Effect of unsaturated acyl chains on structural transformations in triacylglycerols

Polymorphic phase transitions in triglycerides and their mixtures studied by SAXS/WAXS techniques: In bulk and in emulsions

Triacylglycerols (TAGs) exhibit a monotropic polymorphism, forming three main polymorphic forms upon crystallization: α, β' and β. The distinct physicochemical properties of these polymorphs, such as melting temperature, subcell lattice structure, mass density, etc., significantly impact the appearance, texture, and long-term stability of a wide range products in the food and cosmetics industries. Additionally, TAGs are also of special interest in the field of controlled drug delivery and sustained release in pharmaceuticals, being a key material in the preparation of solid lipid nanoparticles. The present article outlines our current understanding of TAG phase behavior in both bulk and emulsified systems. While our primary focus are investigations involving monoacid TAGs and their mixtures, we also include illustrative examples with natural TAG oils, highlighting the knowledge transfer from simple to intricate systems. Special attention is given to recent discoveries via X-ray scattering techniques. The main factors influencing TAG polymorphism are discussed, revealing that a higher occurrence of structural defects in the TAG structure always accelerates the rate of the α → β polymorphic transformation. Diverse approaches can be employed based on the specific system: incorporating foreign molecules or solid particles into bulk TAGs, reducing drop size in dispersed systems, or using surfactants that remain fluid during TAG particle crystallization, ensuring the necessary molecular mobility for the polymorphic transformation. Furthermore, we showcase the role of TAG polymorphism on a recently discovered phenomenon: the creation of nanoparticles as small as 20 nm from initial coarse emulsions without any mechanical energy input. This analysis underscores how the broader understanding of the TAG polymorphism can be effectively applied to comprehend and control previously unexplored processes of notable practical importance.

Co-assembly and Structure of Sodium Dodecylsulfate and other n-Alkyl Sulfates in Glycerol: n-Alkyl Sulfate-Glycerol Crystal Phase

HYPOTHESIS

Following the observation of a microfibrillar phase in sodium dodecylsulfate (SDS)-glycerol mixtures, it is hypothesized that this phase is a crystalline structure containing SDS and glycerol, where the interaction between sulfate and glycerol layers mediates the co-assembly, which also could be universal for similar systems formed by n-alkyl sulfate homologues. Experiment. n-alkyl sulfate glycerol solutions were studied using a combination of optical microscopy, small- and wide-angle X-ray scattering (SAXS/WAXS). Time-resolved SAXS was employed to determine the phase formation in SDS-glycerol-water mixtures.

FINDINGS

The microfibrillar crystalline phase was reproduced in even-chained n-alkyl sulfates with a chain length between 12 and 18 carbon atoms, where the phase lamellar period increased uniformly with the alkyl chain length. Reconstruction of electron density profiles from the diffraction patterns allowed the lamellar structural motif of the phase, the glycerol location and stoichiometry to be determined. When SDS-glycerol-water mixtures with water concentration below 6 wt% are isothermally solidified at 20 °C, SDS-glycerol crystals and/or anhydrous SDS form, where the former is inhibited by the latter at higher water concentrations. The learnings from the SDS-glycerol phase formation allows new gels to be created, utilising the glycerol-sulfate motif generating microfibrils. This expands the knowledge of the applicable formulation space for SDS-water containing mixtures.

An Insight into the Solid-State Miscibility of Triacylglycerol Crystals

The crystallization properties of triacylglycerols (TAGs) strongly determine the functional properties of natural lipids. The polymorphic and mixing phase behavior of TAG molecules have long been, and still are, a hot topic of research with special relevance for the cosmetic, pharmaceutical, and food industry. To avoid the difficulties arising from the study of whole real fats, studies at the molecular level on mixtures of a limited number of TAGs has become an indispensable tool to identify the underlying causes of the physical properties in lipid systems. In particular, phase diagrams of binary mixtures of TAGs exhibiting a different degree of heterogeneity (monoacid or mixed fatty acids; molecular symmetry; the presence of cis or trans double bonds) have resulted in a significant breakthrough in our knowledge about structure-interaction-function relationships. The present work aims to provide an overview of the main reports regarding binary and ternary TAG systems, from the early studies to the most recent developments.

In vitro digestion of emulsions: high spatiotemporal resolution using synchrotron SAXS

Although the biochemical processes of lipid digestion are well-known, the biophysical ones, responsible for the assembly of molecules into functional structures, lack studies resolving both time and space scales. About 35 years ago, the seminal microscopy study of Patton and Carey constituted a major advance to reach this goal. Nowadays, new perspectives arise from the availability of large facilities scattering techniques, able to monitor the dynamics of multi-scale assemblies with unprecedented resolutions. The present small angle X-ray scattering (SAXS) study focused on the roles of the emulsifier and triglyceride in the formation of lipid assemblies during emulsion digestion in vitro. By developing several interpretations of the data in the whole space range (qualitative, shape-dependent and shape-independent models), the characteristic size of the assemblies and their transition times were obtained, which depended on the triglyceride, but not on the emulsifier. The major assembly formed was found to be a spherical mixed micelle, but vesicle was also found to coexist throughout the digestion, although in a lower proportion. The quantitative determination of the sizes and proportions of these assemblies, as well as the evolution of these characteristics during digestion are precious information for nutritional sciences, as these assemblies are the vehicles of lipophilic nutrients and micronutrients towards their absorption site.

Stability of the Metastable α-Polymorph in Solid Triglyceride Drug-Carrier Nanoparticles

Colloidal dispersions of crystalline nonpolar lipids are under intensive investigation as carrier systems in pharmaceutics and nutrition. In this context, the controlled preparation of particles in a metastable polymorphic state is of some interest for the delivery of active substances. In the present study, tristearin particles stabilized with three α-polymorph-preserving emulsifier regimes ((I) sodium glycocholate/saturated long-chain phospholipids, (II) sodium glycocholate, and (III) poly(vinyl alcohol) (PVA)) were investigated concerning the stability of the metastable α-polymorph after controlled crystallization of the particles from the melt. Upon long-term storage, the α-polymorph was preserved best in PVA-stabilized dispersions, followed by those stabilized with the glycocholate/phospholipid mixture and finally those stabilized solely with the bile salt. In particular for rapidly crystallized nanoparticles, the formation of an α-polymorph with highly reduced lamellarity was observed. According to time-/temperature-resolved synchrotron X-ray diffraction analysis with simultaneous DSC (differential scanning calorimetry) studies, this less-ordered α-polymorph transformed into the common, lamellar α-form upon heating. Although the presence of the less-ordered form is probably related to the extraordinarily high stability of the metastable α-polymorph observed in some of the dispersions, it could not completely prevent the transition into the stable β-polymorph. The higher the transition temperature of the less-ordered α-form to the ordered one, the slower was the polymorphic transition to the stable β-polymorph. To estimate the polymorphic stability of the differently stabilized particles upon isothermal long-term storage, standard DSC measurements on samples stored at 23 °C for 4 weeks seem to be of predictive value.

WAXS fat subtraction model to estimate differential linear scattering coefficients of fatless breast tissue: phantom materials evaluation

PURPOSE

Develop a method to subtract fat tissue contributions to wide-angle x-ray scatter (WAXS) signals of breast biopsies in order to estimate the differential linear scattering coefficients μ(s) of fatless tissue. Cancerous and fibroglandular tissue can then be compared independent of fat content. In this work phantom materials with known compositions were used to test the efficacy of the WAXS subtraction model.

METHODS

Each sample 5 mm in diameter and 5 mm thick was interrogated by a 50 kV 2.7 mm diameter beam for 3 min. A 25 mm(2) by 1 mm thick CdTe detector allowed measurements of a portion of the θ = 6° scattered field. A scatter technique provided means to estimate the incident spectrum N(0)(E) needed in the calculations of μ(s)[x(E, θ)] where x is the momentum transfer argument. Values of [Formula: see text] for composite phantoms consisting of three plastic layers were estimated and compared to the values obtained via the sum [Formula: see text], where ν(i) is the fractional volume of the ith plastic component. Water, polystyrene, and a volume mixture of 0.6 water + 0.4 polystyrene labelled as fibphan were chosen to mimic cancer, fat, and fibroglandular tissue, respectively. A WAXS subtraction model was used to remove the polystyrene signal from tissue composite phantoms so that the μ(s) of water and fibphan could be estimated. Although the composite samples were layered, simulations were performed to test the models under nonlayered conditions.

RESULTS

The well known μ(s) signal of water was reproduced effectively between 0.5 < x < 1.6 nm(-1). The [Formula: see text] obtained for the heterogeneous samples agreed with [Formula: see text]. Polystyrene signals were subtracted successfully from composite phantoms. The simulations validated the usefulness of the WAXS models for nonlayered biopsies.

CONCLUSIONS

The methodology to measure μ(s) of homogeneous samples was quantitatively accurate. Simple WAXS models predicted the probabilities for specific x-ray scattering to occur from heterogeneous biopsies. The fat subtraction model can allow μ(s) signals of breast cancer and fibroglandular tissue to be compared without the effects of fat provided there is an independent measurement of the fat volume fraction ν(f). Future work will consist of devising a quantitative x-ray digital imaging method to estimate ν(f) in ex vivo breast samples.

Relationships between molecular structure and kinetic and thermodynamic controls in lipid systems. Part III. Crystallization and phase behavior of 1-palmitoyl-2,3-stearoyl-sn-glycerol (PSS) and tristearoylglycerol (SSS) binary system

The phase behavior of 1-palmitoyl-2,3-distearoyl-sn-glycerol (PSS)/tristearoylglycerol (SSS) binary system was investigated in terms of polymorphism, crystallization and melting behavior, microstructure and solid fat content (SFC) using widely different constant cooling rates. Kinetic phase diagrams were experimentally determined from the DSC heating thermograms and analyzed using a thermodynamic model to account for non-ideality of mixing. The kinetic phase diagram presented a typical eutectic behavior with a eutectic point at the 0.5(PSS) mixture with a probable precipitation line from 0.5(PSS) to 1.0(PSS), regardless of the rate at which the sample was cooled. The eutectic temperature decreased only slightly with increasing cooling rate. PSS has a strong effect on the physical properties of the PSS-SSS mixtures. In fact, the overall phase behavior of the PSS-SSS binary system was determined, for a very large part, by the asymmetrical TAG. Moreover, PSS is a key driver of the high stability observed in crystal growth, polymorphism and phase development. Levels as low as 10% PSS, when cooled slowly, and 30% when cooled rapidly, were found to be sufficient to suppress the effect of thermal processing.