Phospholipids proton diffusion

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. 

S., Dynamic studies of proton diffusion in mesoscopic heterogeneous matrix. The interbilayer space between phospholipids membranes. Biophys. J., 1992, 63, 281-290. 

Zhang, J., Unwin, P.R., Proton diffusion at phospholipid assemblies,... 

The manner in which protons diffuse is a reflection of the physical properties of the environment, the geometry of the diffusion space, and the chemical composition of the surface that defines the reaction space. The biomembrane, with heterogeneous surface composition and dielectric discontinuity normal to the surface, markedly alters the dynamics of proton transfer reactions that proceed close to its surface. Time-resolved measurements of fast, diffusion-controlled reactions of protons with chromophores and fluorophores allow us to gauge the physical, chemical, and geometric characteristics of thin water layers enclosed between phospholipid membranes. Combination of the experimental methodology and the mathematical formalism for analysis renders this procedure an accurate tool for evaluating the properties of the special environment of the water-membrane interface, where the proton-coupled energy transformation takes place. 

Permeabilization of polymer membranes is increased by ethanol or mixtures of specific organic solvents. For example, a small amount (< 0.5% v/v) of ethanol [92] or mixtures of ethanol and dimethyl sulfoxide (1 1 v/v) [90] in the system has no influence on the membrane permeability of PMOXA-fc-PDMS-fc-PMOXA copolymers, and therefore much higher amounts of solvents are necessary to destabilize the membrane. However, this permeabilization method is not appropriate when bioapplications are intended. Likewise, a copolymer membrane based on poly(ethylene glycol)-fcZock-poly(D,L-laetic-5to/-glycolic acid) (PEG-fc-PLGA), which exhibits 18-fold less permeability to protons than a liposome membrane (based on a mixture of soybean phospholipids and cholesterol) was rendered more permeable for proton diffiision when increasing amounts of 1,4-dioxane (5-20%) were added (Fig. 11.7) [93]. Upon removal of the 1,4-dioxane by dialysis, the proton diffusion returned to... 

An SECM proton feedback method has also been developed to investigate lateral proton diffusion at phospholipid assemblies, specifically, Langmuir monolayers at the water-air interface [14],... 

The intestinal absorption of dietary cholesterol esters occurs only after hydrolysis by sterol esterase steryl-ester acylhydrolase (cholesterol esterase, EC 3.1.1.13) in the presence of taurocholate [113][114], This enzyme is synthesized and secreted by the pancreas. The free cholesterol so produced then diffuses through the lumen to the plasma membrane of the intestinal epithelial cells, where it is re-esterified. The resulting cholesterol esters are then transported into the intestinal lymph [115]. The mechanism of cholesterol reesterification remained unclear until it was shown that cholesterol esterase EC 3.1.1.13 has both bile-salt-independent and bile-salt-dependent cholesterol ester synthetic activities, and that it may catalyze the net synthesis of cholesterol esters under physiological conditions [116-118], It seems that cholesterol esterase can switch between hydrolytic and synthetic activities, controlled by the bile salt and/or proton concentration in the enzyme s microenvironment. Cholesterol esterase is also found in other tissues, e.g., in the liver and testis [119][120], The enzyme is able to catalyze the hydrolysis of acylglycerols and phospholipids at the micellar interface, but also to act as a cholesterol transfer protein in phospholipid vesicles independently of esterase activity [121],... 

It also combines very rapidly with superoxide anion radical to form peroxynitrite (Eq. 18-62).513 This is another reactive oxidant which, because of its relatively high pfCa of 6.8, is partially protonated and able to diffuse through phospholipids within cells.514 515... 

The fact that uncouplers are lipophilic weak acids (see above) explains their ability to collapse transmembrane pH gradients. Their lipophilic character allows uncouplers to diffuse relatively freely through the phospholipid bilayer. Because they are weak acids, uncouplers can release a proton to the solution on one side of the membrane and then diffuse across the membrane to fetch another proton. The chemiosmotic theory thus provides a simple explanation of the effects of uncouplers on oxidative phosphorylation. 

Figure 4. Schematic presentation of the reaction space for proton-excited pyranine anion recombination in the thin water layer between phospholipid membranes of multilamellar vesicles. The proton release is depicted at the center of the layer and diffuses in concentric shells. When the diffusion radius exceeds the distance to the membrane (dw/2), the shape of the diffusion space deviates from spherical symmetry and approaches cylindrical symmetry. R0 is the reaction radius, R is the unscreened Debye radius of pyranine (R d = 28.3 A ). in this scheme is 30 A, and the size of the water molecules is drawn to...

Finally, structural investigations of a human calcitonin-derived carrier peptide in a membrane enviromnent by solid-state NMR have been reported. The typical axially symmetric powder patterns of NMR spectra were used to confirm the presence of lamellar bilayers in the samples studied. The chemical shift anisotropy of the NMR spectra was monitored in order to reveal weak interaction of the peptide with the lipid headgroups. In addition, paramagnetic enhancement of relaxation rates and NMR order parameters of the phospholipid fatty acid chains in the absence and presence of the carrier peptide were measured. All peptide signals were resolved and fully assigned in 2D proton-driven spin diffusion experiments. The isotropic chemical shifts of CO, C and provided information about the secondary structure of the carrier peptide. In addition, dipolar eoupling measurements indicated rather high amplitudes of motion of the peptide. 

In the present concept lipids participate in at least three additional functions. The first is the establishment of a proton-conductive, water-cation monolayer discussed in Section III, C, I. The second, closely related to the first, is the drawing of cations to the water-cation monolayer the influence of fixed positive-charges on cation movements has been discussed in Section IV, B, 2. Last, but not least, phospholipids provide the necessary ligands for divalent cation bridges between membrane subunits. This role is important, not only to membrane cohesion, but to all forms of transport with the possible exception of simple diffusion (Section IV, B, 2). 

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