ARTICLE pubs.acs.org/Langmuir
Revisiting β-Casein as a Stabilizer for Lipid Liquid Crystalline Nanostructured Particles Jiali Zhai,†,‡ Lynne Waddington,§ Tim J. Wooster,‡ Marie-Isabel Aguilar,*,† and Ben J. Boyd*,|| †
Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia CSIRO Food and Nutritional Sciences, Sneydes Road, Werribee, VIC 3030, Australia § CSIRO Materials Science and Engineering, 343 Royal Pde, Parkville VIC 3052, Australia Drug Delivery, Disposition and Dynamics, Monash Institute of Pharmaceutical Sciences, Monash University (Parkville Campus), 381 Royal Pde, Parkville VIC 3052, Australia
)
‡
bS Supporting Information ABSTRACT: Lipid liquid crystalline nanoparticles such as cubosomes and hexosomes have unique internal nanostructures that have shown great potential in drug and nutrient delivery applications. The triblock copolymer, Pluronic F127, is usually employed as a steric stabilizer in dispersions of lipid nanostructured particles. In this study, we investigated the formation, colloidal stability and internal nanostructure and morphology of glyceryl monooleate (GMO) and phytantriol (PHYT) cubosome dispersions on substituting β-casein with F127 in increasing proportion as the stabilizer. Internal structure and particle morphology were evaluated using small-angle X-ray scattering (SAXS) and cryo-transmission electron microscopy (cryo-TEM), while protein secondary structure was studied using synchrotron radiation circular dichroism (SRCD). The GMO cubosome dispersion stabilized by β-casein alone displayed a V2 (Pn3m) phase structure and a V2 to H2 phase transition at 60 °C. In comparison, F127-stabilized GMO dispersion had a V2 (Im3m) phase structure and the H2 phase only appeared at higher temperature, that is, 70 °C. In the case of PHYT dispersions, only the V2 (Pn3m) phase structure was observed irrespective of the type and concentration of stabilizers. However, β-casein-stabilized PHYT dispersion displayed a V2 to H2 to L2 transition behavior upon heating, whereas F127-stabilized PHYT dispersion displayed only a direct V2 to L2 transition. The protein secondary structure was not disturbed by interaction with GMO or PHYT cubosomes. The results demonstrate that β-casein provides steric stabilization to dispersions of lipid nanostructured particles and avoids the transition to Im3m structure in GMO cubosomes, but also favors the formation of the H2 phase, which has implications in drug formulation and delivery applications.
’ INTRODUCTION The liquid crystalline phase behavior of polar lipids in an aqueous environment has been well studied.15 Some polar lipids, such as glyceryl monooleate (GMO) and phytantriol (PHYT), self-assemble in excess water to form remarkable structures with long-range periodicity, but short-range disorder at atomic distances, termed lyotropic liquid crystalline phases. The chemical structures of GMO and PHYT and illustrations of structures that they form in excess water are given in Figure 1. GMO is the most commonly studied lipid known to form liquid crystalline phases in excess water.4,68 PHYT is a lipid that has received increasing attention because of its relative stability and purity compared to GMO. Studies have shown that PHYT exhibits liquid crystalline phase behavior very similar to that of GMO.9,10 Both GMO and PHYT in excess water can form an inverse bicontinuous cubic (V2) phase at ambient temperatures, an inverse hexagonal (H2) phase and an inverse micellar (L2) phase (see Figure 1) at higher temperatures. There are three different geometries identified for the V2 phases based on infinite periodic minimal surfaces (IPMS). The V2 phase consists of a network of r 2011 American Chemical Society
two nonintersecting water channels separated by a single continuous lipid bilayer. They are the primitive, diamond, and gyroid types with an Im3m, Pn3m, and Ia3d space group, respectively. The proposed structures of these different bicontinuous cubic space groups have been well described by Hyde et al.1,11 and illustrated in recent reviews.12 The H2 phase consists of infinite cylinders of water molecules in a continuous medium of lipid. The cylinders are arranged in a hexagonal array. The L2 phase consists of water-in-oil micelles with head groups sequestered in the micelle core and the hydrocarbon chains extend away. A consequence of the thermodynamic stability of these structures in excess water is that they can be mechanically dispersed to form particles that retain the internal nanostructure of the “parent” phase at high dilution. Dispersions of cubic and hexagonal phases have been termed “cubosomes” and “hexosomes”, respectively. To make kinetically stable dispersions, a steric Received: August 6, 2011 Revised: October 18, 2011 Published: October 25, 2011 14757
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Table 1. Some Physicochemical and Structural Properties of β-Casein and Pluronic F127 Used in This Study surface MW
stabilizer
nature
(Da)
(mg 3 m2)
(μM)47
β-casein
protein
∼24 000
∼244,45
57
Pluronic
synthetic
∼12 600
∼346
96
F127
Figure 1. Chemical structures of GMO and PHYT and the nanostructures formed by the lipids in excess water. Dispersions of lipid particles with the internal nanostructures of bulk phases can be achieved with the aid of steric stabilizers.
stabilizer is required to prevent rapid aggregation of the particles, which is driven by van der Waals and hydrophobic interactions. Larsson first described this phenomenon using β-casein as a steric stabilizer for GMO-based cubosomes.4,13 However, since the discovery that the block copolymer Pluronic F127 is a highly effective stabilizer for these dispersions,14 it has become the stabilizer of choice for studies of cubosomes and hexosomes.10,1518 However, recent studies have indicated that Pluronic F127 is not necessarily the optimal stabilizer for cubosomes.19 Both the nondispersed liquid crystalline phases and the dispersed cubosomes and hexosomes are of particular interest to the pharmaceutical and food industries due to their potential to protect and control the release of cargo molecules.5,6,2028 Given the interest in application of these materials in oral drug and nutrient delivery, the development of a biocompatible stabilizer is important. Angelova et al. have reported the ability of antibody fragments to fracture the cubic phase into dispersed cubosomes on chemical coupling to lipids incorporated into the bulk cubic phase, indicating a potential role for biological molecules at cubosome stabilizers.29 The use of simpler peptides and proteins are a logical option for natural stabilizers for such systems based on the early reports on β-casein;4,13 however, they do not appear to have been further explored. β-Casein is highly amphiphilic and has a flexible, linear structure, which has made it an excellent stabilizer in other dispersion systems such as oil-inwater emulsions.3032 Studies using synchrotron radiation circular dichroism (SRCD) spectroscopy and neutron reflectivity have shown that β-casein adsorbs to the oilwater interface of emulsions and adopts a characteristic interfacial structure to provide steric stabilization against oil particle aggregation.33,34 In the dispersion systems of cubosomes and hexosomes, there is little understanding of the interaction between protein and the
critical micelle
chemical
coverage concentration
block copolymer
lipidwater interfaces. Moreover, no X-ray diffraction data supporting internal nanostructures of cubosomes and hexosomes dispersed by β-casein have been reported. Further, Larsson’s early study using freeze fracture transmission electron microscopy (TEM) indicated that the Ia3d gyroid cubic phase structure was present for GMO cubosomes dispersed with caseins, which would be unusual in an excess water environment for GMO systems.13 Consequently, the aims of this study were to investigate the ability of β-casein to replace Pluronic F127 for the dispersion of GMO and PHYT into cubosomes and hexosomes in excess water, and to gain a deeper understanding of its interaction with the internal liquid crystalline structure and its association with the lipidwater interface. Some relevant properties of β-casein and Pluronic F127 are provided in Table 1. Dispersions were prepared at varying ratios of β-casein and Pluronic F127, and the particle size and morphology of the dispersed lipid systems were studied by dynamic light scattering (DLS) and cryo-TEM. Internal liquid crystalline structure was determined using small-angle X-ray scattering (SAXS). The effect of increased temperature on the phase transition behavior was examined to probe the nature of interaction of the stabilizer with the particle structure. Finally, the effect of protein interaction with lipid nanostructured particles on protein secondary conformation was studied using SRCD.
’ MATERIALS AND METHODS Materials. Myverol 18-99K was donated by Kerry Bio-Science (Norwich, NY) and was used for glyceryl monooleate (GMO)-based samples. The analytical data from the supplier indicates that Myverol 1899K (certificate no. 31500455) contains 58.3% glyceryl monooleate (C18:1), 12.2% glyceryl monolinoleate (C18:2), 5.1% glyceryl monolinolenate (C18:3), 3.9% glyceryl monopalmitate (C16:0), 1.7% glyceryl monostearate (C18:0), 0.96% glyceryl monogadoleate (C20:1), 0.2% glyceryl arachidonate (C20:4), 0.1% free fatty acids, and 0.4% glycerol. Trace amounts of unquantified diglycerides are also believed to be present. It should be noted that few studies in this field actually use pure GMO due to cost and stability issues (most use Myverol, Rylo MG90, or similar), on the basis that Myverol demonstrates the same phase behavior as more pure GMO, suggesting that the impurities are not significantly impacting on the relevance of this material. Phytantriol (3,7,11,15-tetramethyl-1,2,3-hexadecanetriol, PHYT) was a gift from DSM (Basel, Switzerland), with a nominal purity of >96.6%. β-Casein (BioUltra, g98% PAGE), Pluronic F127 (F127), and sodium phosphate (monobasic and dibasic) were purchased from Sigma (St. Louis, MO). These chemicals were used without further purification. Milli-Q grade water (Millipore, Billerica, MA) was used for all sample preparation. Sample Preparation. Separate stock solutions of β-casein (1% w/w) and F127 (1% w/w) were prepared in 10 mM sodium phosphate buffer, pH 7. These stock solutions were then mixed at various ratios to obtain the desired ratio of β-casein/F127 (at 1% w/w) for subsequent preparation of lipid-based dispersions. The ratio α represents the mass 14758
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fraction of β-casein in the total stabilizer, hence α¼
½casein ½casein þ ½F127
Lipid dispersions were prepared by dropwise addition of 0.5 g of molten lipid to 5 g of the stabilizer solution containing either β-casein, F127, or mixtures of β-casein and F127. The sample was immediately ultrasonicated for 20 min using a Branson sonifier at a power of approximately 80 W, resulting in milky dispersions. Dynamic Light Scattering and Cryo-TEM Measurements. The particle size distributions for dispersions were measured using a Malvern Zetasizer Nano ZS (Malvern Instruments, U.K.). Samples were diluted with Milli-Q H2O and analyzed in a plastic cuvette at 25 °C. Refractive indices of water and lipids were used for calculations of particle size by the software. For cryo-TEM imaging, a laboratory-built humidity-controlled vitrification system was used to prepare the samples. Humidity was kept close to 80% for all experiments. Copper grids (200 mesh) coated with perforated carbon film (Lacey carbon film, ProSci Tech, Australia) were used. Grids were first glow discharged in nitrogen to render them hydrophilic. The 4 μL aliquots of the sample were applied onto each grid. After 30 s adsorption time, grids were blotted manually using Whatman 541 filter paper for approximately 2 s. Grids were then plunged into liquid ethane cooled by liquid nitrogen. The samples were examined using a Gatan 626 cryoholder (Gatan, Pleasanton, CA) and a Technai 12 transmission electron microscope (FEI, Eindhoven, The Netherlands) at an operating voltage of 120 kV, with a Megaview III CCD camera and AnalySIS camera control software. SAXS Measurements. SAXS measurements were performed on an Anton Paar SAXess instrument with Cu Kα radiation of wavelength 1.542 Å and a line collimation system. The SAXS camera was fitted on a 1D-diode-array detector. The sample-to-detector distance was set to 306.8 mm, which provided a q-range of 00.6 Å1. The SAXS profile of each sample was an average of two frames, and the exposure time for each frame was 1800 s. Samples (20 μL) were contained in a quartz flow cell and temperature controlled by a Peltier system. Temperature was set to 20 °C for measurements except for thermal ramp experiments. The optics and sample chamber were under vacuum to minimize air scatter. The q scale was calibrated using silver behenate. The software program SAXSquant was used to conduct baseline subtraction, integration, and smoothing. The resulting SAXS profiles were a function of intensity versus the scattering vector q, which is defined by q = (4π/λ)(sin 2θ)/2, with λ being the wavelength and 2θ the scattering angle. The mean lattice parameter, a, of each liquid crystalline phase was calculated from the interplanar distance,√ d (d √ = 2π/q), √ a/d(hkl) =√ 2, 3, 4, using√the appropriate scattering law:35 √ √ √ √Pn3m: √ √ 6, √ 8...; Im3m: a/d(hkl) = 2, 4, 6, 8, 10...; H2: 3a/2d(hk) = √ 1, 3, 4... For the L2 phase, which shows only one broad peak, d is termed the characteristic distance.36 SRCD Measurements. SRCD measurements were performed on the CD1 beamline of the SRCD station at the ASTRID storage ring using methods previously described.33,34,37 A Suprasil cell (Hellma GmbH & Co., Germany) of 0.01 cm path length was used for far-UV SRCD measurements at 20 °C. The operating conditions were 1 nm bandwidth, 2.15 s averaging time, and three scans for solution sample or eight scans for dispersion sample. F127-stabilized lipid dispersions were used as the baseline. Spectra of β-casein-stabilized lipid dispersions were baseline subtracted and processed using CDtool software.38 The spectra are presented as mean residue ellipticity [θ], based on a mean residue weight of 114.9 for β-casein calculated from its sequence.
’ RESULTS β-Casein Stabilization of GMO-Based Nanostructured Particles. Dispersions formed with α = 1, where only β-casein
Figure 2. (A) Representative cryo-TEM image of GMO-based cubosomes stabilized by 1% β-casein (α = 1). The scale bar represents 200 nm. (B) SAXS patterns from GMO-based dispersions stabilized by various ratios of β-casein/F127. Total stabilizer concentration in all cases was 1% (w/w). (C) phase identity and lattice parameter of GMObased dispersions derived from SAXS data in (B) as a function of the fraction of β-casein to total stabilizer (α).
was present, were milky high quality emulsion-like systems, with mean particle size = 262.9 ( 16.2 nm and polydispersity index = 0.282 ( 0.014. The overall particle size distribution was larger than that with α = 0, where only F127 was used as the stabilizer (mean particle size = 181.5 ( 0.7 nm, polydispersity index = 0.106 ( 0.019). GMO-based dispersions containing mixtures of β-casein and F127 had average particle sizes of 180280 nm and polydispersity indices of
