Electron-nuclear coupling to the proximal histidine in oxy cobalt-substituted distal histidine mutants of human myoglobin

Electron spin echo envelope modulation (ESEEM) spectroscopy was used to investigate electron-nuclear coupling to the N epsilon of the proximal histidine (F8, His93) imidazole in oxyCo(II)-substituted distal histidine (E7, His64) mutants (His-->Leu, His-->Val, His-->Gly, His-->Gln) and recombinant wild-type human myoglobins (Mbs). Nuclear hyperfine and nuclear quadrupole coupling constants decrease in the order: H64L > H64V > or = H64G approximately H64Q > wild-type. The differences in couplings found for the four mutant proteins are correlated with the differences in polarity of the E7 side chain. On the basis of the relative orientation of the nuclear quadrupole and g tensors, obtained by computer simulation of ESEEM spectra, the Co-O-O bond angle of H64G and H64Q appears to be similar to that of oxyCo sperm whale Mb (and possibly wild-type human Mb) at room temperature [Hori et al. (1982) J. Biol. Chem. 257, 3636], while that in H64V and H64L is more obtuse. ESEEM measurements in D2O demonstrate the presence of a hydrogen bond between the distal histidine and bound O2 in the wild-type protein, as was found in oxyCo sperm whale and horse Mbs [Lee et al. (1992) Biochemistry 31, 7274]. This hydrogen bond leads to a reduction in the N epsilon coupling in the wild-type protein as compared to that in the E7 mutants. No hyperfine-coupled deuterons were found in any of the mutants, and therefore, the proposed hydrogen bond between bound O2 and the distal glutamine in H64Q [Ikeda-Saito et al. (1991) J. Biol. Chem. 266, 23641] could not be substantiated.     

Temperature and composition dependence of the elastic constants of Ni3Al

The stiffness constants, c _ij , of monocrystalline Ni₃Al of three different compositions, 23.2, 24.0, and 25.0 at. pct Al, were measured over the temperature range from 300 to 1100 K using the rectangular parallelepiped resonance (RPR) method. The bulk modulus, as well as the shear modulus, Young’s modulus, and Poisson’s ratio for randomly oriented polycrystalline stoichiometric Ni₃Al, were derived from the stiffness constants. The data indicate that c ₄₄ is essentially independent of composition, decreasing slightly with increasing temperature for all three alloys. The values of c ₁₁ and c ₁₂, however, decrease with increasing aluminum content, the difference being small at room temperature but becoming larger at higher temperatures. We find that c ₁₁ and c ₁₂ are not as sensitive to aluminum concentration as is implied by previous results. A comparison of different shear moduli of Ni₃Al and the saturated Ni-Al solid solution in equilibrium with it indicates that the ordered phase is generally elastically stiffer than the solid solution over the range of temperatures at which coarsening of the Ni₃Al precipitate has been heavily investigated.     

Equilibrium constants and thermodynamics of ionization of aqueous hydrogen sulfide

First (K₁) and second (₂) ionization constants for aqueous hydrogen sulfide reported by various investigators have been reviewed, along with results of related calorimetric measurements leading to corresponding enthalpies of ionization. Following critical analysis of the published results, we have selected what we regard as “best” values for the ionization constants and related thermodynamic quantities. We call particular attention to significant uncertainties in these quantities for the higher (above 25°C) temperatures that are generally most important in hydrometallurgy.