Triads pathways

The catalytic triad consists of the side chains of Asp, His, and Ser close to each other. The Ser residue is reactive and forms a covalent bond with the substrate, thereby providing a specific pathway for the reaction. His has a dual role first, it accepts a proton from Ser to facilitate formation of the covalent bond and, second, it stabilizes the negatively charged transition state. The proton is subsequently transferred to the N atom of the leaving group. Mutations of either of these two residues decrease the catalytic rate by a factor of 10 because they abolish the specific reaction pathway. Asp, by stabilizing the positive charge of His, contributes a rate enhancement of 10. ... 

Several isospecific Ci-symmetry catalysts have also been described including (12-15). When activated with [Ph3C]+ [B(C6F5)4]-, (12) affords highly regioregular i-PP (mmmm = 95%) with the stereochemical defects predominantly being isolated rr triads, consistent with a self-correcting enantiomorphic site-control pathway. 2,73 The isospecificity was therefore explained by a mechanism... 

The first molecule, the Ca2+ channel, is required for coupling at the triad. Skeletal muscle contains higher concentrations of this L-type Ca2+ channel that can be accounted for on the basis of measured voltage-dependent Ca2+ influx because much of the Ca2+ channel protein in the T-tubular membrane does not actively gate calcium ion movement but, rather, acts as a voltage transducer that links depolarization of the T-tubular membrane to Ca2+ release through a receptor protein in the SR membrane. The ryanodine receptor mediates sarcoplasmic reticulum Ca2+ release. The bar-like structures that connect the terminal elements of the SR with the T-tubular membrane in the triad are formed by a large protein that is the principal pathway for Ca2+ release from the SR. This protein, which binds the... 

Figure 13.23 The general mechanistic pathway catalysed by mononuclear non-haem iron enzymes with a 2-His-l-carboxylato facial triad, shown in its resting state in (a). (From Koehntop et al., 2005. With kind permission of Springer Science and Business Media.)...

Observation (iii) above, taken in the context of the triad annihilation in Scheme 12, indicates that the more or less statistical o/p pattern is diagnostic of the homolytic pathway (66) since it will clearly dominate the competition for TOL+- at the high concentrations of added N02 (Scheme 16). Indeed this conclusion is supported by observation (i), in which essentially the same isomeric product distribution (i.e. ortho meta para 70 2 28%) is achieved when the pyridine competition is thwarted for the sterically hindered 2,6-lutidine, an ineffective nucleophile (Schlesener et al., 1984). According to the formulation in Scheme 16, the isomeric product distribution is established from the sterically hindered Me2PyNOj during the homolytic annihilation of TOL+- by N02, most favourably at the ortho and para... 

Finally, we ask, if the reactive triads in Schemes 1 and 19 are common to both electrophilic and charge-transfer nitration, why is the nucleophilic pathway (k 2) apparently not pertinent to the electrophilic activation of toluene and anisole One obvious answer is that the electrophilic nitration of these less reactive [class (ii)] arenes proceeds via a different mechanism, in which N02 is directly transferred from V-nitropyridinium ion in a single step, without the intermediacy of the reactive triad, since such an activation process relates to the more conventional view of electrophilic aromatic substitution. However, the concerted mechanism for toluene, anisole, mesitylene, t-butylbenzene, etc., does not readily accommodate the three unique facets that relate charge-transfer directly to electrophilic nitration, viz., the lutidine syndrome, the added N02 effect, and the TFA neutralization (of Py). Accordingly, let us return to Schemes 10 and 19, and inquire into the nature of thermal (adiabatic) electron transfer in (87) vis-a-vis the (vertical) charge-transfer in (62). 

Since electrophilic and charge-transfer nitrations are both initiated via the same EDA complex and finally lead to the same array of nitration products, we infer that they share the intermediate stages in common. The strength of this inference rests on the variety of aromatic substrates (with widely differing reactivities and distinctive products) to establish the mechanistic criteria by which the identity of the two pathways are exhaustively tested. On this basis, electrophilic nitration is operationally equivalent to charge-transfer nitration in which electron-transfer activation is the obligatory first step. The extent to which the reactive triad in (90) is subject to intermolecu-lar interactions in the first interval (a few picoseconds) following electron transfer will, it is hoped, further define the mechanistic nuances of dissociative electron transfer in adiabatic and vertical systems (Shaik, 1991 Andrieux et al., 1992), especially when inner-sphere pathways are considered (Kochi, 1992). 

In cases of promiscuous activity, one species in the pathway often acts as a common intermediate for both mechanisms leading to different activities. An enam-ine between PLP and the substrate branches off into a decarboxylation or a transamination, a TPP-bound intermediate can react either to decarboxylation or to carboligation, and the triad Ser-His-Asp in hydroxynitrile lyase is responsible for both the main function, oxynitrilation, and the subordinate function, ester hydrolysis. It can be assumed that many more promiscuous functionalities will be discovered in the coming years. 

Pathways in the central nervous system. A shows two relay neurons and two types of inhibitory pathways, recurrent and feed-forward. The inhibitory neurons are shown in black. B shows the pathway responsible for presynaptic inhibition in which the axon of an inhibitory neuron synapses on the axon terminal of an excitatory fiber. C Diagram illustrating that dendrites may be both pre-and postsynaptic to each other, forming reciprocal synapses, two of which are shown between the same dendrite pair. In triads, an axon synapses on two dendrites, and one of these dendrites synapses on the second. In serial synapses, a dendrite may be postsynaptic to one dendrite and presynaptic to another, thus connecting a series of dendrites. Dendrites also interact through low-resistance electrotonic ("gap") junctions (two of which are shown). Except for one axon, all... 

Fig. 3. Transient states of C-P-Q triad 4 and relevant electron transfer pathways...

Since the initial reports of the C-P-Q triads, a number of other molecules of the D-D -A or D -D-A types have been described. Triad 12, prepared by Wasielewski and coworkers, is a relative of the C-P-Q series in which the secondary donor is an aniline derivative (D), rather than a carotenoid [63]. The bicyclic bridges were introduced in order to add rigidity to the system. The fluorescence lifetime of the porphyrin moiety of 12 was found to be <30ps. This result is consistent with rapid electron transfer to the quinone to yield D-P+-QT. This result was confirmed by transient absorption measurements. The absorption results also revealed that this intermediate charge separated state decays with a rate constant of 1.4 x 1010 s-1 to a final charge separated state D+-P-Qr. Thus, the decay pathways are similar to those shown in Fig. 3 for the C-P-Q triads. This final state has a lifetime of 2.45 ps in butyronitrile (which is similar to that found for 4 in acetonitrile) [44], and is formed with a quantum yield of about 0.71. Thus, the efficiency of the transfer analogous to step 4 in Fig. 3 for this molecule is also about 0.71. 

The details of the multistep electron transfers undergone by 40 may best be appreciated by reference to the results for two model compounds 41 and 42. Triad 41 is similar to the tetrad, except that it lacks the final benzoquinone moiety. Excitation of the porphyrin leads to the production of C-P+-QA with a quantum yield of essentially 1, as was observed for 40. In common with other C-P-Q triads, this state goes on to produce a final C+-P-Qx species. However, the quantum yield of this state is only 0.04, and its lifetime is about 70 ns (Fig. 7). The low quantum yield is due to the fact that only a single, relatively inefficient electron transfer step (analogous to step 4 in Fig. 6) competes with charge recombination of C-P+-Qx. With the tetrad 40, a similar pathway is still available, but in addition there is a second, relatively efficient pathway which also competes with charge recombination and is responsible for most of the quantum yield of the final state. 

Because of the presence of a well-defined energy gap between the conduction and the valence band, semiconductors are ideally suited for investigation of the interfacial interactions between immobilized molecular components and solid substrates. In this chapter, interfacial assemblies based on nanocrystalline TiOz modified with metal polypyridyl complexes will be specifically considered. It will be shown that efficient interaction can be obtained between a molecular component and the semiconductor substrate by a matching of their electronic and electrochemical properties. The nature of the interfacial interaction between the two components will be discussed in detail. The application of such assemblies as solar cells will also be considered. The photophysical processes observed for interfacial triads, consisting of nanocrystalline TiO 2 surfaces modified with molecular dyads, will be discussed. Of particular interest in this discussion is how the interaction between the semiconductor surface and the immobilized molecular components modifies the photophysical pathways normally observed for these compounds in solution. 

Figure 6.23 Schematic illustration of an interfacial supramolecular assembly incorporating a Ru-Ru diad, showing the potential electron transfer pathways in the interfacial supramolecular triad T1O2-Ru-Ru...

When the interfacial supramolecular triad is irradiated in the presence of I- under solar cells conditions, appreciable photocurrents are obtained. The profile of the photoaction spectrum shows clearly that photoinjection into TiC>2 takes place upon excitation of the ruthenium center. However, the IPCE values obtained are lower than those observed for the model compound, thus suggesting that injection is less efficient in the heterotriad. Of major interest is the mechanism for charge injection. Two different pathways can be envisaged. First, the charge injection may be a two-step process and takes place via the rhodium center as shown in the following equations ... 

The morphological problems associated with the BHJ solar cells, such as low concentration of percolating pathways which are needed in order to bring the separated charge carriers to their corresponding electrodes, have prompted the utilization of molecules in with the donor and the acceptor moieties were covalently linked. In this connection several examples of Pc-based polymers [161,162], Pc-C6o dyads [85,87,88] and triads [275] have been prepared and tested for photovoltaic applications, but the efficiencies of these systems have been proved to be still low. 

Further functionalizations are obtained via the electron transfer— radical cation fragmentation pathway a typical example is side-chain nitration by irradiation of methyaromatics with tetranitromethane. Aromatics form charge-transfer complexes with C(N02)4 irradiation leads to electron transfer and fragmentation of the C(N02)4 radical anion to yield the triad [Ar + C(NO)J N02], followed by combination between the arene radical cation and the trinitromethanide anion. Thus, cyclohexadienes are formed that generally eliminate and rearomatize at room temperature yielding ring-functionalized products [234] (Sch. 21). 

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