Structure of Ultrafine Bubbles and Their Effects on Protein and Lipid Membrane Structures Studied by Small- and Wide-Angle X-ray Scattering

Ultrafine bubbles (UFBs) are defined as small gas-filled bubbles with a diameter smaller than 1 mu m. UFBs are stable for several weeks in aqueous solutions due to their small size. Although the mechanism of the stability of UFBs remains under intensive investigation, industrial applications of UFBs have recently arisen in various fields such as agricultural and fishery industries and medical therapy. The relevance of ions (protons and hydroxide anions) in UFB solutions has been discussed; however, the mechanism underlying the behavior of UFBs is still ambiguous and there is little direct evidence of the effect of UFBs on biological materials. This study deals with gaseous UFBs in aqueous solutions. Using small- and wide-angle X-ray scattering, we have investigated the structures of UFBs (air-UFBs, O-2-UFBs, and N-2-UFBs) and their effect on protein and lipid membrane structures. X-ray scattering and modeling data suggest that UFBs present a dynamic diffusive boundary (interface) due to the continuous release and absorption of gas. UFBs were found to not affect the structures of proteins at all hierarchal structure levels (from quaternary to tertiary, to internal, to secondary), whereas they did influence the packing and fluctuation of the hydrocarbon chains in the liposomes but not their shapes.