Spread Films of Human Serum Albumin at the Air-Water Interface: Optimization, Morphology, and Durability

It has been known for almost one hundred years that a lower surface tension can be achieved at the air-water interface by spreading protein from a concentrated solution than by adsorption from an equivalent total bulk concentration. Nevertheless, the factors that control this nonequilibrium process have not been fully understood. In the present work, we apply ellipsometry, neutron reflectometry, X-ray reflectometry, and Brewster angle microscopy to elaborate the surface loading of human serum albumin in terms of both the macroscopic film morphology and the spreading dynamics. We show that the dominant contribution to the surface loading mechanism is the Marangoni spreading of protein from the bulk of the droplets rather than the direct transfer of their surface films. The films can be spread on a dilute subphase if the concentration of the spreading solution is sufficient; if not, dissolution of the protein occurs, and only a textured adsorbed layer slowly forms. The morphology of the spread protein films comprises an extended network with regions of less textured material or gaps. Further, mechanical cycling of the surface area of the spread films anneals the network into a membrane that approach constant compressibility and has increased durability. Our work provides a new perspective on an old problem in colloid and interface science. The scope for optimization of the surface loading mechanism in a range of systems leading to its exploitation in deposition-based technologies in the future is discussed.

Human serum albumin binding to silica nanoparticles – effect of protein fatty acid ligand

Fat containing and defatted human serum albumin adsorption to silica nanoparticles have different structures and time dependence to form.

Resistance of β-casein at the air-water interface to enzymatic cleavage

X-ray reflectivity from an air-buffer interfacial β-casein monomolecular film placed on a solution of chymosin (renin) showed unexpectedly slow proteolytic cleavage. To understand this, the separate structures of β-casein and chymosin, the presentation of each molecule to the other at the air/liquid interface, and that of their mixtures is reported. At the air/solution interface, the hydrophobicity of the protein molecules causes orientation and some deformation of the conformation. When β-casein was presented to a chymosin monomolecular interfacial film, the chymosin was largely displaced from the surface, which was accounted for by the different surfactancy of the two molecules at 25 °C. There was no observable proteolysis. In the reverse experiment, a significant enzymatic degradation and the signature of hydrophobic fragments was observed but only at and above an enzyme concentration of 0.015 mg/mL in the substrate. For comparison, the air/solution interface of premixed β-casein with chymosin in phosphate buffer showed that the film was composed of β-casein proteolytic fragments and chymosin.

Mixed-Valent Polynuclear Cobalt Complexes Incorporating Tetradentate Phenoxyamine Ligands

The new potentially tetradentate ligand precursor 2-[(bis(2-hydroxyethyl)amino)methyl]-4,6-bis-tert-butylphenol (H3L) has been synthesized and characterized. The reaction of H3L with cobalt(ii) acetate has afforded the novel mixed-valent tetra- and pentanuclear cobalt complexes [CoII2CoIII2(OAc)2(L)2(HL)] (1) and [CoIICoIII4(OAc)2(L)4] (2). Single-crystal X-ray diffraction studies of these complexes indicate different coordination geometries for the divalent cobalt centres in each complex, with distorted trigonal bipyramidal and distorted tetrahedral coordination evident in 1 and 2, respectively. The variable temperature magnetic susceptibility data for complex 1 reveal behaviour dominated by antiferromagnetic coupling between the two cobalt(ii) centres. Their approximate trigonal bipyramidal coordination geometries give rise to a 4A′2 ground term, allowing a spin-only treatment assuming local spin quantum numbers of Si = 3/2. Fitting the data to the Heisenberg exchange Hamiltonian (Ĥex = –2JS·1S2) results in J = –6.9(1) cm–1 and g = 2.30(5).