Effects of magnetic field and pressure on the intermediate valence state of YbPd.

High field magnetization, magnetoresistance and pressure effects of magnetic susceptibility, thermal expansion and electrical resistivity were examined for the intermediate valence system YbPd, which undergoes two first-order transitions at T(1) = 125 K and T(2) = 105 K. Analyses of high field magnetization suggest that half of the Yb atoms have magnetic moments below 100 K up to 55 T. The Yb valence state and the first-order transitions are stable up to 55 T. On the other hand, T(1) and T(2) decrease with increasing pressure and the first-order transitions disappear at around 4 GPa. Above the critical pressure, the experimental results suggest that the intermediate valence state persists down to the lowest temperature or a heavy fermion state is formed. We will show that most experimental results are explained reasonably by assuming the first-order transitions as a valence order transition of Yb. The magnetic ordering temperature is decreased with increasing pressure, in contrast to other Yb intermediate valence or Kondo systems. This may be correlated with the instability of the valence ordered state in this compound.

Crystal structure of rat heme oxygenase-1 in complex with heme.

Heme oxygenase catalyzes the oxidative cleavage of protoheme to biliverdin, the first step of heme metabolism utilizing O(2) and NADPH. We determined the crystal structures of rat heme oxygenase-1 (HO-1)-heme and selenomethionyl HO-1-heme complexes. Heme is sandwiched between two helices with the delta-meso edge of the heme being exposed to the surface. Gly143N forms a hydrogen bond to the distal ligand of heme, OH(-). The distance between Gly143N and the ligand is shorter than that in the human HO-1-heme complex. This difference may be related to a pH-dependent change of the distal ligand of heme. Flexibility of the distal helix may control the stability of the coordination of the distal ligand to heme iron. The possible role of Gly143 in the heme oxygenase reaction is discussed.