Crystallographic binding conformations of 4a (A), 4b (B), 5a (C), and 5b (D) with rat nNOS. The omit Fo – Fc electron density maps for the inhibitors are shown at 2.5 σ contour level. Major hydrogen bonds are depicted with dashed lines. The atom color schemes are: oxygen, red; nitrogen, blue; sulfur, yellow; fluorine, light cyan; chlorine, green.