The calculations correctly distinguish the charged compounds from the neutral ones (Figure ). The R2 group (Table ) locates in a hydrophilic pocket in which negatively charged residues GLU192, ASP241 and GLU294 form favourable electrostatic interactions with the positively charged R2 group (Figure ). This makes the binding of charged compounds more favourable in general than the electrostatically neutral ones. The two variants at R1 studied here may not affect the binding affinities significantly because the ethyl group and the cyclopropyl ring are similar in their hydrophobic properties and their sizes. The two compounds, S01 and S21, differing only at the R1 group, have similar binding affinities from both experimental measurements and the ESMACS calculations (Figure ). It should be noted that no activity was detected for S08 at the highest concentration (250 µM) in the experiments (Table ), indicating that its binding affinity is likely to be less negative than that presented in Figure . This makes the point deviating even farther from the regression line. Our TIES calculations also show that S08 is an outlier (see details below).