To gain a more realistic insight into the performance of pKa-ANI we considered a small subset of 14 pK a values from the full dataset that did not appear in the pKa-ANI's training set. This set, however, contains only lysine residues from two protein systems. The observed accuracy on this subset was 0.49 ± 0.10 pK with a correlation of -0.18 ± 0.27 (compared to 0.48 ± 0.07 pK and 0.71 ± 0.16 with the MD-based approach) (Figure ).