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Title: Investigations of the supramolecular structure of individual diphenylalanine nano- and microtubes by polarised Raman microspectroscopy
Author: Lekprasert, Banyat
Awarding Body: University of Nottingham
Current Institution: University of Nottingham
Date of Award: 2012
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This work presents the development of an integrated instrument which incorporates two powerful techniques , the atomic force microscopy (AFM) and the polarised Raman micro-spectroscopy (PR~!lS), to determine the molecular organisation of the Phe-Phe (FF) micro- and nanotubes. The diphenylalanine peptidebased nanotubes consist of the FF molecules self-aggregated into a tubular structure exhibiting unique physical, chemical and biological properties. These prominent characteristics promote this nanomaterial to be a potential candidate for numerous applications. The FF peptide-based structure has also been studied extensively in medical research. This is because its self-aggregation mechanism plays a key role in the formation of amyloid fibrils , which are related to various diseases. Understanding such formation mechanism could provide new treatments for these diseases. The structural organisation of the FF tubes became a controversial issue because of the lack of direct evidence, especially at micro- and nanoscale. In this ~work , AFM and polarised Raman spectroscopy was used to determine molecular interaction and orientation for individual FF tubes. As the combined AFM-PRMS system consists of two essential parts operating individually, new control software was designed and developed to synchronise both instruments. Therefore, investigation of the selected FF tubes can be performed using polarised Raman micro-spect;~scopy. Simultaneously, the tube diameter can be measured accurately using the AFM. To determine the molecular organisation of the FF tubes , the studied began with the investigation of the FF samples by conventional Raman measurements for determining the observed Raman bands and their corresponding vibrational modes. Additionally, the DFT calculation was employed to confirm the band assignment. Finally, the polarised Raman micro-spectroscopy was used to determined the anisotropic properties of some key vibrational modes, particularly, the amide I and the amide III bands. The experimental measurements were carried out by two methods. In the first method. the FF samples was studied by investigating the variation of intensity of the selected Raman bands when the polarisation direction of excitation laser changed from 0° to 360° vvith respect to the tube axis. The probable orientation of the selected bands was depicted in polar diagrams of normalised Raman intensity. For the second approach, the FF tubes were investigated with four polarisation configurations using a half-wave plate and an analyser. A dataset of polarised Raman spectra corresponding to the selected bands was mathematically analysed using a uniaxial model to determine quantitatively the orientation distribution function (ODF). Both methods provided consistent results illustrating that the peptide C=O bonds of the FF molecule are preferentially aligned along the tube and the peptide backbone is aligned perpendicular of the tube. From the second method , the angle between the C=O bond and the tube axis was determined to be rv 8° in crystalline structure. Furthermore, the tubes of different diameter showed no noticeable variation in molecular organisation within the experimental error. As the calculation was dependent on various parameters, the uncertainty of the mean orientation angle was also assessed. II
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available