Synergism, description

Ac-Phc-Lys-OH, enzymatic synthesis, 156f Acesulfame K, stnicture, 7f Acetoin, in reactions of aldehydes with ammonium sulfide, 36-54 Acid chelators, 57-58 Acid synergism, description, 58 Acidic amino acid peptides, taste,... 

In many of their complexes PF3 and PPI13 (for example) resemble CO (p. 926) and this at one time encouraged the belief that their bonding capabilities were influenced not only by the factors (p. 198) which affect the stability of the a P M interaction which uses the lone-pair of elecU"ons on p and a vacant orbital on M, but also by the possibility of synergic n back-donation from a nonbonding d , pair of electrons on the metal into a vacant 3d , orbital on P. It is, however, not clear to what extent, if any, the a and n bonds reinforce each other, and more recent descriptions are based on an MO approach which uses all (cr and n) orbitals of appropriate symmeU"y on both the phosphine and the metal-containing moiety. To the extent that a and n bonding effects on the stability of metal-phosphorus bonds can be isolated from each otlier and from steric factors (see below) the accepted sequence of effects is as follows ... 

In the following, several terms used to describe interactions between chemicals are mentioned as well as basic concepts used in the hazard and risk assessment of chemical mixmres. The description of these basic concepts, first outlined by Bliss (1939) and Placket and Hewlett (1952), are based on the publications by Konemann and Pieters (1996), Cassee et al. (1998), and Groten et al. (2001). The definitions of additivity, synergism, antagonism, and potentiation are those of Klaassen (1995) and Seed et al. (1995). 

Although testing of the whole mixture as such seems to be the proper way to approach the risk assessment of exposure to that mixture, it will not provide data on combined actions and/or interactions between the individual components of the mixture. Even if the effect of the mixture is compared with the effects of each individual component at comparable concentrations, this will not allow a description of potential synergism, potentiation, or antagonism, and it is even doubtful that deviations from additivity can be concluded. This can only be achieved if dose-response curves are obtained for each of the single compounds. 

An alluring field of research is the mechanism of action of oxidation inhibitors. This research will undoubtedly yield in the near future a theory for inhibition of undesirable oxidation processes. The relatively stable free radicals observed on such inhibition display extremely interesting properties. Of great interest are the effects of synergism, of inhibitor mixtures, and of mixtures of inhibitors with catalysts. A strictly quantitative and elegant description of all these phenomena may be made within the scope of the chain theory for slow oxidation. 

There are three current theories on sulfiding the CoMo/Al catalyst monolayer, intercalation, and contact synergism. These are schematically depicted in Fig. 4. All start with the assumption of a monolayer model for the oxidized catalyst, differences appearing in the subsequent effect that sulfiding has on the monolayer structure. Space limitations preclude a full description of these models the original references should be consulted for more detail. 

Studies on the solvent extraction of actinide ions by different combinations of extractants have been reviewed. Various equilibria involved in the extraction processes and the formation of the extract-able complexes have been considered along with their equilibrium constant data. Various methods which are useful in establishing the composition and the nature of the extractable complexes are presented. The data on isolation and structural studies of some complexes, involved in synergic extraction, are also included. A brief description of the different areas in which synergic extraction is finding application is also given. Many combinations of extractants, where the studies conducted are very few but, which are likely to yield enhanced extractions are indicated. Areas of research, both from the academic and applied points of view, which require attention are suggested. 

We must admit that our repeated use of the expression "aesthetically pleasing" is something of a descriptive cop-out, for we do not know exactly what causes this synergism among olfactory materials. Nor is it easy to describe, but materials working together in a finely balanced accord seem to produce an olfactory resonance as harmonically satisfying to our sense of smell as the sound of a perfectly balanced orchestra is to our ears. 

A key feature of alkene coordination is the loss of planar symmetry. Indeed, it is this loss of symmetry that allows the vc=c IR absorption of alkene complexes to be observed (IR inactive for the free molecule). Consistent with the synergic bonding description, coordination leads to a decrease in the C=C bond strength and accordingly a decrease in the value of vc=c. The case of prochiral alkenes is of particular importance,... 

A covalent bonding model is needed to account for the effects of Jt-donor or 7i-acceptor ligands and synergic bonding. A molecular orbital description provides a more universal description of bonding within transtion metal complexes. 

These examples of the limitations of bioassay methodology show that comparative interpretations of data from various sources can be confused and so lead to ill-reasoned conclusions of both the extent and description of resistance and synergism. As Yamamoto (1973) wrote. Synergistic effect is greatly influenced by the lest insect, formulation, pyrethroid-synergist ratio, method of administration and choice of response, cither kill or knockdown these experimental differences complicate [the] interpretation of results/ However, it is only on the basis of sound toxicological evidence that any theories concerning the mode of action of PBO, either alone or when in combination with another compound, must be founded. 

In the literature, there is another widespread description of synergism dealing with the degree of surface coverage (0) of individual inhibitors. The synergism index Sq is defined as follows... 

In general, the efficiency of an inhibitor increases with an increase in inhibitor concentration (e.g., a typically good inhibitor would give 95 percent inhibition at a concentration of0.008 percent and 90 percent at a concentration of 0.004 jjercent). There is often some positive synergism between different inhibitors and therefore mixtures are commonly chosen in commercial formulations. The scientific and technical corrosion literature has descriptions and lists of numerous chemical compounds that exhibit inhibitive properties. Of these, only very few are actually used in practice. This is partly because the desirable properties of an inhibitor usually extend beyond those simply related to metal protection. Considerations of cost, toxicity, availability, and environmental friendliness are of considerable importance. 

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