Headgroups rotational motion

Rotational Motion of the Chains (Wobbling Motion of the Chains) and of the Headgroup... 

Table 9.1 A comparison of dynamical properties of lipids. Relaxation times of the wobbling motion of the hydrophobic chain and the rotational motion of the headgroup (PN vector) and lateral diffusion coefficient, D.

The lipids themselves are highly mobile. Steady state and time resolved spectroscopy (absorption, emission, ir, raman, nmr, epr) and anisotropy measurements have revealed rotational, vibration and segmental motions of the headgroups and the hydrocarbon tails of the lipids. Translocation of a lipid from one half of the bilayer to the other, ("flip-flop ) as well as intermembrane... 

The low-temperature gel phase corresponds closely to that of the crystalline dihydrate and is thus denoted the Lc/ phase. The detailed stmcture of this phase and the other phases discussed below is treated in detail elsewhere (see article Lipids, Phase Transitions of). The Lg/ phase is characterized by extended hydrocarbon chains that are tilted slightly with respect to the bilayer normal. These chains are packed very tightly, and rotation about their long axes is very severely restricted. The polar headgroup contains only a few bound water molecules, and its motion is also severely restricted. 

An important aspect of simulations of lipid bilayers systems is the timescale on which different motions occur. The fastest motions are small local rearrangements around bonds and the motion of water molecules (picoseconds to tens of picoseconds). Rotational correlation times for tail dihedrals are of the order of tens to hundreds of picoseconds (see Time Correlation Functions). The dihedral rotational correlation times for certain dihedrals in the headgroup is of the order of hundreds of picoseconds. Important motions like the rotation and lateral diffusion of entire lipids occur on a nanosecond scale and it is therefore not possible to sample these motions adequately within the 0.5-2 ns simulations that are usual in the current practice. This is an important consideration in creating a starting structure for a simulation. Although the precise distribution of tail dihedrals will equilibrate reasonably fast, there will remain a strong correlation of the orientation of entire lipids with the starting structure in simulations of less than several nanoseconds. 

Section II, A deals with the spectra of systems with very slow or very fast rotational rates. In the intermediate region (for diffusion coefficients 10 < / < 10 s and the spectral widths usually encountered with membranes), the 3 P-NMR spectra are sensitive to both type and rate of the motion. Although the lipids of liquid crystalline membranes usually have motional rates in the fast-limit region, this is often not so at lower temperatures (Fig. 9). In such cases, the spectra can be used as a source of information about the nature of the motion and orientation of the headgroup. 

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