Alkyl chain motion

Beaufils, J.P., Heimion, M.C., and Rosset, R., Neutron scattering study of alkyl chain motion on reversed phase liquid chromatographic packings. Anal. Chem., 57, 2593, 1985. 

Evidence for polymorphism and possible mesomorphic phases, but not for liquid crystallinity, has been found in studies on methyl, ethyl and propyl stearate (i.e. molecules in which the length of the acid part is maintained at IS carbon atoms and the length of the alcohol part is shorter than in BS) [43,52-55]. We hypothesize that enhanced rotation of an alkoxy group of moderate length in these carboxy-inserted alkanes (perhaps in combination with dipolar effects) includes sufficient disorder within molecular layers to transform the phases from condis to liquid crystals embedding the carboxy group near the middle of an alkane (as with DD) serves to attenuate alkyl chain motions near a layer interface and thus precludes the formation of liquid crystalline phases. These qualitative assertions can and should be tested experimentally using a wider variety of molecules than have been examined thus far. 

The structure of these globular aggregates is characterized by a micellar core formed by the hydrophilic heads of the surfactant molecules and a surrounding hydrophobic layer constituted by their opportunely arranged alkyl chains whereas their dynamics are characterized by conformational motions of heads and alkyl chains, frequent exchange of surfactant monomers between bulk solvent and micelle, and structural collapse of the aggregate leading to its dissolution, and vice versa [2-7]. 

The most likely way for pardaxin molecules to insert across the membrane in an antiparallel manner is for them to form antiparallel aggregates on the membrane surface that then insert across the membrane. We developed a "raft"model (data not shown) that is similar to the channel model except that adjacent dimers are related to each other by a linear translation instead of a 60 rotation about a channel axis. All of the large hydrophobic side chains of the C-helices are on one side of the "raft" and all hydrophilic side chains are on the other side. We postulate that these "rafts" displace the lipid molecules on one side of the bilayer. When two or more "rafts" meet they can insert across the membrane to form a channel in a way that never exposes the hydrophilic side chains to the lipid alkyl chains. The conformational change from the "raft" to the channel structure primarily involves a pivoting motion about the "ridge" of side chains formed by Thr-17, Ala-21, Ala-25, and Ser-29. These small side chains present few steric barriers for the postulated conformational change. 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

Linear guest molecules are included along these canals in an extended planar zig-zag conformation. Branched molecules are generally excluded unless branching occurs near the end of a long linear chain, but aromatic derivatives can be included if they have a long alkyl chain 38). The review article by Takemoto and Sonoda 21) contains an excellent survey of the types of molecules known to form urea inclusion compounds and of the means used to study their detailed conformations and thermal motion. 

In the membrane lipid alkyl chains of n-SASL and n-PC spin labels undergo rapid rotational motion about the long axis of the spin label and wobble within the confines of a cone imposed by the... 

We would like to point out that an order parameter indicates the static property of the lipid bilayer, whereas the rotational motion, the oxygen transport parameter (Section 4.1), and the chain bending (Section 4.4) characterize membrane dynamics (membrane fluidity) that report on rotational diffusion of alkyl chains, translational diffusion of oxygen molecules, and frequency of alkyl chain bending, respectively. The EPR spin-labeling approach also makes it possible to monitor another bulk property of lipid bilayer membranes, namely local membrane hydrophobicity. 

A lovely recent result is the confinement of a carborane derivative bearing two long alkyl chains within a SWCNT, Figure 15.36.59 The vapourised carborane was allowed to diffuse into the nanotube in a vacuum. Using TEM the molecular motions of the included molecule can be followed in breathtaking... 

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