Computer Representations of Molecular Structures - Design and Use of Relational Databases in Chemistry - page 79

Table 7.1 Differences and Similarities among SMILES and
Graphs for Similar Structures

Structure SMILES Graph graph.hcount

H 2
C

CH 3

CCCC CCCC CCCC.H10

H 3 C

H 2

H 3 C CH 3

H
C

CC(C)C CC(C)C CC(C)C.H10

CH 3

H 2

H 3 C O

C

CCCC=O CCCCO CCCCO.H8

C H

H 2

H
C

H 3 C OH

CCC=CO CCCCO CCCCO.H8

H

C

H 2

H 2

C

CH 2

H 2 C

C1CCCCC1 C1CCCCC1 C1CCCCC1.H12

CH 2

H 2 C

C

H 2

H
C

HC

CH

c1ccccc1 C1CCCCC1 C1CCCCC1.H6

HC CH

C

tautomers must have the same simple graph. Yet, this is not sufficient. For
example, C1CCCCC1 is the simple graph for cyclohexane as well as ben-
zene. Yet they are not tautomers. This is because they do not contain the
same number of hydrogen atoms. If the simple graph of a structure is com-
bined with the count of the hydrogen atoms, an equal comparison of these
strings will reveal when two structures are tautomers. Table 7.1 illustrates
the various examples discussed here. It is useful then to have a simple
graph function to help determine whether two structures are tautomers.
The Appendix shows two implementations of an extension function to pro-
duce a simple graph using FROWNS/plpython and OpenBabel/plpython.