Sequence‐specific ¹H‐NMR assignments and determination of the secondary structure in aqueous solution of the cardiotoxins CTXIIa and CTXIIb from Naja mossambica mossambica

Sequence‐specific assignments of the ¹H‐nuclear magnetic resonance (NMR) spectra of the cardiotoxins CTXIIa and CTXIIb from Naja mossambica mossambica were obtained using two‐dimensional NMR experiments at 500 MHz and the independently determined amino acid sequences. Assignments were obtained from data at 25°C and 45°C for all but one back bone proton of the 60 residues in each protein. Complete or partial assignments are also reported for the side‐chain protons. These assignments supercede those published previously for the toxin preparation V^II2 [Hosur, R. V., Wider, G. & Wüthrich K. (1983) Eur. J. Biochem. 130, 497–508]. The ¹H/²H‐exchange kinetics were measured in ²H₂O at 20°C for the amide protons and the N‐terminal amino group. These and additional NMR data enabled the determination of the secondary structure in aqueous solution, which is virtually identical in CTXIIa and CTXIIb. Both proteins contain a short double‐stranded antiparallel β‐sheet comprising the residues 2–4 and 11–13, and a triple‐stranded antiparallel β‐sheet consisting of the residues 20–26, 35–39, and 49–55. The two peripheral strands of the triple‐stranded β‐structure were found to be connected by a right‐handed cross‐over, and the locations of several tight turns were also identified.