Characteristics of the influence of auxins on physicochemical properties of membrane phospholipids in monolayers at the air/aqueous solution interface

Abstract

Interactions between representatives of plant hormones and selected membrane lipids have been studied in monolayers at the air/aqueous solutions interface with π–A isotherm analysis, microscopic visualization and grazing incidence X-ray diffraction technique (GIXD). Four phytohormones: indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 2-naphthoxyacetic acid (BNOA) and 2,4-dichlorophenoxyacetic acid (2,4-D), belonging to the class of auxins differ as regards the chemical structure of the aromatic molecular fragment. The studied phospholipids have been chosen since they are omnipresent in the biological membranes of plant and animal kingdom. Our results revealed that both natural (IAA and IBA) and synthetic (2,4-D and BNOA) phytohormones modify the physicochemical characteristics of the investigated lipid monolayers. Auxins caused strong diminishing of the monolayer condensation, especially for DPPC and SOPE, which may be attributed to the phase transition in these monolayers. In the performed experiments the key step of auxins action occurs when the molecules interact with monolayers in the expanded state—when the space in the lipid head-group region is large enough to accommodate the molecules of water soluble auxins. The application of GIXD technique confirmed that auxin molecules are also present at the interface at higher surface pressure (30mN/m). The obtained results showed that among the investigated auxins, the largest influence on the lipid monolayers occurred in the case of BNOA, which molecule possesses the largest aromatic fragment. In contrast, 2,4-D, having the smallest aryl group affects the studied lipid systems to the smallest extent.