The structures of the N-terminal domain of Ascaris hemoglobin and of several mutants have been determined at 2.2 A resolution. This perienteric hemoglobin from a parasitic nematode has an exceptionally high affinity for oxygen. It is an octameric protein containing two similar heme-binding domains per subunit. The recombinant monomelic N-tem1inal heme-binding domain (D 1) retains full oxygen avidity. Its structure reveals a ch';racteiistic globin fold. A strong hydrogen bond between tyrosine B 10 and the ligand distal oxygen, combined with a weak hydrogen bond between glutamine E7 and the proximal oxygen, grip the ligand in the bmdmg pocket. A third hydrogen bond between these t-vvo amino acids appears to stabilize the structure. Mutation of B 10 Y->L increases the oxygen dissociation rate of D l about 500 fold. Aninitial difference Fomier indicated negative density at the site of tvrosine B l 0, consistent with substitution by leucine: refinement ~f the model indicated little additional change in structure, confinning the importance of the strong hydrogen bond of the tyrosine hydroxyl to dioxygen. Mutation ofE7 Q->L increases the oxvgen off-rate 5-fold. Analysis of this mutant shows a slight reducti~I~ of the E7 side chain volume and the elimination of the hydrogen bonds both to the oxygen and the tyrosine. In addition, there appears to be a slight reorientation of the heme group toward the mutated side chain and a l.3A movement of the FG loop toward the exposed edge of the heme group. Mutation ofE7 Q->N increases the oxygen off-rate 50-fold. The mutated E7 Asn is too far from the oxyg~n to form a hydrogen bond, but a hydrogen bond to the B 10

No abstract

The reaction of [Pt(AuPPh3)8](N0 3 )2 with AgNC>3 under H2-atmosphere yields two new hydride-containing P t-A u -A g cluster compounds: [Pt(H)(AgN0 3 )(AuPPh3)8](N0 3 ) (1) and [Pt(H)(AgN03)2(AuPPh3)8](N03) (2). These reactions show intimate details on cluster growth: the nine-metal-atom particle [Pt(AuPPh3)8](N0 3)2 grows via oxidative addition of H2, deprotonation and electrophilic addition o f Ag+ to a 10-and then to an 11-metal-atom particle, 2. Cluster compounds 1 and 2 were characterized by elemental analyses, ICP analyses, IR and NM R ( lH, 3,P and ,95Pt) spectroscopy, and electrical conductivity measurements. The crystal and molecular structures of 1 and 2 have been determined by means of single-crystal X-ray analyses. The compound [Pt(H)(AgNC>3)- (AuPPh3)

A partially known structure, which may be a fragment of a molecule found by direct methods, can be solved completely by applying a weighted tangent refinement procedure on difference structure factors, similar to the authors' procedure for heavy-atom structures. Often the molecular fragment is found to be misplaced with respect to the symmetry elements: then, the space-group symmetry is reduced to P 1, and the procedure is used to locate the symmetry elements.

No abstract