Fig. 6. Comparison between the scopolamine derivatives.

a Chemical structures of the scopolamine derivative ligands (Scopolamine (Scop), N-butylscopolamine (NBS), Oxitropium (Oxitro), and N-methylscopolamine (NMS)) used in the current FTIR spectroscopic studies, and each ligand bound spectra in the 1800–1450 cm⁻¹ region. The group of quaternary ammonium derivative positions are highlighted. Ligand binding-induced difference ATR-FTIR spectra are measured in H₂O at 293 K. Positive and negative bands correspond to ligand-bound and ligand-unbound states, respectively. One division of the y-axis corresponds to 0.0025 absorbance unit. b Comparison between Atro-bound structure in M₁R (green, PDB: 6WJC)⁴⁷ and NMS-bound structure in M₂R (cyan, PDB: 5YC8)¹⁵ at the view from extracellular side. The amino acid residues and ligands are depicted by sticks, and TM helices are depicted by ribbons. Hydrogen bond between Asp103^3.32 and amine group of both ligands are shown by red dotted lines with hydrogen bond length as labels.