Fig 1. Network of protonation and tautomeric microstates for SAMPL6 pKa challenge target SM07.
Only the subset of possible microstates identified as being of interest in the SAMPL6 challenge are shown. The protons of interest are denoted by blue balls on top of the nitrogen which is the proton acceptor. Each column has a different number of dissociable protons, indicated at the top. All microstates in a column are tautomers with the same total charge; their vertical order is arbitrary. All the tautomers in a column contribute to the macrostates states with 4 (4H) to zero (0H) dissociable protons. Black double-headed arrows indicate pKas that were reported in the SAMPL6 Challenge; red arrows are the transitions between tautomers. Twenty four pKas can be defined between all microstates in one column and all in the neighboring columns as they differ by one proton. Some transitions, such as between 13 and 7 required tautomer changes. Likewise, transitions between tautomers in the top and bottom rows (e.g. between microstates 2 and 3) are also well-defined, but are not shown here for clarity. The numbers associated with each microstate simplify the microstate IDs assigned by the SAMPL6 pKa Challenge, which have the form: SM07_microXXX, where XXX are three digits. For example, microstate 4 in this figure corresponds to SM07_micro004.