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Biomedical subjects

C W Frank

Publications and source records attributed to C W Frank.

At least 55 records · Page 3Linked to original sources
PubMed1972

Optical versus thermal transitions in solids at high pressure.

Pressure-induced electronic transitions have been observed in a wide variety of materials. In particular, Mössbauer resonance studies under high pressure have revealed changes of oxidation state and spin state of iron as a function of pressure. These processes involve the thermal transfer of an electron to a new ground state of the system. The difference in energy between states is frequently measured by optical absorption. In this paper, we relate the energy of the optical absorption peak and its half-width to the difference in thermal energy between the two states as a function of pressure. We show that the optical peak will broaden with pressure only if there is a difference between the force constants of the ground and excited states. These relationships permit the prediction from optical absorption data of the pressure at which a new ground state will be obtained. We demonstrate that the predictions are qualitatively correct for the reduction of Fe(III) in a series of ferric hydroxamates, and for the low-spin to high-spin transition of Fe(II) in ferrous phenanthroline complexes.

Journal Article↗
PubMedDate not supplied

The polymer-supported phospholipid bilayer: tethering as a new approach to substrate-membrane stabilization.

We present a new molecular engineering approach in which a polymer-supported phospholipid bilayer is vertically stabilized by controlled covalent tethering at both the polymer-substrate and polymer-bilayer interfaces. This approach is based on lipopolymer molecules, which not only form a polymer cushion between the phospholipid bilayer and a solid glass substrate but also act as covalent connections (tethers) between the bilayer and cushion. Our approach involves Langmuir-Blodgett transfer of a phospholipid-lipopolymer monolayer followed by Schaefer transfer of a pure phospholipid monolayer and is capable of varying the tethering density between the polymer layer and the phospholipid bilayer in a very controlled manner. Further stabilization is achieved if the glass substrate is surface-functionalized with a benzophenone silane. In this case, a photocross-linking reaction between the polymer and benzophenone group allows for the covalent attachment of the polymer cushion to the glass substrate. This approach is similar to that recently reported by Wagner and Tamm in which double tethering is achieved via lipopolymer silanes (Wagner, M. L.; Tamm, L. K. Biophys. J. 2000, 79, 1400). To obtain a deeper understanding of how the covalent tethering affects the lateral mobility of the bilayer, we performed fluorescence recovery after photobleaching (FRAP) experiments on polymer-tethered bilayers at different tethering densities (lipopolymer/phospholipid molar ratios). The FRAP data clearly indicate that the hydrophobic lipopolymer moieties act as rather immobile obstacles within the phospholipid bilayer, thereby leading to hindered diffusion of phospholipids. Whereas the high lateral diffusion coefficient of D = 17.7 mum(2)/s measured at low tethering density (5 mol % lipopolymer) indicates rather unrestricted motion within the bilayer, corresponding values at moderate (10 mol % lipopolymer) and high (30 mol % lipopolymer) tethering densities of D = 9.7 mum(2)/s and D = 1.1 mum(2)/s, respectively, show significant hindered diffusion. These results are contrary to the recent findings on similar membrane systems reported by Wagner and Tamm in which no significant change in phospholipid diffusion was found between 0 and 10 mol % lipopolymer. Our experimental report leads to a deeper understanding of the complex problem of interlayer coupling and offers a path toward a compromise between stability of the whole system and lateral mobility within the bilayer. Furthermore, the FRAP measurements show that polymer-tethered membranes are very interesting model systems for studying problems of restricted diffusion within two-dimensional fluids.

Diffusion↗
PubMedDate not supplied

Determination of nitrate and nitrite in blood and urine by chemiluminescence.

Methodology has been developed for the rapid determination of nitrate and nitrite in blood and urine. In solution, these species are selectively reduced to nitric oxide, which is outgassed and determined via its gas phase chemiluminescence reaction with ozone. Differentiation between nitrate and nitrite is obtained by careful selection of reducing agents and conditions.

Humans↗