Ligands cubic

While the extended ternary complex model accounts for the presence of constitutive receptor activity in the absence of ligands, it is thermodynamically incomplete from the standpoint of the interaction of receptor and G-protein species. Specifically, it must be possible from a thermodynamic point of view for the inactive state receptor (ligand bound and unbound) to interact with G-proteins. The cubic ternary complex model accommodates this possibility [23-25]. From a practical point of view, it allows for the potential of receptors (whether unbound or bound by inverse agonists) to sequester G-proteins into a nonsignaling state. 

There are some specific differences between the cubic and extended ternary complex models in terms of predictions of system and drug behavior. The first is that the receptor, either ligand bound or not bound, can form a complex with the G-protein and that this complex need not signal (i.e., [ARiG] and [RjG]). Under these circumstances an inverse agonist (one that stabilizes the inactive state of the receptor) theoretically can form inactive ternary complexes and thus sequester G-proteins away from signaling pathways. There is evidence that this can occur with cannabi-noid receptor [26]. The cubic ternary complex model also... 

The cubic ternary complex model takes into the account the fact that both the active and inactive receptor species must have a finite affinity for G-proteins [26-28], The two receptor species are denoted [Ra] (active state receptor able to activate G-proteins) and [RJ (inactive state receptors). These can form species [R,G] and [RaG] spontaneously, and species [ARiG] and [ARaG] in the presence of ligand. 

Cubic ternary complex model, a molecular model (J. Their. Biol 178, 151-167, 1996a 178, 169-182, 1996b 181, 381-397, 1996c) describing the coexistence of two receptor states that can interact with both G-proteins and ligands. The receptor/G-protein complexes may or may not produce a physiological response see Chapter 3.11. 

First, consider an octahedral nickel(ii) complex. The strong-field ground configuration is 2g g- The repulsive interaction between the filled 2g subshell and the six octahedrally disposed bonds is cubically isotropic. That is to say, interactions between the t2g electrons and the bonding electrons are the same with respect to x, y and z directions. The same is true of the interactions between the six ligands and the exactly half-full gg subset. So, while the d electrons in octahedrally coordinated nickel(ii) complexes will repel all bonding electrons, no differentiation between bonds is to be expected. Octahedral d coordination, per se, is stable in this regard. 

Halet )-F, Saillard )-Y (1997) Electron Count Versus Structural Arrangement in Clusters Based on a Cubic Transition Metal Core with Bridging Main Group Elements. 87 81-110 Hall DI, Ling JH, Nyholm RS (1973) Metal Complexes of Chelating Olefin-Group V Ligands. 15 3-51... 

Most complexes showing spin-state transitions are in fact of low symmetry. In order to describe their electronic structure it is convenient to employ term symbols appropriate to cubic symmetry and this practice will be followed below. The most common transition-metal ions for which spin-state transitions have been observed are Fe " (3d ), Fe " (3d ) and Co (3d ), a minor role being played by Co " (3d ), Mn " (3d ), as well as Cr " and Mn " (3d ). The relevant ground states for an octahedral disposition of the ligands are LS Ui,(t ,) and HS r2,(t ,e ) for iron(II), LS and HS Ai,(t, e ) for... 

Figure 2 Representation of a "cubic ternary complex" model of allosteric interaction R, the inactive state of the receptor R, the active state of the receptor A, ligand X, allosteric agent. (From Ref. 14.)...

NIR region (see Chapter 9.13), complexes of 1,2-dithiolene (DT) and related ligands have attracted considerable attention for their (largely cubic) NLO properties. The complex (156) (a.k.a. BDN) is a highly photochemically stable, saturable absorber and has hence found extensive applications in laser Q-switching. The cubic NLO properties of (156) have been studied by DFWM148,403-407 and more recently, Z-scan.408 Time-resolved DFWM has been applied to square planar Co, Ni, Cu, or Pt complexes of 1,2-benzenedithiolate (BDT) or 1,2-aminobenzenethiolate ligands by Lindle and co-workers.409,410... 

Winter, Underhill, and co-workers have published extensively on the cubic NLO properties of complexes of DT and related ligands,411 22 particularly those containing formally Ni11 centers. For example, time-resolved 1,064 nm DFWM was used to obtain resonantly enhanced values for group 10 complexes such as (157).411 15 The smaller of (157) compared with (156) is largely due to resonance effects since the absorption maximum of (157) is somewhat removed from the laser fundamental. However, figures of merit derived from measurements of 2 and linear and two-photon absorption (TPA) coefficients show that low optical losses render complexes such as (157) superior to (156)413 for potential all-optical switching applications.411 14... 

This suggests that the g-strain in EPR spectra contains detailed geometric and energetic information on the 3-D position of ligand atoms for a given metalloprotein conformation with respect to a (virtual) cubic coordination, and therefore, of position and interconversion energy (by strain) of the 3-D position of ligand atoms in, say, two different protein conformations. This is an unexplored area of research. 

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