Flexible Topology: A new method for dynamic drug design

Ligand-induced conformational changes form the underpinnings of most essential biomolecular processes, however they are often neglected in screening and design applications, due to the high computational cost of simulating each candidate ligand individually. We propose a method called "Flexible Topology", where a ligand is comprised of a set of shapeshifting "ghost" atoms, whose atomic identities and connectivity can dynamically change over the course of a simulation. Ghost atoms are guided toward their target positions using a translation-, rotation-, and index-invariant restraint potential. When implemented with a large set possible targets, this can simulate trajectories in a coupled chemical-conformational space that allows solutions to molecular design problems to simply emerge over the course of a trajectory. It also provides a mechanism for observing ligand-induced conformational change, by allowing for the surrounding environment and the ghost atoms to respond to each other during the design process. This builds on a substantial history of alchemy in the field of molecular dynamics simulation, including the Lambda dynamics method developed by Brooks and coworkers [X. Kong and C.L. Brooks III, J. Chem. Phys. 105, 2414 (1996)], but takes it to an extreme by associating a set of four dynamical attributes with each shapeshifting atom that control not only its presence but its atomic identity. Here we outline the theoretical details of this method, its implementation using the OpenMM simulation package, and some preliminary studies of ghost particle assembly simulations in vaccum. We examine a set of 10 small molecules, ranging in size from 6 to 50 atoms, and show that Flexible Topology is able to consistently assemble all of these molecules to high accuracy, beginning from randomly initialized positions and attributes.