Conformation of erabutoxins a and b in aqueous solution as studied by nuclear magnetic resonance and circular dichroism.

The 270-MHZ nuclear magnetic resonance (NMR) spectra of erabutoxins a and b have been observed in 2H2O solution. By the use of convolution difference and double resonance techniques, proton signals in the aromatic and methyl regions have been assigned. From the pH dependence of NMR chemical shifts, the pKa value of His-26 of erabutoxin b is found to be 5.8, whereas His-7 of erabutoxins a and b is not protonated at pH above 3. The imidazole ring of His-7 is protonated upon the denaturation at pH 2.85. The acid denaturation process has been followed by the His-26 and methyl proton signals and is found to be reversible but is slow as compared with NMR chemical shift time scale. The circular dichroism (CD) of erabutoxin b has also been observed. The denaturation is found to involve a major change from the beta-rich conformation to a disordered one. The NMR and CD changes upon acid denaturation are satisfactorily explained by the two-state process. The deuterium exchange rates of the C-2 protons of His-26 and His-7 of erabutoxin b indicate that His-26 is exposed to the solvent whereas His-7 is tightly buried in the interior of the protein globule. The pKa value of Tyr-25 is as high as about 12.0, possibly due to the hydrogen bond formation between the hydroxyl group of Tyr-25 and a carboxylate group. The hydroxyl group of Tyr-25 is reversibly titrated so that this group is not buried tightly in the interior of the protein globule. The line width of the aromatic proton signals of Tyr-25 is significantly broad at room temperature, suggesting a restricted rotation of the aromatic ring. The aromatic proton signals of Trp-29 are fairly sharp; this aromatic ring is exposed and mobile. Except for His-7, the micro-environments of Tyr-25, His-26, and Trp-29 residues and methyl proton signals of valine and isoleucine are consistent with the locations of alpha carbon atoms as elucidated by X-ray crystal analyses.