Protein docking can produce a long list of plausible-looking poses. When the binding site is already known—or at least suspected—searching every possible orientation may spend time on regions that are unlikely to matter. The challenge is to narrow the search without constraining it so tightly that useful poses are excluded.
Hex, available through its SAMSON Extension, provides a practical way to do this with range angles. These settings limit the angular orientations sampled for the receptor and ligand. Used thoughtfully, they can focus a docking run around a starting arrangement and reduce the search domain.
Start with a structure you can trust
Before configuring docking, prepare the receptor and ligand. Remove atoms with alternate locations, water, monatomic ions, and existing ligands when appropriate, and add hydrogens if they are absent. If hydrogens already reflect a chosen protonation state, do not modify them casually. Missing residues or heavy atoms may require protein repair; the SAMSON documentation points to PDBFixer for this task.
This step matters because docking scores and pose assessment depend on the structures being modeled. Preparation does not guarantee a correct result, but it helps avoid interpreting problems in the input as meaningful docking outcomes.
Use what you know about the binding site
In SAMSON, open Hex and assign one structure as the receptor and the other as the ligand. If you have evidence about where binding occurs, first move and orient the ligand near that site. The range-angle constraints are defined around the intermolecular axis connecting the origins of the two proteins, so a useful starting orientation gives the constraints context.
Set the sampling method to Range angles, then open Advanced parameters. A range angle of 180 degrees imposes no angular restriction. A smaller value limits the orientations sampled to a cone. The tutorial uses 45 degrees for both receptor and ligand as an example of a loose constraint around the starting orientation.
Think of 45 degrees as a starting point to evaluate, not a universal setting. If the binding location or approach direction is uncertain, an overly narrow cone can exclude relevant orientations. If the site and approximate orientation are supported by prior knowledge, restricting the range may reduce unnecessary search. The ligand twist angle around the intermolecular axis can also be limited, if appropriate.
Balance speed and search detail
Hex can use a fast, low-resolution Steric scan before the higher-resolution Final search. The scan discards most orientations and passes a smaller set to the final stage. The documentation describes values such as N=16 for the scan and N=25 for the final search as a commonly useful combination; N=20 and N=30 are another option. A higher final-search order can be appropriate for high-resolution crystal structures when binding-related conformational changes are expected to be small. More flexible or model-built structures may call for a lower order.
Post-processing can help sort candidate solutions. A bumps counter reports steric clashes between non-bonded heavy atoms, which can help flag poses for inspection. Molecular-mechanics refinement is described as alpha-stage in Hex and is intended for cases where conformational changes are expected to be small; it should not be treated as a universal filter.
Inspect poses, not just rankings
After the run, review results by cluster and inspect the best-scoring mode from each cluster, rather than assuming that one score identifies the correct biological pose. Selecting a result restores its ligand conformation; you can also create a conformation in the document for further analysis. Visual inspection, distance measurements, and interaction analysis can help assess whether a candidate is consistent with what is known about the system.
The useful habit is to treat constraints as a record of your assumptions: note why you chose the initial orientation and angular ranges, then compare the resulting pose clusters. This makes it easier to tell whether a focused search answered the question you intended—or whether you should broaden the search and try again.
For the complete workflow and parameter details, see the Protein docking with Hex tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.