Amaranth 7S globulin Langmuir films and its interaction with l-α-dipalmitoilphosphatidilcholine at the air-fluid interface.

Amaranth seeds are one of the more promising food ingredients, due to their high protein content, among which the most important are storage proteins known as globulins. However, little is known about the physicochemical of the globulin proteins. In this work, we study the physicochemical behavior of films made of amaranth 7S globulin and its interaction with a model membrane made of L-α-dipalmitoylphosphatidylcholine (L-α-DPPC) at the air-liquid interface. The study was done by means of Langmuir balance, Brewster angle microscopy (BAM), fluorescence microscopy, and atomic force microscopy (AFM). We found that isotherms of pure 7S globulin directly deposited on either water or buffer subphases behave similarly and globulin forms a condensed film made of globular and denature structures, which was confirmed by BAM observations. Good mixtures of the protein with L-α-DPPC are formed at low surface pressure. However, they phase separate from moderate to high surface pressure as observed by BAM. Isotherms detect the presence of the protein in the mixture with L-α-DPPC, but we were unable to detect it through BAM or AFM. We show that fluorescence microscopy is a very good technique to detect the presence of the protein when it is well-mixed within the LE phase of the lipid. AFM images clearly show the formation of protein mono- and multilayers, and in phase mode, we detected domains that are formed by protein and LE lipid phase, which were corroborated by fluorescence microscopy. We have shown that globulin 7S mix well with lipid phases, which could be important in food applications as stabilizers or emulsifiers, but we also show that they can phase separate with a moderate to high surface pressure.

Globulin 11S and its mixture with L-dipalmitoylphosphatidylcholine at the air/liquid interface.

Langmuir films of globulin 11S protein, l-dipalmitoylphosphatidylcholine (L-DPPC), and mixtures of both on water and on buffer subphases were studied. Brewster angle microscopy (BAM) was used to characterize in situ the films morphology along Pi-A isotherms at the air/liquid interface. The L-DPPC monolayer on water behaved as has been reported extensively in the literature but a slight increase on surface pressure and a notable change in domain morphology is observed on buffer. This difference in domain behavior is due to the stabilization interaction of the LE phase by the buffer ions. On the other hand, the protein monolayer was prepared by direct deposit or injection below the surface. Both methods formed mostly a condensed film, with a multilayer formed by globular aggregates in the first method with the two subphases. However, the second method showed different behavior of the protein films depending on the subphase; on water the protein formed a homogeneous film with some globule aggregates, but on buffer a remarkably well-organized monolayer was observed by atomic force microscopy (AFM). Mixtures of globulin 11S and L-DPPC were prepared using both methods for the protein film formation at the air/fluid interface. BAM showed that the mixtures formed coexistence regions between two condensed phases, whose domains of both phases behave like liquids. Fingering phenomena were observed at the interface between protein-rich and L-DPPC-rich domains, which indicates that both phases are fluid. AFM images of the mixtures show the formation of protein- or L-DPPC-rich domains. The liquidlike behavior could be explained due to different sizes of the protein and the L-DPPC, the minority compound in each kind of domain produces defects making them behave as liquids. Interestingly enough, as the monolayer is compressed to higher surface pressure, the lipid molecules are squeezed out and complete separation of the protein and L-DPPC is produced. Furthermore, we present evidence that the protein/L-DPPC mixtures produce films with holes, which might indicate its tendency to form hollow aggregates that could have some relevance in water-channel formation for in vivo seed germination.

Interaction of the cationic peptide bactenecin with phospholipid monolayers at the air-water interface: i interaction with 1,2-dipalmitoyl-sn-glycero-3-phosphatidilcholine.

In this work we have investigated the influence of NaCl on the adsorption of the antimicrobial cationic peptide bactenecin in the monolayer of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air-water interface, as a function of NaCl concentrations in the subphase. We show that the effect of the salt concentration on DPPC monolayers is a monotonic decrease of the liquid-condensed-liquid-expanded (LC-LE) coexistence region. By contrast, the effect of the bactenecin adsorption at the DPPC monolayer not only removed the LC-LE coexistence region plateau, but also shifted the DPPC isotherms to higher pressures and increased the compressibility of the DPPC/bactenecin monolayers with respect to the pure DPPC monolayer around the LC phase. Analysis of the domain structure, obtained by Brewster angle and atomic force microscopes, indicates that the salt concentration in the subphase builds an electrostatic barrier, increasing the rigidity of DPPC monolayers and limiting the bactenecin adsorption at the LC-LE phase coexistence.