Mechanism of Human Apohemoglobin Unfolding

Removal of heme from human hemoglobin (Hb) results in formation of an apoglobin heterodimer. Titration of this apodimer with guanidine hydrochloride (GdnHCl) leads to biphasic unfolding curves indicating two distinct steps. Initially, the heme pocket unfolds and generates a dimeric intermediate in which ∼50% of the original helicity is lost, but the α₁β₁ interface is still intact. At higher GdnHCl concentrations, this intermediate dissociates into unfolded monomers. This structural interpretation was verified by comparing GdnHCl titrations for adult human hemoglobin A (HbA), recombinant fetal human hemoglobin (HbF), recombinant Hb cross-linked with a single glycine linker between the α chains, and recombinant Hbs with apolar heme pocket mutations that markedly stabilize native conformations in both subunits. The first phase of apoHb unfolding is independent of protein concentration, little affected by genetic cross-linking, but significantly shifted toward higher GdnHCl concentrations by the stabilizing distal pocket mutations. The second phase depends on protein concentration and is shifted to higher GdnHCl concentrations by genetic cross-linking. This model for apoHb unfolding allowed us to quantitate subtle differences in stability between apoHbA and apoHbF, which suggest that the β and γ heme pockets have similar stabilities, whereas the α₁γ₁ interface is more resistant to dissociation than the α₁β₁ interface

Iridium, Rhodium, and Ruthenium Catalysts for the “Aldehyde–Water Shift” Reaction

A series of half-sandwich complexes of iridium, rhodium, and ruthenium are shown to be active catalysts for the conversion of aldehydes and water to carboxylic acids. Depending on the catalyst, H₂ is either released (the “aldehyde–water shift”) or transferred to a second equivalent of aldehyde (aldehyde disproportionation). Mechanistic studies suggest hydride transfer to be the selectivity-determining step along the reaction pathway. Using [(<i>p</i>-cymene)­Ru­(bpy)­OH₂]­[OTf]₂ as precatalyst, we demonstrate a novel example of a highly selective aldehyde–water shift in the absence of a hydrogen acceptor or base