Matched molecular pairs
A matched molecular pair (MMP) is two molecules that are identical except for one defined change — say, H → Cl at one position. Because only that single transformation differs, the change in potency between them is the cleanest possible estimate of what *that* change does. MMP analysis collects thousands of such pairs to learn the average effect of a transformation across many series.
Matched pair (transformation: H -> Cl at meta) Compound A: core-[H] IC50 = 900 nM pIC50 = 6.05 Compound B: core-[Cl] IC50 = 60 nM pIC50 = 7.22 ---------------------------------------------------- delta-pIC50 (B - A) = +1.17 -> ~15x more potent Across 40 such H->Cl pairs in this project, median delta-pIC50 = +0.4 (about 2.5x). This one pair is far above the trend -- a candidate activity cliff worth a close look.
Activity cliffs
An activity cliff is a matched pair that are nearly identical in structure yet wildly different in potency — two molecules a chemist would call "the same" that the protein treats as worlds apart. Cliffs are the opposite of flat SAR, where many changes do nothing. They are the richest learning moments because they point straight at a critical interaction.
The classic cause of a cliff is a single contact that switches on or off. Maybe the added group now makes a hydrogen bond that was impossible before; maybe one extra carbon now clashes with a wall and breaks shape complementarity; maybe a methyl locks a productive conformation — the so-called magic methyl effect, where one CH3 jumps potency 100-fold or more.