Comprehensive Examination of Mesophases Formed by DMPC and DHPC Mixtures

Abstract

Mixtures of long- and short-chain phospholipids, specifically 14:0 and 6:0 phosphatidylcholines (DMPC and DHPC), have been used successfully in NMR studies as magnetically alignable substrates for membrane-associated proteins. However, recent publications have shown that the phase behavior of these mixtures is much more complex than originally thought. Using polarized light microscopy and small-angle neutron scattering, phase diagrams of DMPC/DHPC mixtures at molar ratios of 2, 3.2, and 5 have been determined. Generally, at temperatures below the main-chain melting transition of DMPC (T(M) = 23 degrees C), an isotropic phase of disk-like micelles is found. At high temperatures (T > 50 degrees C), a lamellar phase consisting of either multilamellar vesicles (MLV) or extended lamellae is formed, which at low lipid concentrations (e.g., MLV) coexists with an excess of water. At intermediate temperatures and lipid concentrations, a chiral nematic phase made up of worm-like micelles was observed.

“Bicellar” Lipid Mixtures as used in Biochemical and Biophysical Studies

“…From numerous NMR studies, it was initially believed to be bicelles, but was later shown, by SANS, to be consistent with perforated lamellae (Nieh et al 2002). However, most recently SANS, POM and cryo-TEM studies (Nieh et al 2002;Harroun et al 2005;van Dam et al 2004) have shown that the magnetically alignable phase of nondoped bicellar mixtures is in fact characterized by worm-like micelles (Nieh et al 2004a). The nondoped mixtures and their various morphologies have been used extensively in structure-function studies of membrane associated proteins and protein crystallization (see Sect.…”

“…The recent report of Harroun et al (2005) provides a nearly comprehensive study of bicellar mixture mesophases, reconciling a number of conflicting views. The phase diagram shown in Fig.…”

“…In contrast to nondoped bicellar mixtures, a detailed study equivalent to the one by Harroun et al (2005) has not yet been undertaken for doped bicellar mixtures. Presently, the Nieh et al (2002), and whose adapted phase diagram is shown in Fig.…”

“…Using a combination of polarized light microscopy (POM) and SANS (Nieh et al 2004a;Harroun et al 2005), it was concluded that the phase space previously thought to be composed of bicelles (Raffard et al 2000) or extended lamellar sheets (Nieh et al 2002), is in fact a chiral nematic phase made up of worm-like, or ribbon-like, micelles (see Fig. 1B).…”

Applications of neutron and X-ray scattering to the study of biologically relevant model membranes

“…For example, bilayered ribbons (Fig. 12c) and perforated lamellae were found in neutral and charged systems, respectively (Nieh et al, 2004;Harroun et al, 2005;Katsaras et al, 2005). The ribbons transformed into MLVs with increasing temperature, while the charged perforated lamellae remained unaltered.…”