Effect cooperative

3 Cooperative Effect. - The ab initio study of Parra and Zeng194 focused on the cooperative effect in mixed dimers and trimers of methanol and trifluoro-methanol. Cooperativity is the enhancement of of the dipole moment in a HB complex compared to the sum of the dipole moments of the constituents. The authors interpreted the value of pb for the C-O and O-H bonds in four dimers and four trimers in terms of bond strength. A weakening of the O-H bond is seen to be favourable for hydrogen bonding. Another indication of cooperativity is noticed in the strengthening of the HB in the trimer as opposed to the dimer. Their AIM observations provide a consistent picture in support of the cooperative effect. 

Another study which used AIM in the context of the cooperative effect was performed by Masella and Flament.195 In their ab initio computations at the MP2 level on five dimers and five cyclic trimers, drawn from water, ammonia, and formaldehyde, they evaluated the density only at the HB BCPs. The authors mainly use AIM to detect HBs but do not characterise them via topological properties. Instead the cooperative effect is described in terms of geometry 

6 Cooperative Effect. - Gonzalez et studied the structure and the relative stability of the ethanol dimer and the cyclic ethanol trimer using DFT. Cooperative effects are reflected in the electron density of the BCPs. 

Rincon et investigated the energetic, structural, electronic and thermodynamical properties of (HF) , in the range n = 2-8, by ab initio methods and DFT. An AIM analysis reveals a linear correlation between the binding energy per HB and the density at the BCP and a covalent bond order. Based on these correlations HB cooperativity is associated with the electronic delocalization between monomers units. 

In accordance with Eq. (3.4) or Eq. (3.6), the concentration selectivity of ion exchange is variable depending on the degree of ideality of the solution and CP phase. For dilute solutions at a constant ionic strength, it is possible to take into account as a variable only the degree of non-ideality of the CP phase. For the systems considered here, it is convenient to study the effect of the molar fraction of organic counterions (NJ on the concentration selectivity constant. Fig. 14 shows the dependences of Ks on the molar fraction of oxytetracycline in CP. For CP 

For the quantitative description of the cooperative process in the macromolecule-low molecular weight ligand systems, Hill s equation is used. It expresses the dependence of the degree of macromolecule saturation with the ligand (Y) on the equilibrium concentration of the ligand in solution [67]  

The degree of saturation of carboxylic CP with protein (Y) is determined by the ratio of the amount of protein bonded under these conditions (at a predetermined concentration in solution) to the maximum amount Y = m/M. In this case, Hill s equation becomes 

The assumption of the association of Hb in the pores of carboxylic cation exchangers has been advanced in Ref. [47] on the basis of electron microscopy at the maximum filling, almost all the pore surface is filled with Hb associates which are ordered star-shaped structures. Interprotein interaction in the adsorption immobilization of enzymes have been reported in Refs. [74, 75]. 

By using Hill s coefficient, it is possible to draw a conclusion about the character of the process and to determine ligand concentration in one cooperative unit. 

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P, J L Finney, J D Nicholas and J E Quinn 1979. Cooperative Effects in Simulated Water. Nature 2 459-464. 

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. 

These calculations lend theoretical support to the view arrived at earlier on phenomenological grounds, that adsorption in pores of molecular dimensions is sufficiently different from that in coarser pores to justify their assignment to a separate category as micropores. The calculations further indicate that the upper limit of size at which a pore begins to function as a micropore depends on the diameter a of the adsorbate molecule for slit-like pores this limit will lie at a width around I-So, but for pores which approximate to the cylindrical model it lies at a pore diameter around 2 5(t. The exact value of the limit will of course depend on the actual shape of the pore, and may well be raised by cooperative effects. 

In the higher pressure sub-region, which may be extended to relative pressure up to 01 to 0-2, the enhancement of the interaction energy and of the enthalpy of adsorption is relatively small, and the increased adsorption is now the result of a cooperative effect. The nature of this secondary process may be appreciated from the simplified model of a slit in Fig. 4.33. Once a monolayer has been formed on the walls, then if molecules (1) and (2) happen to condense opposite one another, the probability that (3) will condense is increased. The increased residence time of (1), (2) and (3) will promote the condensation of (4) and of still further molecules. Because of the cooperative nature of the mechanism, the separate stages occur in such rapid succession that in effect they constitute a single process. The model is necessarily very crude and the details for any particular pore will depend on the pore geometry. 

The treatment of electrostatics and dielectric effects in molecular mechanics calculations necessary for redox property calculations can be divided into two issues electronic polarization contributions to the dielectric response and reorientational polarization contributions to the dielectric response. Without reorientation, the electronic polarization contribution to e is 2 for the types of atoms found in biological systems. The reorientational contribution is due to the reorientation of polar groups by charges. In the protein, the reorientation is restricted by the bonding between the polar groups, whereas in water the reorientation is enhanced owing to cooperative effects of the freely rotating solvent molecules. 

When two antioxidants are used together, a synergistic improvement in activity usually results. Synergism can arise from three combinations (1) homosynergism — two chemically similar antioxidants (for instance, two hindered phenols) (2) autosynergism — two different antioxidants functions that are present in the same molecule (3) heterosynergism — the cooperative effect between mechanistically different classes of antioxidants, such as the combined effect of primary and secondary antioxidants. Thus, combinations of phenols and phosphites are widely used to stabilize synthetic rubbers. 

Metal basicity and cooperative effects in the reactions of dinuclear pyrazolato rhodium complexes 98PAC779. 

Synergism can also arise from cooperative effects between mechanistically different classes of antioxidants, e.g., the chain breaking antioxidants and peroxide decomposers (heterosynergism) [42]. For example, the synergism between hindered phenols (CB—D) and phosphites or sulphides (PD) is particularly important in thermal oxidation (Table 2). Similarly, effective synergism is achieved between metal dithiolates (PD) and UV-ab-sorbers (e.g., UV 531), as well as between HALS and UV-absorbers, (Table 3). 

The operational model allows simulation of cellular response from receptor activation. In some cases, there may be cooperative effects in the stimulus-response cascades translating activation of receptor to tissue response. This can cause the resulting concentration-response curve to have a Hill coefficient different from unity. In general, there is a standard method for doing this namely, reexpressing the receptor occupancy and/or activation expression (defined by the particular molecular model of receptor function) in terms of the operational model with Hill coefficient not equal to unity. The operational model utilizes the concentration of response-producing receptor as the substrate for a Michaelis-Menten type of reaction, given as... 

Fig. 15. Cooperative effect of bonding of organic ions on Biocarb-T biosorbent A. 0.1 N NaCl in water B, 0.1 N NaCl in methanol. For l) novocain, 2) oleandomycin and 3) oxytetracycline...

Cooperative effects are of considerable interest for high capacity chromatography of BAS, since for practical purposes high-selectivity bonding is possible only in cooperative processes. This is very important for carrying out the sorption, separation and concentration of BAS. 

Cooperative effects in the electronic spectra of inorganic solids. P. Day, Inorg. Chim. Acta, Rev., 1969, 3, 81-97 (84). 

It has been pointed out321-324 that the two groups of solvents differ by some definite structural features. In particular, ED, 1,2-BD, and 1,3-BD possess vicinal OH groups that can form intramolecular hydrogen bonds. For these solvents, the ability of the organic molecule to interact with neighboring molecules is reduced. This results in the possibility of a different orientation at the interface because of different interactions of the OH groups with the Hg surface.323 The different molecular structure leads to different dipolar cooperative effects. As a result, the dependence of C on the bulk permittivity follows two different linear dependencies. 

The resulting newly synthesized Cro protein also binds to the operator region as a dimer, but its order of preference is opposite to that of repressor (Figure 39—7). That is, Cro binds most t hdy to Or3, but there is no cooperative effect of Cro at 0 3 on the binding of Cro to 0 2. At increasingly higher concentrations of Cro, the protein will bind to Op 2 and evenmally to OrI. 

The magnetic properties of electrons arise from a property called spin, which we describe in more detail in Chapter 8. All electrons have spin of the same magnitude, but electron spin can respond to a magnet in two different ways. Most magnetic effects associated with atoms are caused by the spins of their electrons. Iron and nickel are permanent magnets because of the cooperative effect of many electrons. 

Promotional effects of sulfide can evidently be explained, because exposure of reduced metals Is Increased on reduced sulfided catalysts. The role of cobalt Is less clear. It Is normally not fully reduced. It apparently does not promote greater exposure of Mo In any form detected, either In the presence or absence of sulfide. On the contrary. It evidently only decreases the concentration of exposed Mo atoms, although, at concentrations typically used, most. Mo atoms are unaffected by Co. Either some property of Co alone or some local cooperative effect of adjacent Co and Mo must explain promotion. Simple mechanical mixtures will not give the synergism observed, however (1-4). 

The above comparison indicates that the rate acceleration induced by CD is more pronounced than that of the other tertiary nitrogen bases. This fact also indicates that in CD a cooperative effect should exist between the quinuclidine nitrogen and the quinoline ring, The cooperative effect is in force if the modifier is in a shielded form. 

As a increases, the average distance between ionized groups decreases so that these neighbouring groups begin to have an effect. When a exceeds 0-3, individual water spheres begin to overlap and eventually coalesce into a cylindrical form. With further increases in a, a second outer cylindrical sheath of water appears in which water molecules are oriented by the cooperative effect of two or more carboxyl groups. 

Similar results were found in a study of aromatic carboxylates with one to six carboxyl groups (Scott, Jackson Wilson, 1990). Adsorption increased with the number of carboxyl groups and was also dependent on the spacing between the carboxyl groups. With the benzene dicarboxylates, maximum permanent adsorption was obtained with the 1,3-dicarboxylate, while the 1,4-dicarboxylates was not adsorbed at all. This is again evidence of the cooperative effect between carboxyl groups. 

In this system there is a useful cooperative effect between aliuninium, fluoride and calcium, which has been demonstrated by the solution studies of Ellis Wilson (1987). In the absence of aluminium, calcium precipitates as the fluoride at all pHs. Aluminium has the effect of preventing the precipitation of calcium as fluoride, again because it forms strong soluble complexes with fluoride. 

Very recently Chen and co-workers have applied the previously mentioned Ni-based dimetallic pre-catalyst 14 in the Negishi reaction. Remarkable results were obtained even when unactivated aryl chlorides were chosen as reaction partners providing an alternative to the more expensive Pd-based catalysts. The fact that dinuclear pre-catalyst 14 is more active than its mononuclear analogue 13 indicates a possible cooperative effect between the two metal centres [86] (Scheme 6.23). 

It is concluded that the cooperative effect observed is of long-range nature and therefore of elastic rather than of electronic origin. Recently, the additional suggestion has been made [138] that, due to intermolecular interactions in the crystal environment of [Fe(ptz)g](BF4)2, domains of iron(II) complexes interconvert together. The observed kinetics would then correspond to a first- or higher-order phase transition rather than to the kinetics which are characteristic for the conversion of isolated molecules. 

Gonzales, L., Mo, O., Yanez, M., Elguerdo, J., 1996, Cooperative Effects in Water Trimers. The Performance of Density Functional Approaches , J. Mol. Struct. (Theochem), 371, 1. 

A plot of the reciprocal of the measured susceptibility 1 /imol v.v. T is a straight line with slope 1/C, and which crosses the abscissa at T = 0 (Fig. 19.6). For 0 = 0 the equation is simplified to the classic Curie law mol = C/T. Generally, values of 0 (j arc found when cooperative effects arise at low temperatures (ferro-, ferri- or antiferromagnetism). The straight line then has to be extrapolated from high to low temperatures (dashed lines in Fig. 19.6). 

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