Selective elimination of interactions: a method for assessing thermodynamic contributions to ligand binding with application to rhinovirus antivirals

A new method for evaluating the free energy of various physical interactions, such as hydrogen-bond, electrostatic, or van der Waals interactions, is presented. Rather than destroying or creating whole groups, selective (pairwise) interactions are eliminated from the total potential energy and the energy difference with the fully interacting system is evaluated. The exponential ensemble average of such an energy difference is then directly related to the corresponding free energy difference. This procedure is then applied to a rather large protein-ligand system involving the coat proteins of a human rhinovirus and an antiviral ligand. The results seem to indicate that a particular bent hydrogen bond between the ligand and protein system may not be favorable for binding. The method presented gives an estimate of the hydrogen bond free energy contribution with an available trajectory that was previously computed without the expenditure of sizeable computational resources such as recomputing a trajectory. This procedure is effective and efficient for computing the free energy for a given type of physical interaction. It can be used for calculating the binding energy differences for various interactions which can be used to guide the search for isosoluble synthetic targets.