The importance of correct tautomeric structures for biological molecules

The structures of usnic acid and tetracycline are determined using deuterium isotope effects on 13C chemical shifts in a water environment. In case of usnic acid this is achieved by synthesizing a more water soluble usnic acid with a PEG linker. In the usnic acid case an enolic b-triketone (C-1, C-14 and C-3) tautomeric equilibrium is at hand below pH 5. At pH 7.4 it exists as a mono anion. In case of tetracycline equilibrium between a zwitter ion and a neutral form is found together with an amide functional group and a hydrogen bonded enolic b-diketone system shifted strongly towards one tautomer.