Chemical Structure Identifiers (InChI, InChI Key and SMILES)
InChI, InChI Key and SMILES
What is InChI?
InChI (International Chemical Identifier) is a standardized textual representation of a chemical substance developed by the International Union of Pure and Applied Chemistry (IUPAC). It provides a unique, machine-readable description of a molecule’s structure, allowing reliable identification and data exchange between chemical databases, software applications, and scientific publications.
An InChI string contains multiple layers of information, including molecular formula, atom connectivity, hydrogen atoms, stereochemistry, isotopes, and charge.
Example (Maleimide): InChI=1S/C2H6O/c1-2-3/h3H,2H2,1H3
Characteristics:
- Globally standardized by IUPAC
- Unambiguous representation of chemical structures
- Facilitates database searches and data integration
- Supports stereochemical and isotopic information
What is an InChI Key?
InChI Key is a condensed, fixed-length hash generated from an InChI string. It was designed to simplify searching, indexing, and sharing chemical information on the web and in databases.
Unlike the full InChI, the InChI Key cannot be converted back into the complete structure without access to a corresponding database.
Example (Maleimide):LFQSCWFLJHTTHZ-UHFFFAOYSA-N
Characteristics:
- Fixed length (27 characters)
- Easy to use in URLs, search engines, and databases
- Suitable for indexing and rapid structure lookup
- Widely used by chemical suppliers and scientific repositories
What is SMILES?
SMILES (Simplified Molecular Input Line Entry System) is a compact line notation used to describe chemical structures using ASCII characters. It is widely employed in cheminformatics, molecular modeling, drug discovery, and chemical database applications.
A SMILES string represents atoms, bonds, rings, branching, and stereochemistry in a concise format.
Example (Maleimide): O=C1C=CC(O)=N1
Characteristics:
- Human-readable and compact
- Widely supported by cheminformatics software
- Efficient for molecular calculations and storage
- Easily generated from and converted to chemical structures
Key Differences
| Feature | InChI | InChI Key | SMILES |
|---|---|---|---|
| Human readability | Moderate | Low | High |
| Uniqueness | Yes | Yes (hashed form) | Not necessarily |
| Fixed length | No | Yes | No |
| Encodes stereochemistry | Yes | Indirectly | Yes |
| Suitable for web search | Limited | Excellent | Good |
| Can reconstruct structure directly | Yes | No | Yes |
Practical Use
In chemical databases, products and substances are often identified using all three descriptors:
- InChI provides a standardized structural identifier.
- InChI Key enables fast searching and indexing.
- SMILES is commonly used for computational chemistry, molecular modeling, and data exchange between software tools.
Together, these identifiers improve chemical data consistency, interoperability, and discoverability across scientific and regulatory systems.