NMR in soft materials: A study of DMPC/DHPC bicellar system
Introduction
NMR techniques can provide valuable information about ordering and dynamics in soft-matter systems. These techniques have been used for example in the study of collective, overall and local dynamics in aqueous solutions of surfactants [1], [2], gelation processes of pectins [3] or polymer chain mobility in novel nanocomposites [4].
Among soft materials, such as: polymers, gels, colloids and many biomolecular systems (polysaccharides, lipids, proteins or nucleic acids solutions) phospholipids play an important role in pharmaceutical and cosmetic industry [4], [5]. They are the main structural elements of the cell membrane. Phospholipids can form different lyotropic structures in water solution [6]. The type of structure formed depends on temperature, water content and chemical properties of phospholipids [7]. In aqueous solutions the mixtures of phospholipids with long hydrophobic chains and those with short hydrophobic chains can form discotic nematic liquid-crystalline phases also known as bicelles [7], [8], [9], [10]. Bicelles represent an intermediate morphology between lipid vesicles and classical mixed micelles, combining some of the attractive properties of both of the model membrane systems. Like micelles, bicelles are non-compartmentalised, optically transparent and effectively monodisperse [9]. Bicellar structures are formed also by mixtures of phosphatidylcholines. A typical bicelle can be approximated by a core-shell-disc model [11]. The core of the disc consists of the hydrophobic tails of phospholipids and the shell of the disk consists of the polar group of phospholipids. The edge of the bicellar disc consists of short-chain phosphatydylcholine (e.g. DHPC). The top and bottom surface of the bicelle is made of long chain phosphatydylcholine [7]. The molar ratio of phosphatydylcholines with long and short chains and temperature determine the formation of bicellar structure [11], [12].
The bicellar structures are very useful as models of biological membranes in structural studies. Bicelles are used to study the function of peptides [13], their conformational changes and the integral membrane proteins [14], [15], [16], [17]. The most important property of bicelles is their ability to align in the magnetic field. DMPC-rich bicelles spontaneously align in a magnetic field so that the normal to the plane of the lipid bilayer is oriented perpendicularly to the direction of the magnetic field [13], [14], [15], [16], [17].
The aim of the study was to determine the molecular dynamics and structure of a well-defined model phospholipid bicellar system. The bicellar system studied was prepared from 1,2-dihexanoyl-sn-glycero-3-phosphocholine and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC/DHPC) at the molar ratio of 2.8:1 (q = 2.8). It was characterised by the NMR measurements of self-diffusion and T2 dispersion, differential scanning calorimetry (DSC) and small angle scattering of synchrotron radiation (SAXS). This system was chosen as the model for a future study of the effect of different surfactant groups on the stability of the bicellar phase.
Section snippets
Sample preparation
The phospholipids 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were purchased as lyophilized powders from Avanti polar lipids. The appropriate amounts of the phospholipids, with DMPC/DHPC at the molar ratio 2.8:1, were dissolved in a phosphate buffer (10 mM KH2PO4, 0.1 mM NaN3 pH 6.2) according to a standard protocol [18]. The final sample concentration was 15% (w/w).
NMR diffusion
Measurements of the self-diffusion coefficient of 15% (w/w) DMPC/DHPC (q
Results and discussion
The differential scanning calorimetry (DSC) and the small angle X-ray scattering (SAXS) were used for confirmation of the bicellar structure of the system studied.
The DSC measurements were performed to determine the temperature range of phase transition in the DMPC/DHPC system. The DSC results are presented in Fig. 1.
Only one endothermic peak characterised by a peak temperature Tpeak = 295.7 K and an onset temperature Tonset = 290.1 K is observed. This peak could be assigned to the nematic to smectic
Acknowledgements
The work was supported by the Ministry of Scientific Research and Information Technology (Grant No. 3T09A 050 27). The data collection at EMBL c/o DESY Hamburg was supported by EU grant (RII3-CT-2004-506008).
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