Center-of-mass (COM) pulling simulations can provide valuable insights into the molecular interactions and mechanical properties of biological systems. However, setting up such simulations can be tedious, especially when considering factors like system size, boundary conditions, and proper restraints. If you’ve ever found yourself bogged down by the details, tools like the GROMACS Wizard in SAMSON streamline the process and ensure that important considerations aren’t missed. Let’s explore how to set up COM pulling for a system of two chains, using chain A pulled away from chain B as a practical example.
The Basics of Loading your Molecular System
The foundation of any successful molecular dynamics simulation is ensuring your molecular system is correctly loaded. For this tutorial, we use the 2BEG system, which features 5 chains. To pull chain A away from chain B, follow these steps:
- Go to Home > Fetch in SAMSON.
- Enter the PDB code
2BEGand click Load. - Visualize the chains in the Viewport, where separate secondary structures appear. Reorient the system if necessary, using SAMSON’s intuitive orientation tools, such as the Compass.
Once loaded, examine your 2BEG system in the Document view. You’ll notice that chain A and chain B are part of the loaded structure and can be selected separately. These will serve as the COM pulling targets.
Defining Critical Simulation Parameters
With your molecular system loaded, it’s time to ensure the simulation will proceed smoothly. Properly defining the simulation box and system neutrality is crucial in COM pulling:
1. Setting Up the Simulation Box
When pulling chain A from chain B, space in the pulling direction is vital. Use the following settings in SAMSON:
- Choose the Orthorhombic unit cell and set the box dimensions to
6.5 nm x 4.5 nm x 12 nm. This allows sufficient room for a pulling distance of 5 nm. - Adjust the center of mass of the system to
0.5 x 0.5 x 0.2relative to the box size.
2. Neutralizing the Molecular System
To avoid artifacts introduced by charge imbalances, you’ll need to neutralize the system:
- Select Na and Cl ions for neutralization.
- Simulate in the presence of 100 mM NaCl by activating Add additional ions and setting Salt concentration to 0.1 mol/liter.
Adding Index Groups for COM Pulling
Before launching the production molecular dynamics (MD) simulation, create index groups specifying the regions you wish to pull. This ensures GROMACS will recognize chain A and chain B as distinct groups for the COM pulling coordinates:
- Select chain A in the Document view and add it as a new index group named “ChainA.”
- Repeat with chain B and name it “ChainB.”
Executing the Production MD Simulation
Finally, set the simulation duration and pulling parameters for chain A:
- Simulation length: 0.5 ns with a 2 fs timestep.
- Define the pulling force constants: pulling chain A 5 nm away from chain B along the z-axis at a rate of 0.01 nm/ps.
To configure these settings, open the COM pulling parameters and populate the fields accordingly. You can now run your simulation locally or in the cloud based on your computational resources.
Check and Use Results
Upon completion, you can visualize the simulation trajectory in SAMSON and check generated plots for the pulling force and displacement over time. These results can guide further analyses such as umbrella sampling, extending the insights gained in your molecular modeling workflow.
To explore all the steps in greater detail, you can visit the official documentation page.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. You can download SAMSON at https://www.samson-connect.net.