title: Similar compounds versus similar conformers: complementarity between PubChem 2-D and 3-D neighboring sets.
abstract: BACKGROUND
  PubChem is a public repository for biological activities of small molecules. For the efficient use of its vast amount of chemical information, PubChem performs 2-dimensional (2-D) and 3-dimensional (3-D) neighborings, which precompute "neighbor" relationships between molecules in the PubChem Compound database, using the PubChem subgraph fingerprints-based 2-D similarity and the Gaussian-shape overlay-based 3-D similarity, respectively. These neighborings allow PubChem to provide the user with immediate access to the list of 2-D and 3-D neighbors (also called "Similar Compounds" and "Similar Conformers", respectively) for each compound in PubChem. However, because 3-D neighboring is much more time-consuming than 2-D neighboring, how different the results of the two neighboring schemes are is an important question, considering limited computational resources.
  
  
  RESULTS
  The present study analyzed the complementarity between the PubChem 2-D and 3-D neighbors. When all compounds in PubChem were considered, the overlap between 2-D and 3-D neighbors was only 2% of the total neighbors. For the data sets containing compounds with annotated information, the overlap increased as the data sets became smaller. However, it did not exceed 31% and substantial fractions of neighbors were still recognized by either PubChem 2-D or 3-D similarity, but not by both. The Neighbor Preference Index (NPI) of a molecule for a given data set was introduced, which quantified whether a molecule had more 2-D or 3-D neighbors in the data set. The NPI histogram for all PubChem compounds had a bimodal shape with two maxima at NPI = ±1 and a minimum at NPI = 0. However, the NPI histograms for the subsets containing compounds with annotated information had a greater fraction of compounds with a strong preference for one neighboring method to the other (at NPI = ±1) as well as compounds with a neutral preference (at NPI = 0).
  
  
  CONCLUSIONS
  The results of our study indicate that, for the majority of the compounds in PubChem, their structural similarity to other compounds can be recognized predominantly by either 2-D or 3-D neighborings, but not by both, showing a strong complementarity between 2-D and 3-D neighboring results. Therefore, despite its heavy requirements for computational resources, 3-D neighboring provides an alternative way in which the user can instantly access structurally similar molecules that cannot be detected if only 2-D neighboring is used.Graphical AbstractThe binned distribution of the neighbor preference indices (NPIs) for all compounds in PubChem (left) has a bimodal shape with two maxima at NPI = ±1 and a minimum at NPI = 0, indicating that structural similarity between compounds in PubChem can be recognized predominantly by either 2-D or 3-D neighborings, but not by both. The NPI histogram for the drug space (right) has a greater fraction of compounds with a strong preference for one neighboring method to the other (at NPI ≈ ±1) as well as compounds with a neutral preference (at NPI ≈ 0), indicating that the drug space is very different from the PubChem space.
authors.affiliation: National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Department of Health and Human Services, 8600 Rockville Pike, Bethesda, MD 20894 USA.
authors.forename: Sunghwan
authors.identifier: 0000-0001-9828-2074
authors.initials: S
authors.lastname: Kim
authors.affiliation: National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Department of Health and Human Services, 8600 Rockville Pike, Bethesda, MD 20894 USA.
authors.forename: Evan E
authors.initials: EE
authors.lastname: Bolton
authors.affiliation: National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Department of Health and Human Services, 8600 Rockville Pike, Bethesda, MD 20894 USA.
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Bryant
keywords: Molecular similarity; Neighbor Preference Index (NPI); Neighboring; PubChem; PubChem3D