ALBERT

Notizen: Overtone vibration-laser double resonance measurements determine the vibrational relaxation rates of DF(v=1) and HF(v=1) by the DF dimer. Both monomers are efficiently relaxed by the dimer at a rate that is 20% of the gas kinetic rate. The similarity of the rate constants for the two systems, which have very different energy defects, indicates that the relaxation occurs by collision complex formation and energy redistribution, rather than direct vibration-to-vibration energy transfer from the monomer to the dimer.

Notizen: Overtone vibration–laser double resonance studies of DF(v=1–3) energy transfer yield self-relaxation rate constants for v=1, 2 and 3 of k1=(0.37±0.06)×10−12 cm3 mol−1 s−1, k2=(22.0±2.0)×10−12 cm3 mol−1 s−1, and k3=(17.0±1.8)×10−12 cm3 mol−1 s−1, respectively. The approach also directly measures the relative importance of vibration-to-vibration (V–V) and vibration-to-translation-and-rotation (V–T,R) energy transfer. The fraction of DF(v) molecules relaxing by V–V energy transfer is 1.1±0.1 and 0.72±0.10 for v=2 and v=3, respectively. Essentially all of the vibrational energy transfer in v=2 occurs via the V–V mechanism. The slower relaxation of DF(v=3) compared to DF(v=2), in contrast to simple scaling law predictions, reflects the decreasing influence of the V–V mechanism, even though it is still the primary relaxation pathway for DF(v=3). Comparison with HF self-relaxation qualitatively indicates that V–R energy transfer is important in V–T,R relaxation of DF(v=1).

Notizen: Overtone vibration–laser double resonance measurements determine the vibrational relaxation rate of HF(v=1) by HF dimers. Vibration-to-vibration energy transfer from the excited monomer to the dimer followed by vibrational predissociation of the dimer provides an efficient pathway for vibration-to-translation energy transfer that deexcites the monomer at 40% of the gas kinetic collision rate. Analysis of the pressure dependence of the observed decay constants using a simple kinetic model establishes a rough upper limit of 10 ns on the predissociation lifetime of the collisionally excited dimer.

Notizen: The temperature dependencies of the total self-relaxation rate constants for the vibrational deactivation of HF(v=2) and HF(v=1) and the state-to-state vibration-to-vibration (V–V) and vibration-to-translation-and-rotation (V-T,R) energy transfer components of the HF(v=2) self-relaxation process are measured using the overtone vibration excitation-laser double resonance technique. The total self-relaxation rate constants vary inversely with temperature. The much weaker temperature dependence of HF(v=2) self-relaxation compared to that of HF(v=1) arises from the significant role of the V–V energy transfer route. Competition between energetics and collision duration results in a weaker inverse variation with temperature for the slightly endothermic V–V route than for the exothermic V-T,R route for HF(v=2). The branching ratio for V–V energy transfer increases slightly with temperature and the data suggest that two quantum relaxation processes constitute no more than 10% of the total self-relaxation of HF(v=2). The available temperature dependence data on self-relaxation of HF(v=1–5) form a consistent picture in which the energetics of the V–V and V-T,R relaxation pathways control their relative contributions to the total energy transfer.

Notizen: Robinson, J. M. 1988. Does O2 photoreduction occur within chloroplasts in vivo? -Physiol. Plant. 72: 666–680.This discussion reviews evidence supporting the hypothesis that within intact chloroplasts in vivo, molecular O2 may serve as an alternative Hill oxidant (electron acceptor) on the reducing side of Photosystem I. Depending upon the availability of Hill oxidants such as NADP+ and NO−2, there is the potential within intact plastids in vivo, for photolytically derived reducing equivalents to reduce O2 to O−2 and H2O2 (the Mehler reaction). In chloroplasts of healthy tissues, the products of photosyn-thetic O2 reduction O−2 and H2O2) are rapidly removed by superoxide dismutase (EC 1.15.1.1) and L-ascorbate peroxidase (EC 1.11.1.11) to prevent toxicity. The presence of these two enzymes within chloroplasts in vivo reflects the potential for linear (non-cyclic) photosynthetic electron transport systems to draw upon molecular O2 as a terminal oxidant. In the intact plastid, O2 may act as an electron acceptor in the place of any other physiological Hill oxidant, e.g., NADP+, NO−2, and, presumably, oxidized thioredoxin. Under aerobic, physiological conditions, photo reduced ferre-doxin (Fdred), and/or reduced flavoprotein enzymes, e.g., ferredoxin:NADP+ oxidoreductase (EC 1.18.1.2), can donate electrons to O2; this reductive reaction appears to be non-enzymatic, but it is rapid. Stated from another viewpoint, O2 may serve as a Hill oxidant to support some linear electron flow when reductant supplies are in excess of reductant demands. For example, there are nitrogen assimilatory sites in the chloroplast, i.e., ferredoxin-nitrite reductase (NiR; EC 1.7.7.1) and glutamate synthase (ferredoxin) (GOGAT; EC 1.4.7.1), to which Fdred is allocated as reductant. Because NADH:nitrate reductase (NR; EC 1.6.6.1) is the rate limiting step of nitrogen assimilation, and, because NiR and GOGAT activities are in excess of NR activities by a factor of 2 or more, then an excess of unreacted Fdred could accumulate. Alternatively, the allocated Fdred would reduce the excess NiR and GOGAT sites, but the excess of reduced enzymes would not have substrates (e.g., NO−2, glutamine, and α-ketoglutarate) with which to react. Therefore, if ‘excess’ NiR and GOGAT binding sites were not employed, the available excess Fdred, and/or the reduced NiR and GOGAT proteins, would be susceptible to oxidation by O2. The resulting O2 photoreduction could account for nearly all of the observed in vivo Mehler type reactions. In vivo, apparent foliar O2 photoreduction occurs simultaneously with maximal CO2 photoassimilation, and, in high light, average rates have been determined by direct measurement to range from 10 to 40 μmol O2 consumed (mg Chl)−1 h−1. Therefore O2 reduction would support a low rate of linear (non-cyclic) electron flow which, in turn, could maintain a low, but significant rate of ATP production. However, there is not total agreement among researchers that the physiological role of O2 is that of serving as an alternative Hill oxidant in order to recycle unutilized Fdred or other photoreduced proteins. Also, there continues to be considerable controversy on whether or not O2 reduction supports significant photosynthetic phosphorylation. The total process of O2 photoreduction, and its physiological role(s), requires much more study before absolute functions can be assigned to O2 terminated, linear electron transport.〈section xml:id="abs1-1"〉〈title type="main"〉SummaryMolecular O2 possesses the physico-chemical properties that permit this molecule to serve as an alternative Hill oxidant within chloroplasts in vivo. Additionally, the physical and physiological properties within the chloroplast in vivo favor the potential for O2 to serve as an electron acceptor on the reducing side of Photosystem I. This may reflect an important ‘fail-safe mechanism’ which prevents over-reduction of linear photosynthetic electron transport chain proteins. This review has focused on the possibility that unutilized Fdred and/or other non-utilized, reduced plastid enzymes (e.g., NiR) may be electron donors to O2. It is hypothesized that this oxidation ultimately would be reflected as an in vivo Mehler reaction. However, it remains for future studies to establish without doubt, that in vivo, photoreduced chloroplast enzyme proteins can utilize O2 as a terminal electron acceptor.Further, that O2 photoreduction supports a significant level of photophosphorylation in vivo remains to be firmly established. Certainly, considerable evidence, gained with experiments utilizing isolates of intact chlo-roplasts and reconstituted chloroplast systems, supports the hypothesis that O2-terminated linear electron transport has the potential to support high rates of ATP production. However, in vivo studies e.g., with intact leaf tissues, which actually quantitate the relationship between O2 photoreduction and associated ATP production have not been conducted. These will be difficult experiments to perform, because, in vivo, it will be difficult to separate photosynthetic ATP production mediated by O2 from ATP production mediated by those other, more predominant Hill oxidants (e.g., NADP+, NO−2). Also, it continues to be a possibility that it is cyclic, and not pseudocyclic photophosphorylation that provides additional ATP to support photosynthetic cell metabolism. To establish beyond doubt that an in vivo role of the Mehler reaction is that of supplying ‘additional ATP’, remains for considerable future study.

Notizen: We have used the inherent surface sensitivity of second-harmonic generation to develop an instrument for nonlinear optical microscopy of surfaces and interfaces. This optical technique is ideal for imaging nanometer-thick, chromophoric self-assembled monolayers (SAMs), which have been patterned using photolithographic techniques. In this paper, we demonstrate the application of second-harmonic generation microscopy to patterned SAMs of the noncentrosymmetric molecule calixarene and discuss the resolution and sensitivity limits of the technique. © 1997 American Institute of Physics.

Notizen: Overtone vibration-laser double resonance directly measures the relative importance of vibration-to-vibration and vibration-to-translation-and-rotation energy transfer for HF(v=3 and v=4) at room temperature. The fraction of HF(v) molecules relaxing by V–V energy transfer is 0.44±0.05 and 0.16±0.05 for v=3 and v=4, respectively, compared to 0.59±0.10 for v=2. These measurements show that V–T,R energy transfer is the dominant relaxation mechanism for HF(v≥3) and the observed decreased amount of V–V energy transfer for higher initially excited vibrational levels is in good agreement with a chemiluminescence measurement and several theoretical calculations. The data demonstrate that the magnitude of the energy defects for the component pathways primarily determines the energy transfer mechanism for HF(v=2–4).

Notizen: Ambient atmospheric CO2 concentration ([CO2]a) has apparently declined from values above 200μmol mol−1 to values below 200μmol mol−1 within the last several million years. The lower end of this range is marginal for C3 plants. I hypothesize that: (1) declining [CO2]a imposed a physiological strain on plants, and plant taxa evolving under declining [CO2]a tended to develop compensating mechanisms, including increased stomatal efficiency; (2) angiosperms were better able to adjust to declining [CO2]a than were gymnosperms and pteridophytes; and (3) angiosperm adjustment has been uneven. Fast-evolving taxa (e.g. grasses and herbs) have been better able to adapt to CO2 starvation. If these propositions are true, stomatal adjustment mechanisms should show patterned variation, and a single pattern of stomatal regulation cannot be assumed.

Notizen: Summary Phagocytosis by polymorphonuclear leukocytes triggers a burst of oxidative metabolism resulting in hydrogen peroxide and superoxide production, and these active oxygen species function in the killing of microorganisms. A new cytochemical technique, based on a manganese dependent diaminobenzidine oxidation, has been developed to detect superoxide in these cells. It has been shown that superoxide generation is associated with the plasma membrane in cells activated by particulate (zymosan) and nonparticulate (phorbol myristate acetate) stimuli. This membraned activity is maintained during invagination such that reduced oxygen is generated within the endocytic vacuoles. Reaction product is absent from unstimulated cells; additionally, formation of precipitate is blocked by omission of Mn++, low temperature, glutaraldehyde prefixation, and the presence of superoxide dismutase in the incubation medium.