Observation of Wakefields and Resonances in Coherent Synchrotron Radiation

We report on high resolution measurements of resonances in the spectrum of coherent synchrotron radiation (CSR) at the Canadian Light Source (CLS). The resonances permeate the spectrum at wave number intervals of 0.074  cm(-1), and are highly stable under changes in the machine setup (energy, bucket filling pattern, CSR in bursting or continuous mode). Analogous resonances were predicted long ago in an idealized theory as eigenmodes of a smooth toroidal vacuum chamber driven by a bunched beam moving on a circular orbit. A corollary of peaks in the spectrum is the presence of pulses in the wakefield of the bunch at well-defined spatial intervals. Through experiments and further calculations we elucidate the resonance and wakefield mechanisms in the CLS vacuum chamber, which has a fluted form much different from a smooth torus. The wakefield is observed directly in the 30-110 GHz range by rf diodes, and indirectly by an interferometer in the THz range. The wake pulse sequence found by diodes is less regular than in the toroidal model, and depends on the point of observation, but is accounted for in a simulation of fields in the fluted chamber. Attention is paid to polarization of the observed fields, and possible coherence of fields produced in adjacent bending magnets. Low frequency wakefield production appears to be mainly local in a single bend, but multibend effects cannot be excluded entirely, and could play a role in high frequency resonances. New simulation techniques have been developed, which should be invaluable in further work.

Comparing and correlating solubility parameters governing the self-assembly of molecular gels using 1,3:2,4-dibenzylidene sorbitol as the gelator

Solvent properties play a central role in mediating the aggregation and self-assembly of molecular gelators and their growth into fibers. Numerous attempts have been made to correlate the solubility parameters of solvents and gelation abilities of molecular gelators, but a comprehensive comparison of the most important parameters has yet to appear. Here, the degree to which partition coefficients (log P), Henry's law constants (HLC), dipole moments, static relative permittivities (ε(r)), solvatochromic E(T)(30) parameters, Kamlet-Taft parameters (β, α, and π), Catalan's solvatochromic parameters (SPP, SB, and SA), Hildebrand solubility parameters (δ(i)), and Hansen solubility parameters (δ(p), δ(d), δ(h)) and the associated Hansen distance (R(ij)) of 62 solvents (covering a wide range of properties) can be correlated with the self-assembly and gelation of 1,3:2,4-dibenzylidene sorbitol (DBS) gelation, a classic molecular gelator, is assessed systematically. The approach presented describes the basis for each of the parameters and how it can be applied. As such, it is an instructional blueprint for how to assess the appropriate type of solvent parameter for use with other molecular gelators as well as with molecules forming other types of self-assembled materials. The results also reveal several important insights into the factors favoring the gelation of solvents by DBS. The ability of a solvent to accept or donate a hydrogen bond is much more important than solvent polarity in determining whether mixtures with DBS become solutions, clear gels, or opaque gels. Thermodynamically derived parameters could not be correlated to the physical properties of the molecular gels unless they were dissected into their individual HSPs. The DBS solvent phases tend to cluster in regions of Hansen space and are highly influenced by the hydrogen-bonding HSP, δ(h). It is also found that the fate of this molecular gelator, unlike that of polymers, is influenced not only by the magnitude of the distance between the HSPs for DBS and the HSPs of the solvent, R(ij), but also by the directionality of R(ij): if the solvent has a larger hydrogen-bonding HSP (indicating stronger H-bonding) than that of the DBS, then clear gels are formed; opaque gels form when the solvent has a lower δ(h) than does DBS.

Step-scan IR spectroelectrochemistry with ultramicroelectrodes: nonsurface enhanced detection of near femtomole quantities using synchrotron radiation

The result of interfacing step-scan spectroelectrochemistry with an IR microscope and synchrotron infrared (SIR) radiation is provided here. An external reflectance cell containing a 25 μm gold ultramicroelectrode is employed to achieve an electrochemical time constant less than one microsecond. The use of a prototypical electrochemical system, i.e., the mass-transport controlled reduction of ferricyanide, allows for a proof of principle evaluation of the viability of SIR for step-scan spectroelectrochemistry. An analysis of the importance of accounting for synchrotron source variation over the prolonged duration of a step-scan experiment is provided. Modeling of the material flux in the restricted diffusion space afforded by the external reflectance cell allows the quantitative IR results to be compared to theoretical predictions. The results indicate that only at very short times does linear diffusion within the cavity dominate the electrode response and the majority of the transient signal operates under conditions of quasi-hemispherical diffusion. The analytical information provided by the IR signal is found to be considerably less than that derived from the current response due the latter's pronounced edge effects. The results provide a detection limit of 36 fmol for step-scan SIR measurements of ferrocyanide. Implications for future IR spectroelectrochemical studies in the microsecond domain are discussed.

Interface for time-resolved electrochemical infrared microspectroscopy using synchrotron infrared radiation

A description of a coupled electrochemical and spectrometer interface using synchrotron infrared radiation is provided. The interface described allows for the precise and accurate timing needed for time-resolved IR spectroscopic studies of electrochemical systems. The overall interface uses a series of transistor-transistor logic trigger signals generated from the commercial FTIR spectrometer to regulate the recording of control, electrochemical, and IR signals with reproducible and adjustable timing. The instrument has been tested using a thin-layer electrochemical cell with synchrotron light focused through microscope optics. The time-resolved response of the benzoquinone/dihydroxybenzoquinone redox couple is illustrated as an example of the instrument's capability.

Improved spatial resolution for reflection mode infrared microscopy

Standard commercial infrared microscopes operating in reflection mode use a mirror to direct the reflected light from the sample to the detector. This mirror blocks about half of the incident light, however, and thus degrades the spatial resolution by reducing the numerical aperture of the objective. Here, we replace the mirror with a 50% beamsplitter to allow full illumination of the objective and retain a way to direct the reflected light to the detector. The improved spatial resolution is demonstrated using two different microscopes capable of diffraction-limited resolution: the first microscope is coupled to a synchrotron source and utilizes a single point detector, whereas the second microscope has a standard blackbody source and uses a focal plane array detector.

Photoacoustic infrared spectroscopy at the Canadian Light Source: commissioning experiments

The commissioning of synchrotron radiation (SR) photoacoustic (PA) infrared spectroscopy at the Canadian Light Source is described in this article. Aperture tests demonstrated an exponential relationship between the wavenumber where SR and thermal-source PA intensities are equal and beam diameter. Total PA intensity increased linearly with aperture size up to 1.5 mm (SR) or 3 mm (thermal source). At larger apertures, this intensity approached a limiting value. The SR beam diameter in the spectrometer was estimated to be about 0.8 mm in these tests. The low-frequency noise and dc offset that characterize SR PA spectra are suppressed through the calculation of average interferograms prior to Fourier transformation.

A prospective study of femoral versus subclavian vein catheterization during cardiac arrest

Femoral vein catheterization has advantages over subclavian vein catheterization during cardiac arrest in that there is minimal interference to ongoing CPR. In addition, risks of subclavian catheterization are not a factor in femoral vein catheterization. Few studies have compared the success rate for catheterization of one site with that of the other during cardiac arrest. We conducted a prospective study to compare the success and complication rates for femoral with those of subclavian vein catheterization. Ninety-four patients undergoing CPR had either femoral or subclavian vein catheters placed during the course of the arrest. Catheter placement was verified by injection of radiopaque contrast material. We found that the success rate for femoral catheterization was 77% compared with a success rate of 94% for subclavian vein catheterization (P less than .05). There were no instances of pneumothorax with subclavian vein catheterization. There was no apparent learning curve leading to an increased success rate during the course of the study. We conclude that femoral vein catheterization should not be used except in those instances where attempts at peripheral and central venous cannulation are unsuccessful.

Actions of the insecticide 2(nitromethylene)tetrahydro-1,3-thiazine on insect and vertebrate nicotinic acetylcholine receptors

The nitromethylene heterocyclic compound 2(nitromethylene)tetrahydro)1,3-thiazine (NMTHT) inhibits the binding of [125I]alpha-bungarotoxin to membranes prepared from cockroach (Periplaneta americana) nerve cord and fish (Torpedo californica) electric organ. Electrophysiological studies on the cockroach fast coxal depressor motorneuron (Df) reveal a dose-dependent depolarization in response to bath-applied NMTHT. Responses to ionophoretic application of NMTHT onto the cell-body membrane of motorneuron Df are suppressed by bath-applied mecamylamine (1.0 x 10(-4) M) and alpha-bungarotoxin (1.0 x 10(-7) M). These findings, together with the detection of a reversal potential close to that estimated for acetylcholine, provide evidence for an agonist action of this nitromethylene on an insect neuronal nicotinic acetylcholine receptor. The binding of [3H]H12-histrionicotoxin to Torpedo membranes was enhanced in the presence of NMTHT indicating an agonist action at this vertebrate peripheral nicotinic acetylcholine receptor. NMTHT is ineffective in radioligand binding assays for rat brain GABAA receptors, rat brain L-glutamate receptors and insect (Musca domestica) L-glutamate receptors. Partial block of rat brain muscarinic acetylcholine receptors is detected at millimolar concentrations of NMTHT. Thus nitromethylenes appear to exhibit selectivity for acetylcholine receptors and exhibit an agonist action at nicotinic acetylcholine receptors.

Isolation of a pure toxic polypeptide from the venom of the spider Phoneutria nigriventer and its neurophysiological activity on an insect femur preparation

The venom of the South American spider Phoneutria nigriventer has been separated into eleven fractions by gel filtration. The neurophysiological activity of each fraction was tested by perfusion of a locust femur preparation. Fractions which gave a neurophysiological response on this perfusion were fractionated further by ion-exchange chromatography. The purity of each sub-fraction was monitored by isoelectrofocusing on polyacrylamide gels and isoelectric points determined. From one of the polypeptide-containing fractions, a pure, almost neutral polypeptide was isolated and shown to have a molecular weight between 5500 and 5900. The amino acid composition of the pure polypeptide was: Ala6,7 Arg2,3 Asx3 Cys8 Glx3 Gly4 Ile4 Leu1 Lys5,6 Phe2 Ser5 Thr3 Trp1 Tyr2 Val2. This polypeptide elicited the greatest neurophysiological activity of all fractions tested. When the polypeptide (2 X 10(-7) M) was perfused through the locust femur preparation, action potentials were generated along the length of the axons in the crural nerve, resulting in very rapid and uncontrolled twitching of the skeletal muscles. At higher concentrations, the crural nerve discharged repetitively, both spontaneously and in response to a single electrical stimulus. The other polypeptide fractions, although less pure, had neurophysiological responses similar to those observed with the pure polypeptide; the effects of some fractions could be reversed. Further fractions of low molecular weight were purified by thin-layer or paper chromatography to give two pure components that are probably nucleosides or nucleotides. After the initial gel filtration of the total venom had separated the high molecular weight proteinases from the polypeptides, all of the polypeptides retained their neurophysiological activity in solution for several days. In the presence of the proteinases, the polypeptides were inactivated in solution in a few hours at room temperature.

Pharmacological Studies on a Locust Neuromuscular Preparation

1. The structure-activity relationships of agonists of the locust excitatory neuromuscular synapse have been reinvestigated, paying particular attention to the purity of compounds, and to the characteristics and repeatability of the muscle response. The concentrations of compounds required to stimulate contractions of the retractor unguis muscle equal in force to the neurally evoked contractions provided a measure of the relative potencies.

2. Seven amino acids were capable of stimulating twitch contractions, glutamic acid being the most active, the others being analogues or derivatives of glutamic or aspartic acid. Aspartic acid itself had no excitatory activity.

3. Excitatory activity requires possession of two acidic groups, separated by two or three carbon atoms, and an amino group α to a carboxyl. An L-configuration appears essential. The ω-acidic group may be a carboxyl, sulphinyl or sulphonyl group. Substitution of any of the functional groups generally causes total loss of excitatory activity, but an exception is found in kainic acid in which the nitrogen atom forms part of a ring.

4. The investigation of a wide variety of compounds revealed neuromuscular blocking activity among isoxazoles, hydroxylamines, indolealkylamines, β-carbolines, phenazines and phenothiazines. No specific antagonist of the locust glutamate receptor was found, but synaptic blocking agents of moderately high activity are reported.

Studies on Locust Neuromuscular Physiology in Relation to Glutamic Acid

1. Two nerve-muscle preparations were used to investigate the physiology of the locust retractor unguis muscle in relation to L-glutamic acid. These were an ‘isolated preparation’, in which the muscle and its nerve were dissected out, and a ‘perfusedfemur preparation’, in which the muscle suffered no mechanical disturbance.

2. Exposure of the nerve--muscle preparations to glutamate caused a variety of responses, some of which were shown to be abnormal and due to the experimental conditions.

3. When locust femora were perfused with saline or haemolymph the retractor unguis muscles were much more severely affected by glutamate if the hydrostatic pressure was slightly raised. At raised pressures the perfused-femur preparations were particularly prone to give repetitive and spontaneous contractions.

4. Analysis of haemolymph from adult male locusts showed that it contained, on average, 0-2 mmol/1 L-glutamate, 45 mol/1 total non-peptide amino acids, 5-0 mmol/1 calcium, and 11-6 mmol/1 magnesium. It was calculated that approximately 50% of the calcium and 75% of the magnesium ions are bound to amino acids, and that approximately 25% of the glutamic acid is bound to divalent metal ions.

5. The isolated preparations were severely affected by glutamate at the concentration at which it occurs in haemolymph, and it was concluded that in the intact locust some mechanism must protect the neuromuscular synapses from haemolymphg lutamate. No evidence could be obtained of the sequestration of glutamate by haemocytes, or of binding of glutamate to haemolymph proteins.

6. Calcium and magnesium ions reduced the sensitivity of nerve-muscle preparations to glutamate to a greater extent than could be accounted for by the formation of amino acid-metal complexes. This suggests that the protection afforded by calcium and magnesium involves an interaction of the metal ions with the neuromuscular system itself.

7. The retractor unguis muscle was much less sensitive to glutamate when it was contained within an undissected femur than in an isolated preparation. It was concluded that the muscle is normally protected from haemolymph glutamate by a diffusion barrier which is damaged on dissection.

8. Comparison of the fine structure of retractor unguis muscles, fixed either after dissection or while still contained within the femur, showed that dissection normally caused a partial separation of muscle fibres and damage to the connective tissue sheath, with the resultant exposure of some nerve endings. The connective tissue sheath may constitute the postulated diffusion barrier.

9. The excitatory synapses of the locust retractor unguis muscle are believed to be isolated from haemolymph glutamate by a diffusion barrier, which is tentatively identified with the connective tissue sheath that binds the muscle fibres together. Calcium and magnesium ions reduce the sensitivity of nerve-muscle preparations to glutamate, and may have such a role in the living insect.