An introduction to CAS, SMILES, InChl and Fingerprints of drugs or chemicals

CAS:

• unique numerical identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance.
• Example: caffeine: 58-08-2

SMILES strings:

• Simplified Molecular Input Line Entry System.
• using short ASCII strings.
• it is very common that there are a lot of SMILES strings that represent the same structure.
• unique SMILES string is also called the “canonical SMILES”.
• Example: CN1C=NC2=C1C(=O)N(C(=O)N2C)C

InChI identifiers:

• IUPAC Chemical Identifier
• a unique textual label for any chemical substance.
• comprise different layers and sub‐layers of information separated by slashes (/).
• example: caffeine: InChl=1S/C8H10N4O2/c1–10–4–9–6–5(10)7(13)12(3)8(14)11(6)2/h4H,1–3H3

InChI Keys

• a hashed version of the InChI
• The hash function is a one-way conversion

hashed fingerprints:

• use fixed-length binary values (0/1 bits) to encode molecular features, used to evaluation the similarity or diversity of chemicals.
• one molecule -> one fingerprint; but different molecules may have the same fingerprint.
• cannot confirm the presence of a substructure in a molecule, but confirm the absence of a substructure.
• the structure of a chemical cannot be induced from the fingerprint.

References:

• https://webbook.nist.gov/chemistry/cas-ser/
• https://pubchem.ncbi.nlm.nih.gov/compound/2519
• Sousa, J. (2017). Processing of SMILES, InChI, and Hashed Fingerprints.
• Line Notation (SMILES and InChI)

Updated: 1st Dec. 2021