DPPC, diffusion

Table 3 Diffusion Constants and Rotational CoiTelation Tunes of Water Molecules from an MD Simulation of a Fully Flydrated Fluid Phase DPPC Bilayer ...

Figure 31. Diffusion coefficient of water in a DPPC bilayer as a function of position. Results of MD simulations of Marrink and Berendsen [129]. Redrawn by permission of the American Chemical Society...

Figure 19 shows the pressure effeets on the lateral self diffusion eoeffieient of sonicated DPPC and POPC vesicles. The lateral diffusion coefficient of DPPC in the LC phase decreases with increasing pressure from 1 to 300 bar at 50 °C. A sharp decrease in the D-value occurs at the LC to GI phase transition pressure. From 500 bar to 800 bar in the GI phase, the values of the lateral diffusion coefficient 1 x 10 cm /s) are approximately constant. There is another sharp decrease in the value of the lateral diffusion coefficient at the... 

Fig. 19. Lateral self-diffusion constant D of DPPC (top) and POPC (bottom) in sonicated vesicles as a function of pressure at 50 °C and 35 °C, respectively (after Ref 62).

There is an abrupt decrease in the lateral diffusion coefficient of DPPC upon the phase transition from the GI phase to the Gi phase. This is because the acyl-chain region is being packed even more efficiently in the Gi phase than in the GI phase, and the hydrocarbon volume in the Gi phase is smaller than in the GI phase. Also, in the Gi phase, the lipid acyl-chains from the opposing bilayer leaflets interdigitate. In order for a phospholipid molecule to diffuse it has to circumvent the nearby interdigitated molecules which hinder diffusion. 

The change in the two-state distribution is easily monitored by a convenient one-wavelength measurement of the neutral form fluorescence, and this can be used for probing the membrane. The fairly large differences in wavelengths of excitation (300 nm), fluorescence of the neutral form (360 nm), and fluorescence of the anion form (480 nm) makes the fluorescence free from spectral interference. The variation of the P form fluorescence intensity with temperature showed a maximum at phase-transition temperatures (Tc) for both DMPC (23°C) (Fig. 2) and DPPC (42°C) membranes (Fig. 3). Figures 2 and 3 show a very nice correspondence of this variation with DPH fluorescence polarization and self-diffusion rate [93] of 22Na+. The coexistence of solid gel and fluid liquid-crystalline phases at Tc and the consequent imperfection of the membrane [93] result in a redistribution of... 

Figure 3 Variation of P form fluorescence intensity of 1-naphthol ( - - ) and 22Na+ selfdiffusion rate (A-A-A) with temperature in DPPC liposome membrane. (The curve for 22Na+ self-diffusion rate has been adapted from Ref. 93 with permission from Elsevier Science. Figure reprinted with permission from Ref. 93a. Copyright 1998 American Chemical Society.)...

Pyranine has been used to study the proton dissociation and diffusion dynamics in the aqueous layer of multilamellar phospholipid vesicles [101], There are 3-10 water layers interspacing between the phospholipid membranes of a multilamellar vesicle, and their width gets adjusted by osmotic pressure [102], Pyranine dissolved in these thin layers of DPPC and DPPC+cholesterol multilamellar vesicles were used as a probe for the study. Before the photoreleased proton escapes from the coulombic cage, the probability of a proton excited-anion recombination was found to be higher than in bulk. This was attributed to the diminished water activity in the thin layer. It was found that the effect of local forces on proton diffusion at the timescale of physiological processes is negligible. 

The influence of a cut-off relative to the full treatment of electrostatic interactions by Ewald summation on various water parameters has been investigated by Feller et al. [33], These authors performed simulations of pure water and water-DPPC bilayers and also compared the effect of different truncation methods. In the simulations with Ewald summation, the water polarization profiles were in excellent agreement with experimental values from determinations of the hydration force, while they were significantly higher when a cut-off was employed. In addition, the calculated electrostatic potential profile across the bilayer was in much better agreement with experimental values in case of infinite cut-off. However, the values of surface tension and diffusion coefficient of pure water deviated from experiment in the simulations with Ewald summation, pointing out the necessity to reparameterize the water model for use with Ewald summation. 

Fig. 6.9 Calculated diffusion rate profiles of water, ammonia. and oxygen across the DPPC bilayer.

Fig. 3.113. Hysteresis curves of the diffusion coefficient D of surface adsorbed 5-N-(octadecanoyl)aminofluorescein in common black foam films stabilised by phospholipids (a) - DMPC (b) -DPPC (c) - DPPA r = 200 pm arrows on the lines connecting data points indicate the direction of the temperature change [494],...

We have also conducted a detailed study of the interactions of DMSO with DPPC bilayers (Faller et al., 2003). We have found fhat DMSO is able to swell the bilayers considerably and that it diffuses almost unhindered across the membrane. The nature of the swelling and the diffusion, however, are sfrongly temperature dependent at low temperatures, DMSO exhibits a hydrophilic nature and it is excluded from fhe inferior of fhe bilayer. At intermediate to high temperatures (e.g., 50°C), it exhibits a hydrophobic character and it penetrates the bilayers much more readily. This finding helps explain why DMSO is more toxic at intermediate-to-high temperatures than at low temperatures, and why many empirical cell-freezing protocols advocate adding DMSO only after the cells have been cooled to sub-ambient temperatures. 

Very little is known about the motions of lipid bilayers at elevated pressures. Of particular interest would be the effect of pressure on lateral diffusion, which is related to biological functions such as electron transport and some hormone-receptor interactions. Pressure effects on lateral diffusion of pme lipid molecules and of other membrane components have yet to be carefully studied, however. Figure 9 shows the pressure effects on the lateral self diffusion coefficient of sonicated DPPC and POPC vesicles [86]. The lateral diffusion coefficient of DPPC in the liquid-crystalline (LC) phase decreases, almost exponentially, with increasing pressure from 1 to 300 bar at 50 °C. A sharp decrease in the D-value occurs at the LC to GI phase transition pressure. From 500 bar to 800 bar in the GI phase, the values of the lateral diffusion coefficient ( IT0 cm s ) are approximately constant. There is another sharp decrease in the value of the lateral diffusion coefficient at the GI-Gi phase transition pressure. In the Gi phase, the values of the lateral diffusion coefficient ( 1-10"" cm s ) are again approximately constant. 

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