File Formats

Indigo supports the following molecule and reaction formats:

  • Daylight SMILES with ChemAxon extensions
  • Daylight SMARTS (including reactions)
  • CML (including reactions)
  • MDL (Symyx) Molfile and Rxnfile, both v2000 and v3000
  • MDL (Symyx) SDFile and RDFile
  • All the above in GZip-compressed form

Daylight Formats with ChemAxon and CurlySMILES Extensions

Almost all features of the original Daylight SMILES format are supported, including:

  • Aromatic rings
  • Hydrogen counters
  • Tetrahedral and allene-like stereocenters
  • Cis-trans double bonds
  • Disconnected structures
  • Reaction atom-to-atom mapping (AAM)

The only features that are not supported are:

  • “Up or unspecified”, “down or unspecified” notation (“/?”, “\?”)
  • Square-planar, trigonal-bipyramidal, and octahedral stereo configurations.

Almost all features of the original Daylight SMARTS format are supported, including:

  • Aromatic and aliphatic atoms
  • SSSR (smallest set of smallest rings) calculation
  • Logical operators
  • Atom environments (nested SMARTS)
  • Component-level grouping

The only features that are not supported are:

  • Constraint “h” (implicit hydrogen count). Hydrogens can be stored implicitly or explicitly in the database, and this difference should not affect the search results. “H” (total hydrogen count) may be used instead
  • All features of SMILES format that are not supported

The following ChemAxon SMILES extensions are supported:

  • Radical numbers: monovalent, divalent singlet, and divalent triplet
  • Stereogroups
  • Pseudo-atoms (atom aliases)
  • Fragment level grouping in reactions

The following extensions from the CurlySMILES format are supported:

  • Simple polymers: {-}...{+n}
  • Multiple groups: {-}...{+nn=5}

MDL Formats

MDL (Symyx) Molfiles and Rxnfiles are supported by Indigo. Both format versions (2000 and 3000) are supported. Almost all format features are supported, including:

  • Pseudo-atoms (atom aliases)
  • Stereogroups
  • R-Groups (Markush queries)
  • Query atoms and query bonds
  • Various query flags on atoms and bonds
  • Atom positions
  • 3D constraints in v2000 Molfiles
  • Reaction atom-to-atom mapping (AAM)
  • Reacting centers and stereo inv/ret flags
  • Data SGroups, polymers, superatoms (abbreviations)

The only features that are not supported are:

  • 3D constraints in v3000 Molfiles

Molecules and Query Molecules

Indigo treats “real” molecules different from query molecules in certain ways.

  • Only a query molecule can contain query features like “any” atoms, atom lists, etc.
  • Only a query molecule can contain R-Groups (although R-Sites can be present in “real” molecule too)
  • Only a query molecule can be matched as a substructure
  • Only a “real” molecule can be matched against by the substructure matcher.
  • Only “real” molecules can be subject to MCS/common scaffold computation.
  • Canonical SMILES can be calculated only on “real” molecules.

Similar rules apply to “real” reactions and query reactions:

  • Only a query reaction can be matched as a substructure in the reaction substructure matcher.
  • Only a “real” reaction can be matched against by the reaction substructure matcher.

There is never an ambiguity whether a particular object is a molecule/reaction or query molecule/reaction. While most input formats (Molfile/Rxnfile, SMILES) can store both types, different calls are provided for loading them as “real” or query objects. SMARTS strings always are loaded as query molecules, for obvious reasons.

Substructure and SMARTS Matching

Indigo provides full capabilities for substructure matching of query molecules, including ones loaded from SMARTS expressions. In the Bingo User Manual, you can find examples of substructure matches.

Differences between SMARTS and query SMILES

While a lot of SMARTS notation is allowed when loading SMILES as a query, there are differences between SMARTS and query SMILES:

  • SMARTS fragments $(...) are not allowed in query SMILES.
  • Empty bond designator (like ‘CC’ or ‘cc’) denotes ‘single or aromatic’ bond in SMARTS. In query SMILES, it denotes aromatic bond, if it belongs to a ring and has both end atoms aromatic (lowercase); otherwise, it denotes a single bond.
  • ‘C’ within SMARTS means aliphatic carbon, while ‘C’ within query SMILES means any carbon. The same applies to ‘B’, ‘N’, ‘O’, ‘S’, ‘P’. ‘C1~C~C~C~C~C~1’ won’t match ‘c1ccccc1’ as SMARTS, but it will do so as query SMILES.
  • SMARTS queries are not fed to aromaticity matcher. ‘c1-c=c-c=c-c=1’ won’t match ‘c1ccccc1’ as SMARTS, but it will do so as query SMILES.

Exact Matching

Indigo can perform exact matching of pairs of molecules, or pairs of reactions.

Molecule Similarity

Indigo provides various molecule similarity measures, all based on originally developed fingerprints. In the Bingo User Manual, you can find detailed description and examples.

Canonical SMILES

Canonical SMILES generated by Indigo are, according to Daylight and ChemAxon terminology, unique SMILES with isomeric information, or absolute SMILES. All significant molecular features, such as isotopes, charges, radicals, stereocenters, stereogroups, cis-trans bonds, and aromaticity, are encoded into SMILES in a canonical form. A canonical SMILES string defines the molecule independently of any particular representation (atom renumbering, stereogroup renumbering, explicit/implicit hydrogens). So, the equality of the canonical SMILES of two molecules guarantees that these molecules are the same, and vice versa.

‘Useless’ stereocenters

Stereocenter is not considered useful when it does not provide any information for distinguishing stereoisomers. Such useless stereocenters are ignored in canonical SMILES generated by Indigo.

From the pictures below, you can see that all the three molecules specify the same mixture. This is represented in the fact that Indigo gives identical canonical SMILES for them.

Canonical SMILES:
Canonical SMILES:
Canonical SMILES:

Note: Query features are not supported for canonicalization.

Scaffold Detection

Indigo incorporates two algorithms (exact and approximate) of maximum common substructure (MCS) computation. Each of the algorithms can operate on an arbitrary amount of input structures. Thus, it is possible to pass the found scaffold to the R-Group deconvolution procedure.

Moreover, if the scaffold detection procedure has found more than one MCS, it is possible to obtain all of them.

R-Group Deconvolution

With a collection of structures and a scaffold that is common for these structures, it is possible to perform the R-Group deconvolution (R-Group decomposition). The result of this procedure will be a scaffold with marked R-sites (R1, R2, ...), and the actual substituents for these R-sites for each of the input structures.

Examples are available on a separate page.


Indigo is capable of performing layout (cleanup) of molecules and reactions. After the layout procedure, the average length of the bonds in a molecule will always be around 1.0. The procedure is not sensitive to the present molecular coordinates.


Indigo provides high-quality 2D rendering capabilities for molecule and reactions. All the chemical features (including query features) are rendered properly following the IUPAC recommendations (1, 2) for graphical representation. The features that are not covered by IUPAC (mostly, query features) are drawn in such a way that they do not overlay the primary structure.

With Indigo, it is possible to display highlighted bonds and atoms with specified color and/or with thick lines and bold characters.

The full list of options is available on the options page.

The following output formats are supported for rendering:

  • Adobe PDF
  • Portable Network Graphics (PNG)
  • Scalable Vector Graphics (SVG)

On Windows platforms, Indigo is also able to:

  • Render directly to a given device context (HDC)
  • Produce .NET Bitmap objects
  • Produce Enhanced Metafile Format (EMF) files and .NET Metafile objects

Produced PNGs and Bitmaps are transparent unless the background is set explicitly. Produced SVGs, PDFs, and Metafiles contain no raster fragments.

In the Bingo User Manual, you can find examples of rendered molecules and reactions. All the pictures in this manual were rendered to SVG by Indigo.

Combinatorial Chemistry

Indigo provides a reaction products enumerator, which has the following features:

  • Query features (any bonds, any atoms)
  • Accurate tetrahedral and cis-trans stereochemistry transformation based on atom-to-atom mapping
  • Intramolecular reactions
  • Exhaustive product enumeration from available monomers (products are used as monomers)
  • Two generation modes: grid (monomers are assigned to wells and to reactants) and “one tube” (all monomers are mixed together)
  • Support of functional groups with two attachment points
  • Results are generated without duplicates even if source monomers are symmetrical

The full list of options is available on the options page.

The examples are available on a separate page.

CIP Stereo Descriptors

Indigo provides the CIP stereo descriptors calculations. These calculations correspond to latest chemical nomenclature requirements (Nomenclature of Organic Chemistry - IUPAC Recommendations and Preferred Names (2013)). Current implementation includes some simplifications and supports calculations only R/S and E/Z descriptors.

Please see the CIP Descriptors for detailed examples.