The Insertion Behaviour of Wheat Puroindoline-a into Diacylgalactosylglycerol Films

Abstract

In order to study the surface behaviour of puroindoline-a (PIN-a) in the presence of lipid monolayers, protein solutions were injected beneath films of the polar wheat lipid diacylgalactosylglycerol (MGDG). It was found that injection of PIN-a into the sub-phase caused a surface pressure increase of monolayers consisting of MGDG. When PIN-a was injected beneath an MGDG film at an initial surface pressure (initial) between 9 and 15 mN/m, a relatively fast increase in the surface pressure to 17±2 mN/m was observed (the maximum surface pressure that could be reached by the protein alone). Then a much more gradual increase in was observed. At initialvalues exceeding 35 mN/m, injection of PIN-a resulted in a gradual increase in to 49±1 mN/m, which is equal to the collapse pressure of a pure MGDG monolayer. A gradual increase in the surface pressure was observed after injection of PIN-a beneath an MGDG monolayer with initialvalues between 15 and 35 mN/m, but the final value was lower than the collapse pressure of pure MGDG. Injection of peptide-a (a peptide comprising the tryptophan-rich domain of PIN-a) beneath MGDG monolayers also led to a considerable increase in surface pressure. The data are interpreted as insertion of PIN-a or peptide-a into the MGDG films. In contrast to this, lipid-transfer protein did not show this behaviour under the same conditions. The present data suggest that PIN-a may be present at the gas–cell surface in dough. However, although the elasticity of mixed PIN-a/MGDG films somewhat decreased owing to protein insertion, the resulting monolayer would still retard disproportionation.