Interfacial behaviour of wheat puroindolines: monolayers of puroindolines at the air-water interface

Abstract

Puroindolines are among the major basic and cysteine-rich lipid binding proteins of wheat seeds. The interfacial properties of puroindoline-a (PIN-a) and puroindoline-b (PIN-b) are important both from a biological and a technological point of view. In the work reported here, the interfacial characteristics of spread monolayers of wheat puroindolines at the air–water interface were studied at varying subphase compositions using a Langmuir–Blodgett film balance. The compression isotherms (π–ASp) were recorded at constant barrier speed (3.3 cm/min). It was observed that both PIN-a and PIN-b form stable monolayers at the air–water interface. The stability of the monolayers was found to be dependent on the subphase composition as well as on the concentration of protein in the spreading solution. When the ionic strength of the subphase is below 0.50, the compression isotherms of both PIN-a and PIN-b remains unaffected with the change in the ionic strength of the subphase; however, when the ionic strength is above 0.50, the compression isotherms of both PIN-a and PIN-b undergo significant change with an increase in the ionic strength of the subphase. A gradual increase in the values of the collapse pressure (πC) and the limiting area (A0) was observed due to an increase in the ionic strength of the subphase from 0.5 to 4.0, which may be correlated with the salt-induced conformational changes of the protein molecule. The presence of NaCl and KCl (ionic strength 1.0) in the subphase has a comparable effect on the compression isotherms of both PIN-a and PIN-b; however, the presence of CaCl2 (ionic strength 1.0) in the subphase leads to an increase in the values of πC and A0. A change in the pH of the subphase from 3.0 to 7.2 was to have a significant effect on the values of πC and A0, which may be due to the pH-induced alteration of the protein conformation.