Crystal and molecular structure of stercuronium iodide ethanol solvate: A peripheral muscle relaxant

The crystal structure of the muscle relaxant stercuronium iodide in the form of its ethanol solvate C₂₆H₄₃N₂⁺I^– · C₂H₅OH, has been determined by the heavy-atom method and refined by block-diagonal least-squares toR_o = 0.062 for 1941 observed reflections andR = 0.079 for a total of 2420 reflections measured on a four-circle diffractometer in the /2 scanning mode. The crystals are monoclinic,P2₁,a = 15.386(5),b = 11.377(4),c = 8.307(3) Å, = 98.22(2) °,Z = 2. RingsA andB in the modified conessine skeleton are both highly symmetrical half-chairs, ringC is in the chair conformation, and ringsD andE are highly symmetrical envelopes. The pyrrolidine ringE fused to ringD is roughly perpendicular to the steroid skeleton formed by ringsA toD. The N⁺---N distance of 10.76 Å is within the usual range required for potent neuromuscular blocking. Semiquantitative results are presented which suggest that the site of the tertiary N atom in ringE is not heavily congested by neighboring atoms in the molecule and may therefore be susceptible to protonation under physiological conditions. This could account for the relatively high drug potency of stercuronium.