A minor correction: [H] by itself *is* valid and means a hydrogen atom. The Daylight docs say as much in section 4.1. But in other contexts it means a hydrogen count, so to be safe, always using #1 to mean a hydrogen atom can be a good practice.
If you are ever in doubt about how RDKit is interpreting a SMARTS, I recommend making use of the DescribeQuery function which provides a tree representation of a query atom or bond. For example (comments added): >>> mol = Chem.MolFromSmarts('[H][N,H][N,#1]') >>> print(mol.GetAtomWithIdx(0).DescribeQuery()) # [H] AtomAtomicNum 1 = val # [H] interpreted as a hydrogen atom >>> print(mol.GetAtomWithIdx(1).DescribeQuery()) # [N,H] AtomOr AtomType 7 = val AtomHCount 1 = val # H interpreted as a hydrogen count # Overall query atom means "an aliphatic nitrogen OR (any atom with one hydrogen)! >>> print(mol.GetAtomWithIdx(2).DescribeQuery()) # [N,#1] AtomOr AtomType 7 = val AtomAtomicNum 1 = val # "#1" is atomic number, therefore a hydrogen atom. # Overall query atom means "an aliphatic nitrogen OR a hydrogen" One non-obvious convention in the DescribeQuery output is that AtomType implies aliphatic when the value is a normal atomic number, or aromatic if the atomic number is offset by 1000. For example, [n] is "AtomType 1007". Hope you find this approach useful in the future. Ivan On Tue, Mar 1, 2022 at 6:33 AM David Cosgrove <davidacosgrov...@gmail.com> wrote: > Hi Adam > There are a number of issues here. The key one, I think, is a > misunderstanding about the meaning of H in SMARTS. It means "a single > attached hydrogen", and is a qualifier for another atom, it cannot be used > by itself. So [*H] is valid, [H] isn't. See the table at > https://www.daylight.com/dayhtml/doc/theory/theory.smarts.html. If you > want to refer to an explicit hydrogen, you have to use [#1]. However, that > will only match an explicit hydrogen in the molecule, not an implicit one. > Thus c[#1] doesn't match anything in c1ccccc1. If you have read in a > molecule from a molfile, for example, that has explicit hydrogens then you > will be ok. > > Further to that, your SMARTS strings, at least as they have appeared in > gmail, which may have garbled them, are incorrect. In S1, the brackets > round [N,n,H] make it a substituent, so it will not match the indole > nitrogen. Also, it would probably be better as [N,n;H], which would be > read as "(aliphatic nitrogen OR aromatic nitrogen) AND 1 attached > hydrogen." The [N,n,H] will match a methylated indole nitrogen which I > imagine is not what you want. Similar remarks apply to S2. A SMARTS that > matches both 6CI and PCT > is [C,c]1(Cl)[C,c][C,c;H][C,c]([C,c])[C,c;H][C,c]1, but that won't match > the H atoms themselves if you want to use them in the overlay, and it also > won't work in the aliphatic case of, for example, ClC1CCC(C)CC1 because > there the carbon atoms have 2 attached hydrogens. If you really do want > it to match aliphatic cases as well, then you will need something > like > [C,c]1(Cl)[$([CH2]),$([cH])][$([CH2]),$([cH])][C,c]([C,c])[$([CH2]),$([cH])][$([CH2]),$([cH])]1 > which is quite a mouthful. The carbons at the 2,3,5 and 6 positions on the > ring are specified as either [CH2] or [cH]. > > Jupyter notebook can be really useful for debugging SMARTS patterns like > this. The one I used was variations of > ``` > from rdkit import Chem > from IPython.display import SVG > mol = Chem.MolFromSmiles('C1=CC(=CC2=C1C=CN2C)Cl') > qmol = Chem.MolFromSmarts('[C,c]1(Cl)[C,c][C,c][C,c]([C,c])[C,c][C,c]1') > print(mol.GetSubstructMatches(qmol)) > mol > ``` > which prints the numbers of the matching atoms and also draws the molecule > with the match highlighted. > Regards, > Dave > > > On Tue, Mar 1, 2022 at 1:43 AM Adam Moyer <atom.mo...@gmail.com> wrote: > >> Hello, >> >> I have a baffling case where I am trying to match substructures on two >> ligands for the goal of aligning them. >> >> I have two ligands; one is a 6-chloroindole (6CI) and the other is a >> para-chloro toluene (PCT). >> >> I am attempting to use the following SMARTS (S1) to match >> them: '[C,c]1(Cl)[C,c][C,c]*([N,n,H])*[C,c]([C,c,H])[C,c]([H])[C,c]1'. >> For some reason S1 only finds a match in 6CI. >> >> When I use the following SMARTS (S2) I only match to PCT as expected: >> '[C,c]1(Cl)[C,c][C,c]*([H])*[C,c]([C,c,H])[C,c]([H])[C,c]1'. >> >> How can S1 not match PCT? S1 is strictly a superset of S2 because I am >> using the "or" operation. Do I have a misunderstanding of how explicit >> hydrogens work in RDKit/SMARTS? >> >> Lastly when I use the last SMARTS (S3) I am able to match to both, but I >> cannot use that smarts due to other requirements in my >> project: '[C,c]1(Cl)[C,c][C,c][C,c]([C,c,H])[C,c]([H])[C,c]1' >> >> Thanks! >> Adam >> _______________________________________________ >> Rdkit-discuss mailing list >> Rdkit-discuss@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/rdkit-discuss >> > > > -- > David Cosgrove > Freelance computational chemistry and chemoinformatics developer > http://cozchemix.co.uk > > _______________________________________________ > Rdkit-discuss mailing list > Rdkit-discuss@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/rdkit-discuss >
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