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PubMed1986

The diffusion in vitro of fluoride and chlorhexidine in the enamel of human deciduous and permanent teeth.

The permeability of human dental enamel was studied by following the diffusion of [51Cr]-ethylenediaminetetraacetate (EDTA), F and [14C]-chlorhexidine using two-chamber diffusion cells. The [51Cr]-EDTA served as a marker to control the change in enamel permeability during the diffusion process. An average increase in enamel permeability of about 1.6-fold was recorded following the initial diffusion of the test compounds in the deciduous and permanent enamel. The permeability of deciduous enamel was much higher than that of permanent enamel. For [51Cr]-EDTA and [14C]-chlorhexidine, the average diffusion coefficient was about 30-fold more than in permanent enamel; for F it was 150-fold more. The difference was statistically significant (p less than 0.001). Co-administration of F and [14C]-chlorhexidine showed a higher diffusion rate for each compound than when separately diffused. Whether this is due to a synergetic effect or to increased enamel permeability following the initial diffusion of the compound, or both, is still uncertain.

Adolescent↗

PubMed1995

Rapid diffusion of the lipid phosphorus of phosphatidylglycerol liposomes through polycarbonate membranes is caused by the oxidation of the unsaturated fatty acids.

The lipid phosphorus of phosphatidylglycerol liposomes was found to diffuse extensively, after a lag time of 1 to 2 days, through a 0.1 micron pore size polycarbonate membrane in a two compartment system. Diffusion occurred when either multilamellar or large unilamellar vesicles were studied, even if they were sedimented to eliminate any smaller particles. The lipid of liposomes prepared under sterile conditions also diffused extensively. Diffusion appeared to be related to the age of the vesicles, and could be eliminated by incorporating antioxidants into the liposomes, or by using liposomes prepared from saturated phospholipids (C14 or larger). This indicated that diffusion was caused by phospholipid oxidation, which was confirmed by HPLC analysis. Phospholipid phosphorus that diffused through a membrane appeared more polar, as indicated by its capacity to distribute into the upper phase of a two phase extraction. Phospholipid phosphorus diffusion was preceded by the complete loss of liposomes contents, indicated by the complete diffusion of encapsulated carboxyfluorescein through the membrane. Oxidation of the lipid could be prevented by inclusion of either butylated hydroxytoluene or alpha-tocopherol in the membrane. The best retention of liposomal contents was achieved when both antioxidants and cholesterol were included in the liposome preparation. The antioxidant incorporated in the liposomes remained effective in protecting the phospholipids upon storage at 4 degrees C for 2 months. The inclusion of EDTA in the suspension medium retarded the rapid oxidation, suggesting that the presence of trace amounts of heavy metal ions in the buffer catalyzed the oxidation. Phospholipid oxidation was most effectively inhibited by the presence of serum or chemically defined medium, suggesting that oxidation of liposomal lipids in a biological environment may be minimized if appropriate steps are taken.

Antioxidants↗

PubMed1977

Fatty acids diffusion in lecithin multilayers: hydration and PH effects.

The diffusion of the sodium salt of monocarboxylic fatty acids, from formate to stearate, has been studied as a function of water content and pH in lecithin--water lamellar phases. Evolution of the diffusion coefficients with increasing chain length reflects the different localizations of fatty acids in the system. From formate to butyrate, which are mainly restricted to the hydrophilic layer of the phase, diffusion rates decrease rapidly. From butyrate to stearate, fatty acids (anchored at the hydrophilic--lipophilic interface) undergo lateral diffusion and then the decrease of D with increasing chain length is much slower. The diffusion of stereate is already comparable to the diffusion of the lecithin molecule itself. The diffusion rates strongly depend upon phase hydration and pH: it is shown that both parameters control the fatty acid ionization. The variations in diffusion rates observed may be ascribed to the fact that, depending upon their state of ionization, fatty acids assume a different localization and therefore experience different interactions in the lamellar system.

Biological Transport↗

PubMed1995

High-pressure proton NMR study of lateral self-diffusion of phosphatidylcholines in sonicated unilamellar vesicles.

Effects of pressure on the lateral diffusion of phospholipid molecules in sonicated pure 1,2-dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles (15 wt%) in D2O were examined using the high-pressure proton NMR rotating frame spin-lattice relaxation time (T1rho) method. Proton T1rho were measured at pressures from 1 bar to 5000 bar and at temperatures of 50 degrees C to 70 degrees C for DPPC and 5 degrees C to 35 degrees C for POPC. The T(-1)1rho values were plotted as a function of the square root of the spin-locking field angular frequency (omega1(1/2) and the lateral diffusion coefficient (D) calculated from the slope. Pressure effects on lateral diffusion were observed in the liquid-crystalline (LC) phase. The lateral diffusion coefficient exhibited sharp decreases in response to the various pressure-induced phase transitions encountered. However, pressure had little, if any, effect on lateral diffusion in the pressure-induced gel I (GI) phase and pressure-induced interdigitated gel (Gi) phase. The activation volumes for diffusion were calculated from the slopes from plots of In D versus pressure for both DPPC (37 ml/mol at 50 degrees C, 34 ml/mol at 60 degrees C and 25 ml/mol at 70 degrees C) and POPC (16 ml/mol at 5 degrees C, 9 ml/mol at 20 degrees C and 6 ml/mol at 35 degrees C) sonicated vesicles in the LC phase. The activation energy for diffusion (Ea) was calculated using the slopes from plots of In D versus the inverse of the temperature (1/T) for both DPPC and POPC in the LC phase (3.5 kcal/mol and 3.9 kcal/mol, respectively) and for both DPPC and POPC in the GI phase (6.0 kcal/mol and 4.4 kcal/mol, respectively). From the lateral diffusion coefficient and line width data pressure-temperature phase diagrams for sonicated pure DPPC and POPC vesicles were constructed. The values of the temperature to pressure equivalence of DPPC (dTm/dP) were estimated to be 22.1 degrees C/kbar for the LC to GI phase transition and 28.6 degrees C/kbar for the GI to Gi phase transition. The value of the temperature to pressure equivalence of POPC for the LC to GI phase transition was estimated to be 19.0 degrees C/kbar.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1983

Effect of diffusion boundary layers on the initial uptake of O2 by red cells. Theory versus experiment.

We have applied theories of mass transfer, in laminar and turbulent flow, to red cells in the stopped-flow apparatus in order to estimate the effect of extracellular diffusion boundary layers on the initial rate of O2 uptake. We compared the theoretical predictions with the results of our previous stopped-flow experiments with suspensions of red cells to which bovine serum albumin (BSA) had been added to decrease O2 solubility and diffusivity (Huxley and Kutchai, 1981). Models of red cells in laminar flow (Friedlander, 1957, 1961; Harriott, 1962) predict a significant retardation of the rate of O2 entry into red cells by diffusion boundary layers. Mixing in the stopped-flow apparatus occurs by convective and turbulent mechanisms in addition to simple molecular diffusion. The more complex theory of mass transfer to particles in turbulent flow (Levich, 1962) shows, because the red cells are highly entrained in the flow, that mixing is not complete on the time scale of initial O2 uptake. The O2 permeability of the diffusion boundary layer, predicted by both laminar and turbulent flow theories, approximates the experimentally obtained value. The theories of mass transfer to particles in laminar flow predict the dependence of the rate of O2 uptake on red cell size observed by other investigators. This suggests that these experimental results may be primarily due to the effect of diffusion boundary layers. Our studies are consistent with the interpretation that red cells in rapid mixing devices may be mixed incompletely with the suspending fluid; thus O2 transfer is limited by molecular diffusion in the immediate vicinity of the erythrocyte. We conclude that the rates of respiratory gas exchange by red cells in flowing systems may be partly limited by diffusion boundary layers.

Biophysical Phenomena↗

PubMed2005

Interactions between n-octyl and n-nonyl beta-D-glucosides and alpha- and beta-cyclodextrins as seen by self-diffusion NMR.

In this work (1)H NMR self-diffusion experiments have been performed to determine the self-diffusion coefficients of n-octyl beta-d-glucoside and n-nonyl beta-d-glucoside in alpha-cyclodextrin and beta-cyclodextrin solutions at 25 degrees C. Two questions are addressed. The first concerns the general influence on nonionic surfactant transport properties when cyclodextrins are present in solution. The second question concerns the influence of surfactant-chain length and cyclodextrin cavity volume on the association constant. The self-diffusion coefficients of the alkyl glucosides, in cyclodextrin-containing solutions, depend on the cyclodextrin concentration on account of the formation of complexes. The cyclodextrin diffusion is only mildly influenced, since the complex has similar diffusion coefficients as the free cyclodextrin. There are some obstruction effects at the highest surfactant concentrations which decrease the cyclodextrin diffusion. A set of equations to model the self-diffusion coefficients of the surfactant and cyclodextrin was developed and is presented. On the basis of such equations, properties such as association constants, and micelle and complex diffusion coefficients can be estimated.

Diffusion↗

PubMed2004

Three-dimensional diffusion of non-sorbing species in porous sandstone: computer simulation based on X-ray microtomography using synchrotron radiation.

The diffusion pathways of porous sandstone were examined by a three-dimensional (3-D) imaging technique based on X-ray computed tomography (CT) using the SPring-8 (Super Photon ring-8 GeV, Hyogo, Japan) synchrotron radiation facility. The analysis was undertaken to develop better understanding of the diffusion pathways in natural rock as a key factor in clarifying the detailed mechanism of the diffusion of radionuclides and water molecules through the pore spaces of natural barriers in underground nuclear waste disposal facilities. A cylindrical sample (diameter 4 mm, length 6 mm) of sandstone (porosity 0.14) was imaged to obtain a 3-D image set of 450(3) voxels=2.62(3) mm(3). Through cluster-labeling analysis of the 3-D image set, it was revealed that 89% of the pore space forms a single large pore-cluster responsible for macroscopic diffusive transport, while only 11% of the pore space is made up of isolated pores that are not involved in long-range diffusive transport. Computer simulations of the 3-D diffusion of non-sorbing random walkers in the largest pore cluster were performed to calculate the surface-to-volume ratio of the pore, tortuosity (diffusion coefficient in free space divided by that in porous rock). The results showed that (i) the simulated surface-to-volume ratio is about 60% of the results obtained by conventional pulsed-field-gradient proton nuclear magnetic resonance (NMR) laboratory experiments and (ii) the simulated tortuosity is five to seven times larger than the results of laboratory diffusion experiments using non-sorbing I(-) and Br(-). These discrepancies are probably attributed to the intrinsic sample heterogeneity and limited spatial resolution of the CT system. The permeability was also estimated based on the NMR diffusometry theory using the results of the random walk simulations via the Kozeny-Carman equation. The estimated permeability involved an error of about 20% compared with the permeability measured by the conventional method, suggesting that the diffusometry-based NMR well logging with gradient coils is applicable to the in-situ permeability measurement of strata. The present study demonstrated that X-ray CT using synchrotron radiation is a powerful tool for obtaining 3-D pore structure images without the beam-hardening artifacts inevitable in conventional CT using X-ray tubes.

Absorption↗

PubMed2003

Determination of water self-diffusion coefficient in complex food products by low field 1H PFG-NMR: comparison between the standard spin-echo sequence and the T1-weighted spin-echo sequence.

In 1990, Van Den Enden et al. proposed a method for the determination of water droplet size distributions in emulsions using a pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) T1-weighted stimulated-echo technique. This paper describes both the T1-weighted spin-echo sequence, an improved method based on this earlier work, and, the standard PFG spin-echo sequence. These two methods were compared for water self-diffusion coefficient measurement in the fatty protein concentrate sample used as a 'cheese model.' The transversal and longitudinal relaxation parameters T1 and T2 were determined according to the temperature and investigated for each sample; fat-free protein concentrate sample, pure anhydrous milk fat, and fatty protein concentrate sample. The water self-diffusion in fat-free protein concentrate samples followed a linear behavior. Consequently, the water self-diffusion coefficient could be easily characterized for fat-free protein concentrate samples. However, it seemed more complicated to obtain accurate water self-diffusion in fatty protein concentrate samples since the diffusion-attenuation data were fitted by a bi-exponential function. This paper demonstrates that the implementation of the T1-weighted spin-echo sequence, using the different T1 properties of water and fat phases, allows the accurate determination of water self-diffusion coefficient in a food product. To minimize the contribution of the 1H nuclei in the fat phase on the NMR echo signal, the fat protons were selectively eliminated by an additional 180 degrees pulse. This new method reduces the standard errors of diffusion data obtained with a basic spin-echo technique, by a factor of 10. The effectiveness of the use of the T1-weighted spin-echo sequence to perform accurate water self-diffusion coefficients measurement in fatty products is thus demonstrated.

Animals↗

PubMed2005

Modeling of slow glutamate diffusion and AMPA receptor activation in the cerebellar glomerulus.

Synaptic conductances are influenced markedly by the geometry of the space surrounding the synapse since the transient glutamate concentration in the synaptic cleft is determined by this geometry. Our paper is an attempt to understand the reasons for slow glutamate diffusion in the cerebellar glomerulus, a structure situated around the enlarged mossy fiber terminal in the cerebellum and surrounded by a glial sheath. For this purpose, analytical expressions for glutamate diffusion in the glomerulus were considered in models with two-, three-, and fractional two-three-dimensional (2D-3D) geometry with an absorbing boundary. The time course of average glutamate concentration in the synaptic cleft of the mossy fiber-granule cell connection was calculated for both direct release of glutamate from the same synaptic unit, and for cumulative spillover of glutamate from neighboring release sites. Several kinetic schemes were examined, and the parameters of the diffusion models were estimated by identifying theoretical activation of AMPA receptors with direct release and spillover components of published experimental AMPA receptor-mediated EPSCs. For model selection, the correspondence of simulated paired-pulse ratio and EPSC increase after prevention of desensitization to experimental values were also taken into consideration. Our results suggest at least a 7- to 10-fold lower apparent diffusion coefficient of glutamate in the porous medium of the glomerulus than in water. The modeling of glutamate diffusion in the 2D-3D geometry gives the best fit of experimental EPSCs. We show that it could be only partly explained by normal diffusion of glutamate in the complex geometry of the glomerulus. We assume that anomalous diffusion of glutamate occurs in the glomerulus. A good match of experimental estimations and theoretical parameters, obtained in the simulations that use an approximation of anomalous diffusion by a solution for fractional Brownian motion, confirms our assumption.

Cerebellar Cortex↗

PubMed2004

Diffusion properties of the brain in health and disease.

Extrasynaptic transmission between neurons and communication between neurons and glia are mediated by the diffusion of neuroactive substances in the extracellular space (ECS)--volume transmission. Diffusion in the CNS is inhomogeneous and often not uniform in all directions (anisotropic). Ionic changes and amino acid release result in cellular (particularly glial) swelling, compensated for by ECS shrinkage and a decrease in the apparent diffusion coefficients of neuroactive substances or water (ADCW). The diffusion parameters of the CNS in adult mammals (including humans), ECS volume fraction alpha (alpha = ECS volume/total tissue volume; normally 0.20-0.25) and tortuosity lambda (lambda2 = D/ADC; normally 1.5-1.6), hinder the diffusion of neuroactive substances and water. A significant decrease in ECS volume and an increase in diffusion barriers (tortuosity) and anisoptropy have been observed during stimulation, lactation or learning deficits during aging, due to structural changes such as astrogliosis, the re-arrangement of astrocytic processes and a loss of extracellular matrix. Decreases in the apparent diffusion coefficient of tetramethylammonium (ADCTMA) and ADCW due to astrogliosis and increased proteoglycan expression were found in the brain after injury and in grafts of fetal tissue. Tenascin-R and tenascin C-deficient mice also showed significant changes in ADCTMA and ADCW, suggesting an important role for extracellular matrix molecules in ECS diffusion. Changes in ECS volume, tortuosity and anisotropy significantly affect neuron-glia communication, the spatial relation of glial processes towards synapses, the efficacy of glutamate or GABA 'spillover' and synaptic crosstalk, the migration of cells, the action of hormones and the toxic effects of neuroactive substances and can be important for diagnosis, drug delivery and new treatment strategies.

Aging↗

PubMed2004

Characterization of central nervous system structures by magnetic resonance diffusion anisotropy.

Diffusion-weighted magnetic resonance imaging (MRI) provides information about tissue water diffusion. Diffusion anisotropy, which can be measured with diffusion tensor MRI, is a quantitative measure of the directional dependence of the diffusion restriction that is introduced by biological structures such as nerve fibers. Diffusion tensor MRI data was obtained in the brain, brain stem, and cervical spinal cord. For each region, scans were performed in four normal volunteers. Fractional anisotropy (FA), an index of diffusion anisotropy, was measured within regions of interest located in the corpus callosum, capsula interna, thalamus, caudate nucleus, putamen, brain cortex, pyramidal tract of the medulla, accessory olivary nucleus, dorsal olivary nucleus, inferior olivary nucleus, spinal white and gray matter. The highest FA value was measured in the corpus callosum (81 +/- 3%). The values of the other areas decreased in the following order: pyramidal tract in the medulla (72 +/- 1%), spinal white matter (65 +/- 4%), capsula interna (62 +/- 3%), accessory olivary nucleus (36 +/- 2%), spinal gray matter (35 +/- 5%), dorsal olivary nucleus in the medulla (29 +/- 2%), thalamus (28 +/- 2%), inferior olivary nucleus (15 +/- 2%), putamen (13 +/- 2%), caudate nucleus (13 +/- 2%), and brain cortex (9 +/- 1%). Our results indicate that the underlying fiber architecture, fiber density, and uniformity of nerve fiber direction affect anisotropy values of the various structures. Characterization of various central nervous system structures with diffusion anisotropy is possible and may be useful to monitor degenerative diseases in the central nervous system.

Anisotropy↗

PubMed2005

Diffusion tensor imaging in chronic head injury survivors: correlations with learning and memory indices.

Diffusion tensor imaging (DTI) provides a unique insight into the cellular integrity of the brain. While conventional magnetic resonance imaging underestimates the extent of pathology following closed head injury, diffusion-weighted imaging has been shown to more accurately delineate the extent of cerebral damage. There have only been a few case studies of DTI in chronic head injury survivors. This study used DTI to investigate changes in anisotropy and diffusivity in survivors of head injury at least 6 months after their injury. The relationship between cognition and diffusion abnormality was also investigated. The voxel-based analysis revealed significant bilateral decreases in anisotropy, in major white matter tracts and association fibers in the temporal, frontal, parietal and occipital lobes. Statistically significant increases in diffusivity were also found in widespread areas of the cortex. A significant positive correlation was found between diffusivity and impairment of learning and memory in the left posterior cingulate, left hippocampal formation and left temporal, frontal and occipital cortex. The common pattern of abnormality despite heterogeneous injury mechanism and lesion location in the group suggests that these cellular changes reflect secondary insults. The importance of diffusion abnormalities in head injury outcome is emphasized by the significant correlation between a learning and memory index and diffusivity in areas known to subserve this cognitive function.

Adolescent↗

PubMed2005

Correction for direction-dependent distortions in diffusion tensor imaging using matched magnetic field maps.

Diffusion tensor imaging (DTI) has seen increased usage in clinical and basic science research in the past decade. By assessing the water diffusion anisotropy within biological tissues, e.g. brain, researchers can infer different fiber structures important for neural pathways. A typical DTI data set contains at least one base image and six diffusion-weighted images along non-collinear encoding directions. The resultant images can then be combined to derive the three principal axes of the diffusion tensor and their respective cross terms, which can in turn be used to compute fractional anisotropy (FA) maps, apparent diffusion coefficient (ADC) maps, and to construct axonal fibers. The above operations all assume that DTI images along different diffusion-weighting directions for the same brain register to each other without spatial distortions. This assumption is generally false, as the large diffusion-weighting gradients would usually induce eddy currents to generate diffusion-weighting direction-dependent field gradients, leading to mis-registration within the DTI data set. Traditional methods for correcting magnetic field-induced distortions do not usually take into account these direction-dependent eddy currents unique for DTI, and they are usually time-consuming because multiple phase images need to be acquired. In this report, we describe our theory and implementation of an efficient and effective method to correct for the main field and eddy current-induced direction-dependent distortions for DTI images under a unified framework to facilitate the daily practice of DTI acquisitions.

Anisotropy↗

PubMed2006

Diffusion tensor imaging of the corpus callosum in Autism.

The corpus callosum is the largest commissural white matter pathway that connects the hemispheres of the human brain. In this study, diffusion tensor imaging (DTI) was performed on subject groups with high-functioning autism and controls matched for age, handedness, IQ, and head size. DTI and volumetric measurements of the total corpus callosum and subregions (genu, body and splenium) were made and compared between groups. The results showed that there were significant differences in volume, fractional anisotropy, mean diffusivity, and radial diffusivity between groups. These group differences appeared to be driven by a subgroup of the autism group that had small corpus callosum volumes, high mean diffusivity, low anisotropy, and increased radial diffusivity. This subgroup had significantly lower performance IQ measures than either the other individuals with autism or the control subjects. Measurements of radial diffusivity also appeared to be correlated with processing speed measured during the performance IQ tests. The subgroup of autism subjects with high mean diffusivity and low fractional anisotropy appeared to cluster with the highest radial diffusivities and slowest processing speeds. These results suggest that the microstructure of the corpus callosum is affected in autism, which may be related to nonverbal cognitive performance.

Adolescent↗

PubMed2006

Anomalous diffusion in Purkinje cell dendrites caused by spines.

We combined local photolysis of caged compounds with fluorescence imaging to visualize molecular diffusion within dendrites of cerebellar Purkinje cells. Diffusion of a volume marker, fluorescein dextran, within spiny dendrites was remarkably slow in comparison to its diffusion in smooth dendrites. Computer simulations indicate that this retardation is due to a transient trapping of molecules within dendritic spines, yielding anomalous diffusion. We considered the influence of spine trapping on the diffusion of calcium ions (Ca(2+)) and inositol-1,4,5-triphospate (IP(3)), two synaptic second messengers. Diffusion of IP(3) was strongly influenced by the presence of dendritic spines, while Ca(2+) was removed so rapidly that it could not diffuse far enough to be trapped. We conclude that an important function of dendritic spines may be to trap chemical signals and thereby create slowed anomalous diffusion within dendrites.

Animals↗

PubMed2003

Acid diffusion through extracellular polysaccharides produced by various mutants of Streptococcus mutans.

Mutants of Streptococcus mutans V403, constructed by allelic exchange and altered in their capacity to produce enzymes involved in the production of extracellular polysaccharides from sucrose, were used to study the role of glucans and fructans in the diffusion of ions through cell concentrates. A 4.0mm diameter, 0.75 mm deep diffusion chamber with an ion-sensitive field-effect transistor electrode positioned at the base was used to monitor the diffusion of hydronium ions from a sodium lactate buffer using cell concentrates prepared from bacteria grown in various concentrations of sucrose and glucose. The wild-type strain V403 produced at least seven times as much water-insoluble glucan (ISG) from sucrose as mutants deficient in various combinations of glucosyltransferase B (GTF B), GTF C, GTF D and fructosyltransferase. The fastest diffusion of hydronium ions occurred with sucrose-grown cell concentrates of strain V403, and the time of diffusion to the bottom of the chamber was approximately 2.3 times longer when this strain was grown in glucose. The speed of diffusion with glucose-grown V403 was similar to that obtained with each of the mutants. When cells of strain V403 grown in sucrose and glucose were mixed, increases in diffusion speed were found to be directly related to the proportion of sucrose-grown cells. The mixing of ISG with several strains of S. mutans revealed that increases in diffusion speed were directly related to the quantity of ISG added.

Acids↗

PubMed2000

Protein diffusion in living skeletal muscle fibers: dependence on protein size, fiber type, and contraction.

Sarcoplasmic protein diffusion was studied under different conditions, using microinjection in combination with microspectrophotometry. Six globular proteins with molecular masses between 12 and 3700 kDa, with diameters from 3 to 30 nm, were used for the experiments. Proteins were injected into single, intact skeletal muscle fibers taken from either soleus or extensor digitorum longus (edl) muscle of adult rats. No correlation was found between sarcomere spacing and the sarcoplasmic diffusion coefficient (D) for all proteins studied. D of the smaller proteins cytochrome c (diameter 3.1 nm), myoglobin (diameter 3.5 nm), and hemoglobin (diameter 5.5 nm) amounted to only approximately 1/10 of their value in water and was not increased by auxotonic fiber contractions. D for cytochrome c and myoglobin was significantly higher in fibers from edl (mainly type II fibers) compared to fibers from soleus (mainly type I fibers). Measurements of D for myoglobin at 37 degrees C in addition to 22 degrees C led to a Q(10) of 1.46 for this temperature range. For the larger proteins catalase (diameter 10.5 nm) and ferritin (diameter 12.2 nm), a decrease in D to approximately 1/20 and approximately 1/50 of that in water was observed, whereas no diffusive flux at all of earthworm hemoglobin (diameter 30 nm) along the fiber axis could be detected. We conclude that 1) sarcoplasmic protein diffusion is strongly impaired by the presence of the myofilamental lattice, which also gives rise to differences in diffusivity between different fiber types; 2) contractions do not cause significant convection in sarcoplasm and do not lead to increased diffusional transport; and 3) in addition to the steric hindrance that slows down the diffusion of smaller proteins, diffusion of large proteins is further hindered when their dimensions approach the interfilament distances. This molecular sieve property progressively reduces intracellular diffusion of proteins when the molecular diameter increases to more than approximately 10 nm.

Animals↗

PubMed2002

Diffusion of exchangeable water in cortical bone studied by nuclear magnetic resonance.

The rate-limiting step in the delivery of nutrients to osteocytes and the removal of cellular waste products is likely diffusion. The transport of osteoid water across the mineralized matrix of bone was studied by proton nuclear magnetic resonance spectroscopy and imaging by measuring the diffusion fluxes of tissue water in cortical bone specimens from the midshaft of rabbit tibiae immersed in deuterium oxide. From the diffusion coefficient (D(a) = (7.8 +/- 1.5) x 10(-7) cm(2)/s) measured at 40 degrees C (close to physiological temperature), it can be inferred that diffusive transport of small molecules from the bone vascular system to the osteocytes occurs within minutes. The activation energy for water diffusion, calculated from D(a) measured at four different temperatures, suggests that the interactions between water molecules and matrix pores present significant energy barriers to diffusion. The spatially resolved profile of D(a) perpendicular to the cortical surface of the tibia, obtained using a finite difference model, indicates that diffusion rates are higher close to the endosteal and periosteal surfaces, decreasing toward the center of the cortex. Finally, the data reveal a water component (approximately 30%) diffusing four orders of magnitude more slowly, which is ascribed to water tightly bound to the organic matrix and mineral phase.

Animals↗