Electrochemistry of Pyrroloquinoline Quinone (PQQ) on Multi-Walled Carbon Nanotube-Modified Glassy Carbon Electrodes in Biological Buffers

Abstract

Pyrroloquinoline quinone (PQQ) is an important electrocatalyst and redox cofactor for many enzymes used in bioanalytical applications. Careful selection of electrode modifications and buffer compositions is required due to PQQ's tendency to be irreversibly reduced on bare electrodes and the strong effect of pH on its electrochemistry. Multi-walled carbon nanotubes (MWCNTs) can effectively modify glassy carbon electrodes, but PQQ's behavior at these surfaces has not been investigated. While phosphate buffers have been used extensively to characterize PQQ's electrochemistry, phosphate buffers can act as chelating agents for important metal ions needed by PQQ and biomolecules. Different physiological buffers could overcome these challenges and expand the buffer options for biofuel cells. Using cyclic voltammetry, we studied PQQ in HEPES, MOPS, TRIS, and phosphate buffers at pH 7 with MWCNT-modified electrodes. While there were no significant differences in PQQ's behavior with the various buffers, the ionic composition of the non-phosphate buffers was very important. We observed that K + and Mg 2+ were the most influential ions for the reversible reduction-oxidation of PQQ. PQQ's electrochemistry was also affected by the length of and the functional groups on the MWCNTs, indicating that the electron transfer kinetics were quite sensitive to the surface chemistry of the electrodes. Pyrroloquinoline quinone (PQQ) is a redox cofactor found in the active sites of a number of enzymes. Most of these quinoproteins are dehydrogenases capable of catalyzing the oxidation of a broad range of substrates, such as alcohols, acids, aldehydes and sugars. 1 PQQ-dependent glucose dehydrogenase, for example, is capable of oxidizing several mono-and disaccharides, including glucose, maltose, lactose, galactose, xylose and mannose.1 The uniqueness of the quinoproteins and their broad substrate specificities have led to their use in bioanalytical applications, including biosensors and biofuel cells.In the absence of an enzyme, PQQ itself is capable of catalyzing redox reactions, including the oxidation of thiols, 2,3 amino acids, 4 and the redox cofactor nicotinamide adenine dinucleotide (NAD + ). 5The electrochemistry of PQQ has been studied quite extensively in both solution and immobilized forms via cyclic voltammetry (CV) and potentiometric titration studies. 6,7 These studies can be challenging due to the pH dependence of the redox potentials. 6,8,9 Using a cystamine-monolayer Au-modified electrode, Katz et al.6 studied the electrochemistry of PQQ in solution using CV and reported the change in potential with pH to be about 60 mV pH −1 at pH < 6.2; 30 mV pH −1 at 6.2 < pH < 8.6; and about 60 mV pH −1 at pH > 8.6. In another study using PQQ immobilized on few-walled carbon nanotube (FWCNT)-modified glassy carbon electrodes, the change in PQQ's redox potential was estimated to be approximately 67 mV pH −1 , with the highest reversible reduction-oxidation observed at ∼pH 2. The nature of the modified electrode surface can also affect P...

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“…The redox potentials of photosystem I and II subunits are diverse in the literature and the values reported here are obtained from the following sources (Bottin and Lagoutte, 1992; Semenov et al, 2000; Cassan et al, 2005; Allakhverdiev et al, 2010, 2011; Kothe et al, 2013; Caffarri et al, 2014; Schuurmans et al, 2014). The redox potentials of mediators are taken for neutral aqueous conditions from Nivinskas et al (2002),Schuurmans et al (2014), Lai et al (2016), and Emahi et al (2017). AQDS, 9,10-anthraquinone-2,6-disulfonate; CCCP, carbonyl cyanide m -chlorophenylhydrazone; DCMU, 3-(3,4-Dichlorophenyl)-1,1-dimethyl urea; DCBQ, 2,6-Dichloro-1,4-benzoquinone; DBMIB, 2,5-dibromo-3-methyl-6-isopropyl-P-benzoquinone; DCCD, N-N’ -dicyclohexylcarbodiimide; HNQ, 2-hydroxy-1,4-naphthoquinone; HQNO, 2-heptyl-4-hydroxyquinoline n-oxide; MV, methyl-viologen; NEM, N -ethylmaleimide; PMA, phenylmercuric acetate; p-BQ, p-benzoquinone.…”

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“…The proposed detection mechanism is based on the nonenzymatic oxidative deamination of amines by PQQ, an electron transfer mediator. Studies were conducted to understand the electrochemical properties of both soluble and immobilized PQQ at electrode surfaces [ 91 , 92 ]. PQQ is a redox coenzyme of the PQQ-dependent oxidoreductases (quinoproteins) that undergoes cycles of reduction and regeneration with the transfer of two electrons and protons [ 93 ].…”

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“…Common metal cofactors in oxidoreductases are iron heme cofactors, [9] iron-sulfur clusters (FeS), [10] copper structures (both mono and multi-metallic), [10] as well as more specific structures like the molybdenum iron cofactor (MoFe) of nitrogenase, [11] and the nickel iron (NiFe) cofactor of hydrogenase. [12] Oxidoreductases can also have non-metallic cofactors such as flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) of the flavoproteins, [13][14] and pyrroloquinoline quinone (PQQ) [15] containing enzymes (summary of structures in Figure 2).…”

“…Figure15. Mechanism of a one-pot conversion of an inert hydrocarbon to imines using an electroenzymatic cascade.…”