Bio Reaction Enumeration

Version Compatibility

Reference examples tested with: RDKit 2024.09+, RDChiral 1.1+, mmpdb 3.1+, scikit-learn 1.4+, 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.

Reaction Enumeration

Generate virtual libraries by applying reaction SMARTS to building blocks, enumerate analog series via matched molecular pairs, decompose into R-groups for SAR modeling, or extract transformations from reaction data. Reaction enumeration sits at the intersection of medicinal chemistry, lead optimization, and de novo design; DOGS is one published example of reaction-driven de novo design (Hartenfeller et al. 2012). The two key operations are transform (apply known reactions to make new compounds) and mine (extract rules from observed analog series). RDKit's reaction SMARTS handles the former; mmpdb / Free-Wilson handle analog-series analysis, while mapped-reaction template extraction requires separate tooling such as RDChiral.

For retrosynthetic planning (target-to-starting-material decomposition), see chemoinformatics/retrosynthesis. For ML-driven design, see chemoinformatics/generative-design. For scaffold-based design, see chemoinformatics/scaffold-analysis.

Operation Taxonomy

Reaction SMARTS Basics

A reaction SMARTS is reactants >> products with atom maps [atom:idx] tracking atoms through the transformation:

from rdkit import Chem
from rdkit.Chem import AllChem

amide = AllChem.ReactionFromSmarts(
    '[C:1](=[O:2])O.[N:3]>>[C:1](=[O:2])[N:3]'
)

errors = amide.Validate()
print(errors)

Atom mapping rules:

• Atoms with the same map index [C:1] in both reactant and product are tracked
• Maps must be unique within each reactant/product
• Atoms present in a reactant template but absent from the product template are removed; atoms present only in the product template are created
• Atoms outside the matched reaction-center template are generally carried through with their reactant molecule
• Bond orders may change; map index preserves identity

Common error: Missing or inconsistent maps for atoms intended to survive within the reaction center can delete atoms, create duplicates, or obscure which reactant atom a product atom represents. Mapping alone does not define the transformation; the reactant and product templates do.

Common Reaction Templates

REACTIONS = {
    'amide_coupling': '[C:1](=[O:2])O.[N:3]>>[C:1](=[O:2])[N:3]',
    'reductive_amination': '[C:1](=O).[NH2:2]>>[CH:1][NH:2]',
    'suzuki': '[c:1][Br].[c:2][B](O)O>>[c:1][c:2]',
    'buchwald_hartwig': '[c:1][Br].[NH:2]>>[c:1][N:2]',
    'sn2_substitution': '[CH:1][Br].[N:2]>>[CH:1][N:2]',
    'sonogashira': '[c:1][Br].[CH:2]#[C:3]>>[c:1][C:2]#[C:3]',
    'click_chemistry': '[N-:1]=[N+:2]=[N:3][CH2:4].[CH:5]#[C:6]>>[N:3]1[N:2]=[N:1][C:6]=[C:5]1[CH2:4]',
    'esterification': '[C:1](=[O:2])O.[OH:3][C:4]>>[C:1](=[O:2])[O:3][C:4]',
    'urea_formation': '[N:1]=C=O.[NH:2]>>[N:1]C(=O)[N:2]',
    'sulfonamide': '[S:1](=O)(=O)Cl.[NH:2]>>[S:1](=O)(=O)[N:2]',
}

These are illustrative templates; real reactions need stereo, protecting-group, and chemoselectivity considerations. For production library enumeration, use a separately curated and validated template collection, such as a vendor catalog. RXNMapper maps atoms in reaction records; it does not by itself supply or validate reaction templates.

Combinatorial Library Enumeration

Goal: Generate every (R1, R2, ..., Rn) product combination from sets of building blocks.

Approach: Cartesian product of reactant lists; apply reaction SMARTS; sanitize + deduplicate.

from itertools import product
from rdkit import Chem
from rdkit.Chem import AllChem, Descriptors

def enumerate_library(rxn_smarts, reactant_lists, mw_max=600):
    rxn = AllChem.ReactionFromSmarts(rxn_smarts)
    num_warnings, num_errors = rxn.Validate()
    if num_errors:
        raise ValueError(f'Invalid reaction: {rxn_smarts}')

    seen = set()
    products = []
    for combo in product(*reactant_lists):
        mols = [Chem.MolFromSmiles(s) for s in combo]
        if None in mols:
            continue

        for prod_tuple in rxn.RunReactants(tuple(mols)):
            for prod in prod_tuple:
                try:
                    Chem.SanitizeMol(prod)
                    smi = Chem.MolToSmiles(prod)
                    if smi in seen:
                        continue
                    if Descriptors.MolWt(prod) > mw_max:
                        continue
                    seen.add(smi)
                    products.append(smi)
                except Exception:
                    continue
    return products

Scaling: For a 100x100x100 enumeration (1M products), parallelize with multiprocessing. For 1k x 1k x 1k (1B products), use a streaming approach + filter before materializing.

RECAP Fragmentation

RECAP (Lewell 1998) breaks molecules at retrosynthetically reasonable bonds into reusable fragments.

from rdkit.Chem import Recap

mol = Chem.MolFromSmiles('c1ccc(C(=O)Nc2ccc(F)cc2)cc1')
hier = Recap.RecapDecompose(mol)
fragments = list(hier.GetLeaves().keys())

RDKit's current Recap.reactionDefs contains 12 cleavage definitions. Inspect the installed definitions when exact coverage matters instead of relying on a shortened functional-group list. Use cases include building-block library generation and scaffold-decoration enumeration.

BRICS Fragmentation

BRICS (Degen 2008) is an extension of RECAP with more bond types. Better fragment coverage; more fragments per molecule.

from itertools import islice
from rdkit.Chem import BRICS

mol = Chem.MolFromSmiles('CCN(CC)c1ccc(C(=O)NC2CCCC2)cc1')
fragments = BRICS.BRICSDecompose(mol)

builder = BRICS.BRICSBuild([Chem.MolFromSmiles(f) for f in fragments])
new_mols = list(islice(builder, 10))

BRICSDecompose produces SMILES with isotope/environment-labeled dummy atoms such as [1*], [5*], and [16*]; BRICSBuild uses those labels when recombining compatible fragments.

R-Group Decomposition

Goal: Given a set of compounds sharing a scaffold, extract the R-group at each attachment point into a tabular SAR matrix.

Approach: Define scaffold with [*:1], [*:2] placeholders; RDKit matches each compound and extracts R-groups.

from rdkit.Chem import rdRGroupDecomposition as rgd
from rdkit import Chem

scaffold = Chem.MolFromSmiles('c1ccc(-[*:1])cc1-[*:2]')

mols = [Chem.MolFromSmiles(smi) for smi in [
    'c1ccc(C)cc1F',
    'c1ccc(CC)cc1Cl',
    'c1ccc(CCC)cc1Br',
]]

decomp, _ = rgd.RGroupDecompose([scaffold], mols, asSmiles=True)

decomp is a list of dicts such as {'Core': scaffold_smi, 'R1': r1_smi, 'R2': r2_smi}. It does not contain assay values. Preserve compound identifiers and explicitly join the decomposition to the activity table before Free-Wilson analysis:

import pandas as pd

compound_ids = ['cmpd-1', 'cmpd-2', 'cmpd-3']
activities = pd.DataFrame({
    'compound_id': compound_ids,
    'pIC50': [6.2, 6.8, 7.1],  # example measurements
})
decomp, unmatched = rgd.RGroupDecompose([scaffold], mols, asSmiles=True)
unmatched = set(unmatched)
matched_ids = [cid for i, cid in enumerate(compound_ids) if i not in unmatched]
decomp_df = pd.DataFrame(decomp)
decomp_df.insert(0, 'compound_id', matched_ids)
sar_table = decomp_df.merge(
    activities, on='compound_id', how='inner', validate='one_to_one'
)

Matched Molecular Pairs Analysis (MMPA)

MMPA (Hussain & Rea 2010) extracts SAR rules from compound pairs differing by a single transformation.

mmpdb fragment data.smi -o data.fragments
mmpdb index data.fragments -o data.mmpdb
mmpdb transform --smiles 'COc1ccccc1' data.mmpdb

mmpdb produces a database of transformations + statistics on activity changes. The values below are synthetic examples showing the output schema; they are not observations from a cited dataset.

Use case: Lead optimization. Given a hit, ask "what transformations have improved similar series?" Apply top-ranked transformations to generate analog suggestions.

Context-based MMPA conditions transformation statistics on local chemical context (for example, "Me -> F adjacent to an amide"). Raut and Dixit (2025) applied this approach to identify transformations associated with reduced CYP1A2 inhibition; that endpoint-specific result should not be generalized as universal superiority over classical MMPA.

Free-Wilson Analysis

Goal: Decompose activity into additive R-group contributions.

Approach: Linear regression with R-group identity as binary features.

import pandas as pd
from sklearn.linear_model import Ridge

def free_wilson(decomp_results, activity_col='pIC50'):
    df = pd.DataFrame(decomp_results)
    r_groups = pd.get_dummies(df[['R1', 'R2']], prefix=['R1', 'R2'])
    X = r_groups.values
    y = df[activity_col].values
    model = Ridge(alpha=0.1).fit(X, y)
    contributions = dict(zip(r_groups.columns, model.coef_))
    return contributions, model.intercept_

Trade-off: Free-Wilson assumes additivity (R1 contribution independent of R2). Real SAR has interactions; Free-Wilson predictions for un-synthesized combinations are biased when synergy exists. Use as a first-pass model for analog prioritization; validate with QSAR.

Template Extraction from Reaction Data

Goal: Given an atom-mapped reaction SMILES, extract a generalizable SMARTS template.

Approach: Use rxnmapper (Schwaller et al. 2021) for atom mapping, then a template extractor such as RDChiral (Coley et al. 2019).

from rxnmapper import RXNMapper

mapper = RXNMapper()
rxns = ['CCO.OC(=O)c1ccccc1>>CCOC(=O)c1ccccc1']
results = mapper.get_attention_guided_atom_maps(rxns)
mapped_smiles = results[0]['mapped_rxn']

RDKit does not provide a ChemicalReaction.GetReactionTemplateFromMappedReaction method. Pass the mapped reaction to RDChiral's published template-extraction workflow, checking the installed package's interface and expected reaction-record schema, or use a separately implemented and validated extractor.

Per-Tool Failure Modes

Reaction SMARTS -- atom mapping mismatch

Trigger: An atom intended to survive the reaction center is absent from, or inconsistently mapped in, the product template.

Mechanism: RDKit constructs products from the reaction templates. Reactant-template atoms omitted from the product template are deleted, product-only atoms are created, and inconsistent maps can prevent intended atom identity from being carried across the transformation.

Symptom: Products missing expected atoms; valences wrong; sanitize fails.

Fix: Validate with rxn.Validate(), inspect warnings separately from errors, and manually verify that every reaction-center atom intended to survive has one consistent map number on both sides.

RECAP/BRICS -- over-fragmentation

Trigger: Highly substituted molecule with many breakable bonds.

Mechanism: Default bond list breaks at every retrosynthetic position; one molecule yields tens of fragments.

Symptom: Building-block enumeration explodes; many small irrelevant fragments.

Fix: Filter fragments by MW (>=80 Da), heavy atom count (>=4); use only meaningful fragments downstream.

MMPA -- insufficient pair count

Trigger: The dataset yields few matched pairs for a transformation in the relevant chemical context.

Mechanism: Sparse or heterogeneous pairs give imprecise, context-confounded estimates. There is no universal minimum dataset size or pair count that guarantees a meaningful effect.

Symptom: Transformations report with N=1-3 pairs; effect sizes erratic.

Fix: Report pair counts and uncertainty, examine local contexts, use a project-justified precision threshold, and supplement with experimental or literature SAR knowledge.

Free-Wilson -- non-additive interactions

Trigger: R1 and R2 interact through hydrogen bonding, steric clash, or electronic effects.

Mechanism: Free-Wilson is purely additive; cannot capture R1+R2 synergy.

Symptom: Predicted activities for un-synthesized combinations are biased low for synergistic pairs.

Fix: Use Free-Wilson as first-pass screen; validate predictions with QSAR (random forest, chemprop) which captures interactions.

R-group decomposition -- multiple scaffolds

Trigger: Compound matches multiple scaffold templates.

Mechanism: RDKit accepts cores ordered from most to least specific and exposes parameters for multi-core matching and alignment. Ambiguous or inconsistently specified cores can change the resulting labels and SAR table.

Symptom: Same compound's R-groups differ between runs.

Fix: Order cores from most to least specific, label attachment points explicitly, inspect unmatched compounds, and keep a compound only when its selected core assignment matches the intended SAR series.

Reaction enumeration -- combinatorial explosion

Trigger: Large building-block sets (1k x 1k = 1M products).

Mechanism: Cartesian product * RunReactants is O(N^d) where d is reactant count.

Symptom: Memory blowup, multi-hour runtime.

Fix: Pre-filter building blocks; stream products to file rather than list; use mmpdb-style sparse enumeration only for valid pairings.

Reconciliation: Free-Wilson vs MMPA

Both methods derive R-group rules but from different perspectives:

• Free-Wilson: linear regression on assembled SAR table; gives R-group contributions
• MMPA: transformation-based; gives delta(activity) for each substitution

If they agree on direction (Me->F improves activity), high confidence. If they disagree, investigate non-additive interactions or look for context dependence in MMPA.

Common Errors

References

• Hartenfeller M et al. "DOGS: Reaction-Driven de novo Design of Bioactive Compounds." PLoS Comput. Biol. 8:e1002380 (2012). DOI: 10.1371/journal.pcbi.1002380.
• Lewell XQ, Judd DB, Watson SP, Hann MM. "RECAP—Retrosynthetic Combinatorial Analysis Procedure." J. Chem. Inf. Comput. Sci. 38:511–522 (1998). DOI: 10.1021/ci970429i.
• Degen J, Wegscheid-Gerlach C, Zaliani A, Rarey M. "On the Art of Compiling and Using 'Drug-Like' Chemical Fragment Spaces." ChemMedChem 3:1503–1507 (2008). DOI: 10.1002/cmdc.200800178.
• Hussain J, Rea C. "Computationally Efficient Algorithm to Identify Matched Molecular Pairs (MMPs) in Large Data Sets." J. Chem. Inf. Model. 50:339–348 (2010). DOI: 10.1021/ci900450m.
• Dossetter AG, Griffen EJ, Leach AG. "Matched molecular pair analysis in drug discovery." Drug Discov. Today 18:724–731 (2013). DOI: 10.1016/j.drudis.2013.03.003.
• Free SM, Wilson JW. "A Mathematical Contribution to Structure-Activity Studies." J. Med. Chem. 7:395–399 (1964). DOI: 10.1021/jm00334a001.
• Schwaller P et al. "Unsupervised attention-guided atom-mapping." Sci. Adv. 7:eabe4166 (2021). DOI: 10.1126/sciadv.abe4166.
• Raut JA, Dixit VA. "A context-based matched molecular pair analysis identifies structural transformations that reduce CYP1A2 inhibition." RSC Med. Chem. 16:3281–3290 (2025). DOI: 10.1039/D4MD01012D.
• Coley CW, Green WH, Jensen KF. "RDChiral: An RDKit Wrapper for Handling Stereochemistry in Retrosynthetic Template Extraction and Application." J. Chem. Inf. Model. 59:2529–2537 (2019). DOI: 10.1021/acs.jcim.9b00286.
• Dalke A, Hert J, Kramer C. "mmpdb: An Open-Source Matched Molecular Pair Platform for Large Multiproperty Data Sets." J. Chem. Inf. Model. 58:902–910 (2018). DOI: 10.1021/acs.jcim.8b00173.
• RDKit Book, reaction SMARTS and reaction handling: https://www.rdkit.org/docs/RDKit_Book.html.
• RDKit R-group decomposition API: https://www.rdkit.org/docs/source/rdkit.Chem.rdRGroupDecomposition.html.

Related Skills

• chemoinformatics/molecular-io - Read/write reaction SMILES
• chemoinformatics/substructure-search - SMARTS pattern matching
• chemoinformatics/scaffold-analysis - Bemis-Murcko scaffolds for R-decomp
• chemoinformatics/molecular-descriptors - Featurize products
• chemoinformatics/admet-prediction - Filter enumerated products
• chemoinformatics/retrosynthesis - Reverse direction (target -> starting materials)
• chemoinformatics/generative-design - Generative alternatives to template enumeration
• chemoinformatics/qsar-modeling - Validate Free-Wilson predictions