The thesis is mainly concerned with the structure of the protein in the egg case of the praying mantis. It continues the work of Rudall (1956), who showed that the protein was organised into lamellae and had an α-helical structure. He put forward two models: hexagonally packed α-helices, and two-stranded coiled coils arranged in layers. In the latter model, each coil is staggered from its neighbours by a quarter of the pitch, so as to allow optimum packing. Sections on the X-ray diffraction camera, analysis of errors in reciprocal lattice coordinates, analysis of sequences of fibrous proteins, and unsuccessful structural studies on sequential polyheptapeptides are also included. Lamellae may be removed from fresh egg cases, and ribbons from egg cases a few or more hours after production. Electron microscopy of negatively stained lamellae, positively stained lamellae and sectioned ribbons, and X-ray diffraction studies on the ribbons make it possible to postulate a model for the structure. The model is more detailed than Rudall's and has differences. It has layers of two-stranded α-helical coiled coils about 18Å thick packed in register to form 460Å thick lamellae about 1μm x 10μm. The layers are perpendicular to those in Rudall's model. The lamellae pack into ribbons with considerable disorientation. Coiled coils within an 18Å thick layer probably pack in a way similar to that in the layers of Rudall's model with a stagger of 2.5 x the pitch and a rotation of 90° between adjacent coiled coils. Each coiled coil has chains about 730 Å long staggered by about 55Å. Coiled coils interact end to end with an overlap of about 40Å, so as to leave gaps of about 15Å. The molecular length of 730Å is consistent with the molecular weight of 53,000 estimated from SDS gel electrophoresis. The molecular axis is parallel to the principal shear plane in the lamella. Fibres of this protein drawn from the first order gland give an X-ray diffraction pattern indicating a very similar structure. In the presence of water, uranyl acetate solution, neutralised phosphotungstic acid or a mixture of chlorobenzene and bromobenzene, the volume of the unit cell changes. X-ray diffraction evidence indicates that sliding occurs between coiled coils as the volume changes. The amount of lateral swelling is greater in the direction perpendicular to the surface of the lamella than in the direction parallel to it. Together with the different extents of the lattice in these directions, this is consistent with the model, which is different in these two directions. A discussion is included on the literature relating to X-ray diffraction cameras using a Frank s mirror and a quartz crystal monochromator. To this has been added calculations concerning focal size, resolution and intensity of the X-ray beam. The results of these are embodied in a discussion of camera design. An appendix is devoted to a method of estimating standard errors of reciprocal lattice coordinates. The analysis of the sequences of fibrous proteins and the statistical basis of this analysis are discussed in appendices. Preliminary calculations on tropomyosin have revealed periodicities which can be related to a 14 residue stagger between chains, in agreement with Hodges et al (1973), and to the repeat of the actin helix. Hydrophobia residues and possibly positively charged residues are arranged in a pattern which, on the present limited data, appears to be important in interactions with actin. Residues generally considered to disrupt helical structures are arranged in a band coiling around the α-helix. A technique enabling the D stagger in collagen to be identified is described. This also gives information which suggests how hydrophobic interactions are important in defining the pitch of the collagen molecules and the interactions between them. Arguments based on this technique show how pairs of opposite charges lead to interactions consistent with a five-stranded microfibril. There is also an appendix on attempts to produce oriented arrays of sequential polyheptapeptides, in order to obtain X-ray diffraction patterns. Another appendix describes how the colony of praying mantids was raised.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:470038 |
| Date | January 1974 |
| Creators | Rattew, Christopher John |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | https://ora.ox.ac.uk/objects/uuid:afe4e4b4-dd54-41ef-9b80-8d1e3a1f0530 |
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