No description available.

The human saposins are four homologous activator proteins that are essential for the lysosomal degradation of sphingolipids (SLs) with small headgroups. They function in part to increase the solvent accessibility of these SLs to specific acid hydrolases by two proposed modes of action. In the solubilizer model, saposins extract SLs into soluble saposin-lipid complexes, and in the liftase model, the saposins reorganize lipids within membranes. The four saposins, called saposins A, B, C and D, have dual characters and exist as soluble proteins and in membrane-associated states. In this thesis, I first present the crystal structure of saposin D in a lipid-free state. The structure exhibits a closed, monomeric fold as previously described for saposins A and C. Next, I examine the lipid interaction properties of saposin A and determine a crystal structure of SapA in a lipid-bound complex. The complex forms a discoidal lipoprotein particle composed of highly ordered bilayer-like hydrophobic core surrounded by a protein belt consisting of two copies of saposin A in an open conformation. The saposin A-lipid discs are most likely the effective substrate-presenting particles in galactosylceramide hydrolysis. Finally, I compare the lipid interaction properties of the four saposins and address membrane perturbation, liposome binding, lipid solubilization and lipoprotein particle formation for each protein. Each saposin displays a unique behavior in the presence of liposomes under conditions that mimic the lysosomal environment. In particular, saposin D reveals simultaneous formation of different sized protein-lipid complexes, which appear to be primarily dependent on the lipid to protein molar ratio. A comparison of the available structures of saposins A, B and C in the “closed” and “open” conformations reveals structural hinge regions that likely shape the different types of saposin self-association. These states are directly related to the protein-lipid solubilization and/or membrane association properties of the saposins. Collectively, these findings present a more complete understanding of the lipid interaction properties of the saposin proteins and provide new insights into their role as activator proteins.

The objective of this project was to develop a prototype of an automated positioning system for future APXS instruments. The current instruments on Mars rely solely upon the rover arm to place them against rocks or soils, making consecutive measurements with small offsets challenging and resource intensive to conduct. The prototype consists of an x-ray detector and an x-ray tube mounted to a computer controlled three-axis positioning system. Passive surface scans were completed over 55Fe and 244Cm sources to determine raster parameters and to characterize the detector field of view. Active XRF scans of a sample tray containing geological specimens, as well as heterogeneous natural rocks, were acquired to evaluate the system under a variety of field conditions. The results demonstrate that a microcontroller, similar to that used by the APXS for signal processing, is capable of automated scanning and rudimentary decision making based on short duration spectra. This involved using the elemental distribution within the scan field to automatically position the instrument to the region of greatest scientific interest in the xy plane, and adjusting the detector stand-off for optimal data acquisition. The achievable spatial resolution was sufficient to distinguish regions of distinct elemental composition as small as 5 mm, although smaller dimensions are possible. While several aspects of the system must be developed further in order to accommodate more complex sample geometries and enable more advanced decision making, the results clearly demonstrate the potential of such a system to significantly improve the scientific return of future instruments.

The human saposins are four homologous activator proteins that are essential for the lysosomal degradation of sphingolipids (SLs) with small headgroups. They function in part to increase the solvent accessibility of these SLs to specific acid hydrolases by two proposed modes of action. In the solubilizer model, saposins extract SLs into soluble saposin-lipid complexes, and in the liftase model, the saposins reorganize lipids within membranes. The four saposins, called saposins A, B, C and D, have dual characters and exist as soluble proteins and in membrane-associated states. In this thesis, I first present the crystal structure of saposin D in a lipid-free state. The structure exhibits a closed, monomeric fold as previously described for saposins A and C. Next, I examine the lipid interaction properties of saposin A and determine a crystal structure of SapA in a lipid-bound complex. The complex forms a discoidal lipoprotein particle composed of highly ordered bilayer-like hydrophobic core surrounded by a protein belt consisting of two copies of saposin A in an open conformation. The saposin A-lipid discs are most likely the effective substrate-presenting particles in galactosylceramide hydrolysis. Finally, I compare the lipid interaction properties of the four saposins and address membrane perturbation, liposome binding, lipid solubilization and lipoprotein particle formation for each protein. Each saposin displays a unique behavior in the presence of liposomes under conditions that mimic the lysosomal environment. In particular, saposin D reveals simultaneous formation of different sized protein-lipid complexes, which appear to be primarily dependent on the lipid to protein molar ratio. A comparison of the available structures of saposins A, B and C in the “closed” and “open” conformations reveals structural hinge regions that likely shape the different types of saposin self-association. These states are directly related to the protein-lipid solubilization and/or membrane association properties of the saposins. Collectively, these findings present a more complete understanding of the lipid interaction properties of the saposin proteins and provide new insights into their role as activator proteins.

Catalase HPII of Escherichia coli is similar in sequence and structure to other catalases including the conservation of several residues on both the distal and proximal sides of the active center heme. The roles of many residues on the distal side of the heme have been well characterized. By contrast, very few residues on the proximal side of the heme or in the plane of the heme have been investigated. The primary goal of this thesis is to develop a better understanding of the role of the residues and structural features at the core of catalases and in the lateral access channel. The results demonstrate that a break in molecular symmetry does not have any functional significance. Replacing Ile274 with a Cys resulted in the heme being covalently linked to the protein through a Cys-vinyl bond which is hypersensitive to X-ray irradiation being largely degraded within seconds of exposure to the X-ray beam.

No description available.

No description available.

Neutron stars in mass-transferring binaries accrete hydrogen and helium rich matter from their companions. Unstable nuclear burning of this matter results in Type I X-ray bursts every few hours to days. Theoretical models propose three regimes of unstable burning on neutron star surfaces and also define the critical accretion rate above which the neutron star burns fuel stably without bursting. Currently, the theoretical value for this stability boundary is comparable to the Eddington limit whereas the observational limit is estimated to 0.3 m˙Edd. In this dissertation, we examine the boundary between stable and unstable thermonuclear burning at the surface of an accreting neutron star for a range of initial compositions. We compute different steady-state equilibrium models and use them to conduct a linear perturbation analysis. The main results of this analysis are plots of the critical accretion rates as a function of the initial helium mass fraction which were never produced before.

Ligand–mediated coupling between metal centres is of fundamental importance in inorganic and materials chemistry. Bridging ligands involving azo groups as coordinating π–acceptors can yield complexes with interesting properties. This thesis describes the synthesis of a series of N–heterocyclic compounds containing the azo functionality, designed for potential coordination to the metal through the azo nitrogen and a N–heterocyclic ring. The azo ligands are divided into four categories; ligands based on azobispyridines, ligands containing pyrimidine and fused aromatic azine groups and ligands capable of coordinating in a bis–tridentate fashion to the metal centre. Ligands containing flexible imine subunits connected directly, or through different spacers, are also discussed. Overall twenty one ligands were synthesised, six of which are new compounds. The coordination and metallosupramolecular chemistry of these ligands with ruthenium(II) and silver(I) metal atoms was investigated. A total of thirty five ruthenium(II) and eleven silver(I) complexes were prepared, of which thirty eight were characterised by X–ray crystallography. Mononuclear and dinuclear ruthenium(II) complexes were synthesised and characterised by a combination of spectroscopic and structural techniques. UV/Visible absorption studies and electrochemical methods were used to investigate the nature of metal–ligand and metal–metal interactions. In the mononuclear Ru(II) complexes, N–heterocyclic azo ligands act as chelating ligands forming five–membered chelate rings involving azo–N and heterocyclic–N atoms. The non–coordinated pyridine ring of the azo ligand is twisted with respect to the azo–N atom and is directed towards the adjacent bipyridine rings. Studies reveal that these azo ligands posses extremely low–lying π*–orbitals and are electron deficient. X–Ray structural analysis of the dinuclear complexes revealed short inter–metal separations of ca. 4.9 Å and electrochemical studies indicate that these ligands mediate very strong interactions between the metal centres , due to the excellent π*–acceptor properties of the azo functionality. Varying the pyridine ring of the azo ligand to pyrimidines and fused N–aromatic rings has a considerable effect on the electronic properties of these complexes. Incorporation of a pyrimidine ring facilitates the stabilisation of azo anion radicals and leads to the formation of diruthenium(II) species, bridged by radical species. The X–ray crystal structures of both these complexes were determined. The use of the hexadentate ligands coordinating in a bis–tridentate manner mediate even stronger communication between the two ruthenium centres. Ligands containing bis–pyridylimines result in weaker coupling between the metal centres in dinuclear ruthenium(II) species. A complete absence in the inter–metal communication was observed with increasing the distance and/or flexibility between the two pyridylimine units, contrary to a previous reported claim. Reaction with different silver(I) salts afforded an array of one–dimensional coordination polymers and a discrete dinuclear complex depending on the coordination strengths of the anions. The metallosupramolecular assemblies obtained were characterised mainly by X–ray crystallography, elemental analysis and mass spectrometry.

Ligand–mediated coupling between metal centres is of fundamental importance in inorganic and materials chemistry. Bridging ligands involving azo groups as coordinating π–acceptors can yield complexes with interesting properties. This thesis describes the synthesis of a series of N–heterocyclic compounds containing the azo functionality, designed for potential coordination to the metal through the azo nitrogen and a N–heterocyclic ring. The azo ligands are divided into four categories; ligands based on azobispyridines, ligands containing pyrimidine and fused aromatic azine groups and ligands capable of coordinating in a bis–tridentate fashion to the metal centre. Ligands containing flexible imine subunits connected directly, or through different spacers, are also discussed. Overall twenty one ligands were synthesised, six of which are new compounds. The coordination and metallosupramolecular chemistry of these ligands with ruthenium(II) and silver(I) metal atoms was investigated. A total of thirty five ruthenium(II) and eleven silver(I) complexes were prepared, of which thirty eight were characterised by X–ray crystallography. Mononuclear and dinuclear ruthenium(II) complexes were synthesised and characterised by a combination of spectroscopic and structural techniques. UV/Visible absorption studies and electrochemical methods were used to investigate the nature of metal–ligand and metal–metal interactions. In the mononuclear Ru(II) complexes, N–heterocyclic azo ligands act as chelating ligands forming five–membered chelate rings involving azo–N and heterocyclic–N atoms. The non–coordinated pyridine ring of the azo ligand is twisted with respect to the azo–N atom and is directed towards the adjacent bipyridine rings. Studies reveal that these azo ligands posses extremely low–lying π*–orbitals and are electron deficient. X–Ray structural analysis of the dinuclear complexes revealed short inter–metal separations of ca. 4.9 Å and electrochemical studies indicate that these ligands mediate very strong interactions between the metal centres , due to the excellent π*–acceptor properties of the azo functionality. Varying the pyridine ring of the azo ligand to pyrimidines and fused N–aromatic rings has a considerable effect on the electronic properties of these complexes. Incorporation of a pyrimidine ring facilitates the stabilisation of azo anion radicals and leads to the formation of diruthenium(II) species, bridged by radical species. The X–ray crystal structures of both these complexes were determined. The use of the hexadentate ligands coordinating in a bis–tridentate manner mediate even stronger communication between the two ruthenium centres. Ligands containing bis–pyridylimines result in weaker coupling between the metal centres in dinuclear ruthenium(II) species. A complete absence in the inter–metal communication was observed with increasing the distance and/or flexibility between the two pyridylimine units, contrary to a previous reported claim. Reaction with different silver(I) salts afforded an array of one–dimensional coordination polymers and a discrete dinuclear complex depending on the coordination strengths of the anions. The metallosupramolecular assemblies obtained were characterised mainly by X–ray crystallography, elemental analysis and mass spectrometry.