Conformational Studies of Neurotoxin II from Naja naja oxiana

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Abstract
After treatment of neurotoxin II, a component part of the venom of the Middle Asian cobra Naja naja oxiana, with acetoxysuccinimide all five possible ∼‐acetylated‐lysyl derivatives were obtained and the position of the label was established. Trifluoroacetylation of both the derivatives and the parent toxin yielded, respectively, the five acetyl‐penta(trifluoroacetyl)‐neurotoxins II and the hexa(trifluoroacetyl)‐neurotoxin II, which were studied by circular dichroism (CD), 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. The availability of this series of compounds made possible assignment of all six fluorine signals (from the N‐terminal and the five ɛ‐amino groups) in the hexa(trifluoroacetyl)‐neurotoxin II NMR spectra and disclosure of the proximity of the Lys‐26 and Lys‐46 trifluoroacetyl groups. The pH dependence of the 19F NMR signals was determined and the pK values of the groups affecting the signal chemical shifts were calculated by a computer iterative program. In order to ascertain the relative accessibility of the Iysyl side chains, the change in halfwidths of the hexatrifluoroacetylated neurotoxin II 19F signals, with addition of varying amounts of an iminoxyl spin probe, was determined. The data obtained are compared with the X‐ray data on sea snake neurotoxins and the significance of the side chain interactions observed in solution is discussed.