P dimeric

The recognition2 that the a, p-dimer 3 is formed in equilibrium with 1 and not the a,a-dimer 2 was interpreted as a result of the smaller steric strain in 3 than in 23 Also the known strong influence of p-substituents on the equilibrium constants between substituted trityl radicals and their dimers6 found an obvious explanation in this way. The earlier observation that not only those phenoxy radicals 4 carrying three conjugating phenyl substituents 4 (R = C6H5)7a are persistent8 but also their... 

In particular, the rotation and swinging of the extracellular domains coupled with rotation and tilting of the transmembrane domain might occur during dissolution of the receptor homodimers (as is very schematically illustrated by Fig. 5) and formation of heterodimers and can be important at the initial and final stages of interaction of the TGF-P dimer with both receptor dimers. Such motions and conformational transitions of the extracellular and transmembrane domains cannot occur without similar motions of the intracellular domains, resulting in changes of their mutual orientation, the points of contact between them, and hence of the distance between the... 

Specifically Netzel et al. ( - ), in studies of face-to-face , covalently-linked MgP-P dimers, found evidence for the formation within 6 psecs of a low-lying, relatively long-lived intramolecular CT state of the type MgP -P" in polarizable or highly polar solvents and in solvents where chloride ion coordinates with the magnesium ion of the MgP-macrocycle. These workers also observed the formation of benzoquinone anion radicals as stable photoproducts of the CT formation process when the experiments were carried out in the presence of benzoquinone ( ). This approach provides a more direct test for the formation of an intramolecular CT state, and the results are in sharp contrast to those typically observed when porphyrin Ktt, ) states are quenched in the presence of benzoquinone (23). 

Due to the complexity of MgP-P dimer systems, ah initio quantum mechanical studies designed to ascertain the importance of geometric relaxations must be delayed for future investigations. In this regard Warshel (36, rf. ) has applied a variety of semi-empirical and semi-... 

Figure 2. Estimated transition energies of (a) iCTi in isolated MgP-P dimers, (b) the lowest four states in isolated dimers, and (c) ICTi in...

Cl -MgP-P dimers, for the geometries shown in Figure 1. Reproduced with permission from Ref. 8. Copyright 1986 American Institute of Physics. 

Figure 3. Orbital energies of the four highest occupied and four lowest unoccupied molecular orbitals in isolated monomers (MgP, PO) and in face-to-face MgP-P and Cl"-MgP-P dimers with iterring separations of...

Isolated MgP-P Dimers. The spectroscopic consequences of dimer formation are depicted in Figure 4. As shown in the figure, the four lowest-lying states of MgP-P dimers are essentially identical to the corresponding monomeric states of MgP and P (, 37), except for... 

Figure 4. Estimated transition energies of low-lying excited singlet and triplet states in the 5.35A MgP-P dimer. S1-S4 and T1-T9 correspond to localized states on the indicated monomers. 85 and Tio correspond...

Solvent Effects. As noted earlier, Netzel et al. (M- ) observed striking effects on the photodynamics of electronically excited MgP-P dimers depending on the solvent used. For example, in their earlier work (20) they showed that the photodynamics in CH2CI2 were consistent with the formation of a long-lived singlet CT state which decayed by two pathways, while in THF only a single short-lived (tt, tt ) state was observed. 

Internal Electric Field Effects d -MgP-P Dimers. Experimentally Netzel and co-workers addressed the question of the effect of... 

Fig. 2.2.3.B SbHNL beiongs to the o/P-hydroiases. The heterotetramer is formed by two o/P-dimers, which are stabiiized by two intermoiecuiar disuifide bonds. Each subunit carries a giycosyiation site. Both subunits are coded by one gene. Cieavage into a- and P-subunit is assumed to occur after foiding of the pre-protein.

NtrC-P dimerizes and binds to the enhancer sequence, where it appears to catalyze an ATP-depen-dent isomerization of the closed to open forms of the transcription initiation complex (Eq. 28-1).153/154 The isomerization may depend upon looping.152 Other operons that utilize the oN subunit of RNAP often also have upstream or downstream enhancers.155156... 

Biphospholenes have also been prepared. The [4+2] dimerization of transient 2,5-diphenyl-5-//-phosphole yields exclusively the P-P dimer 201, which reacts with Mel to give the monoquaternized product 202 (Scheme 63) <20030M1356>. The nucleophilic attack of EtO induces the cleavage of the P-P bond with formation of the / -bridged bisphospholene 203 (Scheme 63). This compound acts as a 1,4-chelate toward transition metals <20030M1356>. 

Figure 13.15. Inhibition of mitosis by vincristine, a The mitotic spindle consists of tubulin fibers, which are helical polymers of a,P tubulin dimers, b Structure of vincristine, an inhibitor of tubulin polymerization, c Mode of action of vincristine. The drug binds to an a,P dimer. These dimers retain the ability to associate with growing filaments however, the further attachment of dimers is inhibited. At high concentrations, vincristin-associated tubulin is also sequestrated within mis-assembled polymers.

Colchicine and compounds that bind in the vicinity of the colchicine-binding domain of tubulin comprise another class of dmgs with potential for treatment of cancer. Colchicine binds to tubulin at a P dimer interface, and acts by being incorporated with low stoichiometry at the ends of the microtubules as a tubulin-colchicine complex (39). Although colchicine has... 

The three-dimensional structure of hemoglobin is best described as a pair of identical a P dimers (a i P i and (X 2 P 2)... 

How does oxygen binding lead to the structural transition from the T state to the R state When the iron ion moves into the plane of the porphyrin, the histidine residue bound in the fifth coordination site moves with it. This histidine residue is part of an a helix, which also moves (Figure 10.22). The carboxyl terminal end of this a helix lies in the interface between the two a P dimers. Consequently, the structural transition at the iron ion is directly transmitted to the other subunits. The rearrangement of the dimer interface provides a pathway for communication between subunits, enabling the cooperative binding of oxygen. 

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