Cooperativity/cooperative effect positive

Copper(I) tends towards a tetrahedral coordination geometry in complexes. With 2,2 -bipyr-idine as a chelate ligand a distorted tetrahedral coordination with almost orthogonal ligands results. 2,2 -Bipyridine oligomers with flexible 6,6 -links therefore form double helices with two 2,2 -bipyridine units per copper(I) ion (J. M. Lehn, 1987,1988). J. M. Lehn (1990 U. Koert, 1990) has also prepared such helicates with nucleosides, e.g., thymidine, covalently attached to suitable spacers to obtain water-soluble double helix complexes, so-called inverted DNA , with internal positive charges and external nucleic bases. Cooperative effects lead preferentially to two identical strands in these helicates when copper(I) ions are added to a mixture of two different homooligomers. 

Both secondary active transport and positive cooperativity effects enhance carrier-mediated solute flux, in contrast to negative cooperativity and inhibition phenomena, which depress this flux. Most secondary active transport in intestinal epithelia is driven by transmembrane ion gradients in which an inorganic cation is cotransported with the solute (usually a nutrient or inorganic anion). Carriers which translocate more than one solute species in the same direction across the membrane are referred to as cotransporters. Carriers which translocate different solutes in opposite directions across the membrane are called countertransporters or exchangers (Figs. 10 and 11). 

On silica, the rate was lower than in the case of the free complex. The zeolite, however, shows a cooperative effect on the rate and on enantioselectivity of the reaction. The positive effect on the rate can be related to the strong capacity of the zeolite to adsorb H2. These zeolite-anchored complexes can also be recycled without loss of activity. 

Frumkin isotherms for a few different values of the interaction parameter g. Positive values of g broaden the isotherm because the adsorbed particles repel each other for negative values of g the isotherms are narrow because adsorption is then a cooperative effect. The case g = 0 corresponds to the Langmuir isotherm. 

The bisanthraceno-crown ether E-l (Figure 10.26) exhibits a fluorescence spectrum composed of the characteristic monomer and excimer bands. Gradual addition of sodium perchlorate to a solution in methanol induces a decrease in the monomer band and an increase in the excimer band. Complexation is indeed expected to bring closer together the two anthracene units, which favors excimer formation. A 2 1 (metabligand) complex is formed with Na+ in methanol and acetonitrile with a positive cooperative effect (see Appendix B). Interestingly, the overall stability constant obtained from absorption data was found to be lower than that... 

The fact that ATP and CTP bind to the same site follows from the observation that adding ATP to the inhibited enzyme by CTP reduces or reverses the inhibition, presumably because ATP competes with CTP for the same site. The fact that CTP binds to an allosteric site (i.e., it is not a competitive inhibitor) follows from the so-called desensitization effect. Addition of mercurials [e.g., p-mercuribenzoate (PMB)] reduces or eliminates the inhibition by CTP. However, it has no effect on the enzymatic activity of ATCase, presumably because the mercurials affect the regulatory subunits but not the catalytic site. As for the mechanism of cooperativity (both positive and negative), it is known that CTP does induce changes in the quaternary structure of the enzyme. 

A positive cooperative effect of formamidine and acetic acid is possible because we have found that acetic acid acts as a promotor in various types of hydrogenations. 

The remarkable protonation features of 21 led to the formulation of the diproto-nated species as the water cryptate, [H20 cr 21, 2H+] 25, in which the water molecule accepts two +N-H---0 bonds from the protonated nitrogens and donates two 0-H---N bonds to the unprotonated ones [2.17, 2.96], The second protonation of 21 is facilitated by the substrate it represents a positive cooperativity effect, mediated by H20, in which the first proton and the effector molecule water set the stage both structurally and energetically for the fixation of a second proton. When 21 is tetraprotonated it forms the chloride cryptate cryptate [Cl c 21,4H+] 26, in which the included anion is bound by four +N-H---X- hydrogen bonds [2.97] (see also Chapt. 3). 

Cooperative effects in which, for example, a reagent is activated by coordination to one metal center and then induced to react with a ligand on an adjacent metal center or in a bridging position can occur. A search for such model reactions has been initiated. 

One molecule of 391 displays a positive cooperative effect in binding two sodium cations when 391 is treated with sodium perchlorate, whereas 391 generates a 1 1 complex with K+ [360], The photocyclomerization was demonstrated... 

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