NMR in soft materials: A study of DMPC/DHPC bicellar system Maciej Kozak, Marek Kempka, Kamil Szpotkowski, Stefan Jurga * Department of Macromolecular Physics. A. Mickiewicz University, Umultowska 85, 61-614 Poznan ´ , Poland Available online 1 October 2007 Abstract The DMPC/DHPC bicellar system at the molar ratio of 2.8:1 has been characterised by measurements of self-diffusion coefficient (using PFGSE and PFGSTE NMR sequences), differential scanning calorimetry (DSC) and small angle scattering of synchrotron radi- ation (SAXS). The DSC curve shows only one endothermic peak characterised by the peak temperature T peak = 295.7 K and the onset temperature T onset = 290.1 K. This peak can be assigned to the nematic to smectic phase transition. Below the phase transition temper- ature, NMR diffusion experiments indicate a two-exponential decay of the spin echo amplitude allowing two diffusion coefficients D 1 and D 2 to be extracted from the experimental data. The maximum size (D max ) of the bicelle determined from SAXS data using the pair dis- tance distribution function p(r) is 11.2 nm and the bilayer thickness is 5 nm. Ó 2007 Published by Elsevier B.V. PACS: 61.10.Eq; 87.16.Dg; 87.15.Vv; 82.56.Pp Keywords: Biological systems; Biomaterials; Sychrotron radiation; Nuclear spin relaxation; Calorimetry 1. 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 col- lective, overall and local dynamics in aqueous solutions of surfactants [1,2], gelation processes of pectins [3] or poly- mer 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 chem- ical 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 [7–10]. Bicelles represent an intermediate morphol- ogy 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 effec- tively 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 phospholip- ids 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 phosphat- ydylcholines with long and short chains and temperature determine the formation of bicellar structure [11,12]. The bicellar structures are very useful as models of bio- logical membranes in structural studies. Bicelles are used to study the function of peptides [13], their conformational changes and the integral membrane proteins [14–17]. The 0022-3093/$ - see front matter Ó 2007 Published by Elsevier B.V. doi:10.1016/j.jnoncrysol.2007.02.068 * Corresponding author. Tel.: +48 61 8295266; fax: +48 61 8257758. E-mail address: stjurga@amu.edu.pl (S. Jurga). www.elsevier.com/locate/jnoncrysol Available online at www.sciencedirect.com Journal of Non-Crystalline Solids 353 (2007) 4246–4251