Background: Of the polymethylene bismethonium congeners (C5-C12 and C18), decamethonium (C10) is the most potent neuromuscular blocking agent. We tested the hypothesis that these congeners act as straight molecules and will not bend easily in spite of the flexible connecting chain between the methonium heads. For congeners higher than C10, we also hypothesized that the relative difficulty with which the molecules to bend to conform to the interonium distance of C10 proportionately reduces their neuromuscular blocking potency.
Methods: Each congener was modelled and subjected to computer searches for representative low-energy molecular conformers. The conformation-potency relationship of the congeners was examined.
Results: For all congeners, we found that the lowest energy conformer (the 'global minimum') has a straight-chain conformation. Reduction of the interonium distance (by bending) incurs a steep energy penalty linearly related to the distance reduced. The global minimum of C10 has an interonium distance of 14.03 A and a total molecular length of 20.10 A. For other congeners, the interonium distance differential from that of C10 and the energy penalty required to conform to the interonium distance of C10 (where applicable) correlate with the reported logarithmic (mmol kg(-1)) dose requirement for neuromuscular block.
Conclusions: The C10 congeners strongly prefer a straight conformation. Their molecular length and resistance to bending is key to their neuromuscular blocking potency. A molecular length of approximately 20 A should best fit the space available to neuromuscular blocking agents between the two receptive sites of the endplate acetylcholine receptor.