Bidirectional PCET

This chapter is organized about the four PCET geometries illustrated in Fig. 

Section 17.2 will describe the experiments applied to gain mechanistic infor- 

For fast reactions, the simplest kinetics experiment is to resolve the disappearance of the reactants, for example, by transient emission if the reaction can be photoinitiated and a reactant is luminescent. If there are multiple reaction channels available for reactant decay, the kinetics are described by a mono-exponential decay according to the sum of these rates of which PCET is only one. A more powerful experiment is to observe the disappearance of PCET reactants and growth of PCET products directly. In photoinitiated optical experiments, this means probing by transient absorption (TA) spectroscopy rather than transient emission. If PCET proceeds in a concerted fashion then concomitant mono-exponential disappearance of reactant and growth of product will be observed. If a stepwise mechanism operates, the growth of the products will be delayed (and fit by a bi-exponential function), however, this observation does not reveal the sequence in which the electron and proton were transferred. Moreover, in the limit where one of the steps is significantly faster then the other, the bi-exponential character of the kinetics trace will not be discernible, and the reaction may appear as if it were concerted. 

Direct observation of intermediates (or lack thereof) provides credence to any mechanistic assignment. Integrated rate expressions for the intermediates will generally be less convoluted than the products since they are further upstream in the kinetic cascade. However, it is often difficult to observe independent spectroscopic signatures for each of the four PCET states. This is partly a consequence of the inherent coupling between electronic states and protonic states in PCET systems. In addition, PCET systems have not incorporated design elements for independent spectroscopic signatures of the proton and the electron. 

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Figure 17.20 Schematic depiction of metal-centered bidirectional PCET. Oxidation (or reduction) at the M-OH center is coupled to loss (or gain) of a proton and an increase (or decrease) in the M-O bond order.

Other oxidases also derive function from bidirectional PCET pathways at the enzyme active site. The recent crystal structures of PSII [206, 207] support suggestions that as the oxygen evolving complex (OEC) steps through its various S-states [208, 209], substrate derived protons are shuttled to the lumen via a proton exit channel, the headwater of which appears to be the Dgi residue hydrogen-bonded to Mn-bound water [210]. The protons are liberated with the proton-coupled oxidation of the Mn-OH2 site. As shown by the structure reproduced in Fig. 17.23, Dgj is diametrically opposite to Y, which has long been known [148, 151, 152] to be the electron relay between the PS II reaction center and OEC. Notwithstanding,... 

Bidirectional PCET is also featured on the reduction side of the photosynthetic apparatus. In the bacterial photosynthetic reaction center, two sequential photo-induced ET reactions from the P680 excited state to a quinone molecule (Qg) are coupled to the uptake of two protons to form the hydroquinone [213-215]. This diffuses into the inter-membrane quinone pool and is re-oxidized at the Qq binding site of the cytochrome bcj and coupled to translocation of the protons across the membrane, thereby driving ATP production. These PCET reactions are best described by a Type D mechanism because the PCET of Qg appears to involve specifically engineered PT coordinates among amino acid residues [215]. In this case PT ultimately takes place to and from the bulk solvent. Coupling remains tight in... 

Bidirectional PCET also manifests itself in reductases. Crystal structures of hydrogenases [216-218] indicate that the mechanism for hydrogen production occurs by transporting protons into the active site along pathways distinct from those traversed by the electron equivalents. Electrons are putatively injected into the active site via a chain of [FeS] clusters, while proton channels and acid-base residues at the active site manage the substrate inventory. 

The model employs both uni- and bidirectional PCET steps. The mode of transport at the interface (between Y356 and 731) is undefined. Figure adapted from Ref [30]. 

Many definitions and descriptions of HAT, prior to the emergence of PCET as a field of study, did not adequately take into account the complexity embodied by Fig. 17.1. HAT is traditionally defined as the transfer of an electron and a proton from one location to another along a spatially coincidental pathway. In this case, the electron and proton are donated from one atom and they are accepted by another atom. These transfers are well described mechanistically as the diagonal pathway of Fig. 17.1 and they have been treated formally by a number of investigators [3, 4]. However, many reactions treated within a formalism of HAT are more complex as ET and PT are site-differentiated either along uni- or bidirectional pathways. As will be discussed in this chapter, traditional descriptions of HAT do not address how the electron and proton transfer events are coordinated mechanistically in these more complicated reactions and more general treatments of PCET are warranted. 

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