Protonation electron transfer

Most of the Moco enzymes catalyze oxygen atom addition or removal from their substrates. Molybdenum usually alternates between oxidation states VI and IV. The Mo(V) state forms as an intermediate as the active site is reconstituted by coupled proton—electron transfer processes (62). The working of the Moco enzymes depends on the 0x0 chemistry of Mo (VI), Mo(V), and Mo (TV). 

The sequential electron-proton-electron transfer mechanism is in agreement with the experimental observation by Ohno et al. [141]. The mechanism was confirmed by Selvaraju and Ramamurthy [142] from photophysical and photochemical study of a NADH model compound, 1,8-acridinedione dyes in micelles. 

Scheme 30 Sequential electron-proton-electron transfer...

Infrared spectroscopy has also been employed to follow the formation of acetaldehyde and acetic acid on Pt during ethanol electro-oxidation. On the basal planes, acetaldehyde could be observed starting at about 0.4 V (vs. RHE), well before the onset of CO oxidation, while the onset of acetic acid formation closely follows CO2 formation [Chang et al., 1990 Xia et al., 1997]. This is readily explained by the fact that both CO oxidation and acetic acid formation require a common adsorbed co-reactant, OHads, whereas the formation of acetaldehyde from ethanol merely involves a relatively simple proton-electron transfer. 

Based on the EPR, ENDOR, ESEEM and some other spectroscopic results models for Yz function in PS II have been developed (see references 385, 386 for recent reviews). There seems to be consensus that this amino acid is involved in proton-coupled electron-transfer. As proton acceptor His (Dl-190) was identified which is also found to be close to Yz in the recent X-ray structure.19 The postulated direct involvement of Yz in the water splitting process as a hydrogen atom abstractor or in separate or coupled proton/electron-transfer is still controversially discussed.385 386... 

Similar studies have been carried out in the 1,4-dihydropyridine series. Nevertheless, the detailed mechanism remains questionable and evidence in support of both possible reaction pathways, direct hydride transfer and electron-proton-electron transfer, were presented. 

These interfacial pH effects have been investigated by probing the voltammetry in buffered solutions. Figure 5.16 shows that for 1.0 < pFl < 10.6, E0 depends linearly on pH, with a slope of 63 3 mV. This value is indistinguishable from the slope of 59 mV pH-1 expected for a coupled proton/electron transfer and indicates that the H2Q species is produced when the monolayer is reduced. Between pH 10.6 and 12.0, the slope decreases to 25 4 mV pH-1, which compares favorably with the slope expected (29.5 mV pH-1) for a two-electron, one-proton transfer reaction. Therefore, over this pH range Q is reduced to HQ- and the p/12.0, E0 is independent of pH, thus indicating that the pKa of the HQ-/Q2- couple is 12.0 0.2. 

Net transfer of a proton and two electrons may occur other than by direct transfer of hydride ion between the bonding sites. Single electron transfer followed by hydrogen atom transfer, e/H1, (3), or indeed the reverse sequence, H /e, (4), achieves the same result, as do sequential electron-proton-electron transfers, e/H + /e, (5). 

Mechanistic speculations about the molybdoenzymes must be considered to be in their infancy with the possible exception of those for xanthine oxidase. Although the detailed structural nature of the molybdenum site is unknown, there is sufficient information from biochemical and coordination chemistry studies to allow informed arguments to be drawn. Here we first discuss evidence for the nuclearity of the molybdenum site and then discuss both oxo-transfer and proton-electron transfer mechanisms for molybdenum enzymes. A final discussion considers the unique aspects of nitrogenase and the possible reasons for the use of molybdenum in enzymes. 

Figure 5. Proposed coupled proton-electron transfer scheme for xanthine oxidase activity (66, 67, 68)...

Many experimental observations on xanthine oxidase activity are correlated by this scheme, and at present, there appear to be no major inconsistencies. The coupled proton-electron transfer scheme (66, 67, 68) has been successfully incorporated into an overall mechanistic scheme (69) which explains, with great economy, a large amount of rather demanding data, from both kinetic and electron uptake experiments. 

The coupled proton-electron transfer mechanism can also be applied to the molybdenum reductases. For nitrate reductase, a scheme such as Reaction 20 is possible. A Mo (IV)-Mo (VI) couple is used to illustrate this, and while such a couple is viable for some nitrate reductases, the Mo(II)-Mo(IV) or the Mo(III)-Mo(V) couple could also be accommodated... 

This process can be contrasted directly with the oxo transfer scheme (Reaction 16) discussed above. In either case, the cleavage of the N-O bond is assisted by the binding of oxygen to an electrophile (to molybdenum itself in the oxo transfer mechanism or to proton(s) in the coupled proton-electron transfer scheme). Although the coupled proton-electron transfer mechanism would possibly have the advantage of leaving an open site on molybdenum to restart the cycle, there is no strong data to support either of these mechanisms at present. 

Changes in the energy gap, AE, and the nonadiabatic transition probability, P10, in the aqueous solution simulations are dominated in the initial stages by the coupled proton-electron transfer event and the subsequent relaxation of the system into the excited CT state. Similar to the gas phase, variations in AE and P10 at longer time-scales were found to depend strongly on the out-of-plane motions of the system (for instance the dihedral angles 0 and ). However, the presence of... 

Protium/deuterium/tritium kinetic isotope effects are often used to support hydride transfer mechanisms over single electron transfer mechanisms. However, sequential electron/proton/electron transfer mechanisms can easily show isotope effects as well. Even though the rate limiting step in the overall two electron reduction of flavin or NADH may be the isotope independent endergonic electron tunneling to form a radical intermediate state, once formed, this radical state can return the electron to recreate the... 

A range of chemical analogs of the catalytic centers of Mo and W dithiolene-containing enzymes (pterins) have been prepared. In particular, the rich chemistry of multisulfur transition metal systems allows ligand redox, internal electron transfer, and intermediate redox states. Such redox flexibility may facihtate coupled proton/electron transfer and/or 0x0-transfer mechanisms, which are employed by Mo and W enzymes. 

Very stable intermediates reside near the equihbrium potential for adsorbed oxygen and hydroxyl, and coupled proton/electron transfer to and dominates the overall reaction kinetics. In step 3 of Eq. (4), hydrogen peroxide may also form instead of water, and this "associative" hydrogen peroxide mechanism is believed to dominate for most noble metals. 

Figure 19.15. A Plausible Scheme for Oxygen Evolution from the Manganese Center. A possible partial structure for the manganese center is shown. The center is oxidized, one electron at a time, until two bound H2O molecules are linked to form a molecule of O2, which is then released from the center. A tyrosine residue (not shown) also participates in the coupled proton-electron transfer steps. The structures are designated Sq through S4 to indicate the number of electrons that have been removed.

---