A Study of Molecular Adsorption and Transport at Cell Membrane and Biologically Relevant Surfaces by Second Harmonic Generation

Most of the biological processes in living systems involve molecular adsorption and transport at biomembranes. It is highly desired to study the time-resolved transport kinetics through living cell membranes. In this thesis, an experimental means based on a nonlinear optical phenomenon, Second Harmonic Generation (SHG) has been demonstrated to detect the molecular adsorption and transport through living cell membranes in real time and to evaluate the salt ion effects on adsorption processes in biologically relevant colloidal systems. In the case of gram-negative bacteria, E.coli, a hydrophobic cation, Malachite Green (MG) has been observed to adsorb onto the cell surface and then sequentially transport across the double bilayer structures, the bacterial outer membrane and the cytoplasmic membrane. The adsorption characteristics as well as the transport rate constant at each of the membranes have been determined. In contrast to the prokaryotic E.coli cell, the molecular ion can adsorb onto the eukaryotic Murine Erythroleukemia (MEL) cell but cannot penetrate its membrane which has no hydrophobic ion permeable channels and is more tightly packed. MG cation has been used as a SHG indicator to probe the effects of solvent ionic strength and ion specificity on molecular adsorption at model protein systems. Polystyrene sulfate (PSS) microspheres and polystyrene carboxyl (PSC) microspheres have been examined. The electrostatic force dominated molecule-surface interaction between MG cations and the sulfate terminations at PSS surface is largely affected by the ionic strength of the solution but is not sensitive to the ion identity. On the other hand, the hydrophobic force dominated molecule-surface interaction between hydrophobic regions of the MG dye and PSC microsphere shows pronounced specific ion effects but is less affected by ionic strength of the solution. Degree Type Dissertation Degree Name Doctor of Philosophy (PhD) Graduate Group Chemistry First Advisor Hai-Lung Dai