On the interactions and interfacial behaviour of biopolymers. An AFM study: Dissertation

Surface forces and interactions are a key issue in colloid and surface science, including biopolymer systems. Covalent and ionic bonds determine the structure and composition of materials, but the weaker non-covalent interactions define their functions. This thesis deals with the surface forces and interactions occurring between biopolymer surfaces and affecting the self-assembly and interfacial behaviour of biopolymers. The research was aimed at deepening the understanding of molecular interactions and the nature and strength of surface forces in the studied biopolymer systems. The main research tool was atomic force microscopy (AFM). This technique allows imaging of the sample topography in either gas or liquid environments at high resolution. Data on intra- and intermolecular interactions can also be obtained. Interesting phenomena revealed by AFM were supported and confirmed by other relevant surface analytical techniques. The nanomechanical force measurements focused on interactions relevant in papermaking, i.e. between cellulose and xylan, and food technology, i.e. between gliadins (wheat gluten proteins). In the cellulose-xylan interaction work the colloidal probe technique was exploited by attaching cellulose beads to the tip and to the sample surface. The interaction between these beads was measured in different xylan solutions. The main result of the cellulosic systems provided a new perspective on the role of xylan in papermaking. It has been reported previously that the adsorption of xylan increases paper strength and that this is due to formation of hydrogen bonds. Our results indicate that the increase in paper strength cannot originate from such bonds in wet paper, but must be due to effects of xylan on fibre bonds during drying of paper. The viscous and elastic properties of gliadins and glutenins facilitate the production of bread, pasta and many other food products from wheat flour. Gliadin proteins (alfa- and omega-gliadins) were attached to both thetip and the sample surfaces, and the interaction forces between monomeric gliadins (alfa-alfa, omega-omega, and alfa-omega) were measured. On the basis of the nanomechanical force measurements, different roles of different types of gliadins were proposed: whereas omega-gliadins still have acompact structure and are responsible for the viscous flow, alfa-gliadins have already started to participate in forming the network in dough. This may provide a new viewpoint in understanding the interfacial properties of gliadins in relation to baking. The studies of interfacial behaviour of biopolymers focused on hydrophobins, which are very surface active proteins. Hydrophobins are amphiphilic proteins which self-assemble due to the interplay of various surface forces and interactions in solution and at interfaces. Films of Class II hydrophobins, HFBI and HFBII, at the air-water interface were transferred to solid supports and imaged by AFM. The interfacial films of hydrophobins were imaged at nanometer resolution. The results showed that both HFBI and HFBII form organised structures at the air-water interface. Moreover, the nanostructured films formed spontaneously. The HFBI films were imaged and the organised pattern was seen both on the hydrophobic and the hydrophilic side. The dimensions were similar to those of hydrophobin tetramers in solution obtained by small angle X-ray scattering. Protein engineering enabled assignment of a specific functionality to HFBI. The results confirmed the expected orientation of hydrophobins at the air-water interface, and indicated that the hydrophobin retained its capability to form organised films and the covalently attached molecule its functionality.