Optical activity and conformation of cobra neurotoxin

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Abstract
Cobra neurotoxin from Formosan cobra (Naja naja atra) venom is a compact globular protein having an intrinsic viscosity of 4.5 ml/g. The protein is stable in 7.5 M urea but can be denatured in 4.1 M guanidine hydrochloride or at elevated temperature (above 70.degree. C). Its conformation remains virtually the same in solvents of lower polarity than water such as 1,2-ethanediol or a mixed solvent of 1-propanol-1,2-ethanediol-water (5:1:1 by volume). The circular dichroism spectrum is atypical in water in that the peptide chromophores show a small negative circular dichroic (CD) band at 215 nm, a large positive one at 199 nm, and another large negative one below 190 nm. The CD pattern resembles to some extent that of a .beta. form but differs in both positions and magnitudes from the latter. It agrees qualitatively with the theoretical calculations of the reverse .beta. bends, suggesting that cobra toxin contains a considerable amount of .beta. turns and possibly a mixture of .beta. form and .beta. turns.