Spectroscopic detection of macromolecular interactions focusing on protein-protein interactions in food

Susanne Wrang Bruun

    Research output: Book/ReportPh.D. thesis

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    It is essential to improve the knowledge of macromolecular interactions in foods, as the food quality is highly influenced by these interactions. Thus, new methods for detection and characterisation of macromolecule interactions are seeked. In this thesis, near-infrared spectroscopy (NIR) is examined as a tool for this type of analysis, as NIR has the advantages of being non-destructive, fast, flexible and applicable on a wide range of sample types. The attention in this work is to protein interactions and conformations. A part of the work dealt with measurements of aqueous protein solutions, whereas most of the previous studies have used proteins in the dry state to show the sensitivity of NIR to protein secondary structures. The preliminary experiments showed NIR to be sensitive to β-sheet and α-helix contents also for measurements of the dilute protein solutions (10 mg/ml). The structure sensitive reference method mid-infrared spectroscopy (MIR) was applied to confirm this. However, further studies, involving more samples, are necessary in order to survey the possibility to obtain quantitative structure information from NIR spectra. The experiments gave indication that changes in amino acid side chain interactions and their microenvironments influence the spectra and thus indicated that conformational changes other than secondary structure changes may be detected by NIR as well. This was seen in studies of a protein, which adopted monomer or polymer forms depending on the buffer type (Paper IV). Studies of protein conformations and interactions in the gluten-water system were done with the purpose of evaluating the ability of NIR to give information of more complex systems, for which several different constituents as well as light scattering influence the spectra (Paper I). The gluten system is useful for demonstrating the capacity of NIR in structure-functionality studies, as there is a well-known relation between protein structures and the functionality of gluten. The spectral changes were partly interpretable by means of the MIR reference measurements, but a full explanation of the NIR variations will require other types of measurements for monitoring the protein hydration, the hydrophobic interactions etc. Even though NIR was shown insensitive to the intermolecular β-sheet, which is of importance in the network structure of hydrated gluten, the experiments indicated that NIR could give information on the gluten functionality due to its sensitivity towards amino acid side chain hydrations and interactions. Additional experiments involving gluten and other model systems would be necessary in order to confirm this hypothesis and to show how general these results are. Water is a major ingredient in many foods and has a great influence on the obtained spectra. Most times, the variations in the water spectrum are irrelevant and their dominance in the NIR spectra of food proteins can impair the analysis. An empirical model was in this work shown capable of removing these variations from MIR spectra (Paper III), and the same method is considered suitable for correction of NIR-spectra. A similar method has been employed for removal of the water vapour and CO2 absorptions from MIR spectra, as a way to improve the spectroscopic analysis (Paper II).
    Original languageEnglish
    Place of PublicationKgs. Lyngby
    PublisherTechnical University of Denmark
    Number of pages218
    ISBN (Print)8791494184
    Publication statusPublished - Sep 2006

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