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Institute
Enzyme electrode for aromatic compounds exploiting the catalytic activities of microperoxidase-11
(2011)
Microperoxidase-11 (MR-11) which has been immobilised in a matrix of chitosan-embedded gold nanoparticles on the surface of a glassy carbon electrode catalyzes the conversion of aromatic substances. This peroxide-dependent catalysis of microperoxidase has been applied in an enzyme electrode for the first time to indicate aromatic compounds such as aniline. 4-fluoroaniline, catechol and p-aminophenol. The electrode signal is generated by the cathodic reduction of the quinone or quinoneimine which is formed in the presence of both MP-II and peroxide from the substrate. The same sensor principle will be extended to aromatic drugs.
Surface-Tuned Electron Transfer and Electrocatalysis of Hexameric Tyrosine-Coordinated Heme Protein
(2015)
Molecular modeling, electrochemical methods, and quartz crystal microbalance were used to characterize immobilized hexameric tyrosine-coordinated heme protein (HTHP) on bare carbon or on gold electrodes modified with positively and negatively charged self-assembled monolayers (SAMs), respectively. HTHP binds to the positively charged surface but no direct electron transfer (DET) is found due to the long distance of the active sites from the electrode surfaces. At carboxyl-terminated surfaces, the neutrally charged bottom of HTHP can bind to the SAM. For this "disc" orientation all six hemes are close to the electrode and their direct electron transfer should be efficient. HTHP on all negatively charged SAMs showed a quasi-reversible redox behavior with rate constant k(s) values between 0.93 and 2.86 s(-1) and apparent formal potentials E-app(0)' between -131.1 and -249.1 mV. On the MUA/MU-modified electrode, the maximum surface concentration corresponds to a complete monolayer of the hexameric HTHP in the disc orientation. HTHP electrostatically immobilized on negatively charged SAMs shows electrocatalysis of peroxide reduction and enzymatic oxidation of NADH.
Effective recognition of enzymatically active tetrameric acetylcholinesterase (AChE) is accomplished by a hybrid nanofilm composed of a propidium-terminated self-assembled monolayer (Prop-SAM) which binds AChE via its peripheral anionic site (PAS) and an ultrathin electrosynthesized molecularly imprinted polymer (MIP) cover layer of a novel carboxylate-modified derivative of 3,4-propylenedioxythiophene. The rebinding of the AChE to the MIP/Prop-SAM nanofilm covered electrode is detected by measuring in situ the enzymatic activity. The oxidative current of the released thiocholine is dependent on the AChE concentration from approximate to 0.04 x 10(-6) to 0.4 x 10(-6)m. An imprinting factor of 9.9 is obtained for the hybrid MIP, which is among the best values reported for protein imprinting. The dissociation constant characterizing the strength of the MIP-AChE binding is 4.2 x 10(-7)m indicating the dominant role of the PAS-Prop-SAM interaction, while the benefit of the MIP nanofilm covering the Prop-SAM layer is the effective suppression of the cross-reactivity toward competing proteins as compared with the Prop-SAM. The threefold selectivity gain provided by i) the shape-specific MIP filter, ii) the propidium-SAM, iii) signal generation only by the AChE bound to the nanofilm shows promise for assessing AChE activity levels in cerebrospinal fluid.
We report on the redox behaviour of the microperoxidase-11 (MP-11) which has been electrostatically immobilized in a matrix of chitosan-embedded gold nanoparticles on the surface of a glassy carbon electrode. MP-11 contains a covalently bound heme c as the redox active group that exchanges electrons with the electrode via the gold nanoparticles. Electroactive surface concentration of MP-11 at high scan rate is between 350+/-50 pmol cm(-2), which reflects a multilayer process. The formal potential (E degrees') of MP-11 in the gold nanoparticles-chitosan film was estimated to be -(267.7+/-2.9) mV at pH 7.0. The heterogeneous electron transfer rate constant (k(s)) starts at 1.21 s(-1) and levels off at 6.45 s(-1) in the scan rate range from 0.1 to 2.0 V s(-1). Oxidation and reduction of MP-11 by hydrogen peroxide and superoxide, respectively have been coupled to the direct electron transfer of MP-11.
We present an electrochemical MIP sensor for tamoxifen (TAM)-a nonsteroidal anti-estrogen-which is based on the electropolymerisation of an O-phenylenediamine. resorcinol mixture directly on the electrode surface in the presence of the template molecule. Up to now only. bulk. MIPs for TAM have been described in literature, which are applied for separation in chromatography columns. Electro-polymerisation of the monomers in the presence of TAM generated a film which completely suppressed the reduction of ferricyanide. Removal of the template gave a markedly increased ferricyanide signal, which was again suppressed after rebinding as expected for filling of the cavities by target binding. The decrease of the ferricyanide peak of the MIP electrode depended linearly on the TAM concentration between 1 and 100 nM. The TAM-imprinted electrode showed a 2.3 times higher recognition of the template molecule itself as compared to its metabolite 4-hydroxytamoxifen and no cross-reactivity with the anticancer drug doxorubucin was found. Measurements at + 1.1 V caused a fouling of the electrode surface, whilst pretreatment of TAM with peroxide in presence of HRP generated an oxidation product which was reducible at 0 mV, thus circumventing the polymer formation and electrochemical interferences.
Binding of heme to the amyloid peptides A beta 40/42 is thought to be an initial step in the development of symptoms in the early stages of Alzheimer's disease by enhancing the intrinsic peroxidatic activity of heme. We found considerably higher acceleration of the reaction for the physiologically relevant neurotransmitters dopamine and serotonin than reported earlier for the artificial substrate 3,3',5,5'-tetramethylbenzidine (TMB). Thus, the binding of hemin to A beta peptides might play an even more crucial role in the early stages of Alzheimer's disease than deduced from these earlier results. To mimic complex formation, a new surface architecture has been developed: The interaction between the truncated amyloid peptide A beta 1-16 and hemin immobilized on an aminohexanethiol spacer on a gold electrode has been analyzed by cyclic voltammetry. The resulting complex has a redox pair with a 25 mV more cathodic formal potential than hemin alone.
The aromatic peroxygenase (APO; EC 1.11.2.1) from the agraric basidomycete Marasmius rotula (MroAPO) immobilized at the chitosan-capped gold-nanoparticle-modified glassy carbon electrode displayed a pair of redox peaks with a midpoint potential of -278.5 mV vs. AgCl/AgCl (1 M KCl) for the Fe(2+)/Fe(3+) redox couple of the heme-thiolate-containing protein. MroAPO oxidizes aromatic substrates such as aniline, p-aminophenol, hydroquinone, resorcinol, catechol, and paracetamol by means of hydrogen peroxide. The substrate spectrum overlaps with those of cytochrome P450s and plant peroxidases which are relevant in environmental analysis and drug monitoring. In M. rotula peroxygenase-based enzyme electrodes, the signal is generated by the reduction of electrode-active reaction products (e.g., p-benzoquinone and p-quinoneimine) with electro-enzymatic recycling of the analyte. In these enzyme electrodes, the signal reflects the conversion of all substrates thus representing an overall parameter in complex media. The performance of these sensors and their further development are discussed.
Molecularly imprinted polymers (MIPs) for the recognition of proteins are expected to possess high affinity through the establishment of multiple interactions between the polymer matrix and the large number of functional groups of the target. However, while highly affine recognition sites need building blocks rich in complementary functionalities to their target, such units are likely to generate high levels of nonspecific binding. This paradox, that nature solved by evolution for biological receptors, needs to be addressed by the implementation of new concepts in molecular imprinting of proteins. Additionally, the structural variability, large size and incompatibility with a range of monomers made the development of protein MIPs to take a slow start. While the majority of MIP preparation methods are variants of chemical polymerization, the polymerization of electroactive functional monomers emerged as a particularly advantageous approach for chemical sensing application. Electropolymerization can be performed from aqueous solutions to preserve the natural conformation of the protein templates, with high spatial resolution and electrochemical control of the polymerization process. This review compiles the latest results, identifying major trends and providing an outlook on the perspectives of electrosynthesised protein-imprinted MIPs for chemical sensing. (C) 2016 Elsevier B.V. All rights reserved.
A new electrochemical MIP sensor for the most frequently used drug paracetamol (PAR) was prepared by electropolymerization of mixtures containing the template molecule and the functional monomers ophenylenediamine, resorcinol and aniline. The imprinting factor of 12 reflects the effective target binding to the MIP as compared with the non-imprinted electropolymer. Combination of the MIP with a nonspecific esterase allows the measurement of phenacetin - another analgesic drug. In the second approach the PAR containing sample solution was pretreated with tyrosinase in order to prevent electrochemical interferences by ascorbic acid and uric acid. Interference-free indication at a very low electrode potential without fouling of the electrode surface was achieved with the o-phenylenediamine: resorcinol-based MIP.
Hybrid architectures which combine a MIP with an immobilized affinity ligand or a biocatalyst sum up the advantages of both components. In this paper, hybrid architectures combining a layer of a molecularly imprinted electropolymer with a mini-enzyme or a self-assembled monolayer will be presented. (i) Microperoxidase-11 (MP-11) catalyzed oxidation of the drug aminopyrine on a product-imprinted sublayer: The peroxide dependent conversion of the analyte aminopyrine takes place in the MP-11 containing layer on top of a product-imprinted electropolymer on the indicator electrode. The hierarchical architecture resulted in the elimination of interfering signals for ascorbic acid and uric acid. An advantage of the new hierarchical structure is the separation of MIP formation by electropolymerization and immobilization of the catalyst. In this way it was for the first time possible to integrate an enzyme with a MIP layer in a sensor configuration. This combination has the potential to be transferred to other enzymes, e.g. P450, opening the way to clinically important analytes. (ii) Epitope-imprinted poly-scopoletin layer for binding of the C-terminal peptide and cytochrome c (Cyt c): The MIP binds both the target peptide and the parent protein almost eight times stronger than the non-imprinted polymer with affinities in the lower micromolar range. Exchange of only one amino acid in the peptide decreases the binding by a factor of five. (iii) MUA-poly-scopoletin MIP for cytochrome c: Cyt c bound to the MIP covered gold electrode exhibits direct electron transfer with a redox potential and rate constant typical for the native protein. The MIP cover layer suppresses the displacement of the target protein by BSA or myoglobin. The combination of protein imprinted polymers with an efficient electron transfer is a new concept for characterizing electroactive proteins such as Cyt c. The competition with other proteins shows that the MIP binds its target Cyt c preferentially and that molecular shape and the charge of protein determine the binding of interfering proteins.