No description available.

Propanediol metabolism occurs within a proteinaceous organelle in several bacterial species including Citrobacter freundii and Lactobacillus reuteri. The propanediol utilisation (Pdu) microcompartment shell is built from thousands of hexagonal shaped protein oligomers made from seven different types of protein subunits. In this Thesis, I investigate and analyse the structure and assembly of the bacterial microcompartment shell proteins. One of the shell proteins characterised in this work, PduT, has a tandem canonical bacterial microcompartment (BMC) repeat within the subunit and forms trimers with pseudo-hexagonal symmetry. This trimeric assembly forms a flat approximately hexagonally shaped disc with a central pore that is suitable for binding a 4Fe-4S cluster. The essentially cubic shaped 4Fe-4S cluster conforms to the threefold symmetry of the trimer with one free iron, the role of which could be to supply electrons to an associated microcompartment enzyme, PduS. The major shell protein PduB has a tandem permuted BMC repeat within the subunit and also forms trimers with pseudo-hexagonal symmetry. This shell protein closely resembles its homologous counterpart, EtuB; both possess three small pores formed within the subunits rather than a single pore at the centre of the pseudo-hexameric disc. The crystal structure of PduB provides insights into how substrates such as glycerol are able to use these pores as substrate channels. PduB appears to be able to pack within a sheet of PduA molecules, suggesting how the facet of the shell may be assembled. The higher order packing of shell proteins was investigated using PduA. Residues important for the packing of molecules into sheets were mutated and the effects on crystal morphology and on the shape of structures formed within the bacterial cell were assessed. PduA appears to assemble into straws in the bacterial cell and mutation of these residues has a profound influence on the structures produced.

Synthetic, biological and mineralogical carbonate-containing apatites have been studied, principally by infra-red absorption spectroscopy (using polarised or unpolarised radiation), The substitution of a carbonate ion for two hydroxyl ions in the apatite lattice has been confirmed, by chemical analyses, and the variation of the lattice constants with the degree of substitution determined. The orientation of the carbonate ion has been determined by using polarised infra-red radiation; its plane is approximately parallel with the c-axis. The carbonate ions in dental enamel are in two different environments; the lane of the carbonate ion in these two environments is nearly parallel and perpendicular to the c-axis. One of these environments is very probably the substitution for hydroxyl ions discussed above; this accounts for only a small fraction of the carbonate ions, the remainder are probably adsorbed on the surface of the apatite crystals. A number of carbonate-containing apatites prepared in aqueous systems have been studied. These show similarities to enamel but with the carbonate ions in different relative proportions in the two different environments present in enamel. Some apatites have been prepared which have absorption bands due to molecular carbon dioxide. This has been confirmed by preparing apatites containing the heavy isotope of carbon, carbon-13. The orientation of the carbonate ion in francolite has been determined and is consistent with it occupying the sloping face (with respect to the basal plane) of the space left by a phosphate ion. The environment of the carbonate ion in francolite is different from that in enamel, The hydroxyl ion in enamel and francolite is oriented 'with the 0-H bond parallel with the c-axis of the apatite lattice. The assignment of the hydroxyl band in the infra-red spectrum of enamel has been confirmed by deuteration experiments.

No description available.

No description available.

The aim of this thesis is to investigate structure-property relationships in several types of non-conventional liquid crystalline materials. First, supramolecular hydrogen bonded complexes involving either acid or phenol-based donors with pyridyl-based acceptors were studied using FT-IRM. These studies were made to investigate the nature and temperature dependence of the hydrogen bond between the donor and acceptor moieties. Second, a study of carbohydrate liquid crystals was undertaken. In this investigation two different types of carbohydrate-based materials were synthesised; (a) a homologous series of mono-tailed carbohydrates, the methyl-6-O-(<i>n-</i>acyl)-a-D-glucopyranosides were prepared to study the effect of alkyl chain length on phase behaviour and to elucidate how stereochemistry and the position of the alkyl chain affects liquid crystallinity; (b) a novel non-symmetric dimer containing a methoxyazobenzene moiety and a carbohydrate headgroup to study the behaviour of a dimer consisting of two incompatible mesogenic groups. Third, the effect of terminal alkyl chain branching on the molecular organisation in the intercalated smectic. A phase, was studied with a view to exploring the nature of the driving force for smectic phase formation in symmetric and non-symmetric liquid crystal dimers. The symmetric dimer chosen was 4.060.4, while the non-symmetric dimer was CB.060.4. In both systems the <i>n</i>-butyl terminal chains were replaced by either <i>sec-</i>butyl or <i>tert</i>-butyl chains. To establish how these structural changes effect smectic phase formation, binary phase diagrams of <i>n</i>-butyl-based compound with the corresponding branched material have been constructed. Finally, a comparison of the mesogenic properties of a series of monomers, dimers and side-chain liquid crystal polymers containing a methoxyazobenzene mesogenic unit was made. There data were used to examine the virtual trimer model as a means of rationalising the mesogenic behaviour of the polymers.

No description available.

The results of optical studies on some crystal surfaces - cleavage and etched, with special reference to diamonds are reported in the thesis. It is divided into four parts. In the part I, a brief historical review of the studies on crystal growth is given as a general introduction. The existing information on diamond is also described. All the experimental techniques used in the course of this work are given in part II. The techniques used were (1) Multiple beam interferometry (2) Phase contrast microscopy (3) Light profile microscopy (4) Optical goniometry in part III is given a description of the optical studies of cleavage surfaces of topaz, calcite, mica and diamonds type I and type II. Both the surfaces obtained on cleaving a crystal were examined and the counter parts were matched for comparison. In the case of topaz, calcite and mica the matching was exact within limits of interferometric measurements, where as in the case of diamonds there were marked differences. The cleavage surface of type I diamond is very rough compared to that of type II diamond which is a strong difference in the cleavage. In the case of one pair of type II diamond triangular depressions were noticed. An explanation for these is offered. In the last part the evolution of etch phenomena on natural octahedron faces, cleavage (111) face, polished dodecahedron faces and polished cube faces, produced by immersion in hot potassium nitrate in temperatures ranging from 500&deg; to 700&deg;C is reported. In the case of natural (111) faces the etching process though a continuous one, is divided into three stages. The first stage begins with preferential attack on any surface flaws and then develops into random distribution of small etch pits. The concentration of these pits is less within growth trigons than elsewhere. In the second stage relatively small number of etch pits grow steadily and devour their smaller members. The corners become rounded and flat bottomed pits are frequent. In this stage, growth trigons tend to become hexagonal in outline and this mechanism is unfolded. In the third stage the whole of the original face has been eaten away. Triangular pyramidal depression begins to grow deeper and ultimately striking block formation results. The pile of blocks has plane surfaces which are mainly (221), (212), (122) and less prominent (331), (313), (133) which tend to become (334) but then rounding sets in. In the case of etching of cleavage (111), polished dodecahedron and polished cube faces, the etch pits observed are triangular for (111), boat-shaped for (110) and square for (100). Some of these are evaluated and discussed. Appearance of strongly marked rectilinear pattern proves each of the diamond to have laminated structure.(111) face is crossed with lines parallel to (111) edges, the (110) face is filled with beautifully regular set of concentric hexagons with sides parallel to (111) edges and the cube face reveals an equally striking set of concentric rectangles filling most of the face. The etch patterns reveal a sectional history of the growth of diamond. The observations can be accounted for by postulating increased resistance to etching at the on-set (or end) of each layer. The proposed growth by sheet layers is in conformity with the failure to find growth spirals on natural octahedron diamond faces.

Crystal growth from solution has been studied optically and interferometrieally. The two dimensional diffusion phenomena for sodium chlorate crystal grown from aqueous solution has been studied using multiple beam white light fringes of equal chromatic order. The results are found to agree with the assumption of the existence of a quasi-stationary state theory. According to this theory, the radial gradient of concentration is analogous to a static potential field multiplied by a Gaussian cut off factor; the law relating the concentration at a point in the diffusion field to the distance of that point from the growing new phase is logarithmic near the crystal and exponential at larger distances. The growth in the light of the diffusion theory is briefly discussed. The more recent theory of dislocations, which accounts for growth properties of the crystal itself has been confirmed on chrysene crystals grown from solution. They were examined at high magnifications using optical techniques and spiral growth pictures in agreement with those predicted by the dislocation theory were observed. Measurements of step heights of the spirals were carried out using multiple beam inter-ferometric techniques. One origin of such dislocations has been investigated by studying the deformation of cadmium iodide crystals growing in thin plates from solution. They were subjected to a mechanical deformation by an indentation process, as a result of which the growth properties of the crystal have been altered. It is concluded from this experiment that the mechanical deformation is the vital factor for the growth effects here observed. Slip process leading to the creation of dislocations or increase of dislocations already present in the crystal may account for the spiral nature of the growth sometimes observed.

A directional effect has been observed when indentation hardness tests were made with a Double-Cone indenter on single crystals of Tin and Bismuth, The distortion around the impressions made during, the teats was studied by multiple beam interferometric techniques. The surface contours shown by the fringes revealed deformation of an unsymmetrical nature, with "piling-up" and "sinking-in occurring in directions mutually at right angles. The fringes also revealed slip traces, of step heights of the order 200 A.u. or less, crossing the hill formations. As the direction of indentation on the surface was changed, the observed interference patterns varied continuously, the amount of deformation passing from a maximum to a minimum and returning to the original form when the indenter had rotated through 180&deg;. Because of the presence of the slip traces an attempt was made to explain the directional effects observed, in terms of the plastic deformation properties of the crystals and the direction of the forces exerted on the crystal during indentation. As the Indenter was rotated with respect to the crystal, it was found that the value of the shear stress t component on the activated slip planes varied in a similar manner as the measured lengths of the indentations. It is suggested that the hardness variation is a measure of the change in yield stress in the crystal, as the, deformation takes place under strainless conditions.