Substructure Search System IDENT

Finally, for routine applications, our software provides a database management system called BASIS for storage and manipulation of chemical information. BASIS can access generally available spectral libraries from three different spectroscopic techniques (MS, H-NMR and F13C-NMR, IR), and permits the creation of new libraries. For structure elucidation and substructure search of unknown compounds, library search algorithms allow the retrieval of identical and structurally similar spectra. 

After the disconnection strategy is defined, the systems indicate the strategic bond together with their ranks. The user can now analyze the precursor or can verify the disconnection by performing a reaction substructure search in any of the interfaced reaction databases. To perform a search in the reaction database, the user can define the bond sphere to be considered as identity criterion. The first sphere, for instance, includes bonds attached to the atoms of the strategic bond. A hit is presented as a reaction with additional information from the reaction database, such as reaction condition, yield, and references. 

Two methods are commonly applied for library searches. Identity or retrieval searches assume that the spectrum of the unknown compound is present in the reference library, and only experimental variability prevents a perfect match of the unknown and reference spectra. When no similar spectra are retrieved the only information provided is that the unknown spectrum is not in the library. Similarity or interpretive searches assume that the reference library does not contain a spectrum of the unknown compound, and are designed to produce structural information from which identity might be inferred. Interpretive methods typically employ a predetermined set of spectral features, designed to correlate with the presence of chemical substructures. Searching identifies the library spectra that have features most similar to those of the unknown spectrum. The frequency of occurrence of a substructure in the hit list is used to estimate the probability that it is present in the unknown compound. Two well-developed interpretative search algorithms are SISCOM (Search of Identical and Similar Compounds) and STIRS (the Self-Training Interpretive and Retrieval System) [174-177]. Normally a retrieval search is performed first, and when the results are inconclusive, an interpretive search is implemented. In both cases, success depends on the availability of comprehensive libraries of high-quality reference spectra [178]. 

A prominent example of a structure database (see Structure Databases) is the REGISTRY File offered by the host STN. This database contains both structure and nomenclature from the Registry System. This file is searched using the Messenger software developed by Chemical Abstracts Service, which provides both identity, full structure, and substructure searching as well as nomenclature searching (see Chemical Abstracts Service Information System). Structure queries are entered as structure diagrams created with text... 

Computer information systems for chemistry need to include representations of chemical structures, which show the way in which the atoms and bonds of a molecule are connected together. This is necessary if they are to compare Structures for identity (thus allowing a check on whether or not a particular compound is contained in a database), or to search for all compounds in a database containing a particular substructure, or group of atoms and bonds. Such representations may also allow display of conventional two-dimensional structure diagrams on computer screens, and the performance of a wide variety of other analyses on individual molecules or sets of molecules. Several different types of representation are used in computer systems, some of them adapted from structure representations developed before the computer age. This article describes the principal ones, and discusses some of the special problems that arise in the representation of chemical structures, and the feasibility of interconverting different representations. 

The usefulness to the WRAIR Staff of searching for substructures on-line has yet to be determined. Since the chemistry retrieval system uses the screen and connection teible generated by the query to search to screen index file for either a whole or a sub-structure no more information (disk-drives) is needed on-line for sub-structure searching than for identity matches, so one is no more costly than the other, in terms of disk resources. But the elapsed time needed for such sub-structure searches varies greatly and may be large. The time needed may be kept small if the query is specific and many screen bits are set. 

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