Folding and Unfolding of pH Sensitive Peptides: The Role of Interfaces

Abstract

GALA is a 30 amino acid synthetic peptide consisting of a Glu-Ala-Leu-Ala repeat, known to undergo a reversible structural transition from an unordered to an α-helical structure when changing the pH from 7 to 5. In its helical state GALA is amphiphilic and can insert into and permeabilize membranes. This effect has generated much interest because of potential applications for pH triggered targeted drug delivery. GALA also serves as a well-defined model system to understand cell penetration mechanisms and protein folding triggered by external stimuli. The structural reconfiguration of GALA in solution has been studied extensively. However, cell penetration is an interfacial effect and occurs at the membrane surface. Owing to experimental challenges in determining peptide confirmation at an interface, GALAs interaction with surfaces is still unknown. A key question is: What is the secondary structure of GALA specifically at interfaces such as the air-water or water-lipid interfaces? In other words: how does the presence of an interface affect the intricate balance of forces governing folding and unfolding of GALAs Glu-Ala-Leu-Ala motive? We have used sum frequency generation vibrational spectroscopy to probe the structural response of GALA at the air-water interface. SFG is highly interface specific and only probes peptides directly, i.e. within a few Angstrom, at the water-air or water-lipid surface. The data reveal that a large fraction of the GALA population at the water-air interface remains helical above pH 5, while a complete reconfiguration into the unordered state was observed in solution by transmission infrared and circular dichroism spectroscopy. We attribute this effect to the stabilizing interactions of hydrophobic leucine side chains with air. Molecular dynamics simulations support the view that the surface plays a key role in the balance of structure-building forces at the interface.