A binding site can look perfectly well defined in a static structure—and still be difficult to work with. A pocket may be too closed to inspect, or a nearby conformation may be needed to understand how a molecule could enter. The challenge is not simply to move atoms, but to explore coordinated motion without losing track of which changes are meaningful.
SAMSON’s Normal Modes Advanced extension offers one way to investigate this problem. It computes nonlinear normal modes for biomolecular structures and lets you explore combinations of those motions interactively. Rather than manually dragging parts of a protein, you can inspect how collective modes move the structure and define a target structure or pocket to guide the search.
Begin with the structure and a focused calculation
Open a protein, RNA, or DNA structure in SAMSON. The tutorial uses the PDB entry 1VPK, but you can use your own model. Before calculating, decide whether you need modes for the whole structure or for a region of interest. SAMSON’s selection tools let you select residues before starting the calculation, so you can compute on a subset when that suits the question.
In the NMA module, choose the number of modes, the interaction cutoff distance, and the potential function. The interface currently provides the elastic network model potential. During the calculation, progress is shown in the status bar; results appear in the Output box.
Explore before you commit to a motion
Each mode has a slider that lets you preview its motion on the structure. This is useful when the goal is to understand which motions might affect a pocket: move through the modes, observe what changes around the site, and reset individual sliders when you want to compare another motion.
Modes can also be combined. Check the modes you want to apply, then use play/pause to animate them together. You can still adjust unchecked sliders during motion; their changes are displayed in the trajectory too. If the resulting movement needs to be moderated or explored at a different amplitude, adjust the scaling factor. The transformation can be set to linear, using translations only, or nonlinear, using translations and rotations. Motion speed and harmonic behavior are also adjustable.
Real-time minimization can be activated during motion, with one of the available minimization algorithms. Treat this as an exploration aid: it changes how the structure is handled as it moves, so compare the resulting motion with and without minimization when that distinction matters to your analysis.
Turn a pocket-opening question into a target
When visual exploration is not enough, use the Structure Definition tab. The target definition can contain residues or atoms. Set the target structure, then search for a combination of modes that reaches it. For a binding-site question, this provides a way to frame the task in terms of the desired pocket state rather than relying only on trial-and-error slider adjustments.
It helps to keep the question specific: which part of the pocket should open or close, and which residues or atoms should define that change? A focused target makes it easier to inspect whether the resulting movement addresses the structural feature you care about. The computed motion is a way to explore conformations; it does not, by itself, establish that a conformation is biologically populated or experimentally validated.
Keep the conformations you want to revisit
Once you find a useful state, you can save a conformation in the document with the S shortcut. For comparing multiple states, create a SAMSON structural model from the entire structure or a selected part, or export the current structure as a PDB file. The Save frames tab can store sampled points along the motion as conformations or export them as PDB files. You can also store the complete trajectory in a SAMSON trajectory node.
Before saving a trajectory, check the calculation settings: the stored motion reflects the checked modes, their current slider values, the scaling factor, the motion type, and the saving interval. Recording these choices helps you reproduce and interpret the states later.
For the complete walkthrough, visit the Normal Modes Advanced tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON at samson-connect.net.