Properties of proteins at interfaces are important to a many processes as well as in soft matter materials such as aqueous foam. Particularly, the protein interfacial behavior is strongly linked to different factors like the solution pH or the presence of electrolytes. Here the nature of the electrolyte ions can significantly modify the interfacial properties of proteins. Therefore, molecular level studies on interfacial structures and charging states are needed. In this work, we addressed the effects of Y 3+ and Nd 3+ cations on the adsorption of the whey protein β-lactoglobulin (BLG) at airwater interfaces as a function of electrolyte concentration. Both cations caused very similar but dramatic changes at the interface and in the bulk solution. Here, measurements of the electrophoretic mobility and vibrational sum-frequency generation spectroscopy (SFG) were applied and consistently showed a reversal of the BLG net charge at remarkably low ion concentrations of 30
Proteins
at interfaces are important for protein formulations and
in soft materials such as foam. Here, interfacial stability and physicochemical
properties are key elements, which drive macroscopic foam properties
through structure–property relations. Native and fluorescein
isothiocyanate-labeled bovine serum albumin (BSA) were used to modify
air–water interfaces as a function of pH. Characterizations
were performed with tensiometry and sum-frequency generation (SFG).
SFG spectra of O–H stretching vibrations reveal a phase reversal
and a pronounced minimum in O–H intensity at pH values of 5.3
and 4.7 for native and labeled BSA, respectively. This minimum is
attributed to the interfacial isoelectric point (IEP) and is accompanied
by a minimum in surface tension and negligible ζ-potentials
in the bulk. Interfacial proteins at pH values close to the IEP can
promote macroscopic foam stability and are predominately located in
the lamellae between individual gas bubbles as evidenced by confocal
fluorescence microscopy. Different from the classical stabilization
mechanisms, for example, via the electrostatic disjoining pressure,
we propose that the presence of more close-packed BSA, because of
negligible net charges, inside the foam lamellae is more effective
in reducing foam drainage as compared to a situation with strong repulsive
electrostatic interactions.
A new transition metal complex of the mono-protonated ligand (dimethylphosphoryl)methanamine (dpmaH+) was obtained by equimolar reaction of copper(II) chloride dihydrate and dpma in concentrated hydrochloric acid. The asymmetric unit of the title structure, [CuCl2(C3H11NOP)4][CuCl4]2, consists of one half of a fourfold charged trans-dichloridotetrakis[(dimethylphosphoryl)methanaminium]copper(II) complex with the copper atom located on an inversion centre and one tetrachloridocuprate(II) dianion found in a general position. The copper centre in the cationic complex shows a tetragonally distorted octahedral environment composed of four oxygen atoms in a square plane and two trans-coordinated chlorido ligands. This 4+2-coordination causes elongated Cu-Cl distances because of the Jahn-Teller effect. The geometry of the tetrachloridocuprate(II) dianion is best described as a seriously distorted tetrahedron. Analysis of the hydrogen bonding scheme by graph-set theory shows three patterns of rings in the title compound. The cationic copper complex reveals intramolecular hydrogen bonds between two aminium groups and the two axial chlorido ligands. Further hydrogen bonding among the cations and anions, more precisely between four aminium groups and the chlorido ligands of four adjacent tetrachloridocuprate(II) anions, lead to a chain-type structure. Comparing the coordination chemistry of the title structure with an analogue cobalt(II) compound only disclose differences in hydrogen bonding pattern resulting in an unusual chain propagation. Besides the crystal structure received spectroscopic data are in accordance with appropriate literature.
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