Towards a semi-quantitative description of a bimolecular association involving weak interactions in aqueous solution

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Abstract
We present a description of a bimolecular association in aqueous solution by dividing the free energy of binding into a number of terms representing ‘costs’ and ‘benefits'. Using data on the binding of cell-wall peptide analogues to vancomycin-group antibiotics, we have established an experimental basis for the adverse free energy of restricting internal rotations, the benefit in free energy from hydrophobic interactions and polar functional group interactions (amide-amide hydrogen bonds). Collating data from the literature on weak associations in non-polar solvents, a relation between the electrostatics of binding (enthalpy) and the dynamics (entropy) is presented. This provides an approximate, but useful, relation between the magnitude of the enthalpic barrier to dissociation for complexes in aqueous solution, and the cost in translational and rotational free energy of the reverse bimolecular association.

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