Adsorption of the High Molecular Weight Glutenin Subunit 1Dx5 Compared to the 58-kDa Central Repetitive Domain and α-gliadins

Abstract

The adsorption of the high molecular weight glutenin subunit 1Dx5 onto a hydrophobic surface was measured using null ellipsometry. Interactions between subunit 1Dx5 and α-gliadins, and between subunit 1Dx5 and the 58-kDa central repetitive domain from the subunit, were investigated by sequential adsorption experiments. Subunit 1Dx5 and α-gliadins were also studied by competitive adsorption experiments. All measurements were carried out in 0·01 m acetate buffer, pH 4·0, both with and without 0·1 m NaCl. In the absence of NaCl, the apparent adsorption isotherm reached a semi-plateau at 6·4 mg/m2, compared to 14·0 mg/m2in the presence of NaCl. No desorption upon rinsing was detected in experiments without NaCl, whereas there was some desorption at the highest concentrations in experiments with NaCl present. An overshoot in the adsorbed amount was found for adsorption at high protein concentration and high ionic strength. This was attributed to an initial adsorption of aggregates followed by dissociation at the surface. Sequential adsorption experiments showed that 1Dx5 adsorbed onto a 58-kDa peptide layer at low ionic strength, and probably displaced the peptide at high ionic strength. Subunit 1Dx5 blocked adsorption of the peptide at high ionic strength, but in buffer without NaCl, adsorption of the peptide to a 1Dx5 layer was evident. The amount of peptide adsorbed was much less than for 1Dx5, and independent of the ionic strength. The sequential adsorption of 1Dx5 and α-gliadins at high ionic strength suggested partial removal of 1Dx5 by the α-gliadins, and a slight adsorption of 1Dx5 with the reversed adsorption order (i.e. adsorption of α-gliadins followed by addition of 1Dx5). In NaCl-free buffer, 1Dx5 blocked the adsorption of α-gliadins and vice versa. The competitive adsorption of α-gliadins and 1Dx5 indicated the formation of a mixed film, both at high and low ionic strength. These results may be relevant to an understanding of the behaviour of gluten proteins at hydrophilic-hydrophobic interfaces in dough.