Biomedical subjects

Stephen G Weber

Publications and source records attributed to Stephen G Weber.

40 records · Page 3Linked to original sources

PubMed2002

Fabrication of microchannel structures in fluorinated ethylene propylene.

A new technique for fabrication of channel structures with diameters down to 13 microm in fluorinated ethylene propylene (also known as poly(tetrafluoroethylene-co-hexafluoropropylene), FEP) is described. The technique is based on the unique property of a dual-layer fluoropolymer tubing consisting of an outer layer of poly(tetrafluoroethylene) (PTFE) and an inner layer of FEP. When heated (>350 degrees C), the outer PTFE layer shrinks while the inner FEP layer melts, resulting in filling of all empty space inside the tubing with FEP. The channel structures are formed using tungsten wires as templates that are pulled out after completion of the shrinking and melting process. While several analytical devices have been reproducibly prepared and shown to function, this report describes a single example. A microreactor coupled to an electrochemical flow cell detects the biuret complex of the natively electroinactive peptide des-Tyr-Leu-enkephalin.

Biuret Reaction↗

PubMed2002

Steady-state concentration distribution of artificial receptor and target analyte in plasticized PVC membrane between solutions differing in target analyte concentration.

A barbiturate receptor has proven effective in improving selectivity in solid-phase microextraction of barbiturates when doped into plasticized poly(vinyl chloride) (PVC). It would be beneficial to have selective extractions for any given organic species; however, the receptors do not exist. They will be found by screening of libraries of potential receptors; thus, a screening method is needed. It is important to screen the receptors in the medium in which they will work: plasticized PVC. We hypothesize that we can make receptors move in solution in response to the presence of a solute to which they bind. This work examines whether we can establish a sufficient free energy gradient for a good receptor to move to a predetermined place in space. A difference in the barbiturate solute (substrate or guest) concentration in solutions bathing the two sides of a plasticized PVC membrane containing the barbiturate receptor (or host) creates a spatial concentration gradient of the substrate in the membrane. This causes the receptor's chemical potential to vary across the membrane. Upon binding to the analyte, the receptor undergoes a local activity drop, which decreases its free energy. This process produces a flux of receptor to accumulate at place where there is a high substrate concentration. A concentration gradient of substrate can be maintained across the membrane at steady state. In membranes for which the formation of the complex is favored, the receptor responds to the gradient of substrate. In membranes for which binding is not favored, a gradient of substrate is completely ignored by the receptor. Thus, the receptor does respond to the gradient but only if the concentration gradient of guest corresponds to a chemical potential gradient.

Barbiturates↗

PubMed2002

Nonaqueous affinity capillary electrophoresis investigation of small molecule molecular recognition.

The conditional binding constants for a bis-guanidinium-like receptor and a series of dicarboxylate ligands have been determined in two buffer/solvent systems, namely 25 mM ammonium acetate/1% acetic acid in acetonitrile/methanol (7:3 v:v) and 30 mM N-methyl morpholine/15 mM methanesulfonic acid in acetonitrile/methanol (9:1 v:v). The latter buffer has not been applied before in capillary electrophoresis. The binding constants in both solvent systems decrease as the dicarboxylate length increases. The binding constants are larger in the less competitive N-methyl morpholine buffer. The dicarboxylates associate only weakly with a dicationic analog of the receptor, p-xylyl trimethylammonium, which is not a hydrogen bond donor.

Acetates↗

PubMed2002

Kinetic analysis of a photosensitive chelator and its complex with Zn(II).

The reversible sequestration and release of metal ions is an important objective in biological and environmental research. Unfortunately, although there have been dramatic examples of metal ion activity control, there are very few quantitative investigations of stoichiometry, equilibria and kinetics. A significant contributor to this lack of quantitative work is the complexity of many photochromic systems. Therefore, we have attempted to create a simple, reversible photochromic metal-ion chelator that can be analyzed quantitatively. The chelator should have certain other attributes as well, namely, that it binds to divalent metal ions (because of their extreme biological importance) and that it binds metal ions in the dark so that light is used to release metal ions rather than sequester them. The photochromic chelator (1) binds to divalent metal ions [Zn(II), Cu(II), Pb(II), Hg(II), Fe(II), Co(II) and Cd(II); other metal ions have not yet been tested] in the dark with a significant binding strength. In both methanol (by spectrophotometry) and methanol-water (by voltammetry), the stoichiometry of the 1-Zn(II) complex is 2:1. The binding constant (K1K2) is on the order of 10(12)-10(14) M(-2) in methanol and 5.0 x 10(8) M(-2) in 50% aqueous methanol. The chelator 1 is photolabile, yielding 2 with a quantum efficiency of 0.91. In a solution containing excess Zn(II), so that over 99% of the ligand exists as the monodentate complex, photolysis produces 2 with a quantum efficiency of 0.15. A kinetic analysis leads to the conclusion that the complex itself is photolabile.

Chelating Agents↗