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E Greenbaum

Publications and source records attributed to E Greenbaum.

35 records · Page 2Linked to original sources
PubMed1995

Molecular electronics of a single photosystem I reaction center: studies with scanning tunneling microscopy and spectroscopy.

Thylakoids and photosystem I (PSI) reaction centers were imaged by scanning tunneling microscopy. The thylakoids were isolated from spinach chloroplasts, and PSI reaction centers were extracted from thylakoid membranes. Because thylakoids are relatively thick nonconductors, they were sputter-coated with Pd/Au before imaging. PSI photosynthetic centers and chemically platinized PSI were investigated without sputter-coating. They were mounted on flat gold substrates that had been treated with mercaptoacetic acid to help bind the proteins. With tunneling spectroscopy, the PSI centers displayed a semiconductor-like response with a band gap of 1.8 eV. Lightly platinized (platinized for 1 hr) centers displayed diode-like conduction that resulted in dramatic contrast changes between images taken with opposite bias voltages. The electronic properties of this system were stable under long-term storage.

Journal Article↗
PubMed1995

Chemical platinization and its effect on excitation transfer dynamics and P700 photooxidation kinetics in isolated photosystem I.

Isolated photosystem I (PSI) reaction center/core antenna complexes (PSI-40) were platinized by reduction of [PtCl6]2- at 20 degrees C and neutral pH. PSI particles were visualized directly on a gold surface by scanning tunneling microscopy (STM) before and after platinization. STM results showed that PSI particles were monomeric and roughly ellipsoidal with major and minor axes of 6 and 5 nm, respectively. Platinization deposited approximately 1000 platinum atoms on each PSI particle and made the average size significantly larger (9 x 7 nm). In addition to direct STM visualization, the presence of metallic platinum on the PSI complexes was detected by its effect of actinic shading and electrostatic shielding on P700 photooxidation and P700+ reduction. The reaction centers (P700) in both platinized and nonplatinized PSI-40 were photooxidized by light and reduced by ascorbate repeatedly, although at somewhat slower rates in platinized PSI because of the presence of platinum. The effect of platinization on excitation transfer and trapping dynamics was examined by measuring picosecond fluorescence decay kinetics in PSI-40. The fluorescence decay kinetics in both platinized and control samples can be described as a sum of three exponential components. The dominant (amplitude 0.98) and photochemically limited excitation lifetime remained the same (16 ps) before and after platinization. The excitation transfer and trapping in platinized PSI-40 was essentially as efficient as that in the control (without platinization) PSI. The platinization also did not affect the intermediate-lifetime (400-600 ps) and long-lifetime (> 2500 ps) components, which likely are related to intrinsic electron transport and to functionally uncoupled chlorophylls, respectively. The amplitudes of these two components were exceptionally small in both of the samples. These results provide direct evidence that although platinization dramatically alters the photocatalytic properties of PSI, it does not alter the intrinsic excitation dynamics and initial electron transfer reactions in PSI.

Biophysical Phenomena↗
PubMed1990

Effect of oxygen on photoautotrophic and heterotrophic growth of Chlamydomonas reinhardtii in an anoxic atmosphere.

Photoautotrophic growth of Chlamydomonas reinhardtii was shown to be independent of the presence of atmospheric oxygen. Under constant light and photoautotrophic conditions, C. reinhardtii grew equally well in either air or 367 PPM CO2-in-He. During 12-h light-dark cycles, the cells in air grew substantially faster than those grown in CO2-in-He, indicating a significant role for O2 in dark metabolism. Although cells grown under CO2-in-He were not supplied any exogenous O2, photosynthetic water splitting resulted in the liberation of O2. The effect of photoevolved O2 on the growth of C. reinhardtii was examined (1) by measuring the amount of O2 consumed by photosynthesizing algae, (2) by growing the algae heterotrophically on acetate in the dark and supplied with O2 generated by photoautotrophically grown cells, and (3) by determining the minimum level of O2 needed to stimulate CO2 evolution from cells suspended in minimal medium supplemented with acetate. The results from these investigations indicated that exogenous O2 was not required for photoautotrophic growth by C. reinhardtii and that this alga grew in an anoxic environment if supplied with CO2 and light.

Anaerobiosis↗
PubMed1989

A simple apparatus for screening absolute photosynthetic rates of single algal colonies in an anoxic atmosphere.

Photosynthetically generated O(2) was measured from single algal colonies in a He atmosphere, using an enhanced Hersch galvanic cell. The enhancement consisted of using ultrapure potassium hydroxide as the electrolyte and ultrapure lead as the anode. The galvanic cell was placed in a regulated helium-flow system containing a reaction cuvette with the colonies and an electrolysis cell for calibration. Colonies were individually irradiated using a He-Ne laser. Data collection and laser positioning for colony irradiation were microcomputer controlled. This assay system was capable of detecting O(2) production rates of 500 femtomoles per second with a signal to noise ratio of 2, a level of sensitivity that permitted the detection of photoevolved O(2) from single algal colonies. This capability provides, for the first time, an approach for quantitatively measuring the absolute rate of photosynthetic O(2) evolution from a single algal colony.

Journal Article↗
PubMed1988

Establishment of control parameters for in situ, automated screening of sustained hydrogen photoproduction by individual algal colonies.

An apparatus was constructed which allowed automated screening of individual microalgal colonies for sustained ability to photoevolve H(2) during anaerobic photosynthesis. The main components of this apparatus were a microcomputer, a He-Ne laser mounted on a computer-controlled X-Y translation stage, a flow-through chamber which contained an agar plate of colonies, and a H(2) detector which interfaced with the microcomputer for data collection. The system was capable of detecting a minimum production rate of 1 nanomole of H(2) per hour per colony and provided an efficient means of screening relatively large numbers of algal colonies. Examination of the effect of the spacing of colonies on the agar plate, light intensity, stability of colonies within a screening period, colony age, chlorophyll content, and colony size on H(2) yield indicated that, under optimum conditions, yields from genetically uniform colonies varied by no more than a factor of 2 in their H(2)-producing ability. Therefore, colonies of algae whose H(2) yields lie outside this intrinsic twofold variability can be identified and selected as natural variants or mutants. A description of the construction and of the apparatus is presented, and the experimental results used to establish the control parameters for Chlamydomonas reinhardtii colonies are discussed.

Journal Article↗
PubMed1984

Sclerotherapy induced pseudo-carcinoma.

Endoscopic sclerotherapy has been suggested as the therapy of choice for acute hemorrhage from esophageal varices. It not only controls such episodes, but also prevents rebleeding, and prolongs life. After treatment, patients may present with endoscopic and radiographic findings which mimic esophageal carcinoma including ulceration, non-pliable walls, irregular mucosal pattern and overhanging edges.

Diagnosis, Differential↗
PubMed1972

Photoreduction of NADP + sensitized by synthetic pigment systems.

Two synthetic pigment systems capable of enzymatically photoreducing NADP(+) are described. One system contains proflavine; the other, acridine. The complete system consists of ethylene diamine tetraacetic acid (EDTA), proflavine (or acridine), ferredoxin-NADP reductase (EC 1.6.99.4), and NADP(+). The two pigments initiate the photoreduction of NADP(+) in different portions of the electromagnetic spectrum. Proflavine photosensitizes in the visible portion; acridine, in the ultraviolet. Neither proflavine nor acridine is structurally related to chlorophyll. The acridine system has the attractive property that the enzyme, ferredoxin-NADP reductase, is the only component of the system that absorbs appreciably in the visible region of the spectrum.

Acridines↗