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Biomedical subjects

D Cahen

Publications and source records attributed to D Cahen.

16 recordsLinked to original sources
PubMed2005

Endoscopic drainage of pancreatic pseudocysts: long-term outcome and procedural factors associated with safe and successful treatment.

BACKGROUND AND STUDY AIMS: Endoscopic drainage is a widely used treatment modality for pancreatic pseudocysts and has challenged more traditional drainage techniques. This retrospective study evaluates the short-term and long-term results with this technique and aims to identify procedural modifications that may improve its safety and efficacy. PATIENTS AND METHODS: All consecutive patients who underwent endoscopic drainage of pancreatic pseudocysts in our hospital between 1983 and 2000 were included in the study. The patients' charts were reviewed, and long-term follow-up data were obtained by written questionnaires sent to the patients at the end of the follow-up period in November 2002. RESULTS: A total of 92 patients were included (66 men, 26 women; median age 49 years). The technical success rate of the drainage procedure was 97 % and the mortality rate was 1 %. Complications occurred in 31 patients (34 %), eight of which (9 %) were major and required surgery: hemorrhage in four cases (three of which were caused by erosion of a straight endoprosthesis through the cyst wall), secondary infection in three, and perforation in one. During a median follow-up period of 43 months, 10 patients (11 %) underwent additional (nonendoscopic) treatment for a persistent cyst and five (5 %) for a recurrent cyst. Overall, endoscopic drainage was successful in 65 patients (71 %). CONCLUSIONS: Endoscopic drainage is an effective treatment for pancreatic pseudocysts and offers a definitive solution in almost three-quarters of the cases. The majority of major complications might have been prevented by using pigtail stents instead of straight stents and by taking a more aggressive approach to the prevention and treatment of secondary cyst infection.

Adolescent↗
PubMed2001

Direct detection of low-concentration NO in physiological solutions by a new GaAs-based sensor.

Nitric oxide (NO) acts as a signal molecule in the nervous system, as a defense against infections, as a regulator of blood pressure, and as a gate keeper of blood flow to different organs. In vivo, it is thought to have a lifetime of a few seconds. Therefore, its direct detection at low concentrations is difficult. We report on a new type of hybrid, organic-semiconductor, electronic sensor that makes detection of nitric oxide in physiological solution possible. The mode of action of the device is described to explain how its electrical resistivity changes as a result of NO binding to a layer of native hemin molecules. These molecules are self-assembled on a GaAs surface to which they are attached through a carboxylate binding group. The new sensor provides a fast and simple method for directly detecting NO at concentrations down to 1 microM in physiological aqueous (pH=7.4) solution at room temperature.

Algorithms↗
PubMed2000

Molecular control over Au/GaAs diodes

The use of molecules to control electron transport is an interesting possibility, not least because of the anticipated role of molecules in future electronic devices. But physical implementations using discrete molecules are neither conceptually simple nor technically straightforward (difficulties arise in connecting the molecules to the macroscopic environment). But the use of molecules in electronic devices is not limited to single molecules, molecular wires or bulk material. Here we demonstrate that molecules can control the electrical characteristics of conventional metal-semiconductor junctions, apparently without the need for electrons to be transferred onto and through the molecules. We modify diodes by adsorbing small molecules onto single crystals of n-type GaAs semiconductor. Gold contacts were deposited onto the modified surface, using a 'soft' method to avoid damaging the molecules. By using a series of multifunctional molecules whose dipole is varied systematically, we produce diodes with an effective barrier height that is tuned by the molecule's dipole moment. These barrier heights correlate well with the change in work function of the GaAs surface after molecular modification. This behaviour is consistent with that of unmodified metal-semiconductor diodes, in which the barrier height can depend on the metal's work function.

Journal Article↗
PubMed1984

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. Simulations are performed for various values of the rate constants and energetic changes. It is shown that the net heat uptake during a low, final step postulated by some workers should be reflected in the photoacoustic magnitude frequency spectrum, giving rise to a characteristic maximum. However, this effect, which has been observed experimentally, may also be produced by a fast, strongly endothermic step occurring earlier. The precise chronology of an endothermic transition cannot be resolved unambiguously by magnitude measurements alone, although they are free from assumptions regarding difficult-to-measure phase relationships. Hence, the published photoacoustic observations showing the effect are consistent with a cyclic sequence of events in which the bacteriorhodopsin system first undergoes an increase of entropy, followed by a decrease on returning to the initial state, as well as the reverse. It is argued that the molecular disorder-order sequence is more probable.

Journal Article↗
PubMed1982

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). This effect is interpreted in terms of cation binding to the protein, conformational changes of which are thought to be connected with the endothermic process. Wavelength spectra are used to obtain heat dissipation spectra, which allow identification of wavelength regions with varying photoactivity, and estimation of the amounts of enthalpy stored in the photointermediates. Because of bleaching and accumulation of intermediates, however, and because of the small fraction of light energy stored during photocycle, quantitative information cannot be obtained. From photoacoustic wavelength spectra of purple membrane fragments equilibrated at 63% relative humidity, rise and decay times of the bR570 and M412 intermediates are calculated.

Acoustic Stimulation↗
PubMed1980

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. The effect of the background light was saturable and was absent when non-active (e.g. heat-treated) samples were used, showing that the normal smaller signal in the absence of background light is a genuine reflection of the loss of heat due to the competing photochemistry. The effect of the background light is to close the reaction-centers and hence to inhibit the photochemical process. The percent difference of the photoacoustic signal (+/- background light) is taken as a measure of the photochemical activity ('photochemical loss'). Initial results demonstrate the wavelength dependence of the 'photochemical loss'. As expected there was a 'red-drop' decrease of the 'photochemical loss' for lambda greater than 690 nm, when the cofactor methyl viologen was present. Surprisingly, however, there was a 'red-rise' increase for lambda greater than 690 nm when no cofactor was present. These findings indicate that under the last conditions there is an unsuspected photoactivity of PS I which was not detected hitherto by the conventional techniques. The dependence on the background light intensity confirms this result. This photoactivity can be explained tentatively as a cyclic electron flow around PS I, present without any added cofactor. Initial results on the modulation frequency dependence in the presence of electron acceptors are also demonstrated.

Chloroplasts↗
PubMed1980

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. The dependence of the PA signal on the thermal properties of the sample is particularly useful when heterogeneous samples are studied. As a photocalorimetric method the technique shows considerable promise in the study of photo-bioenergetics, especially photosynthesis. Only in special cases can analytical applications of the PA method compete with fluorescence measurements for detection, and with increasingly sophisticated optical transmission and reflectance techniques (for identification).. However, the PA method may find important uses in fundamental research and in applied areas such as biomedicine and agricultural biochemistry.

Calorimetry↗
PubMed1978

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. The necessity for proteins from the chloroplast translational machinery for this formation is evidenced by the inhibition of synthesis of the PSII units in chloramphenicol-treated cells. The effect of this drug, both during the dark and second light periods, can be summarized as follows: (a) disruption of the electron transfer connection to the plastoquinone pool, or decrease in the pool size; (b) loss of excitation energy transfer efficiency in the second light period; (c) impairment of the O(2) evolution appratus, as shown by comparison of the efficiency of DCP and H(2)O as electron donors.These conclusions are based on the use of a previously developed method of measurement and analysis of data (Cahen et al. 1976 Plant Physiol 58: 257-267).

Journal Article↗
PubMed1977

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.In the present work an attempt was made to study these two processes independently. This was achieved using two experimental systems: (a) cells of the y-1 mutant, greened in the presence of chloramphenicol (CAP), whose activity is repaired under conditions which prevent further chlorophyll synthesis and membrane growth (dark or light with addition of cycloheximide [CHI]); (b) greening cells of a pale mutant derived from y-1 in which chloroplast membranes are formed gradually during several cell divisions.In both systems different parameters of 2,6-dichlorophenolindophenol reduction, as well as fluorescence, were measured. The results are interpreted using a kinetic model reported previously. It is concluded that during repair of photosynthetic activity in inactive membranes which were formed previously in the presence of chloramphenicol, photosystem II reaction centers are formed and integrated in preexisting membranes. In the pale green there is complete de novo synthesis of units of constant composition which are added during formation of new chloroplast membranes.

Journal Article↗
PubMed1976

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. The ratio of active centers to Chl remains practically constant throughout the process while the degree of connectivity between the active center and the plastoquinone pool was doubled during the early phase of the greening. In addition the degree of connectivity between the plastoquinone pool and the rest of the electron transport chain increases as demonstrated by a 10- to 20-fold rise in the quantum yield and a 10-fold rise in the maximal rate and the flash yield. The ratio of light harvesting Chl to active centers remains apparently constant during the second phase of the greening as indicated by light saturation experiments and by the constancy of the apparent photosynthetic unit size. Electron donation from H(2)O seems to develop slower than the activity of the rest of the complex as demonstrated by measurements of 2,6-dichlorophenolindophenol photoreduction using 1,5-diphenylcarbazide as the electron donor. The value of all the above parameters which remain constant during the second phase of the greening are comparable to those obtained with membranes of light-grown cells.

Journal Article↗