Abstract

Antimicrobial peptides (AMPs) have been proposed as promising alternatives to conventional antibiotics. They are highly selective and efficient bactericidal agents that are already present as elements of innate immunity. GL13K is a synthetic peptide, derived from residues 141-153 of the human parotid secretory protein, and it is an AMP that is bactericidal against Gram-positive and Gram-negative bacteria. Previous biophysical studies with this peptide showed that it selectively forms beta-sheets in the presence of anionic membranes and targets membranes via the carpet method.
In this thesis, initial studies focused on the surface behaviour of GL13K to determine whether it has the propensity to form amyloidic structures. Once it was established that GL13K does not aggregate into amyloidic fibrils at the air/water interface or when transferred to solid support, studies with anionic monolayers of varied membrane fluidity were conducted using dioleoylphosphatidylglycerol (DOPG) and mixed DOPG:cholesterol and DOPG:diphytanoylphosphatidylglycerol (DPhPG) films. Both cholesterol and DPhPG, a branched, anionic lipid, lower the permeability of membranes, but only cholesterol increases membrane viscosity. These studies showed that membrane viscosity plays a greater role in the prevention of peptide insertion into membranes. This suggests that cholesterol may contribute to the protection of eukayotic cells from AMPs by attenuating peptide insertion.
Some bacteria have developed resistance to AMPs by upregulating the production of lysyl-phosphatidylglycerol (LPG) to mask the negative charge on their membranes. Model membranes consisting of dipalmitoylphosphatidylglycerol (DPPG) and mixtures of DPPG and DP3AdLPG, a stable analogue of LPG, were used to determine how this lysylation alters GL13K behaviour at the air/water interface. The functionalization of the headgroup attenuates the formation of crystalline beta-sheets by disrupting the hydrogen bonding network. Peptide crystallinity appears to increase when the peptide is bound to the headgroup region which could either attenuate activity or alter the mechanism of activity. This highlights the need for further research in this area to determine if a direct relationship between peptide crystallinity and function exists.