SLN

SLN (SYBYL Line Notation, also called Tripos SLN) is an ASCII line notation for encoding chemical structures as compact single-line strings. It was developed at Tripos, Inc. (St. Louis, Missouri) for its SYBYL molecular-modeling software; the notation was introduced by Ash et al. in 1997 and substantially extended by Homer, Swanson, Jilek, Hurst, and Clark in a 2008 Journal of Chemical Information and Modeling paper. Like format:smiles, it belongs to the family of linear chemical notations that serialize a molecular graph—atoms as nodes, bonds as edges—into text suitable for databases and network transmission, but it was designed as a single unified syntax spanning discrete structures, molecular fragments, formulations, queries, reactions, and combinatorial (Markush/R-group) virtual libraries.

Structurally, atoms are written with standard element symbols beginning with an uppercase letter; unlike SMILES, hydrogens are not inferred but written explicitly (methane is CH4, ethane CH3CH3), which the authors argue yields more robust, less ambiguous parsing. Bonds use the symbols - (single), = (double), # (triple), : (aromatic), and . (zero-order), with single bonds usually implicit. Rings close by tagging an atom with a numeric label and referring back to it, e.g. benzene as C[1]H:CH:CH:CH:CH:CH:@1. A defining feature is the bracketed attribute list [key=value,...] attached to atoms and bonds, carrying charge, formal/partial charge, isotope, stereochemistry (s=), spin/radical state, and query constraints. Notably, aromaticity in SLN is treated as a property of bonds rather than of both atoms and bonds as in SMILES, and the notation captures relative stereochemistry and enantiomer mixtures more explicitly.

In the graph-data and format-conversion landscape, SLN overlaps in purpose with format:smiles, format:smarts (its query-language counterpart), format:smirks, and connection-table formats such as format:molfile and format:sdf; toolkits like Open Babel and RDKit handle the broader ecosystem, though SLN support outside Tripos software is limited.

Its main strength is expressive breadth—structures, substructure queries comparable to SMARTS, reactions, and enumerated libraries all in one grammar—together with explicit hydrogen and stereochemistry handling. Its principal limitation is ecosystem reach: SLN is closely tied to the commercial Tripos/SYBYL platform (later Certara) and never achieved the open, cross-vendor adoption of SMILES or InChI (format:inchi), so tool support, public documentation, and round-tripping remain comparatively sparse.