Wheat Storage Proteins at Interfaces
Sammanfattning: This thesis deals with the properties of wheat storage proteins at interfaces. These proteins are the main constituents of gluten and are recognised for their importance in breadmaking. The components studied were gliadin fractions (a-gliadins, b-gliadins, g-gliadins and w-gliadins), the high-molecular-weight glutenin subunit 1Dx5, and a 58 kDa peptide derived from the central repetitive domain of subunit 1Dx5. The amount of protein adsorbed onto a solid hydrophobic surface vs. time was studied with ellipsometry. The surface film balance (Langmuir trough) was employed to measure surface pressure-area relationships of spread films. The technique of axisymmetric bubble shape analysis (ABSA) was used to follow the surface tension vs. time upon adsorption to the air-aqueous interface. The surface rheological properties were also measured with ABSA. Emphasis was placed on the following factors. (a) The establishment of structure-function relationships by comparing subunit 1Dx5 with the 58 kDa peptide. (b) The influence of ionic strength on the surface properties. (c) The effects on layer composition when introducing a second component to an existing layer. (d) The changes of layer structure at the air-aqueous interface upon repeated compression-expansion cycles. (e) The effect of residence time at the air-aqueous interface on the layer properties. The interfacial behaviour of subunit 1Dx5 and the 58 kDa peptide was found to be very different. Upon adsorption to a solid hydrophobic surface the amount of subunit 1Dx5 adsorbed increased considerably when the ionic strength was increased from 0.01 M to 0.1 M (pH 4.0). The 58 kDa peptide, on the other hand, was unaffected by the ionic strength. The strong effect of the electrolyte concentration on the amount of subunit 1Dx5 adsorbed suggests that the ionic strength is an important factor in the functional properties of 1Dx5 layers. Irrespective of the ionic strength, higher amounts of the subunit than the peptide were adsorbed. A likely reason for the difference is that the subunit and peptide had a different orientation at the surface, i.e. that the subunit was oriented more perpendicular to the surface. The N-terminal domain of subunit 1Dx5 are more hydrophobic than the peptide and is therefore expected to be preferentially attached to the hydrophobic surface. Measurements of surface pressure vs. area also showed large differences between subunit 1Dx5 and the 58 kDa peptide, even at apparently corresponding surface concentrations (moles/area unit). The 58 kDa peptide showed a semi-plateau at low surface pressure (2.2 – 2.8 mN/m), indicating a phase transition. Since an analogous process was not observed for subunit 1Dx5, the terminal domains apparently hindered the transformation. The importance of ionic strength on layer composition was demonstrated by sequential adsorption experiments. It is suggested that the a-gliadins displaced subunit 1Dx5 at high, but not at low, ionic strength. This can be explained by increased energy gain upon exchange of subunit 1Dx5 with a-gliadins, and/or a reduction in the a-gliadin adsorption barrier, at high ionic strength. Increased ionic strength improved the stability of a-gliadin and 1Dx5 layers during compression-expansion cycles. This effect was most evident for the a-gliadin layer, which could withstand a surface pressure of 60 mN/m without significant desorption. Although proteins were forced to leave the surface, they apparently became re-attached upon expansion. When subunit 1Dx5 and a-gliadin were kept at a constant surface pressure of 15 and 20 mN/m, respectively, the elasticity (?g/?lnA) of the layers, upon subsequent compression above these pressures, increased.
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