title: Binding of monovalent alkali metal ions with negatively charged phospholipid membranes.
abstract: We have systematically investigated the effect of various alkali metal ions with negatively charged phospholipid membranes. Size distributions of large unilamellar vesicles have been confirmed using dynamic light scattering. Zeta potential and effective charges per vesicle in the presence of various alkali metal ions have been estimated from the measured electrophoretic mobility. We have determined the intrinsic binding constant from the zeta potential using electrostatic double layer theory. The reasonable and consistent value of the intrinsic binding constant of Na(+), found at moderate NaCl concentration (10-100 mM), indicates that the Gouy-Chapman theory cannot be applied for very high (> 100mM) and very low (< 10 mM) electrolyte concentrations. The isothermal titration calorimetry study has revealed that the net binding heat of interaction of the negatively charged vesicles with monovalent alkali metal ions is small and comparable to those obtained from neutral phosphatidylcholine vesicles. The overall endothermic response of binding heat suggests that interaction is primarily entropy driven. The entropy gain might arise due to the release of water molecules from the hydration layer vicinity of the membranes. Therefore, the partition model which does not include the electrostatic contribution suffices to describe the interaction. The binding constant of Na(+) (2.4 ± 0.1 M(-1)), obtained from the ITC, is in agreement with that estimated from the zeta potential (-2.0 M(-1)) at moderate salt concentrations. Our results suggest that hydration dynamics may play a vital role in the membrane solution interface which strongly affects the ion-membrane interaction.
authors.affiliation: Department of Physics, Jadavpur University, 188 Raja S. C. Mullick Road, Kolkata 700 032, India.
authors.forename: Pabitra
authors.initials: P
authors.lastname: Maity
authors.affiliation: Biophysical Chemistry Laboratory, Organic and Medicinal Chemistry Division, CSIR-Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Kolkata 700 032, India.
authors.forename: Baishakhi
authors.initials: B
authors.lastname: Saha
authors.affiliation: Biophysical Chemistry Laboratory, Organic and Medicinal Chemistry Division, CSIR-Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Kolkata 700 032, India.
authors.forename: Gopinatha Suresh
authors.initials: GS
authors.lastname: Kumar
authors.affiliation: Department of Physics, Jadavpur University, 188 Raja S. C. Mullick Road, Kolkata 700 032, India. Electronic address: sanat@phys.jdvu.ac.in.
authors.forename: Sanat
authors.initials: S
authors.lastname: Karmakar
mesh_terms: D002151:Calorimetry; D002462:Cell Membrane / Q000737:chemistry* / Q000378:metabolism; D008051:Lipid Bilayers / Q000737:chemistry / Q000378:metabolism; D008672:Metals, Alkali / Q000737:chemistry* / Q000378:metabolism; D010713:Phosphatidylcholines / Q000737:chemistry / Q000378:metabolism*; D055672:Static Electricity; D013816:Thermodynamics; D053835:Unilamellar Liposomes / Q000737:chemistry / Q000378:metabolism
chemical_list: D008051:Lipid Bilayers; D008672:Metals, Alkali; D010713:Phosphatidylcholines; D053835:Unilamellar Liposomes
keywords: Alkali metal ions; Electrostatic double layer; Isothermal titration calorimetry; Phospholipid vesicles; Zeta potential