Violeta Burns Casamayor (West Virginia University)| Blake Mertz (West Virginia University)
pH dependent proteins, such as the pH Low Insertion Peptide (pHLIP), are of great interest due to their capability for inserting into lipid bilayers under acidic conditions. This characteristic allows them to take advantage of the acidic microenvironment surrounding cancer cells, and thus, makes them ideal candidates for targeted drug delivery and early-stage detection of diseases connected to acidosis. pHLIP is a 36-residue peptide that is known to exist in 3 different states based on the pH and environmental conditions: as an intrinsically disordered peptide in solution (state I), bound to the cell membrane surface (state II), and inserted as a transmembrane helix (state III) at acidic pH. While states II and III have been thoroughly studied both through experimental and computational techniques, state I remains a mystery due to its tendency to aggregate in solution. Our hypothesis is that increasing concentrations of salt drive sampling of secondary structures of pHLIP in solution. Specifically, we hypothesize that interactions between NaCl ions and charged residues of pHLIP increase the stability of the secondary structure of the peptide, thus increasing helicity. In this study we characterized the effect of salt concentration on pHLIP in state I using molecular dynamics (MD) simulations. We found that, at higher salt concentrations, pHLIP tends to increase sampling of helical conformation and becomes more compacted, indicating an increase of stabilized secondary structures of pHLIP. Understanding the effects of salt concentration on pHLIP’s structure could be crucial towards further cognizance of pHLIP’s predisposition to bind and insert into the membrane, which will ultimately lead towards optimized variants of the peptide for targeted drug delivery and diagnosis applications.