Conformational flexibility determines selectivity and antibacterial, antiplasmodial, and anticancer potency of cationic α-helical peptides.

We used a combination of fluorescence, circular dichroism (CD), and NMR spectroscopies in conjunction with size exclusion chromatography to help rationalize the relative antibacterial, antiplasmodial, and cytotoxic activities of a series of proline-free and proline-containing model antimicrobial peptides (AMPs) in terms of their structural properties. When compared with proline-free analogs, proline-containing peptides had greater activity against Gram-negative bacteria, two mammalian cancer cell lines, and intraerythrocytic Plasmodium falciparum, which they were capable of killing without causing hemolysis. In contrast, incorporation of proline did not have a consistent effect on peptide activity against Mycobacterium tuberculosis. In membrane-mimicking environments, structures with high α-helix content were adopted by both proline-free and proline-containing peptides. In solution, AMPs generally adopted disordered structures unless their sequences comprised more hydrophobic amino acids or until coordinating phosphate ions were added. Proline-containing peptides resisted ordering induced by either method. The roles of the angle subtended by positively charged amino acids and the positioning of the proline residues were also investigated. Careful positioning of proline residues in AMP sequences is required to enable the peptide to resist ordering and maintain optimal antibacterial activity, whereas varying the angle subtended by positively charged amino acids can attenuate hemolytic potential albeit with a modest reduction in potency. Maintaining conformational flexibility improves AMP potency and selectivity toward bacterial, plasmodial, and cancerous cells while enabling the targeting of intracellular pathogens.

Control of pH responsive peptide self-association during endocytosis is required for effective gene transfer.

Cationic amphipathic histidine rich peptides demonstrate differential nucleic acid binding capabilities at neutral and acidic pH and adopt conformations at acidic pH that enable interaction with endosomal membranes, their subsequent disordering and facilitate entry of cargo to the cell cytosol. To better understand the relative contributions of each stage in the process and consequently the structural requirements of pH responsive peptides for optimal nucleic acid transfer, we used biophysical methods to dissect the series of events that occur during endosomal acidification. Far-UV circular dichroism was used to characterise the solution conformation of a series of peptides, containing either four or six histidine residues, designed to respond at differing pH while a novel application of near-UV circular dichroism was used to determine the binding affinities of the peptides for both DNA and siRNA. The peptide induced disordering of neutral and anionic membranes was investigated using (2)H solid-state NMR. While each of these parameters models key stages in the nucleic acid delivery process and all were affected by increasing the histidine content of the peptide, the effect of a more acidic pH response on peptide self-association was most notable and identified as the most important barrier to further enhancing nucleic acid delivery. Further, the results indicate that Coulombic interactions between the histidine residues modulate protonation and subsequent conformational transitions required for peptide mediated gene transfer activity and are an important factor to consider in future peptide design.

Backbone NMR assignments of HypF-N under conditions generating toxic and non-toxic oligomers.

The HypF protein is involved in the maturation and regulation of hydrogenases. The N-terminal domain of HypF (HypF-N) has served as a key model system to study the pathways of protein amyloid formation and the nature of the toxicity of pre-fibrilar protein oligomers. This domain can aggregate into two forms of oligomers having significantly different toxic effects when added to neuronal cultures. Here, NMR assignments of HypF-N backbone resonances are presented in its native state and under the conditions favouring the formation of toxic and non-toxic oligomers. The analyses of chemical shifts provide insights into the protein conformational state and the possible pathways leading to the formation of different types of oligomers.

Structural differences between toxic and nontoxic HypF-N oligomers.

We have studied two misfolded oligomeric forms of the protein HypF-N, which show similar morphologies but very different toxicities. We measured over 80 intermolecular distance-dependent parameters for each oligomer type using FRET, in conjunction with solution- and solid-state NMR and other biophysical techniques. The results indicate that the formation of a highly organised hydrogen bonded core in the toxic oligomers results in the exposure of a larger number of hydrophobic residues than in the nontoxic species, causing the former to form aberrant interactions with cellular components.

Manipulating the pH response of 2,3-diaminopropionic acid rich peptides to mediate highly effective gene silencing with low-toxicity.

Cationic amphipathic pH responsive peptides possess high in vitro and in vivo nucleic acid delivery capabilities and function by forming a non-covalent complex with cargo, protecting it from nucleases, facilitating uptake via endocytosis and responding to endosomal acidification by being released from the complex and inserting into and disordering endosomal membranes. We have designed and synthesised peptides to show how Coulombic interactions between ionizable 2,3-diaminopropionic acid (Dap) side chains can be manipulated to tune the functional pH response of the peptides to afford optimal nucleic acid transfer and have modified the hydrogen bonding capabilities of the Dap side chains in order to reduce cytotoxicity. When compared with benchmark delivery compounds, the peptides are shown to have low toxicity and are highly effective at mediating gene silencing in adherent MCF-7 and A549 cell lines, primary human umbilical vein endothelial cells and both differentiated macrophage-like and suspension monocyte-like THP-1 cells.