Search

The role of the plasma electrode (PE) bias in the extraction process in a large-volume hybrid multicusp negative-ion source, which is driven by 2.45-GHz microwaves, is studied. Spatially resolved negative-ion and electron density measurements close to the extractor aperture were performed under various pressures (1-4 mtorr) by means of the electrostatic probe and photodetachment technique. As the low positive voltage applied to the PE is slightly increased (from 4 to 7 V), the electron temperature passes through a minimum (0.2-0.6 eV), while simultaneously, both the negative-ion density and the [H.sup.-] extracted current reach a maximum (~1-2 x [10.sup.9] [cm.sup.-3] and ~0.5 mA/c[m.sup.2], respectively). Optimum pressure values for the extracted negative-ion current and the negative-ion density are found between ~1.5 and 3 mtorr. It is deduced that the negative-ion density measured in the center of the source cannot be directly correlated with the ion extracted current. The electron density and the associated extracted electron current linearly decrease as a function of the PE bias. The physical mechanisms explaining the experimental results are discussed. Index Terms--Electrostatic probe, hydrogen, microwaves, negative ions, photodetachment, plasma electrode (PE) bias.

A report is presented on the steady-state Fourier transform infrared difference spectra of the photoreduction of either [Q.sub.A] or [Q.sub.B] measured in native and DN(L210) mutant reaction centres in the presence of potassium ferrocyanide. Report concludes that species [X.sup.+] and X are spectrally indistinguishable from [Q.sub.B] and [Q.sub.B.sup.-] respectively.

P700, the primary electron donor of photosystem I, is an asymmetric dimer made of one molecular of chlorophyll a' (Pa) and one of chlorophyll a (Pb) that are bound to the homologous PsaA and PsaB polypeptides. While the carbonyl groups of Pa are involved in hydrogen-bonding interactions with several surrounding amino acid side chains and a water molecule, Pb does not engage hydrogen bonds with the protein.

Studies done on the effect of replacing Glu-L212 with Asp and Asp-L213 with Glu in the Rhodobacter sphaeroides reaction center by monitoring the light-induced FTIR absorption changes associated with the photoreduction of QB is presented. The findings of the study indicate that there is a proton sharing among carboxylic acids in a system very similar to the native reaction center, which differs in the absorption frequency of protonating carboxylic acids and in the extent of proton sharing.

In rhodobacter sphaeroids light energy triggers a cycle of electron flow. This model is used to investigate the mechanism of solar energy conversion in photosynthesis.

Research has been conducted on Rhodobacter sphaeroides reaction centers.The role of substituting Pro-L209 with Tyr in these reactions centers has been investigated via monitoring light-induced FTIR absorption changes associated with quinone Q (sub)B.

Results point out to a symmetrical charge distribution within the chlorophyll molecules in P700(sup)+ as revealed by the P700(sup)+/P700 FTIR difference spectra of photosystem I core complexes from Synechocystis. The spectral signal corresponds to the downshift due to P700 photooxidation of the C(sub)5-Ntau of the imidazole ring.

Research illustrates the redox difference spectra of bovine cytochrome c oxidase in the unligated and cyanide-bound states using attenuated total reflection Fourier transform infrared difference spectroscopy technique. The redox difference spectrum of heme a(sub)3, to the exclusion of other metal centers, is the difference between the two spectra.

Amino acid replacement studies show that the extent of ionization of Glu-L212 in the Q(sub)B state is larger in the mutant reaction centers of Rhodobacter sphaeroides bacteria than in wild-type. Data indicate that in the wild-type reaction centers, Asp-M17 and Asp-L210 are partially ionized at neutral pH with no changes in the protonation status.

Fourier transform infrared spectroscopic photolysis difference spectra of the carbon monooxide and cyanide compounds of reduced bovine cytochrome c oxidase reveal that the former is bound in a single form and the latter in at least two forms. Data show that photolysis produces spectral changes in the 1000-1800 wavelength region.

Research has been conducted on the quinone (Q (sub)B) reduction in Rhodobacter sphaeroides chromatophores. The effect of the metal ion binding on the electrogenic phases of electron transfer connected with the quinone reduction has been investigated via time-resolved electric potential changes.

Research has been conducted on the secondary quinone Q (sub)B reduction in the reaction centers of Rhodobacter capsulatus. The use of the light-induced Fourier transform infrared difference spectroscopy in investigating this reduction is discussed.

Research was conducted to examine the impact of global 15N or 2H labeling on the light-induced P700+/P700 Fourier transform infrared difference spectra in photosystem I samples fro Synechocystis at 90 K. A comparison of small isotope-induced frequency shifts of the carbonyl modes observed in the P700+/P700 spectra with those of isolated chlorophyll a was carried out. Results indicate that P700 is a dimer of two Chl a molecules that differ drastically in the hydrogen bonding state of their 9-keto carbonyl group.

Research was conducted to examine the stable conformations and vibrational spectra of 2-methody-3-methyl-1,4-benzoquinone via density functional methods. The calculations reveal how sensitive the energetics of the methoxy group orientation as well as relative energies of stable conformations can depend on protein-chromophore interactions. Findings confirm previously reported speculations that the different properties of Q(sub A) and Q(sub B) in the photosynthetic reaction center of Rhodobacter sphaeroides could be caused by different methoxy group orientations due to differences in protein-chromophore interactions.

Light in the photosynthetic bacterial reaction center (RC) induces electron and proton transfer reactions leading to double reduction and protonation of secondary quinone QB to QBH2. Current FTIR results from RCs with mutations at the H173 site and results from RCs with mutations at other L subunit sites, showed that Glu H173 and Asp L213 are mostly ionized in both QB and QB- states, making no significant contribution to proton uptake in native RCs at pH 7.

A study was conducted to characterize the Tyrosine(sub z)/Tyr(sub z) Fourier transform infrared difference spectrum determined with Mn-depleted photosystem II (PS II)-enriched membranes of spinach and with the PS II cores of Synechocystis sp. PCC 6803. The infrared sample was thermostated in a N2-cooled cryostat at 8 degrees C. Results indicated that Tyr(sub Z) formed stronger hydrogen bonds than Tyr(sub D).

Fourier transform infrared resonance difference spectroscopy was utilized on pure Tyr(sub D)*/Tyr(sub D) relative to the in vitro model spectra identify the majority of the side chain modes of Tyr(sub D) and Tyr(sub D)* in both wild-type and mutant forms. Data indicated that Tyr(sub D) was bonded ny hydrogen to the imidazole ring of a neutral histidine. Likewise, hydrogen bonds exist in Tyr(sub D), Tyr(sub D)*, D2-His189, amide carbonyl and D2-Gln189 side chains.

The reactive 3Fe cluster in the 7Fe ferredoxin from Desulfovibrio africanus, incorporating Fe and other metals to to form cubanes (M3Fe-4S) has a non-cystein ligation to the added metal (M). Connectivities among interconversion with different M, the influence of pH and the reactivities of certain cluster oxidation levels have been studied using electrochemical and spectroscopic methods.