Chemur

Find the non-covalent interactions in a PDB or mmCIF structure — between proteins, nucleic acids, ligands, metals and solvent, in any combination.

pip install chemur
chemur analyze structure.cif --out interactions.json

• 22 interaction types, each with defined geometry
• Any pair of components. Protein–ligand, protein–protein, protein–DNA/RNA, nucleic acid–ligand, ligand–ligand. Nothing is hard-coded to expect a receptor and a small molecule.
• Chemistry from templates, geometry from coordinates. Ligands are typed from SMILES, an SDF, or an automatic Chemical Component Dictionary lookup.
• MD trajectories — per-frame analysis with occupancy and time series.

A ChimeraX plugin builds on this library for interactive 3D visualisation.

Install

pip install chemur

Wheels are published for CPython 3.10–3.13 on Linux x86_64, macOS (a single universal2 build covering Intel and Apple Silicon) and Windows x86_64. Other platforms build from the source distribution and need a C++17 compiler and CMake ≥ 3.18.

pip install "chemur[all]"

pH-range ligand protonation (--protonate) needs no extra — dimorphite_dl is a hard dependency. It requires rdkit<2026, so a Chemur install is capped there too.

MDAnalysis is LGPL-3.0-or-later (it relicensed from GPL at 2.8.0; the >=2.4 floor still permits older GPL-licensed releases if you pin one). Chemur itself is MIT and neither bundles nor links it, but installing the trajectory extra adds copyleft-licensed code to your environment.

To confirm you got the compiled core rather than the fallback:

python -c "from chemur.core import USING_CPP_CORE; print('C++ core:', USING_CPP_CORE)"

True is expected. False means the pure-Python fallback loaded — correct results, 10–100× slower. From a wheel that should never happen; from a source build it means CMake or the compiler failed. Set CHEMUR_FORCE_PYTHON_CORE=1 to select it deliberately.

Quick start

import chemur

result = chemur.analyze("structure.cif")

for interaction in result.interactions:
    print(interaction.interaction_type, interaction.component_ids)

result.to_json("interactions.json")

chemur analyze structure.cif --out interactions.json --csv interactions.csv

Any biomolecule, not just protein–ligand

analyze returns every interaction it finds in the structure. There is no receptor or ligand role — you pick out the ones you want by component.

import chemur
from collections import Counter

result = chemur.analyze("complex.cif")

# What was found, and between what?
print(Counter(i.interaction_type for i in result.interactions))

A protein–DNA interface, a protein–protein interface and a ligand binding site are all the same query — filter on the components involved:

def between(result, a, b):
    """Interactions with one atom in component `a` and the other in `b`."""
    return [
        i for i in result.interactions
        if {a, b} <= set(i.component_ids)
    ]

interface = between(result, "A:ARG:145", "B:DG:12")   # protein side chain to a base

Components are identified by component_id; result.components lists them all with their names, so you can select a chain, a residue range, or a single ligand. Nucleic acid phosphate backbones are perceived as anions, protein Arg/Lys as cations and Asp/Glu as carboxylate anions, so salt bridges and cation–π across a protein–nucleic acid interface are detected the same way as in a binding site.

What it detects

aliphatic_stack (saturated–saturated ring stacking) ships disabled; enable it in the profile. The σ-hole, stacking and n→π* geometries follow Adhav & Saikrishnan, ACS Omega 2023, 8, 22268.

Each type has its own geometric criteria, all adjustable — see docs/cli.md for overriding cutoffs and docs/chemistry.md for the chemical gates (such as why an ordinary Met sulfur cannot donate a chalcogen bond).

Ligand chemistry

Ligands need a chemical template so protonation, charge, aromaticity and donor/acceptor status are correct. Chemur takes one from, in order of precedence:

chemur analyze structure.cif --ligand-smiles LIG='CC(=O)N'   # explicit SMILES
chemur analyze receptor.pdb --ligand-sdf docked.sdf          # an SDF (coordinates too)
chemur analyze structure.cif                                 # automatic CCD lookup

The automatic lookup resolves each ligand by its component name against the RCSB Chemical Component Dictionary. Add --protonate to set the protonation state for a pH range with Dimorphite-DL, and --debug to print the SMILES actually used.

Full detail in docs/cli.md.

Multiple ligands and docking output

Pass several ligands at once, or point at a directory of sdf files.

chemur analyze receptor.pdb --ligand-sdf a.sdf --ligand-sdf b.sdf   # repeatable
chemur analyze receptor.pdb --ligand-sdf-dir poses/                 # a whole directory

By default all supplied ligands are added to the structure and analysed together, as one system — right for a cofactor plus a substrate, or two ligands sharing a pocket.

Docked poses are the opposite case: each is an alternative for the same site, so analysing them together would stack overlapping copies into one pocket. Use --batch to analyse each ligand independently against the same receptor:

chemur analyze receptor.pdb --ligand-sdf-dir poses/ --batch --out all_poses.json

Either way you also get one filtered file per ligand under ligand_outputs/ — in batch mode under a subdirectory per pose — while --out / --csv write the aggregate.

SDF coordinates are used directly, and ligand residues already present in the structure are skipped so the SDF stays authoritative.

Trajectories

pip install "chemur[trajectory]"
chemur trajectory topology.pdb trajectory.xtc --out frames.json

Analyses each frame and reports per-interaction occupancy and time series.

Documentation

• docs/cli.md — full command-line reference
• docs/chemistry.md — templates, hydrogen policy, σ-hole activation, profiles

Development

git clone https://github.com/SweeneyAaron/chemur
cd chemur

conda create -n chemur-dev -c conda-forge \
  python=3.11 rdkit gemmi numpy pyyaml pandas pytest \
  cmake ninja pybind11 scikit-build-core cxx-compiler
conda activate chemur-dev

pip install -e ".[test,all]" -v
pytest

On macOS you need the Apple command line tools (xcode-select --install). If the build fails with 'cstddef' file not found they are incomplete — repair with sudo rm -rf /Library/Developer/CommandLineTools && xcode-select --install.

This project uses a src/ layout and has no conftest.py on purpose, so pytest always exercises the installed package rather than the source tree. That is what makes the wheel-level checks in CI meaningful.

License

MIT — see LICENSE.

Citation

A manuscript is in preparation. Until then, please cite the repository:

Sweeney, A, Genz, L, Topf, M. Chemur: biomolecular interaction detection.
https://github.com/SweeneyAaron/chemur