FT-IR study of multilamellar lipid dispersions containing cholesteryl linoleate and dipalmitoylphosphatidylcholine

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Abstract

The thermotropic properties and acyl chain packing characteristics of multilamellar lipid dispersions containing deuterated dipalmitoylphosphatidylcholine (DPPC-d62) and cholesteryl linoleate (CL) were examined by Fourier transform infrared spectroscopy. For CL/DPPC-d62 ratios (w/w) lower than 0.3, a phospholipid transition temperature measured through the νsCD2 band was found to be near 35°C and coincident with that observed for CL using its νsCH2 band. This phase transition is primarily associated with the corresponding gel to liquid crystal transition of DPPC-d62 multilamellar liposomes or vesicles presumably saturated with cholesteryl linoleate. At 0.6, 1.0, and 2.0 Cl/DPPC-d62 ratios, however, phase separation was found to be predominant. For these compositions involving relatively high CL concentrations, a ternary system must exist as three separate phases: (1) phospholipid saturated with a small amount of CL; (2) pure CL; and (3) water. These separate phases are consistent with the presence of emulsion particles composed of core CL and surface phospholipid. The phospholipid phase transitions of these CL-rich dispersions, appearing at lower temperatures relative to that of pure DPPC-d62, suggest that interdigitation of the acyl chains of the core located CL with those of the phospholipid layers may occur.

Introduction

Lipoproteins are heterogeneous lipid assemblies composed of mixtures of molecular species varying in head-group and hydrocarbon chain composition. Low density lipoproteins (LDL) are the principal vehicle for the transport of plasma cholesterol in man. They are usually isolated between d 1.019 and 1.068, and their average weight percent composition is ≈19% protein, 1% carbohydrate, 43% cholesteryl ester, 11% unesterified cholesterol, 4% triglycerides, and 22% phospholipid (Schumaker et al., 1994). Thermal analysis of human plasma low density lipoproteins (Deckelbaum et al., 1975, Deckelbaum et al., 1977) reveals a broad reversible transition involving a cooperative liquid crystalline to liquid phase change involving the cholesteryl esters in the lipoprotein. These observations necessitate the existence of a region within LDL particle, rich in cholesteryl esters, and large enough to permit a liquid crystal arrangement capable of a cooperative melting behaviour. This is one of the reasons why LDL proteins are described as emulsion particles containing the nonpolar lipids in an oil droplet at the centre of the LDL (Schumaker et al., 1994). However, a question arises as to whether all of the neutral lipids are located at the LDL centre, interacting or not with phospholipid hydrocarbon chains. Thus, the lower enthalpy observed in LDL relative to that from cholesteryl esters extracted from LDL (Deckelbaum et al., 1975, Deckelbaum et al., 1977) can be due not only to structural limitations arising from the size of lipoprotein particles, but also to interactions of some of the cholesteryl ester molecules with other phospholipid molecular constituents of LDL particles.

LDL functions are to a large degree mediated by protein structure and packing properties of the lipid layers of the lipoprotein particles. Since these exist in an essentially isothermal state, the modulation of the physical properties of the LDL lipid layers necessary to maintain a functional milieu for these lipoproteins may be achieved by regulating the LDL lipid composition. Therefore, information concerning the layer packing and dynamics of LDL lipid molecules becomes critical in elucidating characteristics of these lipoproteins.

In this study we shall examine the thermotropic properties of dipalmitoylphosphatidylcholine (DPPC)/cholesteryl linoleate (CL) mixtures in aqueous medium, based on infrared spectroscopic investigations. We have used an isotopic derivative of DPPC with deuterated acyl chains (DPPC-d62) in order to follow the spectra of the DPPC and CL components independently. In general, infrared spectra of lipids are particularly sensitive to the conformational, packing, and dynamical changes involving the hydrocarbon chains. As far as we know, there are no vibrational spectroscopic study regarding the interaction between DPPC and CL at the molecular level.

Section snippets

Samples

CL and DPPC-d62 were obtained from Sigma and Cambridge Isotope Laboratories, respectively, and were used as received. The pure lipids were dispersed in water (2 wt.%) through sonication until total dispersion. Binary mixture dispersions were prepared as follows. Dry mixtures of CL and DPPC-d62 with 2/1, 1/1, 2/3, 1/3, and 1/4 (w/w) CL/DPPC-d62 ratios were previously prepared by dissolving the appropriate amounts of the lipids in chloroform-ethanol (2/1, v/v) and evaporation of the solvent under

Thermotropic behaviour of pure deuterated dipalmitoylphosphatidylcholine and cholesteryl linoleate

Fig. 1 shows the acyl chain region of the infrared spectrum of pure DPPC-d62 in dispersion at 2% by weight in D2O. The two main bands observed at 2194 and 2089/cm are attributed to the antisymmetric and symmetric CD2 stretching vibrations, respectively. The two weaker bands at 2213 and 2075/cm are due to the asymmetric and symmetric stretching vibrations of the terminal methyl group of the lipid acyl chains, respectively (Sunder et al., 1978, Casal and Mantsch, 1984). In this νC-D spectral

Acknowledgements

Our thanks are due to The University San Pablo for financial support (Project number 21/97).

References (29)

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