validator Module

The validator module provides comprehensive geometric and biophysical validation for protein structures.

Overview

The validator module implements standard checks to ensure generated structures adhere to physical and biological principles. It is often used to "pre-screen" structures before expensive physics-based refinement.

Main Classes

::: synth_pdb.validator.PDBValidator options: show_root_heading: true show_source: true members: - init - validate_all - validate_bond_lengths - validate_bond_angles - validate_ramachandran - validate_steric_clashes - validate_peptide_plane - validate_sequence_improbabilities - validate_side_chain_rotamers - validate_chirality

Validation Checks

Bond Geometry

Standard bond lengths (N-CA, CA-C, C-N, C-O) and angles are checked against equilibrium values with configurable tolerances.

Ramachandran Analysis

Dihedral angles ($\phi, \psi$) are validated against MolProbity-style polygonal regions. The module distinguishes between: - Favored: 98% of high-quality protein structures. - Allowed: 99.8% of high-quality protein structures. - Outlier: Highly improbable conformations.

Specific polygons are used for Glycine, Proline, and Pre-Proline residues.

Steric Clashes

Detects overlapping atoms that are too close in space. - Minimum Distance: Atoms must be ≥ 0.5 Å apart. - VdW Overlap: Atoms are checked against their Van der Waals radii. - CA-CA Spacing: C-alpha atoms of non-consecutive residues must be ≥ 3.0 Å apart.

Side-Chain Rotamers

Validates side-chain conformations against the Dunbrack Backbone-Dependent Rotamer Library. Rotamers that deviate significantly from favored staggered conformations are flagged.

Usage Examples

Basic Validation

from synth_pdb.validator import PDBValidator

# Load PDB content
with open("peptide.pdb") as f:
    pdb_content = f.read()

# Run validator
validator = PDBValidator(pdb_content)
validator.validate_all()

# Get violations
violations = validator.get_violations()
for v in violations:
    print(v)

Educational Notes

Physics of the Ramachandran Plot

The Ramachandran plot is the "map of allowed protein shapes". It is primarily based on hard-sphere sterics:

  1. The Clash: For most $\phi/\psi$ angles, the carbonyl oxygen of residue $i$ clashes with the amide hydrogen of residue $i+1$, or the side-chain $C_\beta$ atom clashes with the backbone.
  2. The Exceptions:
    • Glycine: Has no $C_\beta$ atom, allowing it to access regions that are "illegal" for other amino acids. It serves as a flexible hinge.
    • Proline: Its cyclic side-chain locks its $\phi$ angle to $\sim -65^\circ$, acting as a structural stiffener.

Side-Chain Packing (Rotamers)

Side chains are not free to rotate continuously. They snap into specific discrete conformations called Rotamers (Rotational Isomers) to minimize steric repulsion. These are typically staggered conformations ($gauche^+$, $gauche^-$, $trans$).

References

  • Ramachandran Plot: Ramachandran, G. N., Ramakrishnan, C., & Sasisekharan, V. (1963). "Stereochemistry of polypeptide chain configurations." Journal of Molecular Biology. DOI: 10.1016/S0022-2836(63)80023-6
  • MolProbity (Top8000): Williams, C. J., et al. (2018). "MolProbity: More and better reference data for improved all-atom structure validation." Protein Science. DOI: 10.1002/pro.3330
  • Rotamer Library: Dunbrack, R. L., & Cohen, F. E. (1997). "Bayesian statistical analysis of protein side-chain rotamer preferences." Protein Science. DOI: 10.1002/pro.5560060802
  • Calpha Geometry: Lovell, S. C., et al. (2003). "Structure validation by Calpha geometry: phi,psi and Cbeta deviation." Proteins. DOI: 10.1002/prot.10286

See Also