Lateral diffusion rates

It should be emphasized here that the four major complexes of the electron transport chain operate quite independently in the inner mitochondrial membrane. Each is a multiprotein aggregate maintained by numerous strong associations between peptides of the complex, but there is no evidence that the complexes associate with one another in the membrane. Measurements of the lateral diffusion rates of the four complexes, of coenzyme Q, and of cytochrome c in the inner mitochondrial membrane show that the rates differ considerably, indicating that these complexes do not move together in the membrane. Kinetic studies with reconstituted systems show that electron transport does not operate by means of connected sets of the four complexes. 

Alkyl chain heterogeneities cause cell membrane bilayers to remain in the fluid state over a broad temperature range. This permits rapid lateral diffusion of membrane lipids and proteins within the plane of the bilayer. The lateral diffusion rate for an unconstrained phospholipid in a bilayer is of the order of 1 mm2 s 1 an integral membrane protein such as rhodopsin would diffuse 40nm2 s 1. 

The increase of the lateral diffusion rate with increasing temperature was used to estimate the activation energy for diffusion in the LC and GI phases. The temperature dependence of the correlation-time for molecular diffusion, Xd, can be formulated in terms of the activation energy E ) for the motion affecting Xd in an Arrhenius expression (t > = exp( a/R7 ))- Since D = a ldx ... 

FIGURE 11-17 Measurement of lateral diffusion rates of lipids by... 

Provided the components are completely miscible and hexagonally packed in a mixed film below a molar ratio of 0.25 of diacetylenic lipid, each of the 6 nearest neighbors of a polymerizable lipid molecule is a nonpolymerizable natural lipid. Due to the low lateral diffusion rate in the condensed phase diacetylene polymerization should either become impossible or at least proceed at a considerably lower rate. 

The authors reported that the hydrophobic peptide had little influence on the lipid structure, as lipid lateral diffusion rates, lipid conformations, and head group orientations were identical to a neat bilayer. The distance distribution of the phospholipid atoms surrounding the peptide was rather broad, pointing to the absence of special interactions between the peptide and the surrounding phospholipids. 

Even though some plasma membranes, such as nerve myelin membranes, contain a high concentration of lipids that form gel phase bilayers, the presence of cholesterol keeps these membranes in a fluid phase. However, interaction with the rigid cholesterol ring affects hydrocarbon chains of lipids in the liquid crystal phase (L ) and leads to formation of a new phase, the liquid ordered (Lq) phase (27). The phase is well characterized by a variety of physical methods and does not exist in pure lipids or their mixtures. In the liquid ordered phase, the long axis rotation and lateral diffusion rates are similar to the La phase, but the acyl chains are predominantly in an all-trans conformation and, hence, the order parameters are similar to the Lp phase (see Table 1). Recently, the cholesterol-rich Lq phase has been strongly associated with microdomains in live cells—the so-called lipid rafts. ... 

There is independent physical evidence for non-uniform distribution and restriction from transmembrane diffusion of a-Toc in lipid membranes. Differential scanning calorimetry results indicated that it partitioned into the most fluid domains in lipid vesicles. Fluorescence studies showed that a-Toc has a very high lateral diffusion rate in egg lecithin but it does not take part in transbilayer (flip-flop) migration even over many hours . It is not known if this behavior of a-Toc extends to natural biomembranes where actual structures and conditions may dramatically change migration phenomena. 

Among the dynamical properties the ones most frequently studied are the lateral diffusion coefficient for water motion parallel to the interface, re-orientational motion near the interface, and the residence time of water molecules near the interface. Occasionally the single particle dynamics is further analyzed on the basis of the spectral densities of motion. Benjamin studied the dynamics of ion transfer across liquid/liquid interfaces and calculated the parameters of a kinetic model for these processes [10]. Reaction rate constants for electron transfer reactions were also derived for electron transfer reactions [11-19]. More recently, systematic studies were performed concerning water and ion transport through cylindrical pores [20-24] and water mobility in disordered polymers [25,26]. 

Just how fast can proteins move in a biological membrane Many membrane proteins can move laterally across a membrane at a rate of a few microns per minute. On the other hand, some integral membrane proteins are much more restricted in their lateral movement, with diffusion rates of about 10 nm/sec or even slower. These latter proteins are often found to be anchored to the cytoskeleton (Chapter 17), a complex latticelike structure that maintains the cell s shape and assists in the controlled movement of various substances through the ceil. 

The transition from non-protective internal oxidation to the formation of a protective external alumina layer on nickel aluminium alloys at 1 000-1 300°C was studied by Hindam and Smeltzer . Addition of 2% A1 led to an increase in the oxidation rate compared with pure nickel, and the development of a duplex scale of aluminium-doped nickel oxide and the nickel aluminate spinel with rod-like internal oxide of alumina. During the early stages of oxidation of a 6% A1 alloy somewhat irreproducible behaviour was observed while the a-alumina layer developed by the coalescence of the rodlike internal precipitates and lateral diffusion of aluminium. At a lower temperature (800°C) Stott and Wood observed that the rate of oxidation was reduced by the addition of 0-5-4% A1 which they attributed to the blocking action of internal precipitates accumulating at the scale/alloy interface. At higher temperatures up to 1 200°C, however, an increase in the oxidation rate was observed due to aluminium doping of the nickel oxide and the inability to establish a healing layer of alumina. 

Heterogeneous recombination of active particles and their interaction with molecules of the adlayer are simplest processes of this type. The rates of such reactions as functions of surface coverage by the specified reagents are fully determined by the rate of their surface diffusion towards active centers. In a number of cases, the rate of lateral diffusion is determined not only by the type of diffusing particle, but also (sometimes, predominantly) by the composition and state of the solid substrate surface. Taking into account the role played by the composi-... 

As the working temperature of the substrate was increased, the induction period (the delay time) of increased conductivity decreased due to increased rate of lateral diffusion of hydrogen atoms towards the sensor. The activation energy for surface migration of particles along a Si02 substrate estimated from the tilt of the Arrhenius plot was found to be about 20 kj/mol. 

It is also possible for solute species to diffuse laterally (in a radial direction) across the column, and thus to move from one flow path into another. This effect reduces the amount of dispersion produced by the multiple path effect as it tends to equalise the speed of the solute species in the column. The longer the time a solute species spends in the column, the more lateral diffusion will occur, so that flow dispersion is reduced by using low flow rates of mobile phase. 

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