Co-operative effect

Antioxidants are today often used in combination. One reason for this is that certain combinations can exhibit in a polymer an effect sometimes found with other additives, that known as synergism. In these circumstances two antioxidants may exhibit a co-operative effect and be more powerful than either antioxidant used alone, even when the total antioxidant concentration is the same. Certain terms are sometimes encountered here and these are defined in Table 7.6. 

Additional macrocyclic polyether groups show no co-operative effect when the polyether rings diverge. For instance, the bis(crown ethers) [58] and [59]... 

Co-operative effects of crown units are particularly pronounced in poly(crown ethers) such as [62] (Kimura et al., 1978) and [63] (Kopolow etal,... 

Catalyst 329, prepared from trimethylaluminum and 3,3/-bis(triphenylsily 1)-1,1 /-bi-2-naphthol, allowed the preparation of the endo cycloadduct (2S )-327 with 67% ee. The use of non-polar solvents raised the ee, but lowered the chemical yield213. Recently, it was reported that the reaction to form 327 exhibited autoinduction when mediated by catalyst 326214. This was attributed to a co-operative interaction of the cycloadduct with the catalyst, generating a more selective catalytic species. A wide variety of carbonyl ligands were tested for their co-operative effect on enantioselectivity. Sterically crowded aldehydes such as pivaldehyde provided the best results. Surprisingly, 1,3-dicarbonyl compounds were even more effective than monocarbonyl compounds. The asymmetric induction increased from 82 to 92% ee when di(l-adamantyl)-2,2-dimethylmalonate was added while at the same time the reaction temperature was allowed to increase by 80 °C, from -80 °C to 0°C. 

A second appealing feature of tube model theories is that they provide a natural hierarchy of effects which one can incorporate or ignore at will in a calculation, depending on the accuracy desired. We will see how, in the case of linear polymers, bare reptation in a fixed tube provides a first-order calculation more accurate levels of the theory may incorporate the co-operative effects of constraint release and further refinements such as path-length fluctuation via the Rouse modes of the chains. 

L-monomer. The observed effect was, however, even greater than anticipated. In fact, Idelson and Blout (50), who repeated these experiments in sodium methoxide initiated polymerisation that yields a high-molecular weight polypeptide, found enormous effects which are shown in Fig. 18. Addition of even 5% of D-isomer to an L-monomer reduced the polymerisation rate by a factor of 3, and the 50 50 mixture polymerised 17-times more slowly than either isomer. The co-operative effect of this... 

The ability to bind to DNA was studied by ethidium bromide displacement titrations. The phosphodiester backbone of DNA was fully stable in the presence of the coordination compounds. The apparent binding constants were dependent on the charge of the macrocycles. Apparent binding constants of the metal complexes were even stronger and co-operative effect could be observed for the dinuclear ligands. 

Sharonov, Yu. A., andM. V. Volkenstein Co-operative effects in the annealing and softening range of polyvinyl acetate. Vysokomol. Soed. 4, 917—921... 

In a crystal of polar molecules it may well be as easy to reverse a molecule in its site as to form any other defect. This reversal will cost less in energy the more disorder is already present, and this co-operative effect may give rise to a dear order-disorder transition. The ordered phase may in particular be ferro- or antiferro-electric. The ferroelectric transition in thiourea is essentially of this nature. 

Thus there is evidence from MO theory that a pseudo-co-operative effect is active in water hydrogen-bonding bonds become stronger when more molecules are added to the chain. This is not to say that all bonds are of equal strength in a chain in fact it is very likely that they are not equal. This strengthening of hydrogen-bonds is evidence that chains may be present in liquid water the higher the temperature, the shorter the chains. 

This is an example of a co-operative phenomenon in which the energy needed to carry out an elementary process depends upon the state of the whole system. To take account of this co-operative effect, let us make the simple assumption that the energy needed to exchange a pair of unlike atoms is given by... 

Hydrogen bonds and intramolecular interactions (electrostatic, van der Waals) stabilize the native structure of the protein in a co-operative manner. Upon denaturation, the co-operative effect is lost, resulting in tmfolding of the molecule and exposure of the inner hydrophobic core to the hydrophilic aqueous environment. For small globular proteins, denaturation is an almost all-or-none... 

Photochemical processes in heterogeneous systems, and across micelle boundaries in particular, clearly has great potential. The photolysis of amphipathic alkylcobaloximes in mixed micelles shows a co-operative effect owing to structure.The photoreduction of anthraquinone in aqueous micellar solution has been compared with that in non-aqueous solution.The dimerization of 3-(n-butyl)cyclopentenone is solvent-dependent and the ratio of isomeric products depends on surfactant concentration. It is suggested that this can be used as a means of critical micellar concentration determination. 

Figure 17.22b shows the result obtained by implementing the controllers Q2-li and D4-I3, where only the P component has been considered, with the scaled gains from the CLDG plot. The impurities Ij and I3 can be kept between bounds. The interactions enable to keep equally the impurity I2 between bounds, such as its control is not needed. Hence, the use of only two PI controllers to control three impurities is possible, because it takes profit from the co-operative effect of interactions. Note that using manipulated variables from different units is not a current practice. However, the principle of... 

In a pick-up mechanism, because of the weaker interactions between the monomer and the template, there is no preferential adsorption and polymerisation starts in the bulk. When the growing oligomer has reached a critical length the co-operative effects allow for the adsorption of the chain on the template and the polymerisation proceeds by adding monomer from solution onto the growing chain adsorbed on the template. Information on the mechanism may be gained from kinetic studies (75). 

The co-operative effect of 1,3-interrelated directing groups is a powerful strategy in synthesis. This allows the otherwise difficult preparation of 1,2,3-trisubstituted aromatic compounds. Hence the lithiation of the aromatic compound 129 is directed by both the methoxy and secondary amide groups (1.118). Two equivalents of the... 

The form of isotherm expressed by the Langmuir equation is shown by curve a in Fig. 33. Curve b is an isotherm for a system in which a strong co-operative effect exists between the adsorbed molecules. The field due to adsorbed molecules already on the surface contributes to the holding of fresh adherents. Such effects foreshadow liquefaction or crystallization, but sometimes only remotely. 

Even if temporary stereocontrol at C4 and acceptable yields could be achieved on protected 6-hydroxyethylpenicillanates by Nayler s or Denerley s protocol, for penem synthesis obtained thioethers 125 needed further elaboration, ultimately implicating loss of the penicillanic sulfur atom and new stereo-controlled chemistry at C4. These and other drawbacks were overcome by an innovative penam cleavage promoted by the co-operative effect of thiophilic metal salts and strong, non-nucleophilic bases, which was devised at Farmitalia-Carlo Erba by Alpegiani et al. [81]. 

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