The 1H Nuclear‐Magnetic‐Resonance Spectra of Neurotoxin I and Cardiotoxin VII4 from Naja mossambica mossambica

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Abstract
Two toxins from the venom of N. mossambica mossambica, neurotoxin I and cardiotoxin VII4, were investigated in aqueous solution by high-resolution 1H NMR techniques at 360 MHz. The spectral characterization of the proteins included determination of the number of slowly exchanging amide protons which can be observed in 2H2O solution, measurement of the amide proton chemical shifts and exchange rates, characterization of the aromatic spin systems and the internal mobilities of aromatic rings and studies of the pH dependence of the NMR spectra. For numerous resonances of labile and nonlabile protons quite outstanding pH titration shifts were observed. These NMR parameters provided a useful basis from comparative structural studies of different proteins in the large group of homologous snake toxins. As a 1st application the NMR data presently available in the literature on neurotoxin II from N. naja oxiana, toxin .alpha. from N. nigricollis and erabutoxin a and b from Laticauda semifasciata were used to compare these 3 proteins with neurotoxin I from N. mossambica mossambica. This preliminary comparative study provides evidence that the same type of spatial structure prevails for these 4 homologous neurotoxins and that the folding of the backbone corresponds quite closely to that observed in the crystal structure of erabutoxin b. A 2nd application is the comparison of cardiotoxin VII4 from N. mossambica mossambica with the neurotoxins. The folding of the polypeptide backbone is similar, but the cardiotoxin molecule is markedly more flexible than the neurotoxins.