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Incorporation of Melittin Enhances Interfacial Fluidity of Bicontinuous Microemulsions

Sharma, V. K., Hayes, D. G., Gupta, S., Urban, V. S., O’Neill, H. M., Pingali, S. V., Ohl, M., Mamontov, E.
Journal of physical chemistry 2019 v.123 no.17 pp. 11197-11206
antimicrobial peptides, bending strength, chemical reactions, diffusivity, emulsions, light scattering, melittin, mixing, neutron diffraction, neutrons, physical chemistry, salinity, sodium dodecyl sulfate, spectroscopy, sulfates, surfactants
The effect of an antimicrobial peptide, melittin, on the long-range collective and local undulation motions of bicontinuous microemulsions (BμEs) was evaluated using dynamic light scattering (DLS) and neutron spin echo (NSE) spectroscopy. Small-angle neutron scattering (SANS) was also used to study the effect of melittin on the structure and the correlation length of BμEs. BμEs were isolated from Winsor-III systems formed by mixing aqueous melittin solutions containing sodium dodecyl sulfate with dodecane/1-pentanol mixtures at optimal salinity. An increase of melittin concentration linearly increased the diffusivity of the microemulsions (DDLS) and decreased the bending rigidity (κNSE) up to an aqueous melittin concentration of 2 g/L, thus indicating an increase of interfacial fluidity. Further increase of melittin concentration above 2 g/L did not change DDLS and slightly increased κNSE. The changes reflect differences of interaction between melittin and BμE surfactant monolayers at different melittin concentrations. At low concentrations, melittin is highly associated with the surfactant monolayers, likely ion-paired to the sulfate surfactant head groups and perhaps partially penetrating into the surfactant tail region. Above 2 g/L, an apparent saturation concentration, added melittin is less strongly associated with the monolayers. In contrast, the bare bending constant derived from SANS data (κbₐᵣₑ) underwent only a minor decrease. Noting that κbₐᵣₑ reflects contributions from both the elasticity and saddle-splay moduli, this suggests that κbₐᵣₑ is not an effective indicator of dynamics for the BμEs investigated. This study demonstrates that amphiphilic solubilizates can have a complex impact on dynamics of surfactant monolayers in microemulsions, with implications for transmembrane diffusion and the kinetics of release from the microemulsion phase, which are important characteristics for the use of BμEs as delivery vehicles and host systems for (bio)chemical reactions.