InChI to SMILES

What is InChI to SMILES?

InChI to SMILES is a small-molecule identifier converter. It reads an International Chemical Identifier, or InChI, reconstructs the molecular graph it describes, and writes that graph as a canonical SMILES string. InChI is an open, structure-based identifier developed by IUPAC and the InChI Trust. Its layers can encode atom connectivity, hydrogens, charge, stereochemistry, isotopes, and tautomer-related information.

SMILES is a compact molecular notation used widely by cheminformatics libraries, molecular-property models, similarity search systems, and ligand-preparation workflows. An InChI export from a chemical database can therefore become usable input for a workflow that accepts SMILES, such as SMILES to SDF or SMILES to MOL2.

The converter uses RDKit to parse and sanitize each InChI, then serializes the resulting molecular graph as canonical SMILES. Canonical means RDKit chooses a consistent atom order for a given parsed structure. It does not mean that every cheminformatics program will write the identical SMILES text, because canonicalization conventions are software-specific.

How to use InChI to SMILES online

ProteinIQ converts one to ten InChI identifiers at a time. Paste one identifier per line, or upload a text file, then download a canonical SMILES file for each successfully parsed compound. For a named batch, place the compound name and its InChI on the same line, separated by a tab; the names are retained in the combined batch output.

Inputs

For example, ethanol can be submitted as:

ethanol	InChI=1S/C2H6O/c1-2-3/h3H,2H2,1H3

The full InChI is required. An InChIKey is a 27-character hashed lookup key, not a reversible structure encoding, so it cannot be converted into SMILES. To locate a structure from an InChIKey or compound name first, use a database record through PubChem Download.

Results

A failed entry does not prevent the rest of a batch from being processed. The Run log records the failed-entry count, and the entry is omitted from the per-compound files and combined TSV.

How the conversion works

The conversion has three stages:

• Parse: RDKit reads the InChI and creates a molecular graph. The identifier must be complete and syntactically valid for the InChI library.
• Sanitize: RDKit checks and normalizes the graph's chemical representation, including valence and aromaticity perception. Molecules that cannot be sanitized are reported as failed rather than exported as uncertain SMILES.
• Write canonical SMILES: Explicit hydrogens are removed from the working molecule where they can be implicit, then RDKit writes canonical, isomeric SMILES. Stereochemical markers such as @, /, and \\ appear when the InChI specifies stereochemistry and it can be represented in SMILES.

The conversion does not generate a 3D conformer, calculate protonation at a chosen pH, or optimize geometry. SMILES represents connectivity and specified stereochemistry, not coordinates. A downstream 3D ligand file can be made with SMILES to SDF when a workflow needs a structure file.

Reading the result

The output describes the structure encoded by the submitted InChI, not necessarily the exact original input text in another notation. A few consequences matter in compound-data workflows:

• Text can change while the chemistry does not: Several SMILES strings can represent the same molecular graph. A canonical SMILES provides a stable representation from this RDKit conversion, but it should not be compared character-for-character with SMILES produced by another toolkit.
• Stereo depends on the source: A submitted InChI without stereo information cannot create a stereochemically resolved SMILES. Check @, /, and \\ annotations in the result when chirality or double-bond geometry matters.
• InChI normalizes some chemical choices: InChI layers and toolkit parsing can handle tautomers, charge states, and salts differently from a source database's submitted SMILES. Confirm the structure against the original record before using the result for registration, synthesis planning, or a stereosensitive assay analysis.
• A conversion is not a database identity match: The tool validates that RDKit can interpret an InChI. It does not search for synonyms, assay records, bioactivity, or a preferred parent compound.

InChI to SMILES versus SMILES to InChI

Use InChI to SMILES when a source system supplies a full InChI but the next application requires SMILES. Use SMILES to InChI when a SMILES-based library needs a standardized InChI identifier for matching or registration. Neither direction is a reliable byte-for-byte round-trip test: the two notations have different normalization rules, and canonical SMILES is determined by the software writing it.