When preparing molecular systems for simulation, one of the most common issues molecular modelers face is dealing with water molecules present in crystal structures. While some water molecules are crucial for simulating the biological relevance of a system, many others—especially those positioned outside the active site—are not necessary and may unnecessarily increase computational complexity. This blog post walks you through how to efficiently delete crystal waters outside the active site using SAMSON’s GROMACS Wizard.
The challenge of water handling in molecular modeling
When working with data sourced from the Protein Data Bank (PDB), crystal structures often include water molecules that are not functionally relevant. These extraneous waters can complicate the system and lead to inefficient simulations, slowing down calculations and potentially obscuring meaningful results. However, blindly removing all water molecules can be problematic since tightly bound waters, or those within active sites, play functional roles in many molecular mechanisms. Removing these waters may compromise the accuracy of your simulated model.
To strike the right balance, the GROMACS Wizard in SAMSON offers a selective workflow to identify and remove only the water molecules outside the regions of interest. Below, we summarize the steps to achieve this.
Step-by-step guide
Here’s how you can precisely delete water molecules outside your active site:
- Select the active site: Begin by identifying the active site in your molecular system. Selecting specific structures (such as ligands, residues, or atoms) within this region is key to differentiating between functional and non-functional waters.
- Expand your selection: Within the Document view or Viewport, right-click on your selection and navigate to the context menu. Choose Expand selection > Advanced. A dialog will appear where you can specify criteria for the expansion.
- Focus on waters outside a distance cutoff: Set Water as the node type and define a distance threshold—this determines how far beyond the active site the selection should extend. For example, you might specify waters located beyond 5 Å from the active site. To verify your selection, activate the auto-update option before confirming with OK.
- Verify the selected waters: Ensure that only the undesired waters located outside your active site have been selected. Once you are satisfied, proceed to deletion.
- Erase the selection: Right-click on the highlighted selection in the Document view or Viewport, and use the Erase selection option. The structural waters outside the active site will now be removed from your molecular system.
Why it matters
Removing irrelevant water molecules ensures that computational resources are dedicated to simulating the essential parts of your system. Furthermore, this targeted cleaning process can help avoid errors or inaccuracies in your simulation workflows while preserving functional waters that contribute to the structural and biological integrity of your system.
For the full documentation on this process and additional details about molecular preparation workflows in SAMSON, visit the crystal water preprocessing section of the GROMACS Wizard tutorial.
SAMSON and all SAMSON Extensions are free for non-commercial use. You can get SAMSON here.