No description available.

No description available.

This study is concerned with the reductive cleavage of tetracoordinated organophos-phorus compounds (either quaternary phosphonium salts R4P+ X??? or tertiary phosphine oxides R3P=O) with either the naphthalene radical (naphthalenide) anion or lithium aluminium hy-dride in THF solution at room temperature (RT). Part 1 examines the reaction of lithium naphthalenide with both phosphonium salts and phosphine oxides. The reaction was dem-onstrated to cleave phenyl groups from both bis-salts and bis-oxides in the presence of 1,2-ethylene bridges; based upon this, parallel syntheses of either 1,4-diphosphabicyclo[2.2.2]oc-tane or its P,P'-dioxide were attempted by using the commercially available ethane-1,2-bis-(diphenylphosphine) as the starting material in each case. Examination of the products ofreductive cleavage of the series of benzylphenylphosphonium bromide [PhnP(CH2Ph)4-n]+ Br???(where n = 0 to 3) with lithium naphthalenide leads to the proposal of a mechanism. Part 2 describes hydridic reductions of both quaternary phosphonium salts and ter-tiary phosphine oxides. Examination of the lithium aluminium hydride reduction of qua-ternary phosphonium salts using 31P-NMR has confirmed tetraorganophosphoranes (R4PH; R = Ph, alkyl) as intermediates in the reaction; in addition, two previously unknown classes of compounds, the triorganophosphoranes R3PH2 and the tetraorganophosphoranates R4PH2???, were also found to be intermediates. The behaviour of bis-phosphonium salts where the phosphonium centres are separated by either 1,2-ethylene or 1,3-propylene bridges are also examined. Formation of a monocation exhibiting a bridging hydride occurs when the cyclic bis-phosphonium salt 1,1,5,5-tetraphenyl-1,5-diphosphocanium dibromide is reacted with li-thium aluminium hydride. Mechanisms are proposed which are consistent with the observed experimental results.

Organophosphorus hydrolase (OPH, EC 3.1.8.1) is a homodimeric enzyme that has been observed to unfold via a three-state unfolding pathway (N2* ↔ I2 ↔ 2U) under chemical denaturing conditions. The dimeric intermediate (I2) is catalytically inactive and, although this enzyme has a very large overall conformational stability (~40 kcal/mol), it takes only a small amount of energy (~4 kcal/mol) to unfold this enzyme into its inactive form. So that this enzyme might be engineered as a more effective tool for nerve agent countermeasures and bioremediation purposes, its operational stability (the energy required to unfold the enzyme from its active, dimeric state to its inactive, dimeric state) must be increased. For this purpose, it is necessary to understand how the enzyme unfolds into its inactive, intermediate state. As tryptophan residues are sensitive probes of the microenvironment surrounding the residue, enzyme variants consisting of one tryptophan per subunit were constructed. Unfortunately, these variant enzymes did not fold into active conformations, and so could not be used to develop an accurate unfolding profile for the wild type enzyme. Limited proteolysis of OPH by thermolysin revealed detailed information on the unfolding process of OPH in chemical and thermal denaturing conditions. Mild denaturing conditions induced an initial enhancement of activity with a subsequent loss of catalytic activity upon more aggressive treatment. Under thermal conditions from 35 – 55 °C, the enzyme developed a well populated and active intermediate that displayed maximal activity. Similarly, the enzyme displayed maximal activity when incubated at 1.0 M urea. The regions of the enzyme, which became accessible to proteolysis at 45 °C and 1 M urea, were identical. This suggested that increased flexibility of these regions was coupled with the increase in the enzyme’s catalytic activity. Two regions that were determined by limited proteolysis to be the first to unfold were bridged with a novel disulfide bond. The result was an enzyme with an increased operational stability and resistance to proteolysis. This enzyme retained approximately 70% of its original activity in 8 M urea while no activity remained for the wild type enzyme when incubated in 6.5 M urea.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 142-155). Available also in a digital version from Dissertation Abstracts.

No description available.

No description available.

Photocopy of typescript. / Thesis (Ph. D.)--University of Hawaii at Manoa, 1979. / Bibliography: leaves 172-197. / Microfiche. / xii, 179 leaves ill. 29 cm

This study is concerned with the reductive cleavage of tetracoordinated organophos-phorus compounds (either quaternary phosphonium salts R4P+ X??? or tertiary phosphine oxides R3P=O) with either the naphthalene radical (naphthalenide) anion or lithium aluminium hy-dride in THF solution at room temperature (RT). Part 1 examines the reaction of lithium naphthalenide with both phosphonium salts and phosphine oxides. The reaction was dem-onstrated to cleave phenyl groups from both bis-salts and bis-oxides in the presence of 1,2-ethylene bridges; based upon this, parallel syntheses of either 1,4-diphosphabicyclo[2.2.2]oc-tane or its P,P'-dioxide were attempted by using the commercially available ethane-1,2-bis-(diphenylphosphine) as the starting material in each case. Examination of the products ofreductive cleavage of the series of benzylphenylphosphonium bromide [PhnP(CH2Ph)4-n]+ Br???(where n = 0 to 3) with lithium naphthalenide leads to the proposal of a mechanism. Part 2 describes hydridic reductions of both quaternary phosphonium salts and ter-tiary phosphine oxides. Examination of the lithium aluminium hydride reduction of qua-ternary phosphonium salts using 31P-NMR has confirmed tetraorganophosphoranes (R4PH; R = Ph, alkyl) as intermediates in the reaction; in addition, two previously unknown classes of compounds, the triorganophosphoranes R3PH2 and the tetraorganophosphoranates R4PH2???, were also found to be intermediates. The behaviour of bis-phosphonium salts where the phosphonium centres are separated by either 1,2-ethylene or 1,3-propylene bridges are also examined. Formation of a monocation exhibiting a bridging hydride occurs when the cyclic bis-phosphonium salt 1,1,5,5-tetraphenyl-1,5-diphosphocanium dibromide is reacted with li-thium aluminium hydride. Mechanisms are proposed which are consistent with the observed experimental results.

Thesis (Ph.D. 1971) from the Dept. of Agricultural Biochemistry and Soil Science, Waite Agricultural Research Institute, University of Adelaide.