Anticooperativity

If < 1, then binding is anticooperative, for example when an electrically charged particle adsorbs at an initially neutral surface the accumulated charge repels subsequent arrivals and makes their incorjDoration more difficult [58]. 

An anticooperative mode of interactions was assumed in case of concave-shaped Scatchard plots, as alrea% proposed by other authors (Mattai Kwak, 1986 Gamier et al, 1994). A convexe curvature of the plots indicated a cooperative binding process (figure 4). 

An important consequence of quantal charge transfer between ions and ion pairs (dipoles) is the appearance of non-pairwise-additive cooperative or anticooperative contributions that have no counterpart in the classical theory. These nonlinear effects strongly stabilize closed-CT systems in which each site is balanced with respect to charge transfers in and out of the site, and disfavor open-CT systems in which one or more sites serves as an uncompensated donor or acceptor. This CT cooperativity accounts for the surprising stability of cyclic (LiF) clusters, which are strongly favored compared with linear structures. 

Another such instance of anticooperativity can be seen by comparing 5 with 13. In the latter case, the two n— -nco interactions make competitive use of the same busy nitrogen lone pair, and each such interaction (47.9 kcal mol-1) is thereby weakened relative to the value (59.8 kcal mol-1) in 5. Similarly, the comparison of vinylborane 6 with divinylborane 14 reveals an anticooperative effect involving competition for the ns pi acceptor, with each 7tcc—>-113 interaction in 14 (24.2 kcal mol-1) being slightly weakened compared with its value in 6 (26.2 kcal mol-1). 

The weaker delocalization in the latter case would be expected from the anticooperative nature of competitive interactions with two donors.) These interactions thus exhibit forms and magnitudes that are generally similar to those found in smaller boron hydrides. 

The interactions will be described as cooperative or anticooperative (for the chosen configuration) according to whether the deviations are positive or negative,... 

Figure 5.26 (a) The isomeric anticooperative open (HF)3 structure (fully optimized), and (b) the leading np interaction with one of the two equivalent Lewis-acid monomers (with the second-order stabilization energy in parentheses). The net binding energy is 7.92 kcal mol-1. 

Figure 5.30 Alternative water pentamer isomers having partial anticooperativity, (a)-(c), or higher coordination and ring strain, (d). Labels in (a)-(c) correspond to clockwise monomer numbering from the top (see the text). (Species (a)-(c) have been optimized under the constraint of planar equilateral skeletal geometry to prevent rearrangement to Wsc [Fig. 5.29(a)] and are therefore only near-stationary points on the potential-energy surface.)...

Figure 1. Illustration of lone electron pair preferences in alcohol dimers, cooperative and anticooperative binding sites for a third monomer, ring strain and steric repulsion in alcohol trimers, alternation of residues in alcohol tetramers, and chain, branch, and cyclic hydrogen bond topologies in larger clusters.

In some earlier publications the term cooperativity is used for positive cooperativity and anticoop-erativity is used for negative cooperativity. In this book cooperativity is used whenever g 1. Sometimes, when there is positive cooperativity one says that a ligand at a supports or favors the binding of a ligand at b, and vice versa. 

Cooperative and Anticooperative Energy Effects in Systems with Classical Hydrogen Bonds... 

In contrast to those noted above, theoretical investigations of branched complexes in which two or more hydrogen bonds are formed by one proton-acceptor group, as in Structure 2.4, predict an inverse effect. In this case, mutual polarization weakens the hydrogen bonds, leading to anticooperative effects. The various factors governing the influence of the first hydrogen bond on the formation of... 

Hydrogen bonds can be cooperative or anticooperative based on their mutual polarization. 

One of the important aspect of the use of dihydrogen bonds as driving forces in molecular associations is their cooperativity or anticooperativity when increasing the number of H- H contacts in the self-association of molecular systems leads to additional energy, calculated on the basis of one dihydrogen bond, or vice versa. As we have shown in Chapter 2, classical hydrogen bonds can be highly cooperative, due to their mutual polarization [5]. 

Binding of additional ligands to a small substrate is often hindered for steric reasons (anticooperative binding). In most of the protein associations, however, the first association step favors the following binding steps. This behavior, called cooperative binding, leads to the formation of more highly coordinated species in the analyte. 

Bowser and Chen (10) have calculated some theoretical binding isotherms (/z - /zs = /([L]) for anticooperative, noncooperative, and cooperative complex formation at two equivalent binding sites with arbitrarily chosen microscopic constants see Table 1. 

A smaller secondary microscopic association constant compared to the first binding constant (k, = k2 > k2, = kl2) results in anticooperative binding behavior in cases A and B. If k, = k2 < k2i = kl2, as in cases D and E, the cooperative binding yields a higher amount of higher-order complexes. 

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