Comparison of the interfacial properties of Eugenia uniflora and Triticum vulgaris lectins

We have investigated the interfacial and dielectric properties of EuniSL, a recently purified lectin obtained from seeds of Eugenia uniflora (EuniSL), through surface pressure (Π) and surface potential (ΔV) measurements of its floating monolayers at the 2.0 < pH < 9.0 range, and compared the corresponding results to those obtained for Langmuir films of Triticum vulgaris (WGA). The variation of Π and ΔV as a function of the molecular area reveals that while the interfacial properties of both lectins are strongly dependent upon the pH of bulk phase, in general terms EuniSL monolayers seem to be more structured than those of WGA. At the pH range investigated, the interfacial electric double layer values (Ψ0) calculated from the surface potential are negative, both for EuniSL and WGA. While for EuniSL definite breakpoints in an otherwise linear dependence of Ψ0 and zeta-potential as a function of pH were detected at pH 6.5, similar changes were observed for WGA at pH 8.5, a value close to the isoelectric point (pI) of this lectin. We have then used electrical impedance spectroscopy to investigate the dielectric characteristics of aqueous solutions of the two lectins, assuming a simple Debye relaxation model, and determined the pI of EuniSL as 6.5. While it is well known that the pI of a protein dispersed as a Langmuir film can be determined by surface potential measurements, our results confirm the use of impedance spectroscopy as a valuable and convenient technique that allows the identification of the pI of proteins directly dispersed in aqueous solutions.