Bicells

To reduce the possibility that confinement in gel cavities is somehow responsible for the resistance of long-range structure to urea, the same experiments were carried out on A131A in alkyl-PEG bicelles (Ackerman and Shortle, 2002). The same results were obtained, with almost the same correlation coefficients as those seen in strained polyacrylamide gels. 

Fig. 3. Scatterplots of the Nh residual dipolar couplings of A131A measured in 2, 4, 6, or 8 M urea (y-axes) plotted against the same couplings measured in the absence of urea (x-axis) ris the Pearson correlation coefficient. Alignment was achieved with alkyl PEG bicelles (Ackerman and Shortle, 2002.)...

Hydrophobic interactions, on the other hand, are strong, indifferent to local details, and are relatively long range. If transient direct or water-separated contacts occur between nonpolar side chains, the net effect could be local organization and an overall compaction of the polypeptide chain. Whereas the strengths of hydrophobic interactions must be considerably reduced in 8 M urea, they clearly are not eliminated, as evidenced by the persistence of lipid bicelles. Thus hydrophobic interactions probably play some role in persistent global structure, the importance of which can be tested by replacing multiple hydrophobic side chains with similarly shaped polar ones. 

Fig. 5 Membrane models for NMR structure analysis, (a) An isotropic detergent micelle (left) is compared to the dimensions of lipid bilayers (right), (b) Macroscopically oriented membrane samples can be prepared on solid support, as nanodiscs, or as magnetically oriented bicelles. (c) Nomenclature and variability of liposomes small (SUV, 20-40 nm), intermediate (IUV, 40-60 nm), large (LUV, 100-400 nm), and giant unilamellar vesicles (GUV, 1 pm) multi-lamellar (MLV), oligo-lamellar (OLV) and highly heterogeneous multi-oligo-lamellar vesicles (MOLV)...

Marcotte I, Auger M (2005) Bicelles as model membranes for solid- and solution-state NMR studies of membrane peptides and proteins. Concept Magn Reson A 24A 17-37... 

Matsumori N, Murata M (2010) 3D structures of membrane-associated small molecules as determined in isotropic bicelles. Nat Prod Rep 27 1480-1492... 

Void RR, Prosser RS, Deese AJ (1997) Isotropic solutions of phospholipid bicelles a new membrane mimetic for high-resolution NMR studies of polypeptides. J Biomol NMR 9 329-335... 

Diller A, Loudet C, Aussenac F, Raffard G, Fournier S, Laguerre M, Grelard A, Opella SJ, Marassi FM, Dufourc EJ (2009) Bicelles a natural molecular goniometer for structural, dynamical and topological studies of molecules in membranes. Biochimie 91 744-751... 

Nieh MP, Raghunathan VA, Glinka CJ, Harroun TA, Pabst G, Katsaras J (2004) Magnetically alignable phase of phospholipid bicelle mixtures is a chiral nematic made up of wormlike micelles. Langmuir 20 7893-7897... 

Although detailed protocols for preparation of small unilamellar vesicles (SUVs) are published [80-82], their preparation is more time-demanding and complicated. In contrast, bicelles are prepared similarly to micelles with little effort. 

Fig. 8.3 Cartoon illustrating the alignment of the particles of two different orienting media, a Disc particles represent lipid bicelles. b Rods represent viral particles. Bicelles orient with their normal orthogonal to the magnetic field and viruses with their long axis parallel to it. (Reproduced with permission from N. Tjandra, Structure 1999, 7, R205-R211.)...

Other novel bicelle systems composed of a mixture of l,2-di-0-dodecyl-sn-glycero-3-phosphocholine and 3-(chloramidopropyl)dimethylammonio-2-hydroxyl-l-propane sulfonate have been designed to orient proteins at low pH values and over wide temperature ranges [25]. 

Fig. 8.7 Histograms of residual dipolar couplings observed for the C-terminal KH domain of the ri-bonucleoprotein K in 3.2% w/v bicelle solution [J.L. Baber, D. Libutti, D. Levens and N. Tjandra,...

In a multidomain protein whose domains have fixed orientations relative to each other, a unique alignment tensor will represent the preferred orientation of all the domains in the anisotropic environment. Therefore, structure refinement with dipolar couplings is performed as in one-domain proteins (Sect. 8.4). Several examples are reported in the literature of cases with conformational ambiguity due to the lack of NOE contacts between the domains. One example is the determination of subdomain orientation of the riboso-mal protein S4 z)41 [97]. In this work the lack of NOE contacts between the domains produces an ambiguity in interdomain orientation. The authors use two different anisotropic media to obtain dipolar couplings (DMPC/DHPC bicelles and Pfl filamentous bacteriophages). They conclude that subdomain orientation in solution is similar to the one present in the crystal structure. 

The first approach has successfully been applied to the study of amorphous as well as to macroscopically ordered solids. Examples of applications include the determination of backbone geometries in fibrous proteins [4] or the determination of protein-backbone, side-chain, and bound-ligand orientation with respect to the membrane normal in membrane-bound proteins [5-8]. Membranes, bilayers, bicelles, or liposomes are neither solid nor liquid systems but have aspects of both and are sometimes liquid crystalline. In most of these systems, time-independent anisotropic interactions play an important role,... 

---