The 270-MHz proton NMR spectra of the unique long neurotoxins bearing Phe-25, Astrotia stokesii b (As b) and Astrotia stokesii c (As c) from Astrotia stokesii, and Acanthophis antarcticus b (Aa b) from Acanthophis antarcticus, have been analyzed. The aromatic proton resonances of Phe-25 in As b and Aa b were assigned on the basis of the nuclear Overhauser effects observed on irradiation of slowly exchanging amide protons. Phe-25 was found to be involved in hydrophobic interactions with Ile/Val-42, Ala-46 and Ile-58 in As b and As c, and with Ala-46 and Val-58 in Aa b. These hydrophobic interactions, instead of the hydrogen bond between Tyr-25 and Glu-42 found in other neurotoxins, appear to be important for maintenance of the biologically active tertiary structure. The pH dependency of the chemical shift and intensity of the Trp-72 N-1 proton resonance of As b indicates that the indole ring is not fully exposed to the solvent and that the extra tail segment of this long neurotoxin interacts with the main part of the molecule.
Circular Dichroism Elapid Venoms