Scaffold Analysis

Scaffold analysis extracts the core ring system from molecules using the Murcko framework method, enabling structure-activity relationship (SAR) analysis and scaffold hopping.

What Is a Murcko Scaffold?

A Murcko scaffold is the core ring system and linkers of a molecule with all side chains removed.

Example (Ibuprofen):

• Full structure: CC(C)Cc1ccc(cc1)C(C)C(=O)O
• Murcko scaffold: c1ccc(cc1)C
• Generic scaffold: C1CCC(CC1)C

Types of Scaffolds

Murcko Scaffold

Preserves atom types in the ring system and linkers:

• Aromatic vs. aliphatic rings distinguished
• Heteroatoms preserved
• Useful for detailed SAR analysis

Generic Scaffold

Replaces all atoms with carbon and all bonds with single bonds:

• Focuses on topology only
• Groups molecules by ring framework
• Useful for broad scaffold classification

Output

Scaffold analysis returns:

Field	Description
scaffold_smiles	SMILES of Murcko scaffold
generic_scaffold_smiles	SMILES of generic scaffold
has_scaffold	Whether molecule contains ring systems
message	Explanation if no scaffold found

API Usage

curl -X POST http://localhost:8001/api/v1/score \
  -H "Content-Type: application/json" \
  -d '{
    "molecule": "CC(C)Cc1ccc(cc1)C(C)C(=O)O",
    "include": ["scaffold"]
  }'

Response (with scaffold):

{
  "scaffold": {
    "scaffold_smiles": "c1ccc(cc1)C",
    "generic_scaffold_smiles": "C1CCC(CC1)C",
    "has_scaffold": true,
    "message": "Scaffold extracted successfully"
  }
}

Response (no scaffold):

{
  "scaffold": {
    "scaffold_smiles": null,
    "generic_scaffold_smiles": null,
    "has_scaffold": false,
    "message": "No ring system found"
  }
}

Use Cases

SAR Analysis

Group molecules by scaffold to analyze structure-activity relationships:

1. Extract scaffolds from active compounds
2. Group by scaffold SMILES
3. Analyze R-group effects on activity
4. Identify optimal substitution patterns

Scaffold Hopping

Find alternative scaffolds with similar properties:

1. Extract scaffold from lead compound
2. Search for molecules with different scaffolds
3. Test for similar biological activity
4. Expand chemical space explored

Library Diversity

Assess scaffold diversity in compound libraries:

1. Extract generic scaffolds from all molecules
2. Count unique scaffolds
3. Calculate scaffold diversity metrics
4. Identify over-represented scaffolds

Hit Clustering

Cluster screening hits by scaffold:

1. Extract scaffolds from all hits
2. Group by identical scaffolds
3. Prioritize diverse scaffolds
4. Avoid redundant follow-up

Molecules Without Scaffolds

Molecules without ring systems return has_scaffold: false:

Examples:

• Linear molecules (ethanol, hexane)
• Branched aliphatics
• Simple derivatives without rings

These molecules have no Murcko scaffold by definition.

Scaffold Statistics in Batch Processing

In batch mode, scaffold diversity is reported:

{
  "scaffold_statistics": {
    "total_molecules": 1000,
    "molecules_with_scaffolds": 850,
    "unique_scaffolds": 45,
    "unique_generic_scaffolds": 38,
    "scaffold_diversity": 0.053
  }
}

Scaffold diversity = unique scaffolds / total molecules

• Higher diversity = more varied scaffolds
• Lower diversity = repeated scaffold use

Combining with Other Scores

Scaffold + NP-likeness:

Natural products often have complex scaffolds:

has_scaffold = true AND
np_likeness_score > 1.0

Scaffold + Drug-likeness:

Drug-like scaffolds typically:

has_scaffold = true AND
lipinski_passed = true AND
num_rings <= 5

Best Practices

1. Use for grouping: Scaffold is ideal for clustering molecules
2. Generic for broad classes: Use generic scaffold for high-level classification
3. Murcko for SAR: Use Murcko scaffold for detailed SAR
4. Track diversity: Monitor scaffold diversity in libraries
5. Combine with clustering: Use with fingerprint clustering for comprehensive analysis

Limitations

Scaffold definition:

• Only captures ring systems
• Doesn't account for stereochemistry
• Linker atoms may vary between definitions

Edge cases:

• Spiro compounds: Complex scaffold extraction
• Macrocycles: Entire molecule may be the scaffold
• Fused systems: Multiple valid scaffold interpretations

Complex Ring Systems

For molecules with complex or unusual ring systems (spiro, bridged, cage), manually review scaffold extraction results.

Next Steps

• NP-Likeness - Natural product similarity
• Batch Processing - Analyze scaffold diversity
• Scoring Overview - All scoring systems