Conformational Stability Study of a Therapeutic Peptide Plectasin Using Molecular Dynamics Simulations in Combination with NMR

Plectasin is a small, cysteine-rich peptide antibiotic which belongs to the class of antimicrobial peptides and has potential antibacterial activity against various Grampositive bacteria. In the current study, the effect of pH and ionic strength (NaCl) on the conformational stability of plectasin variants has been investigated. At all physiochemical conditions, peptide secondary structures are intact throughout simulations. However, flexibility increases with pH because of the change in electrostatics around the distinct anionic tetrapeptide (9-12) stretch. Furthermore, plectasin interactions with NaCl were measured by determining the preferential interaction coefficients, Gamma(23). Generally, wild-type plectasin has higher preference for sodium ions as 9ASP is mutated in other variants. Overall, the Gamma(23) trend with pH for the two salt conditions remain the same for all variants predominately having accumulation of sodium ions around 10GLU and 12ASP. Insignificant changes in the overall peptide conformational stability are in agreement with the fact that plectasin has three cystines. Thermodynamic integration molecular dynamics simulations supplemented with nuclear magnetic resonance were employed to determine the degree of involvement of three different cystines to the overall structural integrity of the peptide. Both methods show the same order of cystine reduction and complete unfolding is observed only upon reduction of all cystines.