Binding of lanthanide salts to zwitterionic phospholipid micelles

Constantina Sofroniou; Ioanna Chazapi; Epameinondas Leontidis

doi:10.1016/j.jcis.2019.09.048

Binding of lanthanide salts to zwitterionic phospholipid micelles

J Colloid Interface Sci. 2019 Dec 1:557:568-579. doi: 10.1016/j.jcis.2019.09.048. Epub 2019 Sep 16.

Authors

Constantina Sofroniou¹, Ioanna Chazapi², Epameinondas Leontidis³

Affiliations

¹ Department of Chemistry, University of Cyprus, PO Box 20537, Nicosia 1678, Cyprus; Department of Chemistry, University of Florence, Via della Lastruccia, 3-13, 50019 Sesto Fiorentino, Florence, Italy.
² Department of Chemistry, University of Cyprus, PO Box 20537, Nicosia 1678, Cyprus.
³ Department of Chemistry, University of Cyprus, PO Box 20537, Nicosia 1678, Cyprus. Electronic address: psleon@ucy.ac.cy.

PMID: 31550649
DOI: 10.1016/j.jcis.2019.09.048

Abstract

As the use of lanthanide salts in biophysical systems increases and the separation of lanthanides from nuclear and other wastes with extraction processes has become an important technological challenge, a deeper understanding of the behavior of lanthanides at lipid interfaces is urgently needed. In this work the interaction of lanthanide salts with zwitterionic phospholipids is probed using aqueous micelles of the surfactant dodecyl phosphocholine (DPC), which are useful membrane-mimetic model systems, widely used for the solubilization of membrane proteins in aqueous solutions. Because more than one species exists in lanthanide salt solutions, even at the pH value of 4 used in this experiment, the major goal of this investigation is to examine which species are actually binding to the micelles. Using static and time-dependent europium fluorescence, strong indications are obtained that both the Eu³⁺ cation and its 1:1 chloride, nitrate, or sulfate complexes bind to the micelles, whereas the europium species do not appear to interact strongly with DPC molecules below the cmc. From isothermal titration calorimetry (ITC) measurements it is found that the lanthanide interaction with DPC micelles increases to the right of the lanthanide series and is - surprisingly - endothermic, underlying the important role of hydration effects in the interaction. The anion of the lanthanide salt strongly influences the thermodynamics: perchlorate and sulfate salts give extraordinary results, switching the interaction to exothermic. A multi-level phenomenological electrostatic model of the europium fluorescence lifetimes strongly suggests that in the case of nitrate salts both Ln³⁺ and LnNO₃²⁺ ions bind to the micelles. Overall a detailed molecular picture of the complexity of lanthanide-lipid interactions at interfaces is emerging from these experiments and the associated modelling effort.

Keywords: Europium fluorescence; Isothermal titration calorimetry; Lanthanide salts; Lanthanide-lipid interactions; Ligand effect; Zwitterionic phospholipid micelles.