Diffusion measurement membrane

R. J. Cherry, Measurement of protein rotational diffusion in membranes by flash photolysis, Methods Enzymol. L1X, 47-61 (1978). 

The acid-base chemistry of nicotine is now well known and investigations have shown that nicotine in tobacco smoke or in smokeless tobacco prodncts can exist in pH-dependent protonated or nnprotonated free-base forms. In tobacco smoke, only the free-base form can volatilize readily from the smoke particnlate matter to the gas phase, with rapid deposition in the respiratory tract. Using volatility-based analytical measurements, the fraction of nicotine present as the free-base form can be quantitatively determined. For smokeless tobacco products, the situation differs because the tobacco is placed directly in the oral cavity. Hence, the pH of smokeless tobacco prodncts can be measured directly to yield information on the fraction of nicotine available in the nnprotonated free-base form. It is important to characterize the fraction of total nicotine in its conjugate acid-base states as this dramatically affects nicotine bioavailability, because the protonated form is hydrophilic while the nnprotonated free-base form is lipophilic and thus readily diffuses across membranes (Armitage and Turner 1970 Schievelbein et al. 1973). As drug delivery rate and addiction potential are linked (Henningfield and Keenan 1993), increases in delivery rate due to increased free-base levels affect the addiction potential. 

Formulae (49) and (89) have been experimentally tested by some investigators. R. Neihof and K. Sollner (109) compared the transport ratios obtained from B.I.P. s with those obtained from diffusion measurements. In their experiments two counter-ions 1 and 2 occurred at the a side of the membrane, and at the m side there was a third counter-ion. In this case,... 

Diffusion with Electric Current The equations (64), (65), (66), (69) and (70) have been experimentally tested by P. Meares and H. H. Ussing (95) for a Zeo-Karb 315 membrane and NaCl solutions. wNa+ was determined from self-diffusion measurements. At an external concentration up to 0.02 N the flux of the anions could be neglected and ANa+ was directly found from the conductivity of the membrane. 

A system underpinned by commercially made screen-printed electrochemical cells was described by Palmisano et al. [19]. The cells were converted into biosensors for lactate in milk and yoghurt by addition of an electrochemically polymerised barrier to interference and a layer composed of lactate oxidase, glutaraldehyde and BSA. These steps appeared to have been carried out by hand. As there was no outer diffusion-limiting membrane, the linear range of the sensors was quite small (0-0.7 mM). They were incorporated into a FIA with a microdialysis unit based on a planar membrane and a buffer reservoir (earlier work used a microdialysis fibre with a platinum electrode [29]. The concentration of lactate was determined in various milks (0.27-1.64 mM), and in raw milk (c. 0.5-0.9 mM) left to degrade on the laboratory bench. The recovery of the microdialysis unit, 2.6%, implied that the sensor had an ability to return measurable currents for very low concentrations of lactate. A further implication is that the electro-polymerised layer was very effective at preventing interference. 

It is recognized that filtration is operational, that colloidal-bound PCB congeners are not retained by the filter, and that operational dissolved measurements may be biased positively by colloidal material. Techniques to measure truly dissolved PCBs include gas sparging, differential diffusion into membrane-bound lipids (e.g., semipermeable membrane devices, [230]), and selective adsorption (e.g. non-equilibrium solid phase microextraction [231, 232]). Unfortunately, none of these techniques has sufficient sensitivity to reliably and unambiguously measure truly dissolved PCB congeners at the levels present in the Great Lakes. 

One method to extend linear range and to perform measurements in whole blood is to use a further diffusion limiting membrane [55]. Although such an approach slows down the sensor response, measurements in serum and biological fluid are possible. 

Chen Y, Lagerholm BC, Yang B, Jacobson K. Methods to measure the lateral diffusion of membrane lipids and proteins. Methods 2006 39 147-153. 

In recent years, FRAP for diffusion measurements in membranes has been superseded by fluorescence correlation spectroscopy (FCS). FCS is very similar to FRAP in both theoretical and experimental approaches to the observation of diffusion. The difference between these two closely related techniques is that FRAP measures relaxation from an initial nonequilib-rium state after photobleaching, whereas FCS detects stochastic fluctuations that occur even in a system remaining in equilibrium (46). 

Fluorescence recovery after photobleaching (FRAP) for measuring lateral diffusion in membranes Section 12.6... 

Dodecyl phosphocholine micelles in solution are useful and well characterized as a model membrane system for solution NMR studies. To access the membrane-induced conformation and orientation of cardiotoxins, the interaction of the p-type cardiotoxin II from Naja oxiana snake venom with perdeuterated dodecyl phosphocholine was studied by H NMR spectroscopy and diffusion measurements.247 2D NMR is an efficient tool and has been widely used to study the interaction of dodecyl phosphocholine with peptides and protein.248-250 2D... 

It is convenient to distinguish between permeation measurements in which the flux is measured under a known (and constant) pressure gradient and those in which the flux of a component i is driven by a concentration difference between the membrane faces under a constant and equal total pressure at both sides (Wicke-Callenbach [3]). Either of these two main methods may be performed imder steady state or under transient conditions. Whether or not component fluxes cmd diffusivities measured with both methods give similar or different values depends on the conditions and on the type of the dominant diffusion mechanism. 

Use of liposomes to investigate membrane permeability (p. 334) Use of hydropathy plots to locate transmembrane helices fp. 34U) Fluorescence recovery after photnblcaching (FRAP) for measuring lateral diffusion in membranes (p. 342)... 

The measurement of lipophilicity value for all three families of 5-thio-carbopeptides is essential for their potential affinity effect to cell membrane and the level of penetration/diffusion through membrane. The detailed values of measured lipophilicity calculated as log P via molecular modeling will be published separately as collaborative work effort. These particular data are essential for preliminary biological screening for all thioglycomimetics for selective in vitro cell viability assays as reported by us earlier (27)... 

Measurements of the mobility of membrane-associated components, particularly those associating with the cytoplasmic side of the lipid bilayer and positioned to interact with the intracellular domain of the receptor, are few (Bruckert et al., 1992 Kwon and Neubig, 1992 Kwon, 1992). Measurements of the diffusivity of membrane-associated effectors (see Fig. 2), for example, will be important in determining the role of diffusion in receptor/effector coupling and signal transduction. 

R. E. Beck and J. S. Schultz. Hindrance of Solute Diffusion Within Membranes as Measured... 

---