Molecularly Imprinted Overoxidized Polypyrrole Colloids: Promising Materials for Molecular Recognition

This article reviews a novel approach to molecular imprinting technology developed by the authors. Preparation, characterization, and performance of the molecularly imprinted overoxidized polypyrrole colloids is discussed to discriminate amino acid enantiomers and structural isomers of naphthalene compounds by using electrochemical and spectroscopic techniques. An overoxidized colloid, which was prepared with L-lactate as a dopant, shows a higher affinity for L-alanine than for D-alanine, and an uptake ratio (L/D) of as high as 11 was observed under optimum conditions. Furthermore, an overoxidized colloid with a cavity created by 1-naphthalenesulfonate (1-NS) showed a higher uptake for 1-NS than 2-NS. Thus, the shape-complementary cavity created at the colloid surface can precisely recognize the difference in the spatial configuration of enantiomers and structural isomers.     

Enantioselective uptake of amino acids using an electromodulated column packed with carbon fibres modified with overoxidised polypyrrole

A column packed with carbon fibres modified with overoxidized polypyrrole was utilised for the enantioselective uptake of glutamic acid. The polypyrrole prepared with L-glutamate as a dopant was electrochemically overoxidised to create a complementary cavity for the dopant. The target amino acid enantiomers were taken up and released by applying potential to the carbon fibre stationary phase. The uptake performance of an overoxidised polypyrrole modified carbon fibre electrode was investigated by optimising various important overoxidation parameters such as an overoxidation medium and applied potential. Under optimised conditions of overoxidation and with a column wash between the consecutive measurements, the reproducible uptake of L-glutamic acid was almost six times higher than that of D-glutamic acid. Further, in a comparison, the recognition ability of overoxidized polypyrrole with a pseudo-template cavity was examined.     

Highly selective molecularly imprinted overoxidized polypyrrole colloids: one-step preparation technique.

A convenient "one step" preparation process of molecularly imprinted overoxidized-polypyrrole (oPPy) colloids by chemical polymerization and their high uptake ability and enantioselectivity are described. Since an oxidizing agent gives a different standard redox potential and rate of polymerization, the property of the resulting oPPy colloid can be controlled by the kind and concentration of the oxidizing agent. At higher concentrations of (NH4)2S2O8 (0.3 M), the overoxidation of PPy colloid automatically occurred. The extraction of L-lactate as a dopant has created a shape-complementary cavity on the surface of the oPPy colloid through overoxidation following polymerization. The oPPy colloid exhibited an excellent selectivity not only on the alanine enantiomer but also on the difference in the side-chain size of amino acids. The uptake of oPPy colloid towards L-alanine over D-alanine was 11.3 micromol/g-colloid against 3.6 micromol/g. The molecularly imprinted cavity can also recognize the existences of the OH or CH3 substituents.     

Pincer ligands based on α-amino acids: V. New generation chiral receptors for the recognition of amino acid enantiomers in aqueous environment

Selective recognition of enantiomers of amino acids valine, methionine, phenylalanine, serine, and tyrosine by a binuclear copper complex with an azomethine obtained from 2,6-diformyl-4-methylphenol and L-valine was studied by means of spectrophotometry. The binding constants of individual enentiomers were estimated for valine, phenylalanine, methionine, serine, and tyrosine, and the enantioselectivity factors were evaluated. The isomers of serine and tyrosine (with respect to the first stage) were recognized with a considerable enantioselectivity (1.34 and 5.46 respectively), whereas the binding constants of valine and phenylalanine enantiomers are virtually indistinguishable.     

Capillary electrophoresis and molecular modeling of the chiral separation of aromatic amino acids using α/β-cyclodextrin and 18-crown-6

In this work, chiral separation of enantiomers of three amino acids was achieved using capillary electrophoresis technique with α-cyclodextrin (αCD) as a running buffer additive. Only tryptophan has exhibited baseline separation in the presence of αCD, while the enantiomers of the other two amino acids, phenylalanine and tyrosine, were only partially separated. The addition of 18-crown-6 (18C6) as a second additive imparted only slight improvement to the separation of all enantiomers. On the other hand, all three racemic amino acid mixtures demonstrated no indication of separation when the larger cavity cyclodextrin members, β- and γCD, are used as running buffer chiral additives. However, remarkable improvements in the separation of the enantiomers of phenylalanine and tyrosine were obtained when 18C6 is used together with βCD as a running buffer additive. Surprisingly, tryptophan enantiomers were not separated by the dual additive system of cyclodextrin and crown ether. Using electrospray ionization mass spectrometry (ESI-MS), all amino acids were found to form stable binary complexes with individual hosts as well as ternary compounds involving the crown ether and the cyclodextrin. Furthermore, we used molecular dynamics (MD) simulations to build a clear picture about the interaction between the guest and the hosts. Most of these complexes remained stable throughout the simulation times, and the molecular dynamics study allowed better understanding of these supramolecular assemblies.     

Separation of β-blocker and amino acid enantiomers on a mixed chiral sorbent modified with macrocyclic antibiotics eremomycin and vancomycin

A mixed chiral sorbent based on silica with immobilized macrocyclic antibiotics eremomycin and vancomycin was synthesized. A possibility of the separation of enantiomers of β-blockers (metoprolol, pindolol, alprenolol, oxprenolol, labetalol, and atenolol) and amino acids (tryptophan, phenylalanine, DOPA, methionine, and acetyl glutamic acid) on this chiral sorbent by HPLC was studied. The influence of the composition of the mobile phase (pH of buffer solution, its concentration, content of organic modifier, and its nature) on the retention times of β-blocker and amino acid enantiomers, selectivity, and resolution of peaks was studied. It was shown that the mixed chiral sorbent has enantioselectivity to both classes of compounds, while silica modified with vancomycin has no ability to the separation of enantiomers of non-derivatized amino acids, and silica modified with eremomycin has no ability to the separation of β-blocker enantiomers. High values of resolution for amino acids (max Rs > 4) and β-blockers (max Rs > 1) were obtained.     

Pulsed amperometric detection of underivatized amino acids using polypyrrole modified copper electrode in acidic solution

The successful pulsed amperometric detection of underivatized amino acids have been carried out in an acidic media on a polypyrrole (PPy) modified Cu electrode. The formation of PPy film doped with glutamate (glu) on a Cu electrode surface changes the mechanism of Cu dissolution. After application of multistep potential waveform, the PPy film was glu free due to the electro-reduction and overoxidation. High anodic potential polarization treatment yielded partially overoxidized PPy film as long as the Cu surface dissolution and amino acid permeation through the film was well controlled. This overoxidized PPy film acted as a charge and size exclusion barrier in order to improve the selectivity and stability of a Cu electrode. Various process parameters such as film modification time, detection and cleaning potential and pH of solution have been optimized to maximize the beneficial electrocatalytic properties of the electrode surface. At an optimized condition, detection limits for positively charged histidine and arginine are 19 and 22 pg respectively, whereas the neutral amino acids detected in amounts of 0.9–2.3 ng. Furthermore, the PPy coated Cu electrode response was long lived, stable and reproducible.     

Separation of tryptophan enantiomers with molecularly imprinted polypyrrole electrode column

<正>In this study,we have fabricated molecularly imprinted polypyrrole(PPy) packed electrode columns and investigated their effects on separation of tryptophan(Trp) enantiomers by using potential control.The results indicate that the imprinted PPy electrode columns could efficiently enhance the L-Trp uptake and separate Trp enantiomers effectively,implying the great potential for the enantioselective recognition of other amino acids enantiomers.     

Highly selective electrochemical method for the detection of serotonin at carbon fiber microelectrode modified with gold nanoflowers and overoxidized polypyrrole

A gold nanoflowers and overoxidized polypyrrole modified carbon fiber microelectrode (OPPy/Au NFs/CFME) was fabricated using electroless deposition and electrochemical method for highly selective and sensitive detection of 5-HT.     

Highly selective electrochemical method for the detection of serotonin at carbon fiber microelectrode modified with gold nanoflowers and overoxidized polypyrrole

A highly selective and sensitive electrochemical method was developed for the detection of serotonin (5-hydroxytryptamine, 5-HT) at gold nanoflowers (Au NFs) and overoxidized polypyrrole (OPPy) modified carbon fiber microelectrode (CFME). Carbon fiber was firstly modified with gold nanoflowers using electroless deposition method, and then modified with overoxidized polypyrrole using electrochemical polymerization and overoxidization to obtain OPPy/Au NFs/CFME. The obtained OPPy/Au NFs/CFME was characterized by field emission scanning electron microscopy and electrochemical techniques. It was found that the OPPy/Au NFs/CFME showed good sensitivity for the detection of 5-HT in the range of 10 nmol/L − 7.0 μmol/L with detection limit of 2.3 nmol/L, and negligible interferences from ascorbic acid, 5-hydroxyindole acetic acid and uric acid. The OPPy/Au NFs/CFME was successfully applied to the detection of 5-HT in human serum samples with good recovery. The work demonstrates that the electrochemical method, incorporating signal amplification of Au NFs with higher cation selection of OPPy, provides a promising tool for the detection of 5-HT in biological systems     

Enantioselective guest exchange in a chiral resorcin[4]arene cavity

Gas-phase proton-bound complexes between a chiral resorcin[4]arene and some representative amino acids, that is, L- and D-alanine or L- and D-serine, were generated in the source of a Fourier transform ion cyclotron resonance mass spectrometer. Gas-phase exchange of the amino acid from the diastereomeric complexes with the enantiomers of 2-butylamine exhibits a significant enantioselectivity, which depends not only upon the configuration of the leaving guest but also on that of the incoming amine. These findings, coupled with molecular dynamic calculations, point to the observed gas-phase enantioselectivity as determined by the effects of the resorcin[4]arene chiral cavity upon the diastereomeric exchange transition structures.     

Nonenzymatic sensing of glucose using a carbon ceramic electrode modified with a composite film made from copper oxide,overoxidized polypyrrole and multi-walled carbon nanotubes

Microchimica Acta - A carbon ceramic electrode was modified with a thin film composed of overoxidized polypyrrole, CuO and multi-walled carbon nanotubes. The surface morphology, electrochemical...     

Temperature and Solute Molecular Size Effects on the Retention and Enantioselectivity of a Series of D, L Dansyl Amino Acids on a Vancomycin-Based Chiral Stationary Phase

Summary The influence of column temperature (0–28 °C) and solute molecular size on the retention and enantioselectivity of a series of D, L dansyl amino acids with a non-polar side chain (valine, leucine, phenylalanine and tryptophan) were investigated using a vancomycin-based chiral stationary phase (CSP). The enthalpic and entropic terms for the solute-CSP association were determined from the linear vant Hoff plots. Two solute groups were distinguished in relation to these thermodynamic quantities: the solute group I (dansyl valine, dansyl leucine, dansyl phenylalanine) for which large negative values of enthalpic terms were obtained and the solute group II (dansyl tryptophan) for which H value was much less negative. The enthalpy-entropy compensation study revealed that the interaction mechanism was identical for the group I solute enantiomers but changed for D, L dansyl tryptophan. This was further exemplified as the group I compound enantiomers were resolved over the temperature range while the enantiomers of dansyl tryptophan were not separated in the operating conditions. Relationships between both the solute retention factors and apparent enantioselectivity, and the accessible surface area of the amino acid side chain indicated that when the solute molecular size increased (i) the retention was enhanced by the hydrophobic effect and (ii) the chiral discrimination decreased dependent, at least in part, on a steric hindrance phenomenon at the vancomycin aglycone pocket.     

Highly enantioselective discrimination of amino acids using copper deposition on a gold electrode modified with homocysteine monolayer

An extremely enhanced enantioselectivity was achieved for the detection of enantiomers of alanine (Ala), leucine (Leu), and 3,4-dihydroxyphenylalanine (DOPA) based on the voltammograms for the deposition of Cu from Cu complexes of the amino acids at an Au electrode modified with a self-assembled monolayer (SAM) of l-homocysteine (Hcy). The enantioselective current density peak for the Cu deposition was found to change with increasing number of potential cycles after the addition of Cu(II), and the highest enantioselectivity was observed immediately after the addition of Cu(II). Besides, enantioselectivity was not observed with proline, whose five-membered ring contains the nitrogen atom of a secondary amino group, while some amino acids with a primary amine group such as Ala, Leu, and DOPA exhibited enantioselectivity. These results suggest that the chiral ligand exchange reaction at the l-Hcy SAM-modified Au electrode, namely, the enantioselective formation of diastereomeric complexes of Cu(II) with target enantiomers and l-Hcy self-assembled on the Au electrode, plays an important role in the chiral discrimination based on the Cu deposition.     

Selective determination of tryptophan by using a carbon paste electrode modified with an overoxidized polypyrrole film.

We report on the selective determination of tryptophan, using a carbon paste electrode coated with an overoxidized polypyrrole film. Out of 21 protein amino acids, only tryptophan and tyrosine exhibited an oxidative voltammetric response with this electrode. Tryptophan, which was preferentially concentrated to the electrode under an open circuit condition, was determined by the stripping voltammetric technique with a linear response range of 10-100 microM. For the determination of 10 microM tryptophan, interference from a 15-fold excess of tyrosine gave an positive error of 6%, while the other amino acids did not exhibit any detectable interference.     

Chiral and electrokinetic separation of amino acids using polypyrrole-coated adsorbents

An optically active and electroconductive polymeric adsorbent has been developed for the use in chromatographic resolution of nonderivatized amino acids. The chiral selectivity of the adsorbent is based upon ligand exchange of coordinated copper(II) complexes of D or L-amino acids and a molecular imprinting technique by modifying the resin surface with polypyrrole coating. Applying a potential difference of +/-1.5 V to the chiral and conductive column, racemic amino acids are separated according to their charge characteristics, and simultaneously resolved with respect to their optical isomerisms. A pH-controlled mixture of D,L-lysine and D,L-aspartic acid is resolved displaying enantioselectivity values of 1.19 and 2.08, respectively, and a baseline separation of the two amino acids is accomplished by alternating the polarity of the electric field. The synthesized adsorbent also exhibits size exclusion factor discriminating amino acids with larger side chains.     

Engineering a cationic supramolecular charge switch for facile amino acids enantiodiscrimination based on extended-gate field effect transistors

Chiral recognition of essential amino acids (EAAs) is a huge challenge that keeps plaguing analytical scientists due to their cryptochirality and limited steric interaction sites. Inspired by the superior enantioselectivity of functional supramolecular cyclodextrins (CDs) and strong signal amplification ability of field effect transistors (FETs), this work firstly reports a cationic supramolecular charge switch for facile enantiodiscrimination of EAAs based on extended-gate organic FET (EG-OFET). The cationic phenylcarbamoylated-CD single isomer acts as a charge switch via interacting with different enantiomers and the weak stereo-differentiation intermolecular interaction signals between the cationic perphenylcarbamoylated CDs and EAAs on the EG can be strongly and rapidly amplified through an OFET. Efficient chiral differentiation of six EAAs, including phenylalanine, tryptophan, leucine, isoleucine, lysine and valine, are successfully achieved without any derivation process and the detection limit for d-phenylalanine is down to 10^?13 mol/L. We believe that this study provides a new and facile sensing perspective for natural amino acids and may afford deeper understanding of molecular chirality.     

Langmuir monolayers of a cholesterol-armed cyclen complex that can control enantioselectivity of amino acid recognition by surface pressure

The cholesterol-armed cyclen Na(+) complex formed stable Langmuir monolayers at the air-water interface. The π-A isotherm displayed a reasonable limiting molecular area of ～1.57 nm(2) and the areas expanded when amino acids were dissolved in water, indicating the efficient accommodation in the monolayers. Brewster angle microscopy (BAM) images exhibited almost no defects of the compressed Langmuir monolayers regardless of the presence of amino acids. Based on the idea that the increase in the limiting molecular areas corresponds to the amount of the adsorbed amino acid, the binding constants of three enantiomeric amino acids, namely valine (Val), leucine (Leu), and phenylalanine (Phe), were calculated at different surface pressures. Remarkably, a surface pressure dependent enantioselectivity of amino acid recognition was observed. Upon compression of the monolayers, the binding constants of amino acids increased accompanying an inversion of chiral selectivity from the D- to L-form in the case of Val and, conversely, from L- to D-form in the case of Phe.     

Probing chiral amino acids at sub-picomolar level based on bovine serum albumin enantioselective films coupled with silver-enhanced gold nanoparticles

A novel electrochemical sensor with capability of probing chiral amino acids with gold nanoparticle (n-Au) labels using bovine serum albumin (BSA) as a chiral selector and subsequent signal amplification step by silver enhancement is introduced. The assay relies on the stereoselectivity of BSA embedded in ultrathin γ-alumina sol–gel film coated on the surface of the glassy carbon electrode (GCE). The recognition to the n-Au-labeled l- or d-amino acids for BSA-GCE could be monitored by the differential pulse voltammetry (DPV), while the DPV signal was greatly amplified by the anchored silver atoms on the n-Au, leading to a new way of quantitatively analysis of chiral amino acids electrochemically at sub-picomolar level. With l-tryptophan as the probe solute, the linear concentration range was from 1.33 × 10⁻¹² to 1 × 10⁻⁹ mol L⁻¹ and detection limit was 5 × 10⁻¹³ mol L⁻¹. For tryptophan enantiomers, the enantioselectivity coefficient 2.3 was obtained.     

Modified Cyclodextrins as Enantioselective Hosts for Amino Acids

Two modified β‐cyclodextrins, H‐2 and H‐3 , having a flexible appended moiety were studied for the chiral discrimination of the enantiomers of various amino acids by means of fluorescence as signaling option. These hosts quenched the fluorescence intensities of amino acids upon binding. The d‐ enantiomers were better recognized by these hosts. The association constants (K_s) and enantioselectivity factors (α) of the host?guest complexes were calculated.