Triad system

Chymotrypsinogen and related proenzymes have extremely low catalytic activity even though a major part of the substrate binding site as well as the catalytic triad system are already in place. However, the oxyanion hole is created during activation of the proenzyme by a subtle conformational change197,262,271 that involves the chain segment containing Gly 193 (Fig. 12-12). This is further evidence of the importance of this part of the active site structure. 

However, a-chloroketones are powerful alkylating agents and the bound inhibitor attacks His 57 of the catalytic triad system. The reaction is probably more complex than is indicated in the foregoing equation and may involve an epoxy ether intermediate/ Many other peptide chloromethyl ketone inhibitors have been devised/ 1 ... 

These are triad systems, i.e., made of three component units weakly interacting, where a separation of charges occurs over nanometric distances. All systems described here are based on a central Ir(terpy)23+ unit, with functional units appended at the 4 position of the terpy ligands. As discussed above, this allows the construction of linear arrays without complications due to the formation of isomers as is the case for bidentate ligands. 

In addition to the previously described donor-acceptor hybrid dyad systems, a few triads have also been considered and constructed. The fundamentals and principles for designing such triad systems are more complex, as there exist diverse arrangement options for their construction. Thus, the multiple electroactive units of a triad can be arranged in different ways giving rise to different materials [245]. 

Another early triad system is 38, which was reported in 1985 [213]. The triad is similar to 35, but the secondary electron donor is a dimethylaniline species. The bicyclic linkages joining the components preclude large-scale folding of the molecule, and subsequent short circuiting as was observed in 37. The photochemistry of triad 38 is similar to that observed for 35, and the lifetime of the final state in butyronitrile solution at ambient temperatures is 2.45 ps. 

The PTZ unit has been widely used as the electron donor component in triad systems for photoinduced charge separation (see Section 3.4.5). 

Pedersen (39) has presented the theory of protolytic reactions and considers that the mechanisms are usually bimolecular. He makes the seemingly justifiable assumption that a proton cannot move spontaneously from one place in the molecule to another without a catalyst, while valency electrons can move spontaneously within the molecule. For example, in the triad systems,... 

The longer lifetime noted for IS relative to 16 may be ascribed to the enhanced donor-acceptor separation in IS. Direct electron transfer from Qg to C should be slow because of the large distance involved, and a multistep charge recombination such as was observed in some of the triad systems discussed above would require slow endergonic electron transfer to yield either C-P -Qa-Q or C -P-Qa -Qb followed by direct recombination of these states or a second endergonic electron transfer to yield C-P -Qg -Qg. 

The molecular targeting agent (C) selected from a representative sequence of amino acids, resulting in uptake of the triade system particular in the tumor cells. 

Photoinduced charge separation processes in the supramolecular triad systems D -A-A, D -A -A and D -A-A have been investigated using three potential energy surfaces and two reaction coordinates by the stochastic Liouville equation to describe their time evolution. A comparison has l n made between the predictions of this model and results involving charge separation obtained experimentally from bacterial photosynthetic reaction centres. Nitrite anion has been photoreduced to ammonia in aqueous media using [Ni(teta)] " and [Ru(bpy)3] adsorbed on a Nafion membrane. 

Neutron and X-ray diffraction studies have revealed that in the hydrogen-bonded triad system X-H. .. A, where atoms X and A are chemically different, the proton is bound to only one of the two atoms. In systems where the atoms X and A are the same but belong to different groupings, the situation is less clear-cut. Proton disorder may occur. Its extent depends on various factors, such as the X... A distance, the environment about the hydrogen bond, the ionic states of X and A, the molecular conformation, and the temperature. The disorder may be localized within a molecule or involve a pair, a chain, a layer, or a three-dimensional ensemble of molecules. 

The redox-potential diagram for the donor-Ru(bpy)3 -MV triad system in Figure 5 is helpful in understanding the sequence of electron transfer events in these composites. Although excitation of the sensitizer is always the initial step in the overall process, there are several possible pathways for subsequent reactions. Transient absorbance measurements on the donor-sensitizer and... 

Figure 5. Redox potential diagram of the MDESA-Ru(bpy)3-MV triad system, with the important electron transfer steps indicated. All potentials are referenced to SCE in aqueous solution.

Adjusting the EPA s Triad System Conceptual Site Model... 

Recently, the ERA has set forth an approach for environmental remediation called, the Triad System. A key step in this is the development of a Conceptual Site Model. According to the Triad System, the Conceptual Site Model includes the following ... 

Additionally, Hunter and coworker presented another model for the noncovalently linked donor-acceptor complex which was made up of triad components, quinone-cyclophane-porphyrin 144. H NMR measurements indicated the formation of a ternary complex and the association constant of BQ via hydrogen bonds was determined to be 1.4 x 10 M, while the association constant of pyridine of the cyclophane and zinc porphyrin was 3.2 X lO M in dichloromethane. The fluorescence emission from the zinc porphyrin in the presence of the cyclophane was clearly quenched by the addition of BQ, compared with a control experiment, due to diffusional quenching. This evidence indicated that the intracomplex ET occurred within the triad system. 

The simplest supramolecular species capable of performing such type of process are covalently-linked three-component systems ("triads"). Two possible schemes for charge separating triads are shown in Fig. 5. Although the scheme in Fig. 5b is reminiscent of the natural photosynthetic reaction center, that of Fig. 5a seems to be more popular in the field of artificial triad systems. The functioning principles are shown in an orbital-type energy diagram in the lower part of Fig. 5. In both cases, excitation of a chromophoric component (1) is followed by a primary photoinduced electron transfer to a primary acceptor (2). This is followed by a secondary thermal electron transfer process (3) electron transifer from a donor component to the oxidized chromophoric component (case a), or electron transfer from the primary acceptor to a secondary acceptor component (case b). The primary process competes with excited-state... 

Out of many triad systems developed recently, a couple of examples will be shortly discussed here. Compound 6" is one of the first developed triads. It corresponds to the scheme of Fig. 5a with a porphyrin as light absorbing chromophore, naphthoquinone as acceptor, dimethylaniline as donor, and tripticene bridges as connectors. The charge separated state has an energy of 1.39 eV, is reasonably long lived (x=2.5 ms), and can be obtained with high efficiency (0=0.71) in room temperature butyronitrile solution. 

Osuka et al. reported a long lifetime of ion pair state in a triad system of Zn porphyrin(ZnP)-oxodilorin(H2C)-pyromellitdiimide(Im) [567,568]. Excitation of ZnP or H2C led to the ion pair state of (ZnP)+-H2C-(Im) with a long lifetime of 0.24 [IS in DMF [567] (Fig. 55). Interesting series of tetrad and pentad molecules have been studied by the same authors [569-572,574] (32-34). 

Figure 55 Ion pair state with a long lifetime in a triad system of Zn porphyrin(ZnP)-oxochlotin(H2C)-pyromellitdiimide(Im) [567],...

Figure 12 (a) Triad system with accessory quinone incorporated (b) into a liposome vesicle displays photoinduced ET coupled with... 

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