Biomedical subjects

K J Wilkinson

Publications and source records attributed to K J Wilkinson.

5 recordsLinked to original sources

PubMed2006

Bioavailability of trace metals to aquatic microorganisms: importance of chemical, biological and physical processes on biouptake.

An important challenge in environmental biogeochemistry is the determination of the bioavailability of toxic and essential trace compounds in natural media. For trace metals, it is now clear that chemical speciation must be taken into account when predicting bioavailability. Over the past 20 years, equilibrium models (free ion activity model (FIAM), biotic ligand model (BLM)) have been increasingly developed to describe metal bioavailability in environmental systems, despite the fact that environmental systems are always dynamic and rarely at equilibrium. In these simple (relatively successful) models, any reduction in the available, reactive species of the metal due to competition, complexation or other reactions will reduce metal bioaccumulation and thus biological effects. Recently, it has become clear that biological, physical and chemical reactions occurring in the immediate proximity of the biological surface also play an important role in controlling trace metal bioavailability through shifts in the limiting biouptake fluxes. Indeed, for microorganisms, examples of biological (transport across membrane), chemical (dissociation kinetics of metal complexes) and physical (diffusion) limitation can be demonstrated. Furthermore, the organism can employ a number of biological internalization strategies to get around limitations that are imposed on it by the physicochemistry of the medium. The use of a single transport site by several metals or the use of several transport sites by a single metal further complicates the prediction of uptake or effects using the simple chemical models. Finally, once inside the microorganism the cell is able to employ a large number of strategies including complexation, compartmentalization, efflux or the production of extracellular ligands to minimize or optimize the reactivity of the metal. The prediction of trace metal bioavailability will thus require multidisciplinary advances in our understanding of the reactions occurring at and near the biological interface. By taking into account medium constraints and biological adaptability, future bioavailability modeling will certainly become more robust.

Bacteria↗

PubMed2003

Cd bioaccumulation by a freshwater bacterium, Rhodospirillum rubrum.

Cd bioaccumulation by Rhodospirillum rubrum, a Gram-negative freshwater bacterium, was studied in a synthetic medium. The free ion (Cd2+) was the best predictor of the Cd internalization fluxes. Representation of the short-term uptake fluxes as a function of [Cd2+] in the medium demonstrated a linear relationship, as would be expected for a rate-limiting, first-order internalization with a single transporter. Nonetheless, several different accumulation profiles were observed, depending on the Cd concentration. Cd uptake was regulated differently for concentrations above and below 10(-6) M (or was regulated only above [Cd2+] = 10(-6) M). Short-and long-term studies revealed that regulation was rapidly initiated for the highest Cd concentrations examined, effectively decreasing both adsorbed and internalized Cd. Anodic stripping voltammetry demonstrated that a Cd complexing ligand was produced within minutes upon exposure to 5 x 10(-6) M Cd2+ and that an extracellular sequestration of Cd was one mechanism regulating Cd uptake. Competition studies with other cations revealed a competitive inhibition of Cd uptake by Zn and an uptake enhancement in the presence of Mn and Cu.

Cadmium↗

PubMed2003

Diffusion coefficients of humic substances in agarose gel and in water.

Measurements of the diffusion coefficients of five different humic substances (HS) have been performed in water and in agarose hydrogels at several pH values (in the range of 3-10) and gel concentrations (in the range of 0.7-3% w/w). Fluorescence correlation spectroscopy (FCS) and classical diffusion cells were used in parallel to probe diffusion over both microscopic and mesoscopic distance scales. In general, agreement between the techniques was reasonable, which indicated that local nonhomogenities in the gel did not play an important role. Diffusion coefficients (D) in the gel were generally in the range of 0.9-2.5 x 10(-10) m2 s(-1) but were generally only 10-20% lower than in solution. At low pH values, one of the studied humic substances (a peat humic acid, PPHA) formed large aggregates that could not penetrate into the gel and therefore could not be defined by a single D value. The observed decreases of D in the gel for other HS were too large to be explained by the tortuousity and obstructive effects of the gel alone. D decreased slightly with increasing gel concentration and increased slightly with pH. Because modifications of D due to pH were similar in both the gel and the free solution, it is unlikely that complexation with the gel was greatly influenced by the pH. Rather, the main effect that appeared to decrease the diffusive flux in gels was likely small increases in the hydrodynamic radii of the humic macromolecules. An anomalous diffusion model was used to describe the FCS data in the gel. The characteristic exponent determined by fitting the autocorrelation functions with this model decreased only slightly (from 0.96 to 0.90) with increasing gel concentration providing support that HS complexation with the gel fibers was not very important. The results have important implications for our understanding of the fate and behavior of the HS and their associated pollutants and for interpreting metal speciation data obtained using gel-covered analytical sensors.

Diffusion↗

PubMed2001

Determination of electrophoretic mobilities and hydrodynamic radii of three humic substances as a function of pH and ionic strength.

Capillary electrophoresis (CE) and fluorescence correlation spectroscopy (FCS) were employed to determine electrophoretic mobilities and hydrodynamic sizes of three humic substances (IHSS aquatic fulvic acid (FA), IHSS aquatic humic acid (HA), and IHSS peat humic acid (PHA)) as a function of pH and ionic strength. A slight aggregation corresponding to the formation of dimers and trimers was observed at low pH using fluorescence correlation spectroscopy (FCS). For example, for the peat humic acid, diffusion coefficients decreased from 2.1 x 10(-10) m2 s(-1) at pH 4 to 2.4 x 10(-10) m2 s(-1) at pH 11. For all three humic substances, electrophoretic mobilities were also shown to decrease significantly below pH 6. Calculated zeta potentials observed at high pH of -69 mV (FA), -62 mV (HA), and -63 mV (PHA) decreased to -39, -50, and -47 mV, respectively, under slightly acidic pH (4.5-4.8) conditions. No evidence of ionic strength induced aggregation was found using fluorescence correlation spectroscopy (FCS); diffusion coefficients increased slightly (<25%) with increasing ionic strength (up to 1 M). Negative electrophoretic mobilities decreased to a maximum measured ionic strength of 0.18 M. Above this ionic strength, no peaks were observed due to an increased HS adsorption to the capillary wall and an important decrease in electroosmotic flow. Interpretation of electrophoretic mobilities determined by CE is complicated by the fact that under certain conditions, HS appeared to be complexed by CE buffer systems, including MES, BES, and AMPSO.

Alkanesulfonic Acids↗

PubMed2001

Single molecule study of xanthan conformation using atomic force microscopy.

Conformations of individual macromolecules of the biopolymer xanthan were investigated using atomic force microscopy (AFM). Xanthan from very dilute solutions (1 ppm) was allowed to adsorb onto freshly cleaved mica and examined using tapping mode AFM under ambient conditions. The secondary structure of xanthan was probed by heating the polymer and gradually cooling, which denatured and renatured the polymer. When salt was present, renatured xanthan formed a double helical structure, consistent with the structure of native xanthan. In pure water, renaturation was not complete as what appeared to be single helical structures were observed. The number-average contour length (L(n)) of the polymer in its single helical state was 1651 nm. In the double helical state, induced by the addition of salt, L(n) decreased to 450 nm (in 0.5 M KCl). The chains also became less rigid as salt was added. The persistence length decreased from 417 nm in pure water to approximately 150 nm in 0.1 or 0.5 M KCl. This indicated a trend toward more flexible molecules when salt was present. Calculations of end-to-end distances based on equilibrium and projected conformations confirmed that the xanthan chain conformation on the mica surface was at equilibrium and was therefore representative of the conformation of xanthan in solution. The single-molecule AFM technique eliminates one common bias of solution techniques, which is the determination of an average signal between aggregates and dissolved molecules. It is thus a useful complement to solution-based methods for determining physical-chemical properties of biopolymers.

Carbohydrate Conformation↗