Preparing a protein structure for simulation often looks like a cleanup task: remove extra molecules, simplify the structure, then move on. But one cleanup choice deserves more care than it may first appear to: water. Removing every water molecule can also remove tightly bound or active-site waters that are functional parts of the system.
The useful distinction is not simply “water” versus “no water.” It is whether a particular water should remain in the model you intend to simulate. If you are unsure, avoid applying a blanket removal step before checking the active site.
Start with a clean working structure
This workflow uses the 1AKI protein structure as an example. In SAMSON, load it from Home > Fetch by entering 1AKI in the PDB or PDB (mmCIF) field, then click the matching Load button. Import settings can be left at their defaults for this walkthrough. You can also open a structure file you already have using Home > Open.
Before the GROMACS preparation workflow, use Home > Prepare to review and clean the system. Removing alternate locations, unneeded ligands, and monatomic ions can help reduce confusing or unwanted data. Be selective: keep any ligand, cofactor, or other molecule that you plan to simulate. If you retain arbitrary molecules, you will need suitable topology files for them later in the preparation step.
Why blanket water removal can be risky
Removing water is a common cleanup option, but it is not universally appropriate. Some waters may be tightly bound or functional in the system, including waters in or near an active site. Removing all water without checking may discard molecules you meant to preserve.
If you have already decided that no water molecules need to remain, the general cleanup option may suit your workflow. If you want to preserve waters near a ligand or active site while removing more distant waters, use a distance-based selection instead. This makes the cleanup targeted rather than all-or-nothing.
Remove water outside the active site
- Select a reference for the region you want to preserve. This can be a ligand, water, residue, or atom in the active site.
- Right-click the selection in the Document view or Viewport, then choose Expand selection > Advanced.
- In the dialog, set Node type to Water and choose beyond a distance from the active-site selection. The documentation image illustrates a 5 Å example; choose a distance appropriate to your system rather than treating that example as a universal cutoff.
- Use auto-update if you want to preview the selection as you adjust it. Click OK, then inspect the selected waters before deleting anything.
- When you are satisfied with the selection, right-click it and choose Erase selection.
A small check that can prevent a large detour
Before erasing, confirm that the selection contains the waters you intend to remove—not the ones you want to preserve. The reference selection defines the region you are measuring from, while the beyond condition identifies waters outside the chosen distance. Previewing and verifying the selection gives you a chance to catch an unsuitable reference or distance before changing the structure.
Once cleanup is complete, continue to Step 1: Prepare to define the simulation model, box, solvent, ions, and project files. For the complete walkthrough and related guidance, see the GROMACS Wizard pre-processing tutorial.
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