To define the structural similarity 𝑆 𝑠𝑡𝑟𝑢𝑐 between two molecules, the ionized metabolites were represented by a Morgan2 bit-vector (based on ECFP4) using RDKit . Conceptually, each bit in this 2048-bit vector represents the presence (1) or absence (0) of a specific chemical substructure in the molecule. Molecules with many substructures in common have similar bit vectors and therefore yield a Dice similarity score closer to 1. The combination of Morgan2 and Dice similarity scores was chosen as this gave a more uniform spread across structural similarities compared to other fingerprints (e.g. Morgan3) and similarity measures (e.g. Tanimoto) .