Leave Tweedle-Dum alone!

Deleting one substructure match at a time

I recently came across this post on RDKit Discussions about deleting one substructure match when there are multiple in the same molecule.

Specifically, the OP asked how to delete just one Cl atom at a time in his molecule that contains two of them.

This was really interesting for me because I worked on a similar problem recently (but with a twist!).

The problem is, DeleteSubstructs removes all substructures matching the query in a given molecule. Therefore, Greg suggested two ways:

1. converting the Mol to a rewritable molecule RWMol, then exploiting RemoveAtom functionality, and
2. using chemical reactions, which involves encoding the desired reaction into SMARTS.

But here’s the twist - what if you want to delete an entire substructure fragment match, one at a time, when there are multiple in the same molecule? (By substructure fragment, I mean multiple atoms connected to each other.)

As far as I’ve seen, there is not yet any functionality in RWMol that can do this. On the other hand, one could use chemical reactions, but likely need to encode by hand the reaction SMARTS each time you have a different structure…

…which is fine if you’re working with a few molecules you know very well, but if you are working with 1000s of molecules with very diverse structures, you want to be able to remove the same substructure fragment from each molecule, one at a time with minimal fuss.

I worked on this problem for my PhD and came up with a solution after several tries and thought to share it here.

Basically, I use Chem.rdmolops.ReplaceCore().

ReplaceCore, and its counterpart ReplaceSidechains are typically used in medchem applications like R-group decomposition. In a way, it fragments a given molecule according to a substructure query match, then introduces dummy atoms at the sites of fragmentation, represented by a number and asterix like 1*.

The beauty of this is that it ‘removes’ one substructure match at a time if there are multiple in your structure.

I then simply replace the dummy atoms using datamol, a lightweight library built on RDKit. Calling dm.remove_dummies replaces the dummy with a hydrogen, but you could replace with whatever atom you want using dm.replace_dummies_atoms().

Check it out:

It’s perhaps an unorthodox (hacky?) solution for fragmenting molecules and deleting substructures, but it works for what I need.

I came up with this idea thanks to the great documentation and related blogposts of RDKit as well as datamol - super helpful folks in the open source community!

I’m curious if there are better ways to remove one substructure match at a time from a molecule. Leave a comment or Tweet at me!

Solving this problem was part of one of my PhD projects to develop an algorithm to classify homologous series. Check it out here.

Further Reading

• RDKit Blog - R-Group Decomposition
• More R-Group Decomposition