Bio structural biology geometric analysis

Version Compatibility

Reference examples tested with: biopython 1.83+, numpy 1.26+

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.

Geometric Analysis

"Calculate the RMSD between two conformations" -> superimpose on a defined atom correspondence, then report deviation.

• Python: Bio.PDB.Superimposer (SVD/Kabsch); Bio.PDB.qcprot for speed in tight loops "How buried is this residue?" -> compute solvent accessible surface area and normalize to a per-residue maximum.
• Python: Bio.PDB.SASA.ShrakeRupley, then relative SASA against a max-ASA scale

Governing Principle: the metric IS the question

RMSD is NOT a property of two structures. It is a property of two structures GIVEN a superposition AND an atom selection - change either and the number changes. Report what was aligned (CA-only? a defined core? all-atom?) or the number is uninterpretable.

Global all-atom RMSD is a mean of SQUARED per-atom deviations after a least-squares rigid-body fit, so it is dominated by the worst-fitting atoms. A structure whose 150-residue core is essentially identical but whose 10-residue loop or a hinge-rotated domain swings out by 15A reports a "bad" whole-molecule RMSD (often 4-8A) that hides a near-perfect core. RMSD is also length-dependent (longer proteins accumulate larger RMSD for the same local quality) and is NOT a cross-protein similarity metric - it is only meaningful when a genuine 1:1 correspondence exists (same protein, two states; a model vs its native).

Superimposer requires an equal-length, ORDERED atom-to-atom correspondence. Feeding it mismatched or unequal atom lists is the classic error; it computes the optimal fit (Kabsch via SVD), it does NOT solve which atom maps to which. Structures with different sequences need a structure-based alignment FIRST to establish the correspondence (see alignment/structural-alignment), then superposition.

SASA depends on the PROBE RADIUS (1.4A water is a convention, not a constant of nature), the algorithm (Bio.PDB ShrakeRupley is Shrake-Rupley only; freesasa offers Lee-Richards and LCPO), and whether hydrogens are present. A SASA number without its probe radius is meaningless, and absolute SASA in A^2 is not portable across tools. Prefer RELATIVE SASA (residue SASA / max-ASA of that residue type) using the Tien et al 2013 max-ASA scale.

Backbone phi/psi and omega/cis-peptides are a VALIDATION signal, not a description: a residue in a sterically disallowed Ramachandran region usually means a modeling error, not exotic biology. This skill computes the angles; interpreting outliers as quality flags belongs to structural-biology/structure-validation.

Decision: which comparison metric

Metric	Answers (use for)	Caveat	Who reports it
RMSD on a defined core	same molecule, how far did it move after best-fit	needs a real 1:1 correspondence; outlier-dominated (squared mean); length- and selection-dependent; NOT cross-protein	Bio.PDB Superimposer.rms
TM-score (>0.5 = same fold)	different proteins - same fold? fold recognition	length-normalized and outlier-resistant, but asymmetric (state the reference chain); needs an alignment first	TM-align / US-align (alignment/structural-alignment)
GDT-TS / GDT-HA	CASP-style full-model accuracy vs native	superposition-based but fraction-within-cutoff, not a squared mean	LGA / CASP assessors
lDDT (0-100; pLDDT for AlphaFold models)	local model quality, multi-domain, without picking a superposition	superposition-free, immune to domain motion; the quantity pLDDT predicts	OpenStructure lDDT / AlphaFold pLDDT

For cross-protein or fold-similarity work, do not stretch RMSD - route to alignment/structural-alignment (TM-align / Foldseek / DALI). For Ramachandran/omega as a quality gate, route to structural-biology/structure-validation.

Distance Between Atoms

from Bio.PDB import PDBParser
import numpy as np

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

chain = structure[0]['A']
atom1, atom2 = chain[100]['CA'], chain[200]['CA']

distance = atom1 - atom2                              # Atom subtraction returns the distance directly
print(f'Distance: {distance:.2f} A')
print(np.linalg.norm(atom1.coord - atom2.coord))     # Equivalent via numpy on the .coord arrays

Distance Matrix

import numpy as np
from Bio.PDB import PDBParser

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

ca_atoms = [r['CA'] for r in structure.get_residues() if r.has_id('CA') and r.id[0] == ' ']  # id[0]==' ' drops waters/hetero
n = len(ca_atoms)

dist = np.zeros((n, n))
for i in range(n):
    for j in range(i + 1, n):
        dist[i, j] = dist[j, i] = ca_atoms[i] - ca_atoms[j]
print(f'Distance matrix: {dist.shape}')

Bond Angle

import numpy as np
from Bio.PDB import PDBParser, calc_angle

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

res = structure[0]['A'][100]
angle = calc_angle(res['N'].get_vector(), res['CA'].get_vector(), res['C'].get_vector())  # calc_angle needs Vector, not .coord
print(f'N-CA-C angle: {np.degrees(angle):.1f} deg')

Backbone Dihedrals (phi / psi)

import numpy as np
from Bio.PDB import PDBParser, calc_dihedral

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')
chain = structure[0]['A']

prev, curr, nxt = chain[99], chain[100], chain[101]
phi = calc_dihedral(prev['C'].get_vector(), curr['N'].get_vector(), curr['CA'].get_vector(), curr['C'].get_vector())
psi = calc_dihedral(curr['N'].get_vector(), curr['CA'].get_vector(), curr['C'].get_vector(), nxt['N'].get_vector())
print(f'phi={np.degrees(phi):.1f}  psi={np.degrees(psi):.1f}')

Ramachandran Angles for All Residues

import numpy as np
from Bio.PDB import PDBParser, PPBuilder

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

ppb = PPBuilder()                                    # PPBuilder builds peptides from connectivity, so chain breaks split them
rama = []
for pp in ppb.build_peptides(structure):
    for res, (phi, psi) in zip(pp, pp.get_phi_psi_list()):
        if phi is not None and psi is not None:      # None at termini and chain breaks by design - skip, do not fabricate
            rama.append((res.resname, np.degrees(phi), np.degrees(psi)))
print(f'{len(rama)} residues with phi/psi')

Outliers in disallowed regions are usually refinement errors, not biology - interpret them as a quality gate in structural-biology/structure-validation.

Chi Angles (Sidechain Dihedrals)

import numpy as np
from Bio.PDB import PDBParser, calc_dihedral

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')
res = structure[0]['A'][100]

if res.has_id('CB') and res.has_id('CG'):            # Gly/Ala lack CB/CG; chi atom quartets are residue-type specific
    chi1 = calc_dihedral(res['N'].get_vector(), res['CA'].get_vector(), res['CB'].get_vector(), res['CG'].get_vector())
    print(f'Chi1: {np.degrees(chi1):.1f} deg')

Superimposing Structures and RMSD

from Bio.PDB import PDBParser, Superimposer

parser = PDBParser(QUIET=True)
ref = parser.get_structure('ref', 'reference.pdb')
mob = parser.get_structure('mobile', 'mobile.pdb')

ref_ca = [r['CA'] for r in ref.get_residues() if r.has_id('CA') and r.id[0] == ' ']
mob_ca = [r['CA'] for r in mob.get_residues() if r.has_id('CA') and r.id[0] == ' ']
n = min(len(ref_ca), len(mob_ca))                    # Superimposer needs EQUAL-LENGTH ORDERED lists; it does NOT solve correspondence
ref_ca, mob_ca = ref_ca[:n], mob_ca[:n]              # Naive truncation is only valid when residues already correspond 1:1

sup = Superimposer()
sup.set_atoms(ref_ca, mob_ca)                        # Optimal rigid-body fit via SVD/Kabsch
print(f'RMSD (CA): {sup.rms:.2f} A')
rotation, translation = sup.rotran                   # The fitted transform, for reuse on other atoms
sup.apply(mob.get_atoms())                           # Mutates mob in place - copy first if the originals are still needed

QCP alternative for speed in tight loops (MD, all-vs-all): from Bio.PDB.qcprot import QCPSuperimposer (module Bio.PDB.qcprot; historically Bio.PDB.QCPSuperimposer), same set_atoms / .rms / .rotran / apply interface and identical optimum.

Per-Residue Deviation After Superposition

Fitting minimizes the squared mean, so a global scalar hides where the structures actually differ. A per-residue deviation plot exposes the outlier domination directly.

import numpy as np
from Bio.PDB import PDBParser, Superimposer

parser = PDBParser(QUIET=True)
ref = parser.get_structure('ref', 'reference.pdb')
mob = parser.get_structure('mobile', 'mobile.pdb')

ref_ca = [r['CA'] for r in ref.get_residues() if r.has_id('CA') and r.id[0] == ' ']
mob_ca = [r['CA'] for r in mob.get_residues() if r.has_id('CA') and r.id[0] == ' ']
n = min(len(ref_ca), len(mob_ca))
ref_ca, mob_ca = ref_ca[:n], mob_ca[:n]

sup = Superimposer()
sup.set_atoms(ref_ca, mob_ca)
sup.apply([a for a in mob_ca])                        # Move only the paired CA set into the fitted frame
deviation = np.array([r - m for r, m in zip(ref_ca, mob_ca)])
print(f'core (<2A) residues: {(deviation < 2.0).sum()} / {n}')   # 2A is a common rigid-core cutoff, not a law
print(f'max deviation: {deviation.max():.2f} A at index {deviation.argmax()}')

Center of Mass and Radius of Gyration

import numpy as np
from Bio.PDB import PDBParser

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

atoms = list(structure.get_atoms())
coords = np.array([a.coord for a in atoms])
masses = np.array([{'C': 12.0, 'N': 14.0, 'O': 16.0, 'S': 32.0, 'H': 1.0}.get(a.element, 12.0) for a in atoms])

com = (masses[:, None] * coords).sum(axis=0) / masses.sum()
print(f'Center of mass: {com}')

rg = np.sqrt(np.mean(np.sum((coords - coords.mean(axis=0)) ** 2, axis=1)))  # Unweighted radius of gyration
print(f'Radius of gyration: {rg:.2f} A')

Vector Operations

from Bio.PDB import PDBParser

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

v1 = structure[0]['A'][100]['CA'].get_vector()
v2 = structure[0]['A'][101]['CA'].get_vector()

diff = v2 - v1
print(f'length: {diff.norm():.2f}  unit: {diff.normalized()}')
cross = v1 ** v2                                      # ** is cross product on Vector objects
dot = v1 * v2                                         # * is dot product on Vector objects

Solvent Accessible Surface Area (SASA)

from Bio.PDB import PDBParser
from Bio.PDB.SASA import ShrakeRupley

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')

sr = ShrakeRupley(probe_radius=1.40, n_points=100)   # 1.4A = water radius (convention); a SASA number is meaningless without its probe radius
sr.compute(structure, level='R')                     # level R attaches .sasa on each residue; children sum to parents
print(f'total SASA: {sum(r.sasa for r in structure.get_residues() if hasattr(r, "sasa")):.1f} A^2')

Relative SASA and Burial

Absolute SASA is not portable across tools. For burial, normalize to a per-residue maximum (Tien et al 2013 theoretical Gly-X-Gly max-ASA).

from Bio.PDB import PDBParser
from Bio.PDB.SASA import ShrakeRupley

MAX_ASA = {'ALA': 129.0, 'ARG': 274.0, 'ASN': 195.0, 'ASP': 193.0, 'CYS': 167.0, 'GLU': 223.0, 'GLN': 225.0, 'GLY': 104.0, 'HIS': 224.0, 'ILE': 197.0, 'LEU': 201.0, 'LYS': 236.0, 'MET': 224.0, 'PHE': 240.0, 'PRO': 159.0, 'SER': 155.0, 'THR': 172.0, 'TRP': 285.0, 'TYR': 263.0, 'VAL': 174.0}

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')
ShrakeRupley().compute(structure, level='R')

buried = 0
for res in structure.get_residues():
    if res.resname in MAX_ASA and hasattr(res, 'sasa'):
        rsa = res.sasa / MAX_ASA[res.resname]
        if rsa < 0.20:                               # RSA < 0.20 is the common buried heuristic (a rule of thumb, not a law)
            buried += 1
print(f'buried residues (RSA < 0.20): {buried}')

Secondary-Structure Assignment (DSSP)

Secondary structure is an INTERPRETATION, not a value stored in the file: DSSP, STRIDE, and P-SEA legitimately disagree by 1-2 residues at helix and strand termini, so name the tool and version and never mix assignments from two tools in one analysis. DSSP places the backbone amide hydrogen itself and scores an electrostatic H-bond energy, so it needs no explicit hydrogens, and it processes only the FIRST model of an ensemble. The binary was renamed dssp -> mkdssp (v4) and must be installed separately.

from Bio.PDB import PDBParser, DSSP

parser = PDBParser(QUIET=True)
structure = parser.get_structure('protein', 'protein.pdb')
model = structure[0]                                  # DSSP runs on ONE model only

dssp = DSSP(model, 'protein.pdb', dssp='mkdssp')      # pass the current binary name explicitly
codes = [dssp[k][2] for k in dssp.keys()]             # 8-state: H G I helix, E B strand, T S P - other
helix = sum(c in 'HGI' for c in codes)
strand = sum(c in 'EB' for c in codes)
print(f'helix {helix}, strand {strand}, other {len(codes) - helix - strand} of {len(codes)}')

Common Errors

Symptom	Cause	Fix
Superimposer errors on differing list sizes	atom lists unequal length; it needs a 1:1 ordered correspondence	match residues by id, or for sequence-different structures align first (alignment/structural-alignment)
DSSP helix/strand counts differ from another tool	DSSP, STRIDE, P-SEA disagree at element termini; no ground truth	name the tool+version; never mix assignments; compare like-for-like
RMSD is 4-8A for structures that clearly share a fold	global fit dominated by flexible loops/termini/hinge; mean of SQUARED deviations	fit on a defined rigid core, report core vs mobile separately; or use per-residue deviation / TM-score
RMSD differs between runs on the "same" pair	different atom selection (CA vs all-atom) or superposition	state the correspondence and fit selection explicitly and hold it constant
Ranking models of different length by RMSD	RMSD is length-dependent and not a cross-protein metric	use TM-score (length-normalized) or lDDT (superposition-free)
calc_angle/calc_dihedral AttributeError	passed numpy arrays (.coord) not Vector objects	pass atom.get_vector()
phi/psi is None at chain ends	terminal residues lack a preceding C or following N	skip None; get_phi_psi_list returns None at breaks/termini by design
SASA disagrees with a published value	different probe radius, radii set, algorithm, or H atoms present	recompute all structures like-for-like in one tool; report probe_radius; prefer relative SASA
.sasa attribute missing on residues	compute() run at the wrong level or read before it	call sr.compute(entity, level='R') then read residue.sasa
Distance matrix polluted by waters/heteroatoms	iterating residues without filtering the hetflag	filter residue.id[0] == ' '
Chi1 computed for Gly/Ala	those residues have no CB/CG	guard has_id('CB') and has_id('CG'); chi quartets are residue-type specific
Two crystal forms called "different states" at 2A RMSD	difference within coordinate uncertainty / ensemble spread	compare against B-factors, resolution, and NMR ensemble spread before claiming a state change
Calling a predicted model "wrong" where it deviates from a crystal structure	the deviating region may be low-pLDDT, a PAE-uncertain inter-domain float, or the crystal is a different (holo/packing) state	overlay pLDDT/PAE on the deviation before judging (alphafold-predictions); confirm it is not just a state difference
Original coordinates changed unexpectedly	Superimposer.apply and atom.transform mutate in place	copy the structure first if the untransformed coordinates are still needed

Related Skills

• structure-io - Parse and write PDB/mmCIF structure files
• structure-navigation - Walk chains, residues, atoms; handle altlocs and disordered residues
• structure-modification - Transform coordinates and edit structures in place
• structural-biology/interface-analysis - Residue contacts, contact maps, and buried-surface interface analysis (NeighborSearch)
• structural-biology/structure-validation - Ramachandran and omega/cis-peptide outliers as a quality gate
• structural-biology/alphafold-predictions - overlay pLDDT/PAE when a compared structure is a predicted model
• structural-biology/modern-structure-prediction - reconcile predicted models via pLDDT/PAE/pTM before RMSD claims
• alignment/structural-alignment - Cross-protein fold comparison and correspondence (TM-align, Foldseek, DALI)

References

• Cock PJA, et al. (2009) Biopython. Bioinformatics 25(11):1422-1423.
• Kabsch W (1976) A solution for the best rotation to relate two sets of vectors. Acta Crystallogr A 32:922-923.
• Theobald DL (2005) Rapid calculation of RMSDs using a quaternion-based characteristic polynomial. Acta Crystallogr A 61(4):478-480.
• Zhang Y, Skolnick J (2004) Scoring function for automated assessment of protein structure template quality. Proteins 57(4):702-710.
• Xu J, Zhang Y (2010) How significant is a protein structure similarity with TM-score = 0.5? Bioinformatics 26(7):889-895.
• Mariani V, Biasini M, Barbato A, Schwede T (2013) lDDT: a local superposition-free score for comparing protein structures and models. Bioinformatics 29(21):2722-2728.
• Shrake A, Rupley JA (1973) Environment and exposure to solvent of protein atoms. Lysozyme and insulin. J Mol Biol 79(2):351-371.
• Tien MZ, Meyer AG, Sydykova DK, Spielman SJ, Wilke CO (2013) Maximum allowed solvent accessibilities of residues in proteins. PLoS ONE 8(11):e80635.