The notorious neurotoxin Tetrodotoxin is often chemically represented as a zwitterion, shown below as 1 . This idea seems to originate from a famous article written in 1964 by the legendary organic chemist, Robert Burns Woodward.[cite]10.1351/pac196409010049[/cite] This structure has propagated on to Wikipedia and is found in many other sources.
Abstract The crystal and molecular structure of the hydrobromide of tetrodotoxin itself, C11H17O8N3·HBr, have been studied by means of three-dimensional X-ray analysis. The crystals are tri-clinic, with two formula units in a unit cell of the dimensions; a=8.79, b=9.02, c=9.51 A, α=106°51′, β=88°1′, and γ=96°23′; the space group is pl. The intensities of reflections were recorded on equi-inclination Weissenberg photographs taken with CuKα radiation around the b and c axes, and were measured visually with a calibrated intensity scale. The structure was elucidated by means of the Fourier and least-squares methods, and was refined by the block-diagonalmatrix least-squares method, the anisotropic thermal motion being assumed only for bromine atoms. The stereostructures of the two crystallographically-independent molecules of tetrodotoxin thus obtained are almost identical and correspond to that deduced by Tsuda et al., by Hirata et al., and by Woodward et al., all in 1964. It is noteworthy that, in both of the two molecules, the C–O distance for the free hydroxyl group in the ortho-ester part (1.36 and 1.39 Å) is somewhat smaller than the ordinary aliphatic C–O distance, 1.43±0.01 Å, and that it is rather closer to the phenolic C–O distance, 1.36±0.01 Å.