spectroscopy revealed a 1:1 binding stoichiometry for the Ni(2+) and Co(2+) complexes of NTACs in pH 7.4 aqueous buffer. As anticipated, these complexes showed weak intrinsic fluorescence, but addition of a His-tagged peptide (H-(His)(6)-Tyr-NH(2); Tyr was included to allow convenient concentration measurement) in pH 7.4 aqueous buffer resulted in up to a 22-fold increase in the fluorescence quantum yield. We found that the Co(2+) complexes showed superior properties. No fluorescence enhancement was seen in the presence of angiotensin I, which contains two nonadjacent histidine residues; this suggests that the probes are selective for the polyhistidine peptide.     

Capillary electrophoresis coupled with laser-induced fluorescence polarization as a hybrid approach to ultrasensitive immunoassays.

Immunoassays using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) is a powerful approach to the determination of trace amounts of analytes in a complex biological matrix. However, its applicability is limited by the requirement that the free and bound tracer (fluorescently labeled compound) be resolved for their identification and quantitation. Here we show that replacing LIF with laser-induced fluorescence polarization (LIFP) permits ultrasensitive immunoassays to be performed with or without the separation of the free and bound tracer. A binding system involving cyclosporin A (CyA) and monoclonal antibody to CyA was chosen to demonstrate both homogeneous and heterogeneous immunoassay approaches. In the homogeneous scheme where the free and bound tracer were not separated, the fluorescence polarization of the mixture was a quantitative measure of the antibody-bound tracer. The concentration and mass detection limits for CyA using the homogeneous competitive assay were found to be 1 nM and 1 amol (10(-18) mol), respectively. The heterogeneous assay involved a nearly baseline separation of the free and bound tracer using CE with a phosphate running buffer of pH 7.0. The complex of the tracer with the antibody had a fluorescence polarization of approximately 0.24 whereas the free tracer had negligible polarization. The fluorescence polarization was independent of analyte concentration, and the fluorescence intensity of either the free or bound tracer was used for quantitation. Results from both assays suggest that the CE-LIFP approaches may have a wider application than the immunoassays based on either CE-LIF or fluorescence polarization alone.     

Competitive amperometric morphine sensor based on an agarose immobilised molecularly imprinted polymer

A morphine-sensitive device was constructed based on a molecularly imprinted polymer. The imprinted polymer exhibited recognition properties previously. A method of detection based on competitive binding was used to measure morphine in the concentration range 0.1–10 μg/ml. A morphine concentration of 0.5 μg/ml gave a peak current (by oxidation) of 4 nA. The method of morphine detection involves two steps. In the first step, morphine binds selectively to the molecularly imprinted polymer in the sensor. In the second step, an electroinactive competitor (codeine) is added in excess, whence some of the bound morphine is released. The released morphine is detected by an amperometric method. The advantages of this type of sensor compared to biosensors based on antibodies, enzymes or cells are discussed. This sensor, based on an artificial recognition system, demonstrates autoclave compatibility, long-time stability and resistance to harsh chemical environments.     

Affinity capillary electrophoresis and fluorescence spectroscopy for studying enantioselective interactions between omeprazole enantiomer and human serum albumin

Baseline separation of omeprazole (OME) enantiomers was achieved by affinity capillary electrophoresis (ACE), using human serum albumin (HSA) as the chiral selector. The influence of several experimental variables such as HSA concentration, the type and content of organic modifiers, applied voltage and running buffer concentration on the separation was evaluated. The binding of esomeprazole (S‐omeprazole, S‐OME) and its R‐enantiomer (R‐omeprazole, R‐OME) to HSA under simulated physiological conditions was studied by ACE and fluorescence spectroscopy which was considered as a reference method. ACE studies demonstrated that the binding constants of the two enantiomers and HSA were 3.18 × 10³ M⁻¹ and 5.36 × 10³ M⁻¹, respectively. The binding properties including the fluorescence quenching mechanisms, binding constants, binding sites and the number of binding sites were obtained by fluorescence spectroscopy. Though the ACE method could not get enough data when compared with the fluorescence spectrum method, the separation and binding studies of chiral drugs could be achieved simultaneously via this method. This study is of great significance for the investigation and clinical application of chiral drugs.     

Water-compatible molecularly imprinted polymers obtained via high-throughput synthesis and experimental design

A technique allowing high-throughput synthesis and evaluation of molecularly imprinted polymer sorbents at a reduced scale (mini-MIPs) was developed and used for the optimization of MIPs for use in pure aqueous environments. The technique incorporated a 4-port liquid-handling robot for the rapid dispensing of monomers, templates, solvents and initiator into the reaction vessels of a 96-well plate. A library of 80 polymers, each ca. 50 mg, could thus be prepared in 24 h. The MIP rebinding capacity and selectivity could be rapidly assessed in the batch mode by quantifying nonbound fractions in parallel using a UV monochromator plate reader. This allowed a complete evaluation of the binding characteristics of an 80 polymer library in approximately 1 week. With the objective of optimizing a polymer imprinted with the local anaesthetic Bupivacaine for use in pure aqueous systems, a polymer library was prepared by varying the original poly(MAA-co-EDMA) MIP composition. The variable factors were the added amount of the hydrophilic comonomer, 2-hydroxyethyl methacrylate (HEMA), the cross-linking ratio, and the porogen. This optimization resulted in polymers showing high imprinting factors (IF = K(MIP)/K(NIP)) in water as a result, mainly, of reduced binding to the nonimprinted polymer. Normal scale batches of these materials showed strong retention of the template and low nonspecific binding when assessed as chromatographic stationary phases using pure phosphate buffer, pH 7.4, as mobile phase, by equilibrium batch rebinding experiments and as sorbents for extractions of the analyte from blood plasma samples.     

Monitoring solute interactions with poly(ethylene oxide)-modified colloidal silica nanoparticles via fluorescence anisotropy decay

The fluorescence-based nanosize metrology approach, proposed recently by Geddes and Birch (Geddes, C. D.; Birch, D. J. S. J. Non-Cryst. Solids 2000, 270, 191), was used to characterize the extent of binding of a fluorescent cationic solute, rhodamine 6G (R6G), to the surface of silica particles after modification of the surface with the hydrophilic polymer poly(ethylene oxide) (PEO) of various molecular weights. The measurement of the rotational dynamics of R6G in PEO solutions showed the absence of strong interactions between R6G and PEO chains in water and the ability of the dye to sense the presence of polymer clusters in 30 wt % solutions. Time-resolved anisotropy decays of polymer-modified Ludox provided direct evidence for distribution of the dye between bound and free states, with the bound dye showing two decay components: a nanosecond decay component that is consistent with local motions of bound probes and a residual anisotropy component due to slow rotation of large silica particles. The data showed that the dye was strongly adsorbed to unmodified silica nanoparticles, to the extent that less than 1% of the dye was present in the surrounding aqueous solution. Addition of PEO blocked the adsorption of the dye to a significant degree, with up to 50% of the probe being present in the aqueous solution for Ludox samples containing 30 wt % of low molecular weight PEO. The addition of such agents also decreased the value and increased the fractional contribution of the nanosecond rotational correlation time, suggesting that polymer adsorption altered the degree of local motion of the bound probe. Atomic force microscopy imaging studies provided no evidence for a change in the particle size upon surface modification but did suggest interparticle aggregation after polymer adsorption. Thus, this redistribution of the probe is interpreted as being due to coverage of particles with the polymer, resulting in lower adsorption of R6G to the silica. The data clearly show the power of time-resolved fluorescence anisotropy decay measurements for probing the modification of silica surfaces and suggest that this method should prove useful in characterization of new chromatographic stationary phases and nanocomposite materials.     

Investigation of imprinting parameters and their recognition nature for quinine-molecularly imprinted polymers

A series of molecularly imprinted polymers (MIPs) was prepared using quinine as the template molecules by bulk polymerization. The presence of monomer-template solution complexes in non-covalent MIPs systems has been verified by both fluorescence and UV-vis spectrometric detection. The influence of different synthetic conditions (porogen, functional monomer, cross-linkers, initiation methods, monomer-template ratio, etc.) on recognition properties of the polymers was investigated. Scatchard analysis revealed that two classes of binding sites were formed in the imprinted polymer. The corresponding dissociation constants were estimated to be 45.00 micromol l(-1) and 1.42 mmol l(-1), respectively, by utilizing a multi-site recognition model. The binding characteristics of the imprinted polymers were explored in various solvents using equilibrium binding experiments. In the organic media, results suggested that polar interactions (hydrogen bonding, ionic interactions, etc.) between acidic monomer/polymer and template molecules were mainly responsible for the recognition, whereas in aqueous media, hydrophobic interactions had a remarkable non-specific contribution to the overall binding. The specificity of MIP was evaluated by rebinding the other structurally similar compounds. The results indicated that the imprinted polymers exhibited an excellent stereo-selectivity toward quinine.