feat(validation): add isotope tagging utilities

src/macro_lactone_toolkit/validation/isotope_utils.py

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+from __future__ import annotations
+
+from rdkit import Chem
+
+
+def build_fragment_with_isotope(
+    mol: Chem.Mol,
+    side_chain_atoms: list[int],
+    side_chain_start_idx: int,
+    ring_atom_idx: int,
+    cleavage_position: int,
+) -> tuple[str, str, str]:
+    """
+    Build fragment SMILES with isotope tagging.
+
+    Returns:
+        Tuple of (labeled_smiles, plain_smiles, bond_type)
+    """
+    # Get original bond type
+    bond = mol.GetBondBetweenAtoms(ring_atom_idx, side_chain_start_idx)
+    bond_type = bond.GetBondType().name if bond else "SINGLE"
+
+    # Create editable molecule
+    emol = Chem.EditableMol(Chem.Mol(mol))
+
+    # Add dummy atom with isotope = cleavage position
+    dummy_atom = Chem.Atom(0)
+    dummy_atom.SetIsotope(cleavage_position)
+    dummy_idx = emol.AddAtom(dummy_atom)
+
+    # Add bond between dummy and side chain start
+    emol.AddBond(dummy_idx, side_chain_start_idx, bond.GetBondType())
+
+    # Determine atoms to keep
+    atoms_to_keep = set([dummy_idx, side_chain_start_idx] + list(side_chain_atoms))
+
+    # Remove atoms not in keep list
+    # Need to remove in reverse order to maintain valid indices
+    all_atoms = list(range(mol.GetNumAtoms()))
+    atoms_to_remove = [i for i in all_atoms if i not in atoms_to_keep]
+
+    for atom_idx in sorted(atoms_to_remove, reverse=True):
+        emol.RemoveAtom(atom_idx)
+
+    fragment = emol.GetMol()
+    Chem.SanitizeMol(fragment)
+
+    # Get labeled SMILES (with isotope)
+    labeled_smiles = Chem.MolToSmiles(fragment)
+
+    # Get plain SMILES (without isotope)
+    plain_fragment = Chem.Mol(fragment)
+    for atom in plain_fragment.GetAtoms():
+        if atom.GetIsotope() > 0:
+            atom.SetIsotope(0)
+    plain_smiles = Chem.MolToSmiles(plain_fragment)
+
+    return labeled_smiles, plain_smiles, bond_type
+
+
+def extract_isotope_position(fragment_smiles: str) -> int:
+    """Extract cleavage position from fragment SMILES."""
+    mol = Chem.MolFromSmiles(fragment_smiles)
+    if mol is None:
+        return 0
+
+    for atom in mol.GetAtoms():
+        if atom.GetAtomicNum() == 0 and atom.GetIsotope() > 0:
+            return atom.GetIsotope()
+    return 0

tests/validation/test_isotope_utils.py

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+import pytest
+from rdkit import Chem
+
+from macro_lactone_toolkit.validation.isotope_utils import (
+    build_fragment_with_isotope,
+    extract_isotope_position,
+)
+
+
+def test_build_fragment_with_isotope():
+    # Create a simple test molecule: ethyl group attached to position 5
+    mol = Chem.MolFromSmiles("CCCC(CC)CCC")  # Position 4 (0-indexed) has ethyl
+    assert mol is not None
+
+    side_chain_atoms = [4, 5]  # The ethyl group atoms
+    side_chain_start = 4
+    ring_atom = 3
+    cleavage_pos = 5
+
+    labeled, plain, bond_type = build_fragment_with_isotope(
+        mol, side_chain_atoms, side_chain_start, ring_atom, cleavage_pos
+    )
+
+    assert labeled is not None
+    assert plain is not None
+    assert bond_type == "SINGLE"
+
+    # Check isotope was set
+    extracted_pos = extract_isotope_position(labeled)
+    assert extracted_pos == cleavage_pos
+
+    # Plain should have no isotope
+    extracted_plain = extract_isotope_position(plain)
+    assert extracted_plain == 0