Why does demethylallosamidin show increased affinity compared to allosamidin?

Demethylallosamidin is an allosamidin derivative where one of the methyl groups on the allosamizoline moiety is removed. Its apparent IC50 value against human chitinase is 1.9 nM, compared to 40 nM for allosamidin.

The removal of one methyl group on the allosamizoline moiety in demethylallosamidin leads to a significant increase in affinity (approximately 20-fold stronger inhibition compared to allosamidin). This modification allows for altered hydrogen bonding and hydrophobic interactions within the binding pocket. Specifically, the remaining methyl group points towards the oxygen side of the oxazoline ring, creating a small void that allows Glu-140 and Asp-138 to rotate, bringing Asp-138's Oδ2 atom closer to the allosamizoline nitrogen, nearly forming a hydrogen bond.