Interaction mode specific reorganization of gel phase monoglyceride bilayers by beta-lactoglobulin

Improved Bioavailability of Encapsulated Bioactive Nutrients Delivered through Monoglyceride-Structured O/W Emulsions

Effects of monoglyceride (MG) on the properties of WPI-stabilized emulsions and the bioavailability of encapsulated β-carotene were investigated. MG-structured emulsions showed reduced surface charge, higher viscosity, and better creaming stability than an emulsion without MG. Exposure of emulsions to GIT digestion led to significant changes in droplet size and interfacial properties. In vitro bioavailability of β-carotene in 1% MG (63.9%) and 2% MG (77.1%) structured emulsions were higher than that in emulsion without MG (53.4%) (p < 0.05). All MG emulsions demonstrated a better cellular uptake of β-carotene by Caco-2 cells than the emulsion without MG (p < 0.05). A significant increase in the cellular uptake of β-carotene with increasing MG content was observed, increasing from 0.109 μg/well for the 0.5% MG emulsion and up to 0.138 μg/well for 2% MG emulsion. The findings in this study confirm the potential of the MG-structured emulsions as novel carriers for lipophilic nutrients with improved stability and bioavailability.

Probing protein-membrane interactions using solid supported membranes

Tethered bilayer lipid membranes have been used as a model system to mimic the interactions between the whey protein β-lactoglobulin and a lipid interface. The approach allowed for a detailed study of the lipid-protein interactions, the results being of possible importance in food and cosmetic applications. For such applications, lipid-protein interactions and the interfacial behavior are vital factors in controlling and manipulating process conditions such as emulsion stabilization and gelification. Lipid composition as well as the structural properties of the protein governed their interactions, which were probed by a combination of surface plasmon spectroscopy, neutron reflectivity, and electrochemical impedance spectroscopy. Comparison of results obtained using native and a partially unfolded protein indicated that the protein preferentially forms loosely packed layers at the lipid interface.

Protein-lipid interactions at the air-water interface

Protein-lipid interactions play an important role in a variety of fields, for example in pharmaceutical research, biosensing, or food science. However, the underlying fundamental processes that govern the interplay of lipids and proteins are often very complex and are therefore studied using model systems. Here, Langmuir monolayers were used to probe the interaction of a model protein with lipid films at the air-water interface. The protein beta-lactoglobulin (beta lg) is the major component in bovine milk serum, where it coexists with the milk fat globular membrane. During homogenization of milk, beta lg adsorbs to the interface of lipid fat globules and stabilizes the oil-in-water emulsion. pH and ionic strength of the subphase had a significant effect on the surface activity of the protein. Additionally, by using lipids with different charges, it could be shown that the interactions between beta lg and a phospholipid layer were driven by hydrophobic as well as by electrostatic interactions. beta lg preferentially interacted with phospholipids in an unfolded state. This could be either achieved by denaturation at the air-water interface or due to electrostatic interactions that weaken the intramolecular forces of the protein.

Conformational changes of beta-lactoglobulin induced by anionic phospholipid

Conformational changes of beta-lactoglobulin (beta-LG) induced by anionic phospholipid (dimyristoylphosphatidylglycerol, DMPG) at physiological conditions (pH 7.0) have been investigated by UV-VIS, circular dichroism (CD) and fluorescence spectra. The experimental results suggest that beta-LG-DMPG interactions cause beta-LG a structural reorganization of the secondary structure elements accompanied by an increase in alpha-helical content, and a loosening of the protein tertiary structure. The interaction forces between beta-LG and DMPG are further evaluated by fluorescence spectra. The fluorescence spectral data show that conformational changes in the protein are driven by electrostatic interaction at first, then by hydrophobic interaction between a protein with a negative net charge and a negatively charged phospholipid.

Effect of phospholipids and a transmembrane peptide on the stability of the cubic phase of monoolein: implication for protein crystallization from a cubic phase

The cubic phase of monoolein has successfully been used for crystallization of a number of membrane proteins. However, the mechanism of protein crystallization in the cubic phase is still unknown. It was hypothesized, that crystallization occurs at locally formed patches of bilayers. To get insight into the stability of the cubic phase, we investigated the effect of different phospholipids and a model transmembrane peptide on the lipid organization in mixed monoolein systems. Deuterium-labeled 1-oleoyl-rac-[(2)H(5)]-glycerol was used as a selective probe for (2)H NMR. The phase behavior of the phospholipids was followed by (31)P NMR. Upon incorporation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, or phosphatidic acid, the cubic phase of monoolein transformed into the L(alpha) or H(II) phase depending on the phase preference of the phospholipid and its concentration. The ability of phospholipids to destabilize the cubic phase was found to be dependent on the phospholipid packing properties. Electrostatic repulsion facilitated the cubic-to-L(alpha) transition. Incorporation of the transmembrane peptide KALP31 induced formation of the L(alpha) phase with tightly packed lipid molecules. In all cases when phase separation occurs, monoolein and phospholipid participate in both phases. The implications of these findings for protein crystallization are discussed.

The effect of protein stability on protein-monoglyceride interactions

We have previously shown that proteins such as beta-lactoglobulin and lysozyme insert into monoglyceride monolayers and are able to induce an L(beta) to coagel phase transition in monoglyceride bilayers. These studies gave a first indication that protein stability could be an important factor for these interactions. This study therefore aims at further investigating the potential role of protein stability on protein-monoglyceride interactions. To this end we studied the interaction of stable and destabilized alpha-lactalbumin with monostearoylglycerol. Our results show that protein stability is important for the insertion of proteins into a monostearoylglycerol monolayer, such that the lower the stability of the protein the better the protein inserts. In marked contrast to beta-lactoglobulin and lysozyme we found that destabilized alpha-lactalbumin does not induce the L(beta) to coagel phase transition in monoglyceride bilayers. We propose that this is due to an increased surface coverage by the protein which could result from the unfolding of the protein upon binding to the interface.

Thermotropic phase behavior of monoglyceride-dicetylphosphate dispersions and interactions with proteins: a (2)H and (31)P NMR study

The phase behavior of a 1-[(2)H(35)]-stearoyl-rac-glycerol ([(2)H(35)]-MSG)/dicetylphosphate (DCP) mixture and its interaction with beta-lactoglobulin and lysozyme were studied by (2)H and (31)P nuclear magnetic resonance (NMR). The behavior of the lipids was monitored by using deuterium-labeled [(2)H(35)]-MSG as a selective probe for (2)H NMR and DCP for (31)P NMR. Both (2)H and (31)P NMR spectra exhibit characteristic features representative of different phases. In the lamellar phases, (31)P NMR spectra of DCP are different from the spectra of natural phospholipids, which is attributable to differences in the intramolecular motions and the orientation of the shielding tensor of DCP compared with phospholipids. The presence of the negatively charged amphiphile DCP has a large effect on the phase behavior of [(2)H(35)]-MSG. At low temperature, the presence of DCP inhibits crystallization of the gel phase into the coagel. Upon increasing the temperature, the gel phase of [(2)H(35)]-MSG transforms in the liquid-crystalline lamellar phase. In the presence of DCP, the gel phase directly transforms into an isotropic phase. The negatively charged beta-lactoglobulin and the positively charged lysozyme completely neutralize the destabilizing effect of DCP on the monoglyceride liquid-crystalline phase and they even stabilize this phase. Without DCP the proteins do not seem to interact with the monoglyceride. These results suggest that interaction is facilitated by electrostatic interactions between the negatively charged DCP and positively charged residues in the proteins. In addition, the nonbilayer-forming DCP creates insertion sites for proteins in the bilayer.

Conformational rearrangement of beta-lactoglobulin upon interaction with an anionic membrane

Interactions between beta-lactoglobulin (beta-lg) and dimyristoylphosphatidylglycerol (DMPG) bilayers were studied using one- and two-dimensional infrared spectroscopy above (pD 7.4) and below (pD 4.4) the protein's (beta-lg's) isoelectric point (pI=5.2). The aim of the study was threefold: (1) gain a better understanding of beta-lg-phospholipid interaction; (2) provide information relative to the structure of beta-lg as it interacts with membranes; (3) determine whether the conformational modifications of the protein in the presence of lipids are strictly caused by thermal effects or whether they are modulated by the chain-melting phase transition. At pD 7.4, the lipid thermotropism, the acyl-chain order, and the membrane interfacial region were essentially unaffected by the presence of beta-lg, whereas the protein amide I region showed dramatic alterations. The results suggested the predominance of beta-sheets and alpha-helix elements, with a lost of structural integrity. At pD 4.4, beta-lg induced an approximately 2 degrees C downshift of the transition temperature, whereas the conformational order of the lipid chain decreased in the gel phase and increased in the liquid-crystalline phase. The hydration state of the DMPG C==O groups increased in the liquid-crystalline phase. The conformation of beta-lg at pD 4.4 in the presence of DMPG showed similarities with that observed at pD 7.4, but an increase in the alpha-helix content and a reduced thermal stability were noticed. In contrast to the protein alone, beta-lg aggregates in the presence of DMPG at pD 4.4 above 50 degrees C. At both pD values, the charged surface of the membrane seemed to be the main factor for inducing protein conformational changes by altering the intramolecular interactions that stabilize the native structure. However, protein incorporation within the membrane seemed to be involved at pD 4.4. The two-dimensional analysis performed with spectra recorded upon heating showed that spectral intensity changes at pD 4.4 and 7.4 occurred at the same frequencies in the amide I' region. The heat-induced structural changes of beta-lg were not correlated with the conformational modifications of the phospholipids along the phase transition, indicating that the thermal behavior of the protein was not modulated by the lipid chain melting, but rather represented the heat-induced protein rearrangement in the presence of DMPG.

The specificity of monoglyceride-protein interactions and mechanism of the protein induced L(beta) to coagel phase transition

This study aims at gaining insight into the specificity and molecular mechanism of monoglyceride-protein interactions. We used beta-lactoglobulin (beta-LG) and lysozyme as model proteins and both monostearoylglycerol and monopalmitoylglycerol as defined gel phase monoglycerides. The monoglycerides were used in different combinations with the two negatively charged amphiphiles dicetylphosphate and distearylphosphate. The interactions were characterized using the monolayer technique, isothermal titration calorimetry, (2)H-nuclear magnetic resonance (NMR) using deuterium labelled monoglycerides and freeze fracture electron microscopy (EM). Our results show that lysozyme inserts efficiently into all monolayers tested, including pure monoglyceride layers. The insertion of beta-LG depends on the lipid composition of the monolayer and is promoted when the acylchains of the negatively charged amphiphile are shorter than that of the monoglyceride. The binding parameters found for the interaction of beta-LG and lysozyme with monoglyceride bilayers were generally similar. Moreover, in all cases a large exothermic binding enthalpy was observed which was found to depend on the nature of the monoglycerides but not of the proteins. (2)H-NMR and freeze fracture EM showed that this large enthalpy results from a protein mediated catalysis of the monoglyceride L(beta) to coagel phase transition. The mechanism of this phase transition consists of two steps, an initial protein mediated vesicle aggregation step which is followed by stacking and probably fusion of the bilayers.

Lipid organization and dynamics of the monostearoylglycerol-water system. A 2H NMR study

Deuterium labeled monostearoylglycerols with fully ([2H(35)]-MSG) and selectively ([11-(2)H(2)]-MSG) deuterated chains have been synthesized and used as a probe for 2H NMR. At low temperature monoglyceride-water systems form the coagel or crystalline phase, which transforms with increasing temperature subsequently into the gel, liquid crystalline and cubic phase. The 2H NMR spectra exhibit characteristic features representative of these phases. The gel phase is metastable and gradually transforms into the coagel at temperatures below 40 degrees C. The undercooled cubic phase transforms into the liquid crystalline phase during days. In the liquid crystalline phase, the chain order profile indicates an increase of the chain flexibility towards the methyl group. In the liquid crystalline phase, bilayers spontaneously align in a magnetic field with their normal perpendicular to the field. The results demonstrate that 2H NMR can serve as a convenient tool to study both structure and dynamics of different monoglyceride-water phases.