IntraPD model: Leaching of heavy metals from naturally contaminated soils

This paper presents a novel method for predicting the migration of heavy metal contaminants in soils that contain heavy metal pollutants deep within the particles, which may cause delayed leaching. The proposed approach, the intraparticle pore-diffusion (IntraPD) model, incorporates both intraparticle diffusion and sorption equilibrium and is utilized to simulate batch leaching tests of heavy metal-contaminated soils using the finite difference method. The IntraPD model can solve the leakage of heavy metals from contaminated soil particles of arbitrary particle size distribution as a one-dimensional, polar-symmetric problem in the spherical coordinate system, assuming the soil particles to be porous perfect spheres. The findings from the simulation demonstrate a notable impact of soil particle size distribution on both the leaching rate and the amount of contaminants. Specifically, the coefficient of uniformity, which indicates the ratio of the maximum to minimum particle size, was identified as a significant factor influencing the leaching rate. When the coefficient of uniformity is less than or equal to 5, it has a combined effect on the leaching rate. However, when the coefficient of uniformity exceeds 5, the maximum particle size becomes the primary determinant of the leaching rate. The proposed model provides a valuable tool for accurately simulating batch leaching tests of naturally contaminated soils with diverse particle size distributions.

New concept to describe three-phase capillary pressure-degree of saturation relationship in porous media

The Leverett concept is used conventionally to model the relationship between the capillary pressures and the degrees of saturation in the water-nonaqueous phase liquid (NAPL)-air three-phase system in porous media. In this paper, the limitation of the Leverett concept that the concept is not applicable in the case of nonspreading NAPLs is discussed through microscopic consideration. A new concept that can be applied in the case of nonspreading NAPLs as well as spreading NAPLs is then proposed. The validity of the proposed concept is confirmed by comparing with past experimental data and simulation results obtained using the conventional model based on the Leverett concept. It is confirmed that the proposed concept can correctly predict the observed distributions of NAPLs, including those of nonspreading ones.

Mechanical Properties of a Single-Headed Processive Motor, Inner-Arm Dynein Subspecies-c of ChlamydomonasStudied at the Single Molecule Level

Dynein from inner arms of Chlamydomonasflagella contains sevendistinct subspecies, a through g. Several lines of evidence suggest thesesubspecies play important roles in generating flagellar beating and thatthe different subspecies are functionally diverse. To evaluate theirroles and diversity, the mechanical properties of subspecies-c, which isa single-headed motor, were examined using optical trap nanometry. Apolystyrene bead coated with a small number of subspecies-c moleculeswas captured with the optical trap and brought into contact with amicrotubule fixed to a coverslip. Movements of the bead were measured bya quadrant photodiode sensor with sub-nanometer- and millisecond-resolution.Beads carrying a single active subspecies-c molecule moved processivelyalong the microtubules in 8-nm steps but slipped backwards under highloads. Force-velocity relationships of single subspecies-c molecules werealmost linear and the shapes of the normalized curves at 5 μM and 100μM ATP were similar. These results indicate that dynein subspecies-cfunctions in a very different way from conventional motor proteins, suchas myosin and kinesin, and has properties that could produceself-oscillation in vivo.

Real-time observation of the disassembly of stable neuritic microtubules induced by laser transection: possible mechanisms of microtubule stabilization in neurites

By dissolving the membrane with detergent perfusion, we have shown that the established neurites of dorsal root ganglion cells cultured for more than 5 days contained microtubules which persisted outside the cell for a few minutes to more than 1h [Tashiro et al., 1997: J. Neurosci. Res. 50:81-93]. To investigate their stabilization mechanism, we transected the exposed microtubules by laser microbeam irradiation and observed their length changes with video-enhanced differential interference contrast microscopy. Microtubule fragments started to shorten on both sides of the transection site. more rapidly from the newly generated plus ends than from the minus ends. The maximal rate as well as the pattern of shortening correlated with the time of transection; microtubules transected later than 30 min after membrane removal shortened at rates less than 20 microm/min and typically with intermittent pauses, while the more labile microtubules included in the earlier transections shortened continuously at higher rates. Microtubules in neurites were thus stabilized by 1) stopping disassembly at local sites including the plus ends, and 2) slowing disassembly along the length. Transection also suggested the presence of specialized points along microtubules which are involved in anchoring microtubules to the substratum. Cell Motil. Cytoskeleton 42:87-100, 1999.

Comparative single-molecule and ensemble myosin enzymology: sulfoindocyanine ATP and ADP derivatives

Single-molecule and macroscopic reactions of fluorescent nucleotides with myosin have been compared. The single-molecule studies serve as paradigms for enzyme-catalyzed reactions and ligand-receptor interactions analyzed as individual stochastic processes. Fluorescent nucleotides, called Cy3-EDA-ATP and Cy5-EDA-ATP, were derived by coupling the dyes Cy3.29.OH and Cy5.29.OH (compounds XI and XIV, respectively, in, Bioconjug. Chem. 4:105-111)) with 2'(3')-O-[N-(2-aminoethyl)carbamoyl]ATP (EDA-ATP). The ATP(ADP) analogs were separated into their respective 2'- and 3'-O-isomers, the interconversion rate of which was 30[OH(-)] s(-1) (0.016 h(-1) at pH 7.1) at 22 degrees C. Macroscopic studies showed that 2'(3')-O-substituted nucleotides had properties similar to those of ATP and ADP in their interactions with myosin, actomyosin, and muscle fibers, although the ATP analogs did not relax muscle as well as ATP did. Significant differences in the fluorescence intensity of Cy3-nucleotide 2'- and 3'-O-isomers in free solution and when they interacted with myosin were evident. Single-molecule studies using total internal reflection fluorescence microscopy showed that reciprocal mean lifetimes of the nucleotide analogs interacting with myosin filaments were one- to severalfold greater than predicted from macroscopic data. Kinetic and equilibrium data of nucleotide-(acto)myosin interactions derived from single-molecule microscopy now have a biochemical and physiological framework. This is important for single-molecule mechanical studies of motor proteins.

Direct visualization and characterization of stable microtubules from the neurites of cultured dorsal root ganglion cells

We tested the stability of microtubules (MTs) in the neurites of cultured dorsal root ganglion cells by dissolving the cytoplasmic membrane with detergent and exposing them to defined extracellular medium under observation with a video-enhanced differential interference contrast (DIC) microscope. Smooth cytoplasmic filaments visualized after membrane removal were suggested to be MTs by the preservation of all of the filaments in the presence but not in the absence of taxol. They were further confirmed to be MTs by specific immunostaining with anti-tubulin antibody. A significant number of MTs in the established neurites of 6-day-old cultures remained longer than 10 min after membrane removal while MTs in the Schwann cell processes or in the distal regions of the growth cone-bearing neurites of 3-day-old cultures disappeared within 2 min. A population of very stable MTs persisting longer than 30 min was also found specifically in the 6-day-old cultures. Association with other structures or bundling seemed to stabilize the MTs to some degree. The most stable MTs, however, were not associated with some structure along the length but were mainly anchored at points, suggesting that specific point attachments may be another important mechanism operating in MT stabilization. The present method is thus capable of directly demonstrating the unusual stability of neuritic MTs, and provides a new system for further investigation on the mechanism of stabilization.