Molecular modelers often face challenges when preparing protein models for simulations, particularly when dealing with crystal waters outside the active site. These water molecules, if left unchecked, can unnecessarily complicate simulations and lead to inefficiencies. However, carefully preserving or removing water molecules can significantly improve the quality of results. In this post, we’ll go over how to selectively delete crystal waters outside the active site using SAMSON’s GROMACS Wizard.
The importance of handling crystal waters
Protein systems obtained from databases like the Protein Data Bank often include water molecules in the crystal structure. While some water molecules in the active site may be critical for your simulation—for example, water molecules tightly bound or involved in catalytic processes—others can clutter your model and lead to unnecessary computational expenses. The key lies in identifying and deleting only the unnecessary water molecules while keeping the essential ones.
Step-by-step guide for selective water removal
Follow these simple steps to cleanly and accurately remove crystal waters outside of your protein’s active site:
- Select the active site: Begin by selecting the key components of your active site, such as ligands, water molecules, residues, or atoms. This forms the core reference for water deletion.
- Expand the selection: Right-click on your active site selection in the Document view or the Viewport. Choose Expand selection > Advanced from the context menu to refine the selection further.
- Define water for removal: In the dialog that appears, set Water as the Node type and specify a distance outside the active site (e.g., beyond 5 Å). Use the auto-update feature to dynamically preview your selection in real time. When satisfied, click OK.
- Review and erase: Verify the highlighted selection of water molecules. Once confirmed, right-click on the selection in the Document view or the Viewport, and choose Erase selection. This removes the unnecessary waters from your system.
Congratulations! By following these steps, you’ve now prepared a cleaner molecular system, keeping only those water molecules essential to your simulation.
When to be cautious
Care must be exercised when working with water molecules in the active site. Removing critical waters essential for structural integrity or ligand interaction can compromise your simulation’s accuracy. We recommend thoroughly reviewing the literature or structural studies on your system and conducting a visual inspection of the selected water molecules before confirming their removal.
Looking ahead
Effectively preparing your system by managing crystal water molecules lays the groundwork for accurate simulations. After cleaning your system, you can proceed with the GROMACS Preparation step, where you’ll define your simulation model, solvent, ions, and more.
Ready to refine your modeling workflow? Visit the original GROMACS Wizard documentation here to learn more.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. To get started, download SAMSON here.