Preparing molecular systems for simulation can often feel like an odyssey, particularly when working with complex proteins or custom setups. Issues such as alternate atom locations, unnecessary waters, unwanted ligands, or extraneous ions can cause confusion and errors down the line. That’s why cleaning up your molecular system is such a pivotal step before entering the main GROMACS preparation workflow. In this post, we’ll guide you through the process of cleaning up your system using SAMSON’s GROMACS Wizard, ensuring your simulation begins smoothly and effectively.
The Importance of Cleaning Up Before You Simulate
When preparing molecular simulations, overlooking cleanup can lead to problems like erroneous errors, computational inefficiencies, or inaccurate results. For example, alternate atom locations may introduce ambiguity, while unnecessary water molecules and ions can unnecessarily increase the size of the simulated system. Using SAMSON’s GROMACS Wizard, the cleanup process becomes manageable and intuitive, even for complex molecular structures.
Setting Up for Success
Before following this walkthrough, make sure to add the GROMACS Wizard extension to your SAMSON interface. For this demonstration, we’ll use the 1AKI protein structure, but you can substitute it with any protein system you’re working on. Do note: if your system contains ligands, cofactors, or other specific molecules that you intend to simulate, make sure not to remove them during the cleaning process.
Step 1: Loading Your Structure
To begin, you’ll first load your molecular structure:
- Navigate to Home > Fetch in SAMSON.
- Input your desired protein structure ID, such as
1AKI, in the PDB or PDB (mmCIF) field. - Click the Load button. If prompted with an import settings dialog, the default parameters should suffice for most systems.
If you already have your structure file downloaded locally, you can load it via Home > Open (Ctrl/Cmd + O).
Step 2: Pre-processing Your System
SAMSON simplifies the system cleanup process through the Home > Prepare menu. Here’s how to clean your structure effectively:
- Remove Alternate Locations: This step eliminates ambiguity caused by alternate atom locations in structures.
- Remove Ligands (Optional): Use the Remove ligands feature to eliminate molecules or ligands you don’t want in your simulation. Be cautious not to remove any crucial ligands or cofactors.
- Remove Water: Unnecessary water molecules can inflate computational costs. SAMSON provides options to remove all water molecules or selectively delete only those outside functional regions—for example, using Select > Biology > Ligands to identify functional water molecules you want to keep.
- Remove Monatomic Ions: Clear out excess ions unless they are significant to system behavior.
- Optional: Add Hydrogens: While GROMACS can add hydrogens during simulations, SAMSON allows you to perform this step during the cleaning process if desired.
Fine-Tuning Water Removal
In many cases, crystal waters play essential roles in molecular interactions, such as those in active sites. If you wish to keep some while removing others, SAMSON allows you to tailor your selections:
- Select residues, molecules, or atoms in the active site.
- Right-click the selection in the Document view or Viewport, then choose Expand selection > Advanced.
- Set Water as the node type and define a threshold distance (e.g., beyond 5 Angstroms). Use the auto-update to visualize selections.
- Erase unnecessary water molecules by right-clicking your selection and choosing Erase selection.
This selective cleanup ensures important waters are conserved for accurate simulations.
Ready for the Next Step
After cleaning your system, you’re poised to proceed to the Preparation step in GROMACS Wizard. Want to dive deeper into system preparation for GROMACS? Visit the full tutorial at SAMSON’s documentation page.
SAMSON and all SAMSON Extensions are free for non-commercial use. Download SAMSON at https://www.samson-connect.net.