Portland State University. Department of Physics
Date of Publication
Master of Science (M.S.) in Physics
Adsorption, Ions, Membranes (Biology)
1 online resource (3, vi, 65 p.)
Artificial lipid membranes have been used in biophysical studies as well defined models of biological membranes. In the present work we studied adsorption of ions to artificial lipid membranes, composed of phosphatidylcholine (PC) and phosphatidylserine (PS), and to biological membrane from sarcoplasmic reticulum (SR). The studies of ion adsorption and electrokinetic characterization of membranes were done by means of microelectrophoresis of PC/PS liposomes and SR vesicles. The ions of interest were positively charged potassium (K), calcium (Ca), tetraphenylarsonium (TPAs), tetraphenylphosphonium (TPP) and negatively charged pentachlorophenol (PCP). Electrophoretic mobility of PC/PS liposomes and SR vesicles has been measured as a function of pH, ionic strength and the concentration of adsorbing ions. From the data we have determined the isoelectric point and the density of electric charge of the SR membrane. We have shown that the mobility of PC/PS liposomes and SR vesicles can be understood in terms of electrostatic screening of membrane surface charge by the diffuse double layer of counterions and by ion adsorption. The experimental results, with the exception of Ca and SR membranes, have been found consistent with the adsorption model based on Langmuir adsorption isotherm and Grahame's equation relating the membrane electric potential and the membrane surface charge. The adsorption of ions to membranes have been characterized by the ion association constant and the membrane surface area of adsorption site. These quantities have been obtained from the fit of the adsorption model to the electrophoretic mobility data and the results are reported in the thesis. The following two findings are most interesting and important. First, the adsorption of K and lipophilic ions TPAs, TPP, and PCP to both the SR membrane and the artificial PC/PS membrane can be well characterized by the the adsorption model. The theoretical predictions of the adsorption model agree with the experimental results for a single (although different for PC/PS and SR membranes) set of ion adsorption constant and adsorption site area. This indicates a similarity between the biomembrane and the artificial lipid membrane. In contrast, such similarity was not found for calcium. Adsorption properties of SR membrane and artificial lipid membrane for Ca were found to be very different. Whereas the adsorption of Ca on PC/PS membranes could be described by one set of adsorption sites, the results for the SR vesicles indicate the presence of more than two types of Ca adsorption sites in the SR membrane. This finding is of physiological significance since the SR membrane regulates calcium transport in the muscle cell. Second, the adsorption affinity of SR membrane to all ions has been found to be significantly smaller compared to that of the lipid bilayer. This indicates that membrane proteins in the SR membrane reduce ion adsorption. This effect cannot be due to electrostatic interactions because the artificial lipid membranes had similar surface charge density as the biological membrane and the reduced adsorption was observed for both the positively charged TPAs and TPP as well as for the negatively charged PCP.
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Mense, Martin, "Comparison of Ion Adsorption to Phophatidylcholine/Phosphatidylserine and Sarcoplasmic Reticulum Membranes" (1993). Dissertations and Theses. Paper 4619.