EFFECT OF SALTS ON ZWITERIONIC SURFACTANT AGGREGATES WITH VARIABLE POLAR REGION AND CATIONIC SURFACTANTS DERIVED FROM GUANIDINE

• Grant number: 24/20537-3
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)
• Start date: July 01, 2025
• End date: July 31, 2028
• Field of knowledge: Biological Sciences - Biophysics - Molecular Biophysics
• Principal Investigator: Iolanda Midea Cuccovia
• Grantee: Matheus Pires Cortez Favoreto
• Host Institution: Instituto de Química (IQ). Universidade de São Paulo (USP). São Paulo , SP, Brazil

Abstract

Interfaces are common boundaries between two independent and adjacent systems that separate and communicate bodies, substances or phases. Most chemical reactions in biology occur at interfaces between, for example, proteins/solution or membranes/medium. The focus of our projects is the study of biologically relevant interfaces using appropriate models to understand distinct aspects of interfaces in chemical and biological reactivity. Our research group has been contributing to the understanding of these relationships using micelles and vesicles.Membranes are essential components in cells because they separate the contents of cells and organelles from surrounding solutions. At the same time, membranes play an active role in regulating traffic between these open systems and regulating communication between external and internal compartments.Recently, we described, experimentally and theoretically, that the binding of hydrotropic anions to positive micelles depends mainly on the contribution of the nonpolar part of the ion. The distribution of ions inside and outside cells represents another marked asymmetry maintained by a complex system of ion pumps responsible for a significant part of the energy produced in metabolism. Recent simulations and experiments show that ion binding to membranes and models containing exclusively lipids or zwitterionic surfactants is selective.Ion-specific and hydrophobic effects determine properties of complex systems, but are far from being completely understood in molecular detail. Ion-specific effects determine a wide variety of phenomena in physicochemical and biological systems, from the stability of colloidal suspensions to enzymatic activity or ligand-receptor binding. Establishing a rigorous scientific interpretation of the effects has kept scientists busy for more than a century, yet fully describing the complex network of interactions between ions, water and the surface of the solute still represents a challenge.The hydrophobic effect (in the formation of aggregates) depends, almost exclusively, on the difference in free energy of the exposed areas of the hydrocarbon chains of the surfactant (natural as in lipids or synthetic) in solution and in the aggregate. However, the detailed structure, degree of order and viscosity of the nonpolar part of the aggregate critically depend on the detailed molecular structure of the surfactant. The thermodynamics of micellization of zwitterionic detergents will be analyzed using calorimetry in the presence of different ions.We, therefore, intend to study the following systems: zwitterionic and cationic micelles (with a guanidine group in the polar head) under the effect of ions with hydrophobic portions on this surface, using molecular dynamics, calorimetry, and surface tension, varying the hydrophobicity of the polar head of these zwitterionics surfactants aiming to understand the hydrophobic effect on the formation of these aggregates, and the effect of the polar head on the interaction with ions in the case of guanidine surfactant, given the interaction with salts not only in an ionic way (opposite charges attracting each other), but also with the hydrogen interactions that can be carried out by that specific group. From this information, it will be possible to gain a greater understanding of the hydrophobic effect and extrapolate the information acquired to more complex biological systems.The study of selectivity in the association of ions with zwitterionic micelles is known, but recently, we demonstrated that interfacial water is not particularly sensitive to the nature of these ions. This particular and apparent contradiction can be understood by the proposal of this project, which includes the thermodynamic analysis of the association of ions as well as the complete description of the association of these ions using molecular dynamics. (AU)