Masters Theses

Date of Award

8-1986

Degree Type

Thesis

Degree Name

Master of Science

Major

Physics

Major Professor

Solon Georghiou

Committee Members

W. H. Flecher

Abstract

The effect of melittin, a polypeptide that consists of twenty-six amino acid residues and the major component of bee venom, on the order and dynamics of dimyristoylphosphatidylcholine multilamellar vesicles at a protein-to-lipid molar ratio of 1:60 has been investigated through the application of two different techniques: Raman spectroscopy and the enhancement of the 0-0 vibronic transition in the fluorescence spectrum of pyrene which is a measure of the polarity of the microenvironment of the probe.

In the Raman studies, the lipid phase behavior was monitored by following intensity changes of the Raman features present in the 2750-3050 cm-1 acyl chain methylene carbon-hydrogen stretching mode region and the 970-1220 cm-1 acyl chain carbon-carbon skeletal stretching mode intervals as a function of temperature from 5°C to 45°C. Investigating the behavior of the ratios I2880/I2850 and I2930/I2880 which are a measure of both the inter- and intramolecular order of the lipid chains, revealed that melittin decreases the order below and above the gel-to-liquid crystalline phase transition temperature. The behavior of both I1080/I1062 and I1080/I1130 which are a measure of the intramolecular disorder, indicated that melittin increases the number of gauche conformers in the gel state while no effect was detected in the liquid crystalline state. Thus the overall effect of melittin on the lipid order in the gel phase is to increase the number of gauche conformers and decrease the intermolecular order of the chains whereas in the liquid crystalline state the intermolecular order decreases without any significant change in the number of gauche conformers.

In the enhancement of the 0-0 vibronic transition in the fluorescence spectrum of pyrene, melittin has been found to induce a pronounced decrease in the enhancement ratio R in the vicinity of the phase transition temperature of the phospholipid indicating that the protein causes inter- and intramolecular rearrangements in the lipid bilayer that result in a decrease in its packing density with a concomitant increase in its fluidity that facilitates the penetration of the probe into the bilayer. These results are in good agreement with the Raman results for which the most pronounced changes induced by melittin were also observed in the vicinity of the phase transition temperature.

Both of the techniques revealed that the protein causes the phase transition profile to broaden without inducing significant changes in the gel-to-liquid crystalline phase transition temperature.

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