Dynamics, electron transfer pathways

Fig. 13. Stereogram showing the relationship of the PC copper atom with the copper iigand His-87, the Cyt-f residue Phe-4 and the heme-iron ligand Tyr-1 suggestive of short electron-transfer pathways. Figure source Ubbink, Kjdeback, Karlsson and Bendall (1998) The structure of the complex of plestocyanin and cytochrome f, determined by paramagnetic NMR and restrained rigid-body molecular dynamics. Structure 6 331.

Dynamics of Electron Transfer Pathways in Redox Proteins... 

Figure 1 Electron transfer pathways between Bph and Qa- Upper panel the coherence parameter C = G) / ) is close to 1, indicating that dynamical effects on the coupling are small. Lower panel the effective couplings for the crystallographic and the average conformation and T a) close. The coupling is controlled by the overall protein structure, and it is almost insensitive to structural details and nuclear dynamics.

Figure 2 Electron transfer pathways between Qa and Qb- Upper panel the coherence parameter (eq. (4)) is close to zero, indicating that the coupling is controlled dynamically. Lower panel the effective coupling is sensitive to the structural details the couplings for the crystallographic and the average conformations are substantially different. The dynamics increases the coupling by almost three orders of magnitude at the relevant energies.

The mapping of electron transfer pathways has been described in Refs. [44,46,47], The general idea of the approach is to examine dynamics of charge redistribution in the system clamped at the transition state of electron transfer reaction. Briefly, the essence of the method is as follows. The tunneling dynamics of a many-electron system is described by following wave function ... 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

Computed fits of experimental signals probed at different wavelengths allow for the careful investigation of ultrafast electronic pathways (Fig.2). The transient signal at 400 nm is assigned to a very short-lived CTTS state of aqueous hydroxyl ions (OH), . This excited state is instantaneously populated, typically in less than 50 fs and follows a pseudo first order dynamics with a frequency rate of 5 x 1012 s. Semi-quantum MD simulations emphasize that transient excited CTTS states play a crucial role in photoinduced electron transfers [4-6]. 

This investigation shows that the GSR dynamics of Pe + can be used as a probe to monitor the time evolution of the ion pair formed upon electron transfer quenching. Indeed, at short interionic distance, an additional deactivation pathway of the excited cation, probably a back electron transfer to the excited reactant, is operative. Further experiments will be carried out to confirm this hypothesis. First, a three-color pump/pump-probe measurement will allow to probe the Pe (Si) population in order to see if the deactivation pathway 2+3 is really operative. Second, higher quencher concentrations have to be used in order to ensure faster formation of ET product and to make the interpretation of the data easier. 

Dynamic spectral sensitization (16). Based upon the kinetic scheme given above (Scheme 1), the dynamic sensitization pathway leading to the desired product, e.g. by electron transfer, ray be written as shown in Equation 2. 

Electron transfer catalyzed cycloadditions via radical cations show remarkable selectivity that could be exploited for expanded synthetic methodology. As a complement to the neutral Diels-Alder reaction, ET catalysis hlls the void of the electron-rich diene/electron-rich dienophile cyclizations. In attempt to understand the intricate details of the reaction, experimentalists and theorists have uncovered a range of novel factors to control and manipulate these high-energy reactive intermediates. As exemplihed by the cases discussed in this contribution, the charged character of the intermediates and the presence of back electron transfer leading to the biradical reaction manifold opens new pathways to control the chemo-, peri-, and stereochemical patterns in these dynamic species. 

Optical detection of intermediates produced in the reactions of triplet carbonyl compounds with electron donors has some obvious limitations. However, the technique of CIDNP is proving particularly effective at elucidating the reaction pathways in these systems. The outstanding work of Hendriks et al. (1979) illustrates the power of the technique. Not only was the role of radical ions in the reactions of alkyl aryl ketones with aromatic amines defined but the rate constants for many of the processes determined. The technique has been used to show that trifluoracetyl benzene reacts with electron donors such as 1,4-diazabicyclo[2.2.2]octane and 1,4-dimethoxy-benzene by an electron-transfer process (Thomas et al., 1977 Schilling et al., 1977). Chemically induced dynamic electron polarisation (CIDEP) has been... 

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