Structure and organization of albumin molecules forming the shell of air-filled microspheres : evidence for a monolayer of albumin molecules of multiple orientations stabilizing the enclosed air

The structure and organization of albumin molecules in the shell of air-filled microspheres formed by sonication of a 5% albumin solution have been investigated. By limited proteolysis of intact microspheres, it has been shown that every albumin molecule in the shell may be cleaved without disintegration of the microsphere structure. The microsphere shell accordingly appears to be composed of a monolayer of albumin molecules. Most of the main cleavage sites identified after N-terminal sequencing of proteolytic fragments are localized in three distinct regions common to both native and microsphere albumin molecules : the extended region of the first domain, the extended region of the second domain and the first disulphide loop of the third domain. The similarity in the localization of cleavage sites in the native and microsphere albumin molecules suggests that the formation of microspheres implies only a limited degree of conformational change of the albumin molecules. The localization of the cleavage sites in the three-dimensional structure of albumin suggests that the shell may be constituted of albumin molecules in both a native-like heart-shaped form and a more flipped-out elongated form with different orientations.