Screen Fragments Dictionary

Perhaps the most significant feature of this virtual screen set is the incorporation of a rare and/or theoretical screen that is set when any generated fragment is not found in the "formal part of the screen fragment dictionary. The frequency of occurrence of this rare/theoretical screen is very rare. It can be viewed as a fail safe screen since it can logically complete any set of alternative screens for a synthetic screen without significantly detmorating the search speed. 

Assume that a screen set is to be created that contains N screens. Partition the fragment dictionary into N partitions, each of which contains approximately the same number of fragment occurrences. The range of values encompassed within each such partition then corresponds to one of the screens that are available for assignment to database structures or to query substructures. 

To explore ways to utilize better the more precise screens in the dictionary, CAS has developed a screen fragment thesaurus which is a computer program that identified broader and narrower relationships among screen fragments via the manipulation of the textual representations in the dictionary. 

Figure 2 Examples of linear sequence fragments and ring descriptor developed for the CAS ONLINE screen dictionary. An example structure is given at the top. The sequences start from the acyclic carbon on the left and go along the bottom of the ring to the acyclic oxygen on the right. Bonds marked r are explicitly ring. Note that only the bond sequence has completely specific bond types.

The fragments that have been chosen to act as screens are listed in a fragment coding dictionary. When a query or a new molecule is to be processed, the corresponding connection table is analyzed to identify those screens from the coding dictionary that are present in the structure. A database structure or query substructure is then represented by a fixed-length bit string... 

Screens can be yielt from the Screen Dictionary, which contains approx. 6000 codes. To illustrate the criteria for their structure, please refer to the 12 classes of screens below defining certain types of structure fragments. 

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