Membranes rotational motions

The motional characteristics of interest are typically those governed by the phospholipid fatty acyl chains and head-group region and the neutral lipid or protein components of membranes. Rotational motion can be subdivided into a structural component, the order or degree of orientational constraint,... 

The H+ translocates through the membrane in three steps. First it adsorbs into a half channel of the membrane, then it connects with a proton acceptor site on the rotating Fi ring. In a rotation, the proton is back-donated to the empty other half-channel of the membrane, that transfers the proton to the other side of the membrane. Rotational motion will only occur when one half-channel is unoccupied by a proton and the other half is empty. 

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. 

Considering dynamic processes in fluid membranes as an example and starting from the fastest dynamical processes, one may first consider rotational diffusion of individual carbon-hydrogen bonds in CH2 groups in lipid hydrocarbon chains. The time scale of these rotational motions is on the order of picoseconds. The rotational motion of whole lipids around their principal axes of rotation is a slower process and usually takes place over a scale of nanoseconds. Lateral diffusion, in turn, involves diffusion of matter and hence longer time scales. This time scale is characterized by the diffusion length X = (Id D where d is the dimensionality, D is the... 

Experiments on bacteriorhodopsin (BR), which is the basis for a light-driven proton pump in halobacteria, were recently reported [96], The primary photoreaction is believed to be a trans-to-cis isomerization. Absorption of a 620-nm pulse by BR in membranes was followed by measurements of stimulated emission at various probe wavelengths between 695 and 930 nm. The rapid (ti 200 fs) decay of stimulated emission itensity at the bluer wavelengths, slower decay (T2 500 fs) at redder wavelengths, and biexponential decay at intermediate wavelengths were interpreted in terms of partially coherent rotational motion along the Sj potential surface. 

We and other workers have exploited the triplet lifetime to observe the motion of molecules too large to be studied by singlet techniques. Examples are the rotational motion of RNA polymerase (Austin et al., 1983), the rotational diffusion of membrane-bound components (Cherry et al., 1976 Vazetal., 1979 Jovinetal., 1981) and the motion of the DNA helix... 

The ESR spectrum of the aqueous solution is composed of three sharp peaks, which means that the correlation time Tc for the rotational motion of the nitroxide radical group is 6 x 10 s. This is determined from both the peak height and width. On the other hand, the ESR spectrum of the membrane in the dry state is very broad and the it value is determined as 2 x 10 " s from the maximum separation width. The ESR spectra of the membrane in the swollen state (right side of Fig. 32) are complex, indicating the pre.sence of several components from the viewpoint of the motion of the nitroxide radical group in the membrane. There are several kinds of microenvironment around the Tyr side-chain in membranes swollen by water. The relative intensities of the three sharp ESR peaks of the membrane in the swollen state decrease with increasing methanol treatment time. Thus, by assuming that the ESR spectrum consists of three components from the viewpoint of motion in the ESR time scale, the fraction of each component was determined by the nonlinear least squares method from a computer spectrum simulation (Fig. 33). 

The vestibular system in each ear consists of the otolith and saccule (coUectively by caUed the otolithic organs), which are the Hnear motion sensors, and the three semicircular canals (SCCs), which sense rotational motion. The SCCs are oriented in three nearly mutually perpendicular planes so that angular motion about any axis may be sensed. The otoliths and SCCs consist of membranous structures that are situated in hollowed-out sections and passageways in the vestibule of the temporal bone. This hollowed-out... 

With a gentle rotating motion, insert tube through membrane. 

ESR spectra of the rigid spin probe cholestane embedded in model membranes yield information about the molecular order (order parameters and and about the rate of motion of the probe molecules in the membranes (rotational diffusion constants D and Dj ). For a correct analysis of the ESR lineshapes in the slow motion regime (10 stochastic Liouville Equation (SLE) formalism. 

Ine rate of rotational motion can be used to deduce information on the size of the rotating unit , provided the dynamical friction opposina the motion is known. This we have previously achieved by measuring the rotational diffusion of EITC labelled lipid molecules in the membrane of azolectin liposomes (1]). 

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