Bio Pose Validation

Version Compatibility

Reference examples tested with: PoseBusters 0.6+, RDKit 2024.09+, pandas 2.2+, posecheck 0.5+ (optional).

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Pose Validation

Test docked or AI-generated protein-ligand poses for physical plausibility. PoseBusters (Buttenschoen 2024) is the modern gold standard: a suite of geometric, chemical, and energetic checks that flag implausible poses (planar aromatic rings now non-planar, vdW clashes, broken bonds, wrong chirality, unrealistic torsions). The PoseBusters benchmark showed that AI-based docking methods (DiffDock, EquiBind, TANKBind) produce ~50% physically-invalid poses despite reporting good RMSD; classical methods (Vina, GOLD) produce ~5-15% invalid. PB-valid status is therefore essential for downstream SAR, FEP setup, or generative model training.

For docking, see chemoinformatics/virtual-screening. For ML docking specifically, see chemoinformatics/ml-docking-rescoring.

PoseBusters Test Suite

PoseBusters runs ~20 individual checks grouped into:

A pose passing ALL tests is "PB-valid". Combined PB-valid + RMSD <= 2 Å is the modern criterion.

When to Apply PoseBusters

PoseBusters Usage

from posebusters import PoseBusters

bust = PoseBusters(config='redock')

results = bust.bust(
    mol_pred='predicted.sdf',
    mol_true='reference.sdf',
    mol_cond='receptor.pdb',
)

config options and included checks:

Output DataFrame columns include: mol_pred_loaded, sanitization, all_atoms_connected, bond_lengths, bond_angles, internal_steric_clash, aromatic_ring_flatness, double_bond_flatness, internal_energy, protein_flexibility, minimum_distance_to_protein, minimum_distance_to_organic_cofactors, minimum_distance_to_inorganic_cofactors, volume_overlap_with_protein, volume_overlap_with_organic_cofactors, volume_overlap_with_inorganic_cofactors. All bool; True = pass.

Output: DataFrame with one column per check, one row per pose, boolean pass/fail.

Python Library API

Goal: Programmatically validate a docked-pose SDF against a receptor PDB and produce a PB-valid filter.

Approach: Instantiate PoseBusters(config='dock'), call bust() on the SDF + PDB pair, and AND-aggregate all boolean check columns into a single pb_valid flag.

from posebusters import PoseBusters
import pandas as pd

bust = PoseBusters(config='dock')

results = bust.bust(
    mol_pred='/path/to/docked_poses.sdf',
    mol_cond='/path/to/receptor.pdb',
)

results['pb_valid'] = results.iloc[:, 4:].all(axis=1)
valid = results[results['pb_valid']]
print(f'{len(valid)} / {len(results)} poses are PB-valid')

Strain Energy Quantification

Beyond binary PB-valid, quantitative strain energy distinguishes "marginal" from "egregious" poses.

Goal: Quantify how far each docked pose is from its lowest-energy free conformer in MMFF94 energy units.

Approach: Generate a reference conformer ensemble (ETKDGv3 + MMFF94), take the global minimum energy as baseline, and report docked_energy - min_ref_energy as strain.

from rdkit import Chem
from rdkit.Chem import AllChem

def ligand_strain(docked_sdf, n_ref=20):
    suppl = Chem.SDMolSupplier(docked_sdf, removeHs=False)
    strains = []
    for docked in suppl:
        if docked is None:
            continue

        smi = Chem.MolToSmiles(docked)
        ref = Chem.MolFromSmiles(smi)
        ref = Chem.AddHs(ref)
        AllChem.EmbedMultipleConfs(ref, numConfs=n_ref, params=AllChem.ETKDGv3())
        AllChem.MMFFOptimizeMoleculeConfs(ref)

        ref_energies = []
        for c in range(ref.GetNumConformers()):
            ff = AllChem.MMFFGetMoleculeForceField(
                ref,
                AllChem.MMFFGetMoleculeProperties(ref),
                confId=c
            )
            ref_energies.append(ff.CalcEnergy())
        min_ref = min(ref_energies)

        docked_ff = AllChem.MMFFGetMoleculeForceField(
            docked,
            AllChem.MMFFGetMoleculeProperties(docked)
        )
        docked_e = docked_ff.CalcEnergy() if docked_ff else None

        strains.append({
            'min_ref_energy': min_ref,
            'docked_energy': docked_e,
            'strain': docked_e - min_ref if docked_e else None,
        })
    return strains

Strain interpretation:

• < 2 kcal/mol: ideal; close to relaxed
• 2-5 kcal/mol: realistic for bioactive conformer
• 5-10 kcal/mol: marginal; check for plausibility

• 10 kcal/mol: implausible without exceptional binding

(Boström 1998 showed bioactive ligands have median strain ~2-4 kcal/mol.)

vdW Overlap with Protein

PoseBusters limit: protein-ligand vdW overlap < 7.5% of ligand vdW volume. Computed via:

def vdw_overlap_fraction(ligand_pdb, protein_pdb):
    from rdkit import Chem
    from rdkit.Chem import AllChem
    # Use RDKit AllChem.UFFGetMoleculeForceField for vdW radii
    # Compute pairwise distances, sum overlap volume
    # (full implementation in PoseBusters; this is a sketch)
    return overlap_fraction

In practice, PoseBusters' bust(...) does this automatically.

Aromatic Ring Planarity

import numpy as np

def aromatic_planarity(mol):
    deviations = []
    for ring in mol.GetRingInfo().AtomRings():
        ring_atoms = [mol.GetAtomWithIdx(i) for i in ring]
        if not all(a.GetIsAromatic() for a in ring_atoms):
            continue
        coords = np.array([mol.GetConformer().GetAtomPosition(i)
                          for i in ring])
        centroid = coords.mean(axis=0)
        centered = coords - centroid
        _, s, vh = np.linalg.svd(centered)
        normal = vh[-1]
        deviation = np.abs(centered @ normal).max()
        deviations.append(deviation)
    return max(deviations) if deviations else 0

Aromatic ring deviation > 0.25 Å is implausible; flag.

Per-Tool Failure Modes

DiffDock-L -- chirality inversion

Trigger: Default DiffDock-L on chiral ligand.

Mechanism: Diffusion sampling can invert stereocenters; not constrained by training.

Symptom: PoseBusters chirality check fails; SMILES from pose != input SMILES.

Fix: Filter to PB-valid; rerun with constrained stereo if supported.

EquiBind -- planar aromatic violation

Trigger: EquiBind single-shot pose prediction.

Mechanism: Equivariant NN does not preserve ring planarity.

Symptom: Aromatic rings buckled.

Fix: Post-relax with UFF/MMFF94 minimization before downstream use.

TANKBind -- vdW clash

Trigger: TANKBind pose in tight pocket.

Mechanism: Distance prediction not constrained to vdW radii.

Symptom: PoseBusters vdW overlap fails.

Fix: Post-process with constrained energy minimization (UFF + frozen heavy atoms 10% relaxation).

Boltz-1 -- bond length distortion

Trigger: Cofactor-bound complex.

Mechanism: Model trained on bond-length distribution; uncommon ligand pushes outside training distribution.

Symptom: Bond lengths >2 std from RDKit reference.

Fix: Post-relax with MMFF94 (constrained to maintain pose).

High strain after Vina docking

Trigger: Highly constrained pocket; flexible ligand.

Mechanism: Vina exhaustiveness limits sampling; not enough rotation search.

Symptom: Strain energy > 8 kcal/mol but pose passes PoseBusters geometric.

Fix: Re-dock with exhaustiveness=32; or post-relax pose with UFF inside pocket.

Reconciliation: PoseBusters vs RMSD

In DiffDock benchmarks, ~25% of poses are RMSD <= 2A AND PB-valid; ~25% RMSD <= 2A but NOT PB-valid (the "false success" rate of AI methods).

Integration into VS Pipeline

import pandas as pd
from posebusters import PoseBusters

def pose_qc_pipeline(docked_sdfs, receptor_pdb):
    bust = PoseBusters(config='dock')
    all_results = []
    for sdf in docked_sdfs:
        r = bust.bust(mol_pred=sdf, mol_cond=receptor_pdb)
        r['pb_valid'] = r.iloc[:, 4:].all(axis=1)
        r['source'] = sdf
        all_results.append(r)
    df = pd.concat(all_results)

    df['rank'] = df.groupby('source')['pb_valid'].cumsum()
    valid_top = df[df['pb_valid']].groupby('source').head(1)
    return valid_top

Common Errors

References

• Buttenschoen et al., Chem. Sci. 15:3130 (2024) -- PoseBusters benchmark and tool.
• Boström, J. Comput.-Aided Mol. Des. 12:383 (1998) -- bioactive ligand strain.
• Hawkins, J. Chem. Inf. Model. 57:1747 (2017) -- conformer ensemble best practices.
• Krasoulis et al., J. Cheminformatics 14:24 (2022) -- DENVIS pose quality assessment.
• Cole et al., Acta Crystallogr. D 64:144 (2008) -- ligand strain in PDB structures.

Related Skills

• chemoinformatics/virtual-screening - Source of poses to validate
• chemoinformatics/ml-docking-rescoring - DiffDock, EquiBind, TANKBind validation
• chemoinformatics/molecular-io - SDF format handling
• chemoinformatics/conformer-generation - Generate reference conformer ensemble for strain
• chemoinformatics/free-energy-calculations - PoseBusters-valid poses for FEP input
• chemoinformatics/covalent-design - Covalent pose validation