Determination of the Site of Ultrasonic Absorption in Suspensions of Large Unilamellar Vesicles
The site of ultrasonic absorption in suspensions of negatively-charged large unilamellar vesicles (LUVs) was investigated. The ultrasonic absorption per wavelength, $\alpha\lambda$, was determined for suspensions of LUVs (LUVs: 4:1 (w/w) mixture of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG)) in the vicinity of their phase transition temperature (t$\sb{\rm m}$), using a double crystal acoustic interferometer. LUV suspensions absorb ultrasound highly near t$\sb{\rm m}$, and $\alpha\lambda$ shows a maximum, $\alpha\lambda\sb{\rm max}$, as a function of frequency at 2.1 MHz. It was hypothesized that this represents a molecular relaxation phenomenon, taking place in the hydrophobic region of the lipid bilayer, to which ultrasound couples. Therefore, perturbation of the appropriate region of the LUV bilayer should change $\alpha\lambda$ as a function of frequency at t$\sb{\rm m}$. The membrane-aqueous interface of the LUV was investigated by replacing water with deuterium oxide. The polar headgroup region of the LUV was perturbed by the addition of divalent cations to the suspension. In both studies, although the t$\sb{\rm m}$ of the phase transition, as detected by ultrasound, was changed, $\alpha\lambda\sb{\rm max}$ as a function of frequency at t$\sb{\rm m}$ was not changed. The hydrophobic portion of the bilayer was investigated by the incorporation of A23187, a lipophilic ionophore, into the bilayer. Although little change in t$\sb{\rm m}$ of the phase transition was observed with A23187 addition, $\alpha\lambda\sb{\rm max}$ as a function of frequency was decreased for all concentrations of A23187 studied. The hydrophobic region of the bilayer was also studied by replacing DPPC with per-deuterated DPPC (DPPC whose fatty acyl chains were deuterated), which resulted in a change of $\alpha\lambda\sb{\rm max}$ as a function of frequency. As perturbation of the membrane-aqueous interface and the polar headgroup region do not affect $\alpha\lambda$ as a function of frequency, these are probably not regions of ultrasound interaction in LUV suspensions. The A23187 and per-deuterated DPPC, however, changed $\alpha\lambda$ as a function of frequency. As $\alpha\lambda$ is sensitive to these changes in the hydrophobic region of the bilayer, and specifically to changes in the fatty acyl chains of the phospholipids, this is the most probable site of ultrasound interaction with LUVs.
Biophysics, Medical, Biophysics, General
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign