A protein structure can look perfectly usable in a viewer and still cause trouble later. Alternate atom positions, missing atoms, unexpected small molecules, or incomplete residues may complicate docking, simulation, or other modeling tasks. The frustrating part is that these problems often surface only after you have started a workflow.
A useful way to avoid that detour is to prepare and validate the structure in stages: first make routine cleanup decisions, then check for structural issues, and finally use a deeper repair tool if something is missing. The right amount of preparation depends on what you plan to do with the model.
Start with a quick preparation pass
In SAMSON, Home > Prepare brings several common cleanup steps together. It can resolve alternate locations, delete selected ligands and other small molecules, strip water, clear monatomic ions, and add hydrogens. This is a useful starting point when the structure does not appear to need substantial reconstruction.
These actions are not merely cosmetic. For example, a structure may contain multiple positions for an atom. The preparation tool keeps the highest-occupancy atoms; when occupancies are tied or absent, it uses the available alternate-location labels in a defined order. Resolving those alternatives gives downstream tools one selected set of atom positions to work with.
Other cleanup choices depend on the system and the task. A ligand, cofactor, ion, or water molecule may be unwanted for a particular workflow, but the tool’s ability to remove it does not mean it should always be removed. Decide what belongs in the model before deleting it. If you are unsure, inspect the structure and preserve an unmodified copy so you can return to the original.
Check the model instead of assuming it is clean
After quick preparation, use Home > Validate to inspect specific issues. The Structure Validation module can find alternate locations, flag bonds outside expected length ranges, identify non-standard residue aliases, and highlight clashes or contacts. It also includes utilities for merging nodes, reordering atoms in a connected component, and renumbering atom serial numbers.
For alternate locations, open the Alt. locations tab, choose Find alternate locations, review the results, and then select Remove alt. locations if that is appropriate. Reviewing before applying a change helps you understand what the cleanup will affect.
Hydrogen addition is another step where the downstream purpose matters. Edit > Add hydrogens adds standard hydrogens at pH 7, using residue type for standard amino and nucleic acids and valences for other atoms. You can select a sub-structure first to limit where hydrogens are added. If you need hydrogens for a specific pH, the PDBFixer extension offers pH-specific addition. Minimization after adding hydrogens is not automatically required for every task; for instance, the documentation notes that AutoDock Vina needs only polar hydrogens to determine hydrogen bonds.
Know when quick cleanup is not enough
If residues or atoms are missing, routine cleanup cannot supply the missing structural pieces. The PDBFixer extension can add missing residues based on SEQRES records, add missing heavy atoms and side chains, convert non-standard residues to standard equivalents, resolve alternate locations, and add hydrogens at a specified pH. It can also prepare explicit-solvent water boxes or lipid membranes for supported workflows.
For repeated work, Batch Protein Prepare can apply the same cleaning steps to files in a folder or to structures supplied by PDB identifier. It supports PDB, PDBx/mmCIF, MMTF, and MOL2 inputs, and preserves the folder’s internal subfolder structure in the output.
A simple decision sequence
- Identify the downstream task and decide which molecules should remain.
- Use Home > Prepare for routine cleanup and hydrogen addition.
- Use Home > Validate to inspect geometry, alternate locations, residue labels, and clashes.
- If residues or atoms are missing, consider PDBFixer rather than treating the structure as complete.
- Keep the original structure available and review changes before continuing to docking or simulation.
This staged approach helps separate ordinary cleanup from actual structural repair, making it easier to spot what changed and why. For detailed instructions and related workflows, see the Protein Preparation & Validation tutorial.
Note: SAMSON and all SAMSON Extensions are free for non-commercial use. Get SAMSON.