How does InChI represent compounds with mobile H-atoms (tautomerism, for example)?
The Main Layer must be the same for any arrangement of mobile hydrogen atoms. This is achieved by the logical removal of mobile-H atoms and the tagging of H-donor and H-receptor atoms. As an example we shall look at guanine (taken from the InChI Technical Manual), some of whose tautomeric structures are shown below: If we create an InChI for one of those tautomeric forms (with the optional Fixed-H layer selected) we get: Note: Donors and receptors of H and changeable bonds are highlighted. If we take a closer look at the H-atom sub-layer of the Main Layer we see that on generation the InChI program has signified that atom number 1 has one H and that 4H atoms are shared by atoms 6, 7, 8, 9, 10 and 11.
