A protein structure is a snapshot. If a binding pocket is partly closed in that snapshot, its shape alone may not tell you whether nearby motions could open it. For molecular modelers, this creates a practical challenge: how do you explore plausible large-scale movement without manually dragging atoms or relying on a single static conformation?
In SAMSON, the Normal Modes Advanced extension can help you inspect nonlinear normal modes and explore motions such as binding-site opening. Instead of treating the structure as immovable, you can calculate modes, adjust their contributions, and watch how the model changes. The workflow below focuses on a useful goal: investigating how a defined region might open or close.
Start with a structure and a question
Open a biomolecular structure in SAMSON. The tutorial uses the protein structure 1VPK, though the workflow can also be applied to protein, RNA, or DNA models. Before calculating, decide what you want to explore: broad motions across the full structure, or movement in a selected subset of residues.
Add the Normal Modes Advanced extension from SAMSON Connect, then choose the number of modes, interaction cutoff distance, and potential function. The interface currently provides the elastic network model potential. You can calculate on the full structure or select residues first to focus the calculation.
Read the motion, not just the mode number
After calculation, the results appear in the Output box. Moving a mode slider displays that mode’s motion on the structure. This is a useful first pass: move through the modes individually and watch which parts of the structure shift, rather than assuming that a particular mode number will correspond to pocket opening.
Modes can also be combined. Check the modes you want to apply, then use play/pause to animate their combined motion. Sliders for unchecked modes remain adjustable during motion, so you can inspect how changing a contribution affects the displayed trajectory. The scaling factor changes motion amplitude; the transformation can be linear, using translations only, or nonlinear, using translations and rotations. Motion speed and harmonic behavior can also be adjusted.
For a pocket-opening question, this interactive stage helps you identify promising movements and combinations. Real-time minimization can be enabled while the modes are applied, using one of the available minimization algorithms. Treat the resulting motion as an exploratory view of structural flexibility, not as proof that a particular opening occurs biologically.
Define the target instead of guessing
If you have a specific opening or closing objective, use the Structure Definition tab. The target can be defined using residues or atoms. The module can then find a combination of modes that reaches the target structure. This shifts the task from searching through sliders by eye toward asking a clearer question: which combination of the calculated modes moves the model toward the structure I defined?
Keep useful conformations for the next step
Once you find a conformation worth revisiting, save it as a SAMSON conformation using the S shortcut. To compare or superpose structural states, create a SAMSON structural model instead; you can select the whole structure or only part of it. The current structure can also be exported as a PDB file.
If the path itself matters, use the save frames tab to store selected frames as conformations or export them as PDB files. You can also store the full trajectory in a SAMSON trajectory node using the Path button. Remember that saved conformations and trajectories use the current calculation settings, including checked modes, their slider values, scaling factor, motion type, and saving interval.
This gives you a practical loop: calculate modes, inspect likely movements, define a target if you have one, and save states or trajectories for later comparison, path generation, analysis, or simulation. For the complete walkthrough, see the Normal Modes Advanced tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON at samson-connect.net.