Time-Dependent Absorption Evidence of Phenylurea-Derived Herbicides on Model Biomembranes: A DSC Study

Environmental Context. The need to improve the quality of agricultural cultivation and to control the weed growth of non-cultivated area has brought herbicides to wide-scale use. Unfortunately, these compounds have been proved to be toxic for many organisms, humans included. With an aim to a better knowledge of the mobility and fate of environmental pollutants, it becomes very important to have an exact knowledge of the role that the medium in which these products are dissolved or dispersed plays on the absorption of these products by biological membranes as a complement to information on the interaction between these compounds and cells.

Abstract. Phenylurea derivatives are used as herbicides that inhibit photosynthesis. These materials enter plants via their roots and are employed for selective control of germinating grass and broad-leaved weeds in many crops as well as for total weed control of non-cultivated areas such as roads, railways, and parks. The study of the interaction between the herbicide and lipid membrane is interesting in assessing the relevance of the dispersing medium in the absorption processes. Differential scanning calorimetry (DSC), which detects the effect of foreign molecules on the phase transition from an ordered to a disordered lipid structure when submitted to heating, was employed to study such an interaction. Effects exerted by four phenylurea herbicides (difenoxuron, diuron, metoxuron, and linuron) on the thermotropic behavior of model membranes were here investigated. Aqueous dispersions of dimyristoylphosphatidylcholine (DMPC) were used as model membranes to study herbicide–membrane interactions. Experiments carried out by leaving herbicides, in a powdered form, in contact with DMPC-based multilamellar or unilamellar vesicles show that all the examined herbicides are able, but in different ways, to migrate through the aqueous medium and interact with model membranes. Experiments on herbicide-loaded liposomes in contact with empty ones indicate that the compounds are able to migrate from a loaded membrane to an empty one. The obtained data seem to validate the use of the DSC in demonstrating that bioactive and potentially toxic compounds not only to interact with biological membranes but also adsorb into a cell when dispersed in a lipophilic medium.