Martin S. Westwell

Aspects of weak interactions

Weak interactions, such as those non-covalent interactions that occur in biological systems, are less well characterised than their strong, covalent counterparts. Here, we discuss associations between two or more molecules and consider the effect of interactions with solvent molecules (particularly water) and changes in the internal structure of the associating molecules on binding. We go on to discuss some of the progress that has been made in the estimation of binding constants.

Application of a generalised enthalpy–entropy relationship to binding co-operativity and weak associations in solution

Enthalpy–entropy compensations are a consequence of weak associations where the binding enthalpy is much lower than typical covalent bond strengths. The general form of an enthalpy–entropy curve is presented based upon theoretical considerations and justified on the basis of experimental data for associations in the gas phase and for monatomic sublimation. The intrinsic curvature of the enthalpy vs. entropy plot provides the basis for a description of co-operativity and binding phenomena in solution. In the case of cooperativity, we divide the mutual aiding of two interactions into two distinct parts, one of which is entropic in origin and related to the classical chelate enhancement of binding of Jencks; the other is an enthalpic benefit due to improved electrostatic bonding. Since the experimental Gibbs energy for all weak associations in solution is a consequence of competing solute–solvent, solvent–solvent and solute–solute interactions, the summation of these interactions can be usefully described by a vector analysis using the enthalpy–entropy curve. We illustrate the utility of this approach by presenting a qualitative description of the various contributions to binding in the following examples: (i) one-point associations in non-polar solvents, (ii) entropy-driven association of two large discs involving the release of multiple solvent molecules and (iii) enthalpy driven associations involving metal chelation by polyamines. By considering the curvature of the enthalpy–entropy plot for a given interaction, in combination with the possibility of making or breaking multiple interactions on one template, net enthalpies and entropies of association in solution can be explored in an approximate manner.

Cooperativity and anti-cooperativity between ligand binding and the dimerization of ristocetin A: asymmetry of a homodimer complex and implications for signal transduction

BACKGROUND Recent work has indicated that dimerization is important in the mode of action of the vancomycin group of glycopeptide antibiotics. NMR studies have shown that one member of this group, ristocetin A, forms an asymmetric dimer with two physically different binding sites for cell wall peptides. Ligand binding by ristocetin A and dimerization are slightly anti-cooperative. In contrast, for the other glycopeptide antibiotics of the vancomycin group that have been examined so far, binding of cell wall peptides and dimerization are cooperative. RESULTS Here we show that the two halves of the asymmetric homodimer formed by ristocetin A have different affinities for ligand binding. One of these sites is preferentially filled before the other, and binding to this site is cooperative with dimerization. Ligand binding to the other, less favored half of the dimer, is anti-cooperative with dimerization. CONCLUSIONS In dimer complexes, anti-cooperativity of dimerization upon ligand binding can be a result of asymmetry, in which two binding sites have different affinities for ligands. Such a system, in which one binding site is filled preferentially, may be a mechanism by which the cooperativity between ligand binding and dimerization is fine tuned and may thus have relevance to the control of signal transduction in biological systems.

Weak interactions and lessons from crystallization

Ideas derived from the study of the process of crystallization may provide insights into molecular recognition in biological systems. Both processes exploit the cooperativity which arises from the formation of a large array of weak interactions.

Enthalpic (electrostatic) contribution to the chelate effect: a correlation between ligand binding constant and a specific hydrogen bond strength in complexes of glycopeptide antibiotics with cell wall analogues

The 1H NMR chemical shift of amide protons in the binding pocket of glycopeptide antibiotics has been used to monitor the interaction of these amide protons with the carboxylate group of cell wall analogues and related ligands. A good correlation is observed between overall ligand binding energy (ΔG°) and amide NH chemical shift. We conclude that the strength of the electrostatic interaction of the carboxylate group, which is crucial to recognition and binding by the antibiotics, is cooperatively enhanced by adjacent functional groups on the same ligand template. Hydrogen bonding and burial of hydrocarbon in adjacent sites produce an enhancement of electrostatic binding of the carboxylate group. The data provide experimental evidence for an enthalpic contribution to the chelate effect that is distinct from, and works in addition to, the classic entropic chelate effect. The correlation between amide NH chemical shift and overall binding energy has been used to show binding affinity for eremomycin and chloroeremomcin by di-N-Ac-Lys-D-Ala-D-Lac (Lac = lactate), which is a cell wall analogue of bacteria which exhibit vancomycin resistance. Binding constants for this ligand have also been determined by UV difference spectrophotometry (70 dm3 mol–1 and 240 dm3 mol–1 respectively).

Cooperativity in ligand binding expressed at a model cell membrane by the vancomycin group antibiotics

Dimerisation or the use of a membrane anchor enhances the binding of the glycopeptide antibiotics at the surface of a model cell membrane.

Expression of electrostatic binding cooperativity in the recognition of cell-wall peptide analogues by vancomycin group antibiotics

The strength of the binding interaction of the carboxylate group of peptide cell-wall analogues with ristocetin A is shown, by 1H NMR studies of hydrogen bonded NHs, to increase towards a limiting value as the number of interactions on the same ligand template increases; the carboxylate electrostatic binding energy appears to reflect the whole set of linked weak interactions as a cooperative unit, and this cooperative effect is distinct from the classical chelate effect.

The ‘n’ effect in molecular recognition

The cooperativity which exists in crystal melting and many biological molecular recognition phenomena arises from extended arrays of weak interactions. We present a correlation between the melting temperature of a crystal and the intermolecular energy (which is evident only when compounds possessing several or many internal rotors are excluded). The correlation is used as the basis for a model of crystal melting which is capable of estimating the melting temperature of crystals. This model provides the basis for an understanding of the sharpness of the crystal melting transition for organic and inorganic substances. The cooperativity illustrated by the extended arrays of weak interactions, or the ‘n’ effect, is extended to analogous order/disorder transitions in biological systems, such as the ‘melting’ of DNA and RNA duplexes, providing insights into the physical properties of these structures.

Enhancement of Electrostatic Binding Through Cooperative Interactions: Enthalpy/Entropy Compensation and Peptide—Peptide Recognition

In recent work we have established a fundamental relationship between the cost in entropy of an association between two entities A and B, involving one- and two-point interactions in the gas phase, or in non-polar solvents, to give a complex A,B, as a function of the exothermicity of the interaction between them (1). While there is a limit to the adverse entropy of a bimolecular association in terms of the loss of translational and overall rotational freedom (ca 50 to 60 kJ/mol for TΔS° in solution at 298 K) (2), the exothermicity of an interaction can increase far beyond the exothermicity at which the limiting cost in entropy is approached. We have proposed, and subsequently justified on the basis of a large body of experimental data from many laboratories (for example, see Figure 1A), that the enthalpic benefit versus entropie cost of associations of the type stipulated above have the general form shown in Figure 1B (1). We emphasise the approximate form of the curve; the precise entropie cost of an association is dependent on many variables such as mass, density of vibrational states and the shape of the potential energy well in which the associated species lies. In complex systems where the possibility arises that internal rotations are also restricted within the associating species, then the limiting entropie cost will be higher with the curve displaced to the right. The loss of entropy illustrated by Figures 1A and 1B reflects only loss of translational and overall rotational freedom upon association. The data clearly show that associations with small exothermicities can result in remarkably small adverse entropy changes.