OXYGEN QUENCHING OF CHLOROPHYLL FLUORESCENCE IN CHLOROPLASTS

Abstract— Three phases of chlorophyll a fluorescence quenching by O₂ are observed in green plants. The effects of various inhibitors on photosynthetic partial processes in chloroplasts were investigated in attempts to (1) localize the O₂-quenching sites and (2) assess possible physiological significance of O₂-quenching. Our results localize the most sensitive (and presumably functionally important) phase to a site between plastoquinone and the photosystem I acceptor, chlorophyll (P₇₀₀), possibly plastocyanin. It is suggested that PC may transfer electrons to oxygen in addition to P₇₀₀.     

Photoemission study of the growth of the NdF3/Si(111) interface

Photoemission spectroscopy with synchrotron radiation was used to study the NdF₃/Si(111) interface as a function of annealing temperature for NdF₃ films. These films range in thickness from 1–20 monolayers and were deposited at room temperature. Without annealing, both F-Si and Nd-Si bonding is observed, indicating that the planar triangular NdF₃ molecules lie flat on the Si(111) substrate. At annealing temperatures between 400 and 500° C, the NdF₃/Si(111) interface is dominated by Nd-Si bonding as evidenced from a line-shape analysis of the Si 2 p and Nd 4 f core levels. By resonant excitation of the giant 4 d-4 f absorption resonance, the photoemission signal from the partially occupied 4 f orbitals is enhanced and can be distinguished from the photoemission signal of the overlapping F 2 p valence band. At higher temperatures F is completely lost due to the decomposition of NdF₃.     

A highly sensitive and selective hydrogen gas sensor from thick oriented films of MoS2

A new process is developed to fabricate a highly sensitive and selective hydrogen sensor by depositing a partially crystalline and highly oriented film of MoS₂ from its single layer suspension on an alumina substrate. When these films are promoted with some catalysts selected from Pt-group metals (Pt, Pd, Ru or any combination of these metals) they exhibit a high sensitivity and selectivity to hydrogen gas. Unlike other metal oxide sensors which are sensitive to many reducing and oxidizing gases and operate at a temperature of 350 °C or higher; this sensor is highly selective to hydrogen gas and its operating temperature is from 25 to 150°C. The lower operating temperature enhances safety when dealing with hydrogen gas. The sensor response to hydrogen at 120 °C is linear in concentration from 30 to 10⁴ ppm with a 10 to 30 second response time and a 45 to 90 second recovery time. Above 10⁴ ppm the sensor is still linear but the slope of conductance versus hydrogen concentration changes.     

Stabilization of silicon-based devices in ion-containing media using thin Al2O3 platelets and MoS2 oriented thin films

A simplified analogue to an Ion-Sensitive Field-Effect Transistor (ISFET) was tested in the presence of ions. The gate electrode in an ISFET is an aqueous solution, unlike in a Metal-Oxide Field-Effect Transistor (MOSFET) where the gate is a metal. A problem with the ISFET is that the insulating oxide between the silicon and the solution is slowly penetrated by various ions such as OH^– or Na⁺ causing a change in the characteristics of the device.The application of thin alumina platelets and thin MoS₂ films as a protective insulating layer, when deposited over the insulating silicon oxide, was tested against the penetration of ions. It is shown that there is a significant decrease in the ion penetration through to the silicon oxide layer, depending on the applied bias, when the oxide is covered with thin alumina or MoS2 layers. The effect of different ions and ion concentrations are presented. Suggestions for further improvement are made.     

A STUDY OF THE FLASH INDUCED 518 NM ABSORBANCE CHANGE KINETICS UNDER ANAEROBIC CONDITIONS

The preillumination induced acceleration of the flash-induced 518 nm absorbance change (ΔA₅₁₈) decay was studied in lettuce leaves and chloroplasts. In leaves, the acceleration was inhibited by DCMU or reversibly by removal of oxygen. In chloroplasts with added ADP and phosphate and/or reconstructed electron transport, the acceleration was also inhibited by DCMU or the lack of O₂.
Anaerobic inhibition of ΔA₅₁₈ decay acceleration was no longer observed when hydroxylamine replaced water as electron donor to PSII. Anaerobiosis was also shown to reversibly inhibit the initial rate of FeCN reduction in chloroplasts. These results suggest the mechanism of anaerobic inhibition of ΔA₅₁₈ decay acceleration to be associated with the O₂ evolving system.