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General Structure Search
  • Introduction
  • Atom lists, not lists
  • Generic atoms
  • Reaction Search
  • Simple query structure in reaction
  • Query structure in reaction components
  • Atom maps
  • Component identification

  • General Structure Search

    Introduction
    In the simplest case two molecular structures are compared. An exact structure search determines whether they have the same topology or not. Chemists are more often interested in substructure search, that is, whether one molecular structure contains the other one as a substructure or not. By definition, the examined molecule is called a target, the structure we are looking for is called a query, and a target molecule matching the query structure is called a hit (Table 1).

    Table 1 Exact structure search, substructure search
    query target hit
    exact structure search substructure search



    Atom lists, not lists
    It is possible to define the type of an atom in a custom atom list. If the type of the corresponding atom in the target molecular structure is a member of the list, it is considered a matching atom (Table 2). Not lists can be used to specify atoms to be excluded (Table 3).

    Table 2 Atom lists
    target
    query

    Table 3 Atom not lists
    target
    query



    Generic atoms
    Applying atom lists and not lists is a practical solution when the number of included or excluded atoms is small. However, generic atom types are helpful to avoid long atom lists (Table 4). JChem handles two generic atom types, Any and Hetero (the set of available generic atoms will be extended soon).

     Any         any (any atom except hydrogen)
     Q           hetero (any atom except hydrogen and carbon)
    
    

    Table 4 Generic query atoms
    target
    query



    Reaction Search

    Simple query structure in reaction
    Structures to the left of the reaction arrow are reactants (starting materials), structures to the right of the reaction arrow are products, and those molecules drawn just above or below the arrow are agents (ingredients). Corresponding atoms containing changing bonds (created, destroyed or modified) are marked with map numbers both in the reactants and in the products (Table 5).

    Searching for a substructure in a reaction equation does not differ from the classical substructure search process described above. Any matching in any reaction components (reactants, agents, products) is a hit. This is not the case when the query itself is a reaction.

    Table 5 Searching for simple query structure in reaction
    target
    query


    Query structure in reaction components
    Reaction queries are not necessarily complete reactions. Reaction queries sometimes contain reactants only. In this case, the search engine retrieves reactions containing reactants matching to the given structure. When just a product is specified in the query, those reactions will be returned which contain matching products (Table 6).

    Table 6 Searching for query structure in reaction components
    target
    query


    Atom maps
    When a reaction query has mapped atoms, the reaction center of a matching reaction is mapped correspondingly. Although, the actual value of the map numbers might be different in the query and the target, the hit atoms have to be paired exactly as they are in the query (Table 7).

    Table 7 Searching by mapped reaction queries
    target
    query


    Component identification
    A query structure occasionally consists of some disjunct fragments. Since these fragments belong to a single reaction component in the query, their corresponding hits must belong to a single component as well. Two components of a reaction query are matching to two components of a target reaction (Table 13).

    Table 13 Component identification during reaction search
    target
    query


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