Date of Award
Program or Major
Doctor of Philosophy
Sterling A Tomellini
Bile salts play important roles in digestion and the treatment of diseases. Few studies have been performed to determine, at the molecular level, the interactions of individual bile salt species with cellular membranes. Vibrational spectroscopy provides a noninvasive approach for characterizing the structural changes occurring within the lipid matrix due to their association with bioactive molecules. The frequency of the methylene stretching modes in the infrared spectra of the lipids comprising the membrane, as a function of temperature, yields information on the intra- and interchain order/disorder of the lipid matrix, the stability of the membrane, and the cooperativity between lipid molecules during the main phase transition. The methylene wagging modes provide information on the number and type of gauche conformers in the lipid acyl chains.
The interactions which the unconjugated and the glycine conjugated forms of sodium ursodeoxycholate (UDC) and sodium chenodeoxycholate (CDC) have on a lipid bilayer composed of dipalmitoylphosphatidylcholine, DPPC, were investigated. Experiments were performed using KH$\sb2$PO$\sb4$ hydrating solutions having initial pH's of 5, 7, and 8. Results show that the presence of these bile salts decreases the stability of the membrane, decreases the cooperativity between the lipid molecules during the gel to liquid crystalline phase transition and increases the fluidity of the liquid crystalline phase. The degree to which the unconjugated and the glycine conjugated forms of UDC and CDC affect the DPPC bilayer is dependent on the initial pH of the hydrating solution.
Studies were also performed using hydrating solutions containing various concentrations of ethanol, which at high concentrations (0.8 to 1.2 M) causes the acyl chains in the DPPC bilayer to interdigitate. Results indicate that ethanol, at these concentrations, subdues the effects which the bile salts have on the lipid bilayer.
The methylene wagging modes provided information on the effects which the bile salts and ethanol have on the number and type of gauche conformers in the acyl chains. The results obtained for the methylene wagging and symmetric stretching regions were compared. Some differences were noted for the lipid bilayer in the gel phase, while, the results for the liquid crystalline phase were similar.
Parthum, Kimberly Ann, "Interactions of conjugated and unconjugated bile salts with model phospholipid membranes: Vibrational spectroscopic studies" (1996). Doctoral Dissertations. 1920.