A ligand can fit neatly inside a binding pocket and still be difficult to study once it starts moving. The challenge is not just finding a route out: the protein may need to adjust along the way, and the search depends on how the system and search region are set up. Ligand Path Finder in SAMSON offers a way to explore candidate unbinding pathways and inspect them as molecular conformations.
This workflow is useful when a static structure cannot answer a practical question: which routes might connect the bound ligand to the surrounding space? The result is a set of sampled pathways to examine—not a guarantee that a route is biologically used or a direct measurement of unbinding kinetics.
Start with a system you can trust
Before searching, make sure the model is prepared and minimized. The tutorial’s sample is lactose permease with the ligand thiodigalactosid (TDG), but the same setup choices apply to your own system. Depending on the structure, preparation may include removing alternate locations, water, or ions, adding hydrogens, and fixing the model. SAMSON’s Home > Prepare tools and the linked Protein Preparation & Validation tutorial can help with this step.
For energy evaluation, the documented workflow selects Universal Force Field (UFF) and the FIRE state updater. The system should already be minimized; if needed, the documentation points to Edit > Minimize. Set FIRE’s step size to 1 fs and its number of steps to 1, as shown in the tutorial.
Make the search geometry meaningful
Three setup decisions have a particularly direct influence on what the search explores:
- Starting conformation: choose the minimized bound conformation as the starting state.
- Ligand atoms: select the ligand and set those atoms in the app. The remaining atoms are treated as protein atoms.
- Active and fixed atoms: active ARAP atoms control ligand motion; passive ligand atoms follow them. Fixed protein atoms provide positional anchors so the protein does not simply drift with the ligand.
The tutorial uses the ligand’s S1 sulfur atom as an active atom and a CA atom in HIS 205 as a fixed atom. Those are examples for the sample, not universal choices. For another protein, choose active atoms that reasonably control the ligand’s movement, and fixed atoms in protein regions expected to remain relatively static. The placement of fixed atoms can affect the pathways found.
Orientation matters too. The sampling domain is a box in Cartesian coordinates, so orient the system before defining it. Then position and size the box to cover plausible exit space. A box is not just a boundary: its geometry biases where the active atoms can be sampled, and therefore influences the routes the search may find.
Use parameters to make runs interpretable
For a first exploration, the tutorial uses two runs, 20 ARAP-modeling iterations, 20 constrained-minimization iterations, and a 1 Å RRT extension step. A maximum ligand displacement of 40 Å provides a stopping distance. These are sample settings to get started, not values that must suit every system.
If you need to reproduce a run, enable the seed option and record the seed value along with the other settings. Without it, the planner uses a random seed. Keeping a record of the sampling box, atom assignments, force-field and updater choices, and search parameters makes comparisons between runs much more meaningful.
During a run, the Advanced information panel reports elapsed time, tree nodes, run number, and paths found. The search can be paused and resumed, or stopped. More runs can provide additional candidates, but the number of paths alone does not establish which route is most likely in reality.
Inspect routes, not just the table
When a path appears in the Results tab, its table includes the number of states, minimum and maximum energies, and quantities such as Saddle and Barrier. Select a path and move the slider to inspect its conformations in the structural model. You can select multiple paths to compare their energy curves.
Interpret these values in the context of the modeling setup. They describe energies calculated for the sampled conformations under the selected model; they are useful for inspecting and comparing computed paths, but should not be treated on their own as experimental free-energy barriers or rates. Review the molecular motion and the surrounding protein as well as the summary numbers.
You can copy table contents or path energies, export selected paths as trajectories, and export conformations at a chosen interval. For refinement, the documentation recommends the P-NEB app; Pathlines can help visualize ligand center-of-mass motion.
For the full interface walkthrough and sample system, see the Ligand Path Finder documentation.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.