A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science 6 February 2017, Johannesburg. / Polymorphism is not only limited to single component systems. Co-crystals have exhibited polymorphism and various polymorphic co-crystals have been reported. Polymorphism in co-crystals presents an expansion of the optimization space around a pharmaceutical compound and also offers the opportunity to develop novel patentable material. Polymorphism of pharmaceutical co-crystals was investigated by means of an exhaustive data mining survey and the formation of polymorphic co-crystals. The search was performed using the Cambridge Structural Database (CSD). The search aimed to find and tally neutral pharmaceutical co-crystals which are polymorphic. The survey of the CSD showed that 14% of the pharmaceutical co-crystals were polymorphic. The co-crystal of theophylline and 3,4-dihydroxybenzoic acid was found to be polymorphic and the novel polymorph was synthesized and characterized. The co-crystals were characterized by x-ray crystallographic techniques and Differential Scanning Calorimetry. The single crystals of carbamazepine and cinnamic acid was grown and characterized by SCXRD for the first time. The single crystal data was able to show that the hydrogen bonding packing that was modelled in the literature is incorrect. / MT2017

Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / An X-ray diffraction study of tri-(p-aminophenyl) carbonium perchlorate, (H2NC6H4)3C.ClO4, was carried out in order to determine (1) the effects of substituents on the structure of triphenylcarbonium ion, and (2) the effect of crystal symmetry on the configuration of the triarylcarbonium ions. [TRUNCATED] / 2031-01-01

Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / Photographic X-ray diffraction data were used to determine the crystal and molecular structure of dimethyl sulfoxide near 5C. Of the total 868 unique X-ray reflections collected, 507 had measurable intensity, 270 were unobserved, and 91 were extinct. Near 5°C, dimethyl sulfoxide is found to be monoclinic with a = 5.303 +/- 0.005 A, b = 6.829 +/- 0.003 A, c = 11.693 +/- 0.010A, and B = 94°30' +/- 15'. The space group is P21/c with Z = 4. [TRUNCATED] / 2031-01-01

Thesis (M. Ph.)--Chinese University of Hong Kong. / Bibliography: l. 79-84.

This thesis describes the synthesis and crystallisation of a selection of known and novel transition-metal – nitrite complexes and their detailed study using non-ambient X-ray diffraction techniques. These coordination compounds are specifically investigated for their ability to undergo nitro – nitrito linkage isomerisation in the single-crystal, with conversion between isomers achieved on either thermal- or photoactivation. The lifetime of the excited-state isomer is strongly dependent on the experimental temperature and, as such, this thesis is also concerned with time-resolved crystallographic methods. This work relies primarily on the technique of photocrystallography, a growing area of crystallographic research that has enjoyed considerable interest in recent decades. Chapter 1 reviews the seminal progress made in both time-resolved and photocrystallographic research, made possible by dramatic technological advances in the field in recent decades. Chapter 2 outlines the goals of this research project, aiming to obtain a more detailed understanding of the factors that influence the solid-state nitro – nitrito isomerisation. Chapter 3 describes the X-ray methods used in these studies, introducing both fundamental crystallographic principles and detailing the experimental procedures that are undertaken. Chapters 4 and 5 record steady-state photocrystallographic studies conducted for a series of complexes containing bulky, chelating ancillary ligands accompanying a monodentate nitrite group expected to be capable of nitro – nitrito isomerism. The response of these systems to both thermal- and photoactivation is investigated via single-crystal diffraction methods. The analysis focusses on the potential steric and electronic influences from the surrounding crystalline environment, which may have an impact on the progress of the solid-state reaction. Chapter 6 introduces photocystallographic kinetic methods to investigate the progress of nitro – nitrito conversion in real time. These studies are the first of their kind to be conducted on nitrite linkage isomer species and some novel methodology is introduced. Finally, Chapter 7 discusses pseudo-steady-state and trial time-resolved experiments conducted using the new time-resolved diffraction set-up on Beamline I19 at the Diamond Light Source. These early results indicate the power of time-resolved methods for elucidating key information on the nitro – nitrito conversion process and show promise for future experiments.

Bovine pancreatic ribonuclease (RNase A) and its homologues are pyrimidine-specific ribonucleases widely present in mammals, birds, amphibians, reptiles, and fish. RNases recognise a specific sequence – an adenosine 3' to a pyrimidine – on RNA, and cleave the molecule on the 3' side of the 3'-phosphate of the pyrimidine base. Extensive studies have been carried out on the RNase A homologues, including eosinophil-derived neurotoxin (EDN; RNase 2), eosinophil cationic protein (ECP; RNase 3), and angiogenin (ANG; RNase 5), and revealed distinct biological functions: EDN and ECP are involved in neurotoxicity, and ANG possesses angiogenic activity. RNase 4, although being discovered for a long time, is not as well characterised as much as other RNases. RNase 4 has been found in several mammalian species including a few primates, porcine, bovine, and rodents. The mature protein of RNase 4 consists of 119 amino acids, making it the shortest amongst all RNase A homologues. It has a higher inter-species similarity than its homologues, and such high evolutionary conservation suggests that RNase 4 has a more specialised function than RNA degradation. While RNase A, EDN, ECP, and ANG show cytidine preference, RNase 4 has a strong preference for uridine, which can be reversed back to cytidine by a single amino acid substitution of Asp-80, as shown by studies performed with porcine RNase 4 (also known as PL3). In this study, we used PL3 as a model to study the substrate specificity of RNase 4, and have solved four structures, including PL3, PL3 D80A mutant, and these two proteins in complex with dUMP and dCMP respectively. PL3 adopts the classic kidney-shaped RNase A fold, and residues forming the substrate binding subsites occupy similar positions as those in human RNase 4 and the prototypic RNase A. The structure of PL3 D80A mutant resembles that of the wild type iii PL3, and only hydrogen bond interactions between the side chains of Asp-80 and Arg-101 are lost. The structure of PL3·dUMP complex revealed interactions between the dUMP and residues Arg-7, His-12, Thr-44 and Phe-117 of PL3, which were also observed in the structure of human RNase 4 in complex with dUp. The additional hydrogen bonds identified between dUMP and residues Gln-11, Lys-40, Asn-43, and Lys-119 of PL3, as well as the absence of the interactions between Arg-101 of PL3 and the ligand that were present in the hRNase 4·dUp structure, could be due to the flexibility of the mononucleotide ligand. The crystal structure of PL3 D80A·dCMP complex presents a small number of hydrogen bond interactions between the protein and the dCMP ligand, which might be sufficient to stabilise the ligand in the B1 subsite, as the repulsion force on the dCMP ligand from the side chain of Arg-101 is absent in the PL3 D80A mutant. This is because in the D80A mutant, Ala-80 cannot provide hydrogen bonding that would hold the side chain of Arg-101 towards the B1 subsite. The activities of RNases can be inhibited by a 50 kDa cytosolic protein, the natural ribonuclease inhibitor (RI). RI binds to all the members of the RNase A superfamily, thus regulating the cytoplasmic RNA levels and protecting cells from inappropriately secreted RNases. The interactions between RNase and RI are tight, reversible, and in a 1:1 ratio. Several complex structures of RNase·RI from various species have been determined, and the residues in the interfaces between RNase and RI are conserved in all of the complexes. Studies revealed a 17-fold tighter interaction between PL3 and human RI than RNase A, making it very interesting to study the structure of the PL3·RI complex and characterise the interactions between PL3 and RI proteins. iv To date, we have established purification protocols for both proteins, and the next step towards the structure of PL3·RI would be to prepare and purify the protein complex, subject the protein complex to crystallisation experiments, and eventually lead us to the structural determination of PL3·RI complex.

No description available.

No description available.

Trypanosomatid parasites cause a wide range of so-called neglected diseases which affect over 27 million people every year. Current treatments are toxic and prone to resistance; therefore, it is imperative to identify novel protein targets and to develop more efficient treatments. Phosphofructokinase (PFK) is the third enzyme in glycolysis, and its reciprocal enzyme in gluconeogenesis is fructose-1,6-bisphosphatase (FBPase); in trypanosomatid parasites (Trypanosoma brucei [Tb], Trypanosoma cruzi [Tc] and Leishmania [Lm] species), both enzymes are recognised drug targets. This thesis describes biochemical and structural studies on these two allosteric enzymes that have been studied with two main purposes: 1) To understand their intrinsic behaviour. The allosteric mechanism of T. brucei PFK is described with the help of two novel crystal structures: TbPFK with the allosteric activator AMP, and mutant A288D located in the effector site. These studies have provided a better understanding of the effect of evolution on the allostery of PFK; and have introduced the first reproducible crystallisation of TbPFK via its A288D mutant. 2) To find novel inhibitors using in silico and high-throughput methods, and to investigate how the intrinsic behaviour relates to the mechanism of inhibition. Nanomolar selective inhibitors against TbPFK and TcPFK have been obtained and optimised to a novel family with low micromolar inhibitory activity against cultured parasites. Crystal structures with three of these inhibitors on TbPFK have helped us understand the structure-activity relationship. Moreover, novel crystal structures of TcPFK and LmFBPase, as well as reproducible crystallisation conditions for the latter enzyme and a mutant of TbPFK (A288D) will undoubtedly facilitate future drug discovery on these targets. Our long-term aim of finding novel drugs against sleeping sickness has been supported by the Wellcome Trust which has recently granted a Seeding Drug Discovery Award with the name “Optimisation of a trypanosome phosphofructokinase lead series to give candidates for treatment of the trypanosomatid based neglected disease Human African Trypanosomiasis”.

Graduation date: 1994