Lawson Number (LN)

 

Description:

The Lawson Number is a special feature related to the Beilstein Handbook registration system. Using the Lawson Number, it is possible to browse the database in terms of organic structures without involving the Structure Search System. The Lawson numbers are calculated by algorithmically segmenting the structure, whereby each fragment is automatically assigned a Lawson Number based on its structural characteristics. Usually there is more than one Lawson Number for each structure registered.

 

Type of Indexing:

numerical

 

Tips and Hints:

CrossFire offers an opportunity to study the power of searching with the Lawson Number. Since a Lawson Number range defines a structural similarity it is easy to restrict search results to particular substance classes without carrying out complex substructure searches.

 

Searching with Lawson Numbers

 

Sometimes a controlled relaxation of precision in searching the database is desirable and advantageous: the experienced chemist knows that many types of information can be transferred from the particular reported case to other analogous cases. For instance, preparative methods are often equally applicable to classes of compounds, and certain physical data can be used as a starting point for estimating of missing data in the same class of compounds.

 

For this reason Beilstein databases are equipped with a retrieval tool for classification of structures, the LN. This number enables the searcher to investigate a family of structures; in particular the family referred to as "positional isomers". In general it is neither easy nor quick to retrieve positional isomers with traditional substructure searching, thus the LN can be regarded as an additional help to existing tools, not as a substitute.

 

The LN is a two-byte unsigned integer, which can take any value in the range 9 - 32759. Any particular compound may possess several LNs, one for each "chemically significant" fragment in the molecule. In general, most compounds have 2-3 LNs. An analysis of the Beilstein Database gave the following distribution:

 

25.1 % have

1 LN

39.4 % have

2 LNs

24.0 % have

3 LNs

8.5 % have

4 LNs

3.0 % have

> 4 LNs

 

Thus the LN is an economical (but non-unique) Description of structural properties, since each structure is characterized by less than 5 bytes on average.

 

Example I

You have found the hit pyridine-2-carboxylic acid (LN: 26332) and wish to locate positional isomers.

 

Operator

Field Name

Value

 

LN

26332

AND

MF

C6H5NO2

 

 

The Lawson Number is a browsing tool and as with all other browsing methods false drops sometimes occur. Thus the user is sometimes confronted with a spattering of unexpected ring systems. (For instance a query based on pyridines might possibly contain a few methyl pyrroles, among the otherwise sensible hits, which would then be considered as false drops).

 

Note that these can sometimes be filtered out by combining the search with nomenclature terms, but nomenclature alone is rarely capable of retrieving positional isomers in a dependable fashion, because of the non-systematic usage encountered in even the best of databases. For instance, not all the hits of the example above necessarily contain the nomenclature term "pyridine", since "nicotinic acid" and "isonicotinic acid" are perfectly acceptable names.

 

Example II

You have found a dibenzocyclooctenone oxime (LN: 7525) and would like to find similar compounds

 

Operator

Field Name

Value

 

LN

7525

AND

MOFO

C16*

AND

ELC

N1