Bicontinuous microemulsions as a biomembrane mimetic system for melittin.

Antimicrobial peptides effectively kill antibiotic-resistant bacteria by forming pores in prokaryotes' biomembranes via penetration into the biomembranes' interior. Bicontinuous microemulsions, consisting of interdispersed oil and water nanodomains separated by flexible surfactant monolayers, are potentially valuable for hosting membrane-associated peptides and proteins due to their thermodynamic stability, optical transparency, low viscosity, and high interfacial area. Here, we show that bicontinuous microemulsions formed by negatively-charged surfactants are a robust biomembrane mimetic system for the antimicrobial peptide melittin. When encapsulated in bicontinuous microemulsions formed using three-phase (Winsor-III) systems, melittin's helicity increases greatly due to penetration into the surfactant monolayers, mimicking its behavior in biomembranes. But, the threshold melittin concentration required to achieve these trends is lower for the microemulsions. The extent of penetration was decreased when the interfacial fluidity of the microemulsions was increased. These results suggest the utility of bicontinuous microemulsions for isolation, purification, delivery, and host systems for antimicrobial peptides.

In Vivo Protein Dynamics on the Nanometer Length Scale and Nanosecond Time Scale.

Selectively labeled GroEL protein was produced in living deuterated bacterial cells to enhance its neutron scattering signal above that of the intracellular milieu. Quasi-elastic neutron scattering shows that the in-cell diffusion coefficient of GroEL was (4.7 ± 0.3) × 10-12 m2/s, a factor of 4 slower than its diffusion coefficient in buffer solution. Internal protein dynamics showed a relaxation time of (65 ± 6) ps, a factor of 2 slower compared to the protein in solution. Comparison to the literature suggests that the effective diffusivity of proteins depends on the length and time scale being probed. Retardation of in-cell diffusion compared to the buffer becomes more significant with the increasing probe length scale, suggesting that intracellular diffusion of biomolecules is nonuniform over the cellular volume. The approach outlined here enables investigation of protein dynamics within living cells to open up new lines of research using "in-cell neutron scattering" to study the dynamics of complex biomolecular systems.