Simplifying Molecular Simulations: Selecting and Removing Crystal Waters

Molecular modelers often face the challenge of preparing protein systems for simulations, especially when deciding what water molecules to include or exclude. While some waters are functional and critical to simulations, others, like crystal waters, might only add noise. Knowing how to carefully identify and remove unnecessary water molecules outside of active sites can save time and improve simulation clarity.

Why Remove Crystal Waters?

Molecular systems downloaded from repositories such as the Protein Data Bank (PDB) often retain crystal waters used in structure determination. While helpful during X-ray diffraction or cryo-EM studies, many of these waters do not contribute to the biological or chemical function of the system. However, tightly bound water molecules or those in active sites may play essential roles. Removing only the unimportant water molecules preserves the fidelity of your simulation while reducing computational cost.

Step-by-Step Guide to Deleting Non-Essential Waters

SAMSON makes it straightforward to selectively remove crystal waters located outside active sites. Follow these steps:

  1. Identify the Active Site: Select the critical structures in the active site. This can include the ligand, essential waters, or specific residues.
  2. Expand the Selection: Right-click your selection in the Document view or the Viewport. In the context menu, choose Expand selection > Advanced.
  3. Set Selection Parameters:

    4. In the dialog that appears, set the Node type to Water and the distance criteria to waters beyond the active site (e.g., 5 Å). You can enable the auto-update option to preview the selection dynamically. When satisfied, click OK.

    Expand selection: select water beyond 5A

  4. Verify and Remove: Review the expanded selection, ensuring no crucial molecules are included. Once confirmed, right-click the selection and choose Erase selection.

Congratulations! You now have a molecular system free of extraneous waters, optimized for simulation while retaining functional and tightly bound waters.

Additional Resources

If you’re new to molecular modeling or GROMACS workflows, consider exploring the full GROMACS Wizard Pre-processing Tutorial for a deeper dive into system preparation.

SAMSON and all SAMSON Extensions are free for non-commercial use. You can download SAMSON at https://www.samson-connect.net.

Comments are closed.