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Fundamentals of Bioelectrocatalysis

Matteo Grattieri1,2 and Shelley D. Minteer3

1Department of Chemistry, Università degli Studi di Bari Aldo Moro, Bari, Italy

2Istituto per i Processi Chimico Fisici, Consiglio Nazionale delle Ricerche, Bari, Italy

3Kummer Institute Center for Resource Sustainability, Missouri University of Science and Technology, Rolla, MO, USA

1.1 Introduction

Bioelectrocatalysis is a fascinating research field where a biological material, spanning from isolated apparatuses to intact organisms (Box 1.1), is utilized as a catalyst for redox reactions taking place at an electrode. The interest in this research field has its foundations on the combination of the advantages provided by the features of biological systems, such as diverse catalytic functions, mild reaction conditions, high activity and selectivity, and the possibility to control the oxidizing and reducing ability of an electrode by modifying its Fermi level [1]. Nowadays, bioelectrocatalysis has several applications, which are presented and discussed in detail in various chapters of the books, including the development of (i) biofuel cells for power production [2, 3], (ii) biosensors [4-9], (iii) systems for drug release [10], (iv) wearable devices [11, 12], (v) systems performing the treatment of contaminated water [13-15], and (vi) systems performing the electrosynthesis of valuable products [16-19].

At the basis of bioelectrocatalysis is the process of electron transfer (ET) between the biological components and the electrodes. Achieving a rapid and efficient ET defines the success or the failure of a bioelectrocatalytic system. Accordingly, the ET process constitutes the major challenge in bioelectrocatalysis, and this chapter is dedicated to introduce the general aspects governing the interaction between biological catalysts and electrodes, providing the basic concepts to approach the book. The detailed ET processes for enzymes and microorganisms are discussed in Chapters 2 and 3, respectively.

Box 1.1 Enzymes as Biological Catalysts

Enzymes play a pivotal role in life, with more than 99% of all the reactions that are crucial to biological systems being catalyzed by enzymes. Like other catalysts, enzymes allow for an increase in the rate of a chemical reaction without being consumed or altered by such reactions, thus overcoming the kinetic barriers that hinder a specific process. Interestingly, when the term "catalysis" was proposed in 1836 by the Swedish chemist Berzelius (1779-1848) from the Greek kata (wholly) and lyein (to loosen), no distinction was made between a chemical and biological catalysis, and only the term "contact substance" was used for referring to a catalyst [20]. One of the unique properties of enzymes, their high specificity for a specific substrate, was highlighted as early as 1894 by the German chemist Emil Fischer (1852-1919, Nobel Prize in Chemistry in 1902), who hypothesized the popular lock-and-key mechanism of enzyme interaction with a substrate. Shortly later, the kinetics of enzyme catalysis was studied by Victor Henri (in 1903) and followed by Leonor Michaelis and Maud Menten (in 1913), who described the rate equation of enzyme kinetics with the Henri-Michaelis-Menten equation, commonly referred to as the Michaelis-Menten equation:

where v is the rate of enzyme-catalyzed reaction (velocity), Vmax is the limiting maximum value for the velocity, S is the substrate for the reaction, and KM is a constant often referred to as "Michaelis constant" arising from the three rate constants (k1, k-1, and k2) of the following reaction:

where E is the enzyme, S is the substrate, ES is the enzyme-substrate complex that is formed when enzyme and substrate come together, and P is the product. k1 and k2 are the forward rate constants for the first and second steps, respectively, while k-1 and k-2 are the backward rate constants for the first and second steps, respectively. Although not all enzymes follow Michaelis-Menten kinetics, it is usually a good approximation for enzymatic electrochemistry systems.

1.2 Bioelectrocatalysts and Electron Transfer

Enzymes can be considered the most commonly utilized biological catalyst for bioelectrocatalysis. They provide high specificity due to the molecular recognition event that takes place in the binding pocket for their substrates, where their redox active center(s), the cofactors, allow performing the catalytic process, converting the substrate into the products. This process is made possible by the 3D structure of the enzyme. However, different from the commonly known metallic electrocatalysts (i.e. platinum group and non-platinum group catalysts) [21, 22] and nonmetallic electrocatalysts (i.e. 2,2,6,6-tetramethylpiperidinyl-N-oxyl) [23], the 3D structure of biological catalysts constitutes a nonconductive layer surrounding the cofactors [24]. As a result, when aiming to "electrically wire" an enzyme to an electrode, the primary goal is to successfully transfer the electrons across this insulating layer. This ET process can be described by utilizing the semi-classical Marcus theory for ET in nonadiabatic systems. According to Marcus theory, the rate constant of ET (kET) can be expressed as:

where ? is the reduced Planck's constant (1.05457182 × 10-34 Js), HDA is the matrix coupling element (also known as electronic coupling parameter), ? is the nuclear reorganization energy (the energy required to force the reactant into the same nuclear configuration as the products without the ET taking place), and ?G0 is the thermodynamic driving force of the reaction.

Importantly, the electronic coupling parameter exponentially decays with an increasing distance between the electron donor and acceptor sites (rDA) as described by:

where indicates the electronic coupling parameter for the donor (D) and acceptor (A) when they are at the minimum distance, and ß is the electronic coupling attenuation coefficient that varies depending on the medium separating donor and acceptor.

According to Eq. (1.4), and considering that the electron tunneling time for biological redox systems should be in the millisecond to microsecond range for their proper functioning, the maximum center-to-center distance for a single-step electron tunneling should be approximately 24 [25, 26], with lower distances resulting in higher rates of ET (Figure 1.1) [28]. This distance should not be taken as a hard limit, and ET at distances higher than 24 has been reported [29]. Taking this into consideration, it is interesting to underline the center-to-center distance for two widely employed enzymes in bioelectrocatalysis: laccase and glucose oxidase. Laccases (EC 1.10.3.2) [30] are multicopper oxidoreductase where a Type 1 copper center (located about 6.5 Å below the enzyme surface) accepts one electron from diphenols and related substances and transfers it to a trinuclear copper cluster (where the oxygen reduction process takes place) that is located at a distance of 12-13 Å [31, 32]. Glucose oxidase from Aspergillus niger (E.C. 1.1.3.4) is an enzyme that catalyzes the oxidation of D-glucose due to two flavin active sites buried deeply within the enzyme structure, at a distance of approximately 20-27 Å [33-35].

Another important aspect is that the ET rate also depends on the electrochemical overpotential applied. Specifically, knowing that:

Figure 1.1 Dependence of the rate of single-step electron transfer on the distance between donor and acceptor centers based on semi-classical Marcus theory.

Source: Hickey et al. [27] / with permission of The Royal Society of Chemistry.

and that we can apply a specific overpotential (?) by modulating the potential applied to an electrode with respect to the redox potential of the redox species involved in the electrode transfer process:

we can relate the free energy and the overpotential with Eq. (1.7):

By combining Eqs. (1.3) and (1.7), and noticing the quadratic nature of the free energy term in Eq. (1.3), a finite range where an increase in overpotential results in an increase in the ET rate can be defined. Passing this overpotential results in a point of inversion in the ET rate, which starts to decrease for increasing overpotentials, indicating what is known as the "Marcus inverted region" [25, 36, 37]. However, such an "inverted region" is not typically observed in bioelectrochemical systems, likely because at high overpotentials, the reactions become limited by mass transfer or...