Lateral diffusion coefficient, single molecule

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]. 

The above data suggest that a crosslinked bilayer vesicle is essentially a single polymer molecule (really two, one in each half of the bilayer). In other words the polymerization of the lipid monomers exceeded a gel-point. This concept raises the question of what mole fraction of bis-substituted lipid is necessary to achieve a gel-point for a bilayer composed of a crosslinker lipid, i.e. bis-lipid, and a mono-substituted lipid. Approximately 30% of the lipids in a bilayer vesicle of SorbPCs must be bis-SorbPC (4) in order to produce a polymerized vesicle that could not be dissolved by detergent or organic solvent [29], A complementary study of Kolchens et al. found that the lateral diffusion coefficient, D, of a small nonreactive lipid probe in a polymerized bilayer of mono- and bis-AcrylPC was dramatically reduced when the mole fraction of the bis-AcrylPC, was increased from 0.3 to 0.4 [24]. The decreased freedom of motion of the probe molecule indicates the onset of a crosslinked bilayer in a manner consistent with a 2-dimensional gel-point. 

TABLE 10.1. Effects of surfactants on the lateral diffusion coefficient (Di) of single Dil molecules, apparent viscosity (rji) and intrinsic viscosity (jji) at the dodecane/water interface. 

Fig. 13 Single molecule tracking data of dye-labeled PMOx lipopolymers as a function of area per molecule. The plots of the lateral diffusion coefficient, D, vs area per molecule for DiCisPMOx3o and DiCisPMOxso show two different diffusion regions (labeled I and II). Unlike in Region II, D follows Rouse scaling in Region I [31] (reproduced with permission from the American Chemical Society)...

Fig. 23 Lateral diffusion coefficient D as a function of Aiipo using FRAP and single molecule fluorescent microscopy methods [39] (reproduced with permission from the American Chemical Society)...

The most general water transport mechanism is diffusion through lipid bilayers, with a permeability coefficient of 2-5 x 10 4 cm/sec. The diffusion through lipid bilayers depends on lipid structure and the presence of sterol (Subczyhski et al., 1994). It is suggested that the lateral diffusion of the lipid molecules and the water diffusion through the membrane is a single process (Haines, 1994). 

The behavior of a pseudo-gel solution is quite different from the polymer solution from which it is formed. The diffusion coefficient of a pseudo-gel is much smaller than that of the original polymer, and the viscosity of the pseudo-gel solution will be much larger than that of the original polymer solution. The pseudo-gel, in theory, will be compressed closer to the cold wall and will elute out of the channel later than the parent molecules. However, as the size of the pseudo-gel cluster increases, hydrodynamic effects will result in an earlier emergence from the chan-nel. If either of these scenarios occurs in the ThFFF channel, double peaks might be observed for a sample of a single peak without overloading. ... 

Single particle tracking (SPT) is a highly sensitive approach for studying the motion of membrane molecules. This chapter describes the use of nanometer-sized quantum dots (QDs) in single fluorophore optical imaging. QD-based SPT permits to follow molecules over extended time periods and to obtain information about the lateral diffusion of a particle of interest its diffusion coefficient, confinement, residency time in specific submembrane regions. This technique has been used successfidly in cultured neurons to follow the membrane diffusion of, i.e., individual ionotropic and metabotropic receptors, ion channels, ion transporters, neurotransmitter transporters, aquaporins, lipid raft markers, and adhesion molecules. 

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