Tautomers
"Tautomeric bonds" can not be designated in SMILES notations. It is up to the user to enter the correct form of the tautomer that needs to be evaluated. The figure below illustrates the enol form and the keto form of 2-pyridinol. Acceptable SMILES notations are listed for each tautomeric form. The keto form must be entered with upper case letters. The ECOSAR and EPI Suite SMILES interpreter will not convert a keto form tautomer to an aromatic structure; the keto form is evaluated as an aliphatic by intentional design. The CLOGP (BioByte Corp) interpreter will convert selected keto form tautomers to aromatics. The 2-pyridinol example shown in the figure is a tautomer that is comprised of only one ring. The same rules apply to tautomers with multiple rings. For example, in a multiple ring keto form tautomer, the C=O must be entered in upper case letters; in addition, other members of the ring containing the C=O must be entered in upper case unless they are aromatic members of other rings. If a tautomer needs to be evaluated as an aromatic structure, then the enol form of the tautomer should be entered by the user.
The following figure illustrates the enol and keto forms of 2-pyridinol. The enol SMILES is Oc1ccccn1 and the keto SMILES is O=C2C=CC=CN1
Note: The current EPI SMILES interpreter will automatically convert selected aromatic keto SMILES to aliphatic SMILES. For example, if the keto form of 2-pyridinol is entered as: O=c1[nH]cccc1 ...the EPI programs will convert it to: O=C1NC=CC=C1