Min Chul Kim

Spectrum of Membrane Morphological Responses to Antibacterial Fatty Acids and Related Surfactants

Medium-chain saturated fatty acids and related compounds (e.g., monoglycerides) represent one class of membrane-active surfactants with antimicrobial properties. Most related studies have been in vitro evaluations of bacterial growth inhibition, and there is limited knowledge about how the compounds in this class destabilize lipid bilayers, which are the purported target within the bacterial cell membrane. Herein, the interaction between three representative compounds in this class and a supported lipid bilayer platform was investigated using quartz crystal microbalance-dissipation and fluorescence microscopy in order to examine membrane destabilization. The three tested compounds were lauric acid, sodium dodecyl sulfate, and glycerol monolaurate. For each compound, we discovered striking differences in the resulting morphological changes of supported lipid bilayers. The experimental trends indicate that the compounds have membrane-disruptive behavior against supported lipid bilayers principally above the respective critical micelle concentration values. The growth inhibition properties of the compounds against standard and methicillin-resistant Staphylococcus aureus bacterial strains were also tested. Taken together, the findings in this work improve our knowledge about how saturated fatty acids and related compounds destabilize lipid bilayers, offering insight into the corresponding molecular mechanisms that lead to membrane morphological responses.

Supported lipid bilayer repair mediated by AH peptide

The adsorption and fusion of small unilamellar lipid vesicles on silica-based substrates such as glass is a common method used to fabricate supported lipid bilayers. Successful bilayer formation depends on a number of experimental conditions as well as on the quality of the vesicle preparation. Inevitably, a small fraction of unruptured vesicles always remains in a supported bilayer, and this kind of defect can have devastating influences on the morphological and electrical properties of the supported bilayer when used as a biosensing platform. In this paper, a simple method is reported to improve the completeness of supported bilayers by adding a vesicle rupturing peptide as a final step in the fabrication process. Peptide treatment reduces the fraction of unruptured vesicles to less than 1%, as determined by epifluorescence microscopy and quartz crystal microbalance-dissipation experiments. This step can easily be incorporated into existing procedures for preparing high-quality supported lipid bilayers.

Immobilization Strategies for Functional Complement Convertase Assembly at Lipid Membrane Interfaces

The self-assembly formation of complement convertases-essential biomacromolecular complexes that amplify innate immune responses-is triggered by protein adsorption. Herein, a supported lipid bilayer platform was utilized to investigate the effects of covalent and noncovalent tethering strategies on the self-assembly of alternative pathway C3 convertase components, starting with C3b protein adsorption followed bythe addition of factors B and D. Quartz crystal microbalance-dissipation (QCM-D) experiments measured the real-time kinetics of convertase assembly onto supported lipid bilayers. The results demonstrate that the nature of C3b immobilization onto supported lipid bilayers is a key factor governing convertase assembly. The covalent attachment of C3b to maleimide-functionalized supported lipid bilayers promoted the self-assembly of functional C3 convertase in the membrane-associated state and further enabled successful evaluation of a clinically relevant complement inhibitor, compstatin. By contrast, noncovalent attachment of C3b to negatively charged supported lipid bilayers also permitted C3b protein uptake, albeit membrane-associated C3b did not support convertase assembly in this case. Taken together, the findings in this work demonstrate that the attachment scheme for immobilizing C3b protein at lipid membrane interfaces is critical for downstream C3 convertase assembly, thereby offering guidance for the design and evaluation of membrane-associated biomacromolecular complexes.

Brownian Dynamics of Electrostatically Adhering Small Vesicles to a Membrane Surface Induces Domains and Probes Viscosity

Using single-particle tracking, we investigate the interaction of small unilamellar vesicles (SUVs) that are electrostatically tethered to the freestanding membrane of a giant unilamellar vesicle (GUV). We find that the surface mobility of the GUV-riding SUVs is Brownian, insensitive to the bulk viscosity, vesicle size, and vesicle fluidity but strongly altered by the viscosity of the underlying membrane. Analyzing the diffusional behavior of SUVs within the Saffman-Delbrück model for the dynamics of membrane inclusions supports the notion that the mobility of the small vesicles is coupled to that of dynamically induced lipid clusters within the target GUV membrane. The reversible binding also offers a nonperturbative means for measuring the viscosity of biomembranes, which is an important parameter in cell physiology and function.

Elucidating how bamboo salt interacts with supported lipid membranes: influence of alkalinity on membrane fluidity

Bamboo salt is a traditional medicine produced from sea salt. It is widely used in Oriental medicine and is an alkalizing agent with reported antiinflammatory, antimicrobial and chemotherapeutic properties. Notwithstanding, linking specific molecular mechanisms with these properties has been challenging to establish in biological systems. In part, this issue may be related to bamboo salt eliciting nonspecific effects on components found within these systems. Herein, we investigated the effects of bamboo salt solution on supported lipid bilayers as a model system to characterize the interaction between lipid membranes and bamboo salt. The atomic composition of unprocessed and processed bamboo salts was first analyzed by mass spectrometry, and we identified several elements that have not been previously reported in other bamboo salt preparations. The alkalinity of hydrated samples was also measured and determined to be between pH 10 and 11 for bamboo salts. The effect of processed bamboo salt solutions on the fluidic properties of a supported lipid bilayer on glass was next investigated by fluorescence recovery after photobleaching (FRAP) analysis. It was demonstrated that, with increasing ionic strength of the bamboo salt solution, the fluidity of a lipid bilayer increased. On the contrary, increasing the ionic strength of near-neutral buffer solutions with sodium chloride salt diminished fluidity. To reconcile these two observations, we identified that solution alkalinity is critical for the effects of bamboo salt on membrane fluidity, as confirmed using three additional commercial bamboo salt preparations. Extended-DLVO model calculations support that the effects of bamboo salt on lipid membranes are due to the alkalinity imparting a stronger hydration force. Collectively, the results of this work demonstrate that processing of bamboo salt strongly affects its atomic composition and that the alkalinity of bamboo salt solutions contributes to its effect on membrane fluidity.