Definition: A pool of molecules that are neither complexes nor are genetically encoded. Rationale: Identity of small molecules are based on structure, rather than sequence as in the case of DNA, RNA or Protein. A small molecule reference is a grouping of several small molecule entities that have the same chemical structure. Usage : Smalle Molecules can have a cellular location and binding features. They can't have modification features as covalent modifications of small molecules are not considered as state changes but treated as different molecules. Some non-genomic macromolecules, such as large complex carbohydrates are currently covered by small molecules despite they lack a static structure. Better coverage for such molecules require representation of generic stoichiometry and polymerization, currently planned for BioPAX level 4. Examples: glucose, penicillin, phosphatidylinositol
Source:http://www.biopax.org/release/biopax-level3.owl#SmallMolecule