We
present an alchemical enhanced sampling (ACES) method implemented
in the GPU-accelerated AMBER free energy MD engine. The methods hinges
on the creation of an “enhanced sampling state” by reducing
or eliminating selected potential energy terms and interactions that
lead to kinetic traps and conformational barriers while maintaining
those terms that curtail the need to otherwise sample large volumes
of phase space. For example, the enhanced sampling state might involve
transforming regions of a ligand and/or protein side chain into a
noninteracting “dummy state” with internal electrostatics
and torsion angle terms turned off. The enhanced sampling state is
connected to a real-state end point through a Hamiltonian replica
exchange (HREMD) framework that is facilitated by newly developed
alchemical transformation pathways and smoothstep softcore potentials.
This creates a counterdiffusion of real and enhanced-sampling states
along the HREMD network. The effect of a differential response of
the environment to the real and enhanced-sampling states is minimized
by leveraging the dual topology framework in AMBER to construct a
counterbalancing HREMD network in the opposite alchemical direction
with the same (or similar) real and enhanced sampling states at inverted
end points. The method has been demonstrated in a series of test cases
of increasing complexity where traditional MD, and in several cases
alternative REST2-like enhanced sampling methods, are shown to fail.
The hydration free energy for acetic acid was shown to be independent
of the starting conformation, and the values for four additional edge
case molecules from the FreeSolv database were shown to have a significantly
closer agreement with experiment using ACES. The method was further
able to handle different rotamer states in a Cdk2 ligand identified
as fractionally occupied in crystal structures. Finally, ACES was
applied to T4-lysozyme and demonstrated that the side chain distribution
of V111χ1 could be reliably reproduced for the apo
state, bound to p-xylene, and in p-xylene→ benzene transformations. In these cases, the ACES
method is shown to be highly robust and superior to a REST2-like enhanced
sampling implementation alone.