The dependence of electron transfer efficiency on the conformational order in organic monolayers.

The electron transfer through an organized organic monolayer of alkyl chains adsorbed on a silicon wafer has been studied. The silicon was used as an electrode in a three-electrode electrochemical cell, and the current versus voltage response was measured. The results show that when the chains in the monolayer are in the "all trans" configuration, the charge transfer efficiency is higher than when the chains have a "gauche" configuration.

Room-Temperature, Electric Field-Induced Creation of Stable Devices in CulnSe2 Crystals.

Multiple-junction structures were formed, on a microscopic scale, at room temperature, by the application of a strong electric field across originally homogeneous crystals of the ternary chalcopyrite semiconductor CulnSe(2). After removal of the electric field, the structures were examined with electron beam-induced current microscopy and their current-voltage characteristics were measured. Bipolar transistor action was observed, indicating that sharp bulk junctions can form in this way at low ambient temperatures.

The use of photothermal radiometry in assessing leaf photosynthesis: I. General properties and correlation of energy storage to P700 redox state.

Energy storage measurements by modulated photothermal radiometry (PTR) were carried out on intact leaves to assess the value of the PTR method for photosynthesis research. In particular, correlations to the redox state of P700 under various conditions were examined. PTR monitors modulated light conversion to heat by sensing the resulting modulated infra-red radiation emitted from the leaf.

Photoacoustic study of the green alga Trebouxia in the lichen Ramalina duriaei in vivo.

Photosynthetic parameters of the lichen Ramalina duriael were investigated in vivo, by the photoacoustic method using intensity-modulated exciting light (frequency range 5-300 Hz). The photoacoustic signal in intact lichens was similar in its general characteristics to that obtained from intact leaves of higher plants (Poulet et al., Biochim.

Simulations of frequency-dependent photoacoustic magnitude signals and their implications for bacteriorhodopsin photocycle energetics.

The modulation frequency dependence of photoacoustic signals obtained from photoactive samples can provide information on the time-dependent enthalpy changes occurring during the light-induced process. The experimental requirements for this type of calorimetry, and the interpretation ot the results, are critically examined with reference to the light-driven proton pump bacteriorhodopsin. For a three-step unbranched model of the bacteriorhodopsin photocycle we derive an expression for the photoacoustic magnitude signal as a function of frequency.

Photoacoustic photocalorimetry and spectroscopy of Halobacterium halobium purple membranes.

Enthalpy changes associated with intermediates of the photocycle of bacteriorhodopsin (bR) in light-adapted Halobacterium halobium purple membranes, and decay times of these intermediates, are obtained from photoacoustic measurements on purple membrane fragments. Our results, mainly derived from modulation frequency spectra, show changes in the amount of energy stored in the intermediates and in their decay times as a function of pH and/or salt concentration. Especially affected are the slowest step (endothermic) and a spectroscopically unidentified intermediate (both at pH 7).

Photoacoustic detection of photosynthetic activities in isolated broken chloroplasts.

Methodology and demonstration how to utilize the photoacoustic technique in photosynthesis research are presented. Photoacoustic signals were obtained from suspensions of isolated broken chloroplasts. In the presence of strong, continuous (non-modulated) background light the signals were normally larger than without the background light.

Photoacoustics in life sciences.

Photoacoustic (PA) measurements provide, by the very nature of the PA effect, the possibility to obtain information on the optical and thermal properties of samples. In addition they can yield information on the enthalpy changes and characteristic times involved in photo-induced processes as the acoustic signal is proportional to the hear produced following the absorption of the modulated excitation. In the study of optical properties the relative insensitivity to scattered light of the PA signal makes such measurement an attractive way to measure biological samples in vivo, or, at least, without the need to isolate the absorbing compounds.

Simple setup for single and differential photoacoustic spectroscopy.

Simple cells are described for normal and differential measurements in photoacoustic spectroscopy. The differential cell allows for easy background signal correction and for comparison of related samples. The arrangement allows great flexibility in cell design for adaptation to special sample forms.

Development and Repair of Photosystem II Activity in Normal and Chloramphenicol-treated Euglena gracilis Cells.

Photosystem II activity, development, and organization were studied in membranes from dark-adapted Euglena gracilis Klebs var. Z Pringsheim cells during a modulated greening process of light-dark-light cycles. The results obtained from measurements of overlapping partial photosystem II (PSII) reactions (fluorescence induction parameters, quantum yield, flash yield, and maximal rate of H(2)O --> 2,6-dichlorophenolindophenol [DCIP] and 1,5-diphenylcarbazide [DPC] --> DCIP reactions) during these cycles indicate the formation of active PSII units in the dark.

Development of Photosystem II Activity in Chlamydomonas reinhardi Mutants: Insertion of Photosystem II Units into Inactive Preexisting Membranes versus Continuous Formation of New Photosynthetic Membranes.

In a previous work the development of photosystem II activity during the greening process of the y-1 mutant of Chlamydomonas reinhardi was studied (Cahen, Malkin, Shochat, Ohad. Plant Physiol 58: 257-267). Measurements of quantum yield, maximal rate of electron transfer, flash yield, and fluorescence induction indicated that photosystem II development consists of two partially overlapping phases: (a) reorganization and integration of preexisting units; and (b) addition of newly formed units to the growing membranes.

Development of Photosystem II Complex during Greening of Chlamydomonas reinhardi y-1.

The relative content of organized pigment, active centers, and acceptor pools of photosystem II and their interconnection during the development of the photosynthetic membranes of Chlamydomonas reinhardi y-1 have been measured using the fluorescence induction technique. The degree of connectivity and efficiency of the developing system has been assessed also from measurements of maximal rates, quantum yield, and flash yield of 2,6-dichlorophenolindophenol photoreduction using H(2)O as the electron donor. The results obtained indicate that the process of membrane development in this organism consists of two phases: an initial phase of reorganization and connection between pre-existing components, and a second phase of actual accumulation of newly formed, complete, and active units.