Fig. 2

Natural effects contributing to molecular diversity. Implicit hydrogen atoms on carbon atoms are not shown. a Three tautomeric variants of pyrimidin-4-one. The bottom two structures are different Kekulé representations of the same tautomer. b Thioacetic acid as an example for tautomerism (top, in left–right direction), ionization (left and right, in top–bottom direction) and mesomerism (bottom, in left–right direction). Redrawn with permission from Sayle 2010 [32]