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Circular dichroism (CD) measurements help characterize optically active molecules and higher-order biomolecular structures. The harmonization of interlaboratory CD measurements requires minimizing measurement uncertainties and determinate errors. Most CD measurements utilize a single-wavelength intensity measurement at a spectral peak to calibrate the intensities of the ultraviolet wavelength range. However, such a single-wavelength calibration is inherently less precise than using the CD instrument's spectrum over the entire measured spectral range. A more thorough and informative calibration can be achieved by remapping the spectrum into what we call a spectral similarity plot. This process allows a straightforward, quantitative evaluation of the shape congruence between two spectra over the full spectral range. While preliminary analyses suggest spectral similarity plots can be utilized with a variety of different spectrometry methods, here we illustrate the process applied to circular dichroism. Spectral similarity plots are highly sensitive to deviations due to differences in concentration, pathlength, source and detector properties, circular polarization balance, as well as wavelength nonlinearities and shifts. Deviations in these properties can be quantitated by a linear least squares fit of the remapped data. The remapping enables protocols to correct spectra toward congruence between two spectra. The spectra similarity comparison provides an objective, unambiguous test of the CD instrument quality when, for example, compared to a carefully calibrated system as shown in the examples described in the text.

X-ray and neutron scattering have provided insight into the short range (

Proteins are polymers, and yet the language used in describing their thermodynamics and kinetics is most often that of small molecules. Using the terminology and mathematical descriptions of small molecules impedes understanding why proteins have evolved to be big in comparison. Many properties of the proteins should be interpreted as polymer behavior, and these arise because of the longer length scale of polymer dimensions. For example, entropic rubber elasticity arises only because of polymer properties, and understanding the separation of entropic and enthalpic contributions shows that the entropic contributions mostly reside within the polymer and enthalpy originates mostly at the site of small-molecule binding. Recognizing the physical chemistry of polymers in descriptions of proteins’ structure and function can add clarity to what might otherwise appear to be confusing or even paradoxical behavior. Two of these paradoxes include, first, highly selective binding that is, nevertheless, weak, and, second, small perturbations of an enzyme that cause large changes in reaction rates. Further, for larger structures such as proteins every thermodynamic measurement depends on the length scale of the structure. One reason is that the larger molecule can control up to thousands of waters resulting in collective movements with kcal sums of single-calorie-per-molecule solvent energy changes. In addition, the nature of covalent polypeptides commonly leads to multiple binding—i.e., multivalency—and the benefits of multivalent binding can be assessed semiquantitatively drawing from understanding the chelate effect in coordination chemistry. Such approaches clarify the origins, inter alia, of many low energies of protein denaturation, which lie in the range of only a few kcal mol−1, and the difficulties in finding the structures of proteins in the multiple substates postulated within complex kinetic schemes. These models involving longer length scales can be used to elucidate why such observed behavior occurs, and can provide insight and clarity where the phenomena modeled employing experimentally inseparable translational, vibrational, and rotational entropy along with charge, dipole moment, hydration, hydrogen bonding, and van der Waals energies together obscure such origins. The short-distance, long distance separation does not include explaining any enzymatic lowering of activation energies due to stabilization of the intermediate(s) along the reaction path. However, well known small-molecule methods that treat electrostatics and bonding can be used to explain the local chemistry that contributes most of the changes in enthalpy and activation enthalpy for the process.

As has long been understood, the noise on a spectrometric signal can be reduced by averaging over time, and the averaged noise is expected to decrease as t1/2, the square root of the data collection time. However, with contemporary capability for fast data collection and storage, we can retain and access a great deal more information about a signal train than just its average over time. During the same collection time, we can record the signal averaged over much shorter, equal, fixed periods. This is, then, the set of signals over submultiples of the total collection time. With a sufficiently large set of submultiples, the distribution of the signal’s fluctuations over the submultiple periods of the data stream can be acquired at each wavelength (or frequency). From the autocorrelations of submultiple sets, we find only some fraction of these fluctuations consist of stochastic noise. Part of the fluctuations are what we call “fast drift”, which is defined as drift over a time shorter than the complete mea...

Mixtures of light and heavy water are used in NMR, small-angle neutron scattering (SANS), growth media for producing deuterated biological molecules, and analytical methods such as hydrogen–deuterium exchange (HDX) mass spectrometry. It is common to measure the pH of these solutions with a combination glass electrode with all chambers filled with aqueous (H2O) potassium chloride solutions. In the daily measurement of samples containing mixtures of H2O with D2O in some ratio – call this measurement p(H,D) – we generally do not control for all of the contributions to the differences measured in carefully controlled electrochemical experiments. For example, the calibration solutions contain relatively low concentrations of the calibrant buffer with low or no added salt. Meanwhile the tested solutions can contain widely varying levels of any number of different salts as well as both polar and nonpolar organics and polymers and proteins. In this note, the p(H,D) behaviors of 50 mM solutions of five different buffers used in biological in vitro solutions were measured over the full range of H2O : D2O ratios in the open atmosphere. After calibration, pH measurements were made with the buffer solutions alone and with 100 mM KCl added to model a significant ionic strength difference. The solutions consisted of 1 : 1 volume mixtures of the acid and base forms of acetate, monobasic/dibasic phosphate, 2-amino-2-hydroxymethyl-propane-1,3-diol (tris), 2-amino-2-hydroxymethyl-propane-1,3-diol (HEPES), and glycine to span the common, full range for biological buffers. The pH values of the 1 : 1 mixtures mean that the measurements were, in fact, of their formal pKa values. Each of the buffers exhibited a unique pattern of behavior, and none of them exhibited a measured ΔpKa = pKDa − pKHa as large as 0.4, a value that has been suggested to be added to a pH measured in H2O to match the equivalent pD measured in D2O. The results do indicate that when a reasonable, required accuracy for pH measurement is ±0.1 units, three general guidelines apply: (1) where p(H,D) values are less than 8 for any D2O content, no correction is needed for the p(H,D) measurement when comparing it to pHH; (2) for less than 50% D2O, if the 8 8, any corrections required will depend on the specific conditions and the specific buffer. Outside of the range 4 < p(H,D) < 10 or for needed greater accuracy, any corrections required will depend on the specific conditions and the identity of the buffer.

X-ray and neutron scattering have been used to provide insight into the structures of ionic solutions for over a century, but the probes have covered distances shorter than 8 Å. For the non-hydrolyzing salt SrI2 in aqueous solution, a locally ordered lattice of ions exists that scatters slow neutrons coherently down to at least 0.1 mol L(-1) concentration, where the measured average distance between scatterers is over 18 Å. To investigate the motions of these scatterers, coherent quasielastic neutron scattering (CQENS) data on D2O solutions with SrI2 at 1, 0.8, 0.6, and 0.4 mol L(-1) concentrations was obtained to provide an experimental measure of the diffusive transport rate for the motion between pairs of ions relative to each other. Because CQENS measures the motion of one ion relative to another, the frame of reference is centered on an ion, which is unique among all diffusion measurement methods. We call the measured quantity the pairwise diffusive transport rate Dp. In addition to this ion centered frame of reference, the diffusive transport rate can be measured as a function of the momentum transfer q, where q = (4π/λ)sin θ with a scattering angle of 2θ. Since q is related to the interion distance (d = 2π/q), for the experimental range 0.2 Å(-1)≤q≤ 1.0 Å(-1), Dp is, then, measured over interion distances from 40 Å to ≈6 Å. We find the measured diffusional transport rates increase with increasing distance between scatterers over the entire range covered and interpret this behavior to be caused by dynamic coupling among the ions. Within the model of Fickian diffusion, at the longer interionic distances Dp is greater than the Nernst-Hartley value for an infinitely dilute solution. For these nm-distance diffusional transport rates to conform with the lower, macroscopically measured diffusion coefficients, we propose that local, coordinated counter motion of at least pairs of ions is part of the transport process.

The solution structure of the full-length DNA helicase minichromosome maintenance protein from Methanothermobacter thermautotrophicus was determined by small-angle neutron scattering (SANS) data together with all-atom molecular modeling. The data were fit best with a dodecamer (dimer of hexamers). The 12 monomers were linked together by the B/C domains, and the adenosine triphosphatase (AAA+) catalytic regions were found to be freely movable in the full-length dodecamer both in the presence and absence of Mg(2+) and 50-meric single-stranded DNA (ssDNA). In particular, the SANS data and molecular modeling indicate that all 12 AAA+ domains in the dodecamer lie approximately the same distance from the axis of the molecule, but the positions of the helix-turn-helix region at the C-terminus of each monomer differ. In addition, the A domain at the N-terminus of each monomer is tucked up next to the AAA+ domain for all 12 monomers of the dodecamer. Finally, binding of ssDNA does not lock the AAA+ domains in any specific position, which leaves them with the flexibility to move both for helicase function and for binding along the ssDNA.

X-ray and neutron scattering have been used to provide insight into the structures of ionic solutions for over a century. Most of the structures discussed in the literature cover distances less than 8 A. Outside that distance, single scattering bands have, however, been seen. For the non-hydrolyzing salt SrI2 in aqueous solution, a structure sufficient to scatter slow neutrons persists down to a concentration of at least 0.1 mol·L−1 at which the measured average distance between scatterers is over 18 A. Over the concentration range of 1 to 0.1 mol·L−1, the full distribution of the distances between the scatterers remains within only about 10 A, the size of an ion and its first hydration shell. This measurably correlated structure of the ions in the solution appears to hold because changes in hydration (and interior distances along any single spatial dimension) require displacements near the size of a water molecule. Together, these facts support a rotatory mechanism for simultaneous ion transport and water countertransport. A formula is presented for calculating the average, equally spaced interscatterer distance as a function of concentration. Using this relationship, the experimental results are interpreted as showing increasing ion association as the salt concentration is raised.

Heparin is a linear, anionic polysaccharide that is widely used as a clinical anticoagulant. Despite its discovery 100 years ago in 1916, the solution structure of heparin remains unknown. The solution shape of heparin has not previously been examined in water under a range of concentrations, and here is done so in D2O solution using small-angle neutron scattering (SANS). Solutions of 10 kDa heparin-in the millimolar concentration range-were probed with SANS. Our results show that when sodium concentrations are equivalent to the polyelectrolyte's charge or up to a few hundred millimoles higher, the molecular structure of heparin is compact and the shape could be well modeled by a cylinder with a length three to four times its diameter. In the presence of molar concentrations of sodium, the molecule becomes extended to nearly its full length estimated from reported X-ray measurements on stretched fibers. This stretched form is not found in the presence of molar concentrations of potassium ions. In this high-potassium environment, the heparin molecules have the same shape as when its charges were mostly protonated at pD approximate to 0.5, that is, they are compact and approximately half the length of the extended molecules.

Aqueous solutions of poly(ethylene glycol) (PEG) exhibit some remarkable properties, among which is the small changes in water activity compared to the volumes occupied by the PEG: For example, the water in a 20% mass fraction solution of 6000 Da PEG has an activity of 0.9939. We have investigated PEGs with molecular weights 200, 400, 1000, 2000, 4000, and 8000 Da in the concentration range 1% to 17% mass fraction at neutral pH and with added KCl concentrations of 10 mmol L−1 in aqueous solutions–conditions near those for promoting protein crystallization. These solutions exhibit a structural change at around 6% mass fraction as seen in the solution viscosities, compressibilities, and infrared spectra. Raman spectroscopy shows that the PEGs remain in the same structural form over the concentration range, and the infrared spectra indicate that the change must be due to a local shift in the water structure. Modeling of the results from small-angle neutron scattering (SANS) on the solutions suggests that the structures of the PEGs in the molecular mass range 2000 Da to 8000 Da are paired in the solution, and the separation distance decreases with increasing PEG concentration. From the structure, it becomes clear that the small effect on water activity occurs because of screening by the more weakly bound outer layers. From the bulk measurement of aw and with reasonable assumptions, a free energy ΔG° can be assigned to each of the fourth, third, and second hydration layers.

Fluorous liquids are the least polarizable condensed phases known, and their nonpolar members form solutions with conditions the closest to being in vacuo. A soluble salt consisting of a large fluorophilic anion, tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and its counterion, tetra-n-butylammonium, dissolved in perfluoromethylcyclohexane produces ionic solutions with extremely low conductivity. These solutions were subjected to small-angle neutron scattering (SANS) to ascertain the solute structure. At concentrations of 9% mass fraction, the fluorophilic electrolyte forms straight, long (>160 A) self-assembled structures that are, in essence, long, homogeneous cylinders. Molecular models were made assuming a requirement for electroneutrality on the shortest length scale possible. This shows a structure formed from a stack of alternating anions and cations, and the structures fit the experimental scattering well. At the lower concentration of 1%, the stacks of ion pairs are shorter and eventually break up to form solitary ion pairs in the solution. These characteristics suggest such conditions provide an interesting new way to form long, self-assembling ionic nanostructures with single-molecule diameters in free solution onto which various moieties could be attached.

Poly(ethylene glycol)s of all molecular weights are highly soluble in water. Small-Angle Neutron Scattering (SANS) data has been collected on buffered ionic D2O solutions of PEGs of nominal molecular weights 2000, 4000, and 8000 Da at a low concentration of 0.5% (w/v). The radii of gyration Rg vary as approximately Mw0.5, which is consistent with both a Gaussian chain and a flat plate. Numerous types of experiments indicate flaws in the interpretation as a Gaussian chain. When the alternative interpretation is used, the structures in these dilute solutions are found to have a constant area stoichiometry of 1:1 water:–CH2CH2O– and are consistent with the data when monomeric molecules are plates of packed chains one chain thick.

Small-Angle Neutron Scattering (SANS) is a powerful, nondestructive technique that can measure simultaneously macroscopic compressibilities of solutions and overall shapes of macromolecular solutes, as well as their intermolecular structural correlations. We have conducted SANS experiments on aqueous solutions of polyethylene glycols (PEGs) with nominal molecular masses 2000, 4000, and 8000 Da over the q -range 0.03–0.30 A −1 [ q = (2π/ λ )sin θ ]. By incorporating accurate background subtraction and short extrapolations of the intermolecular structure factor S ( q ) down to q = 0, the isothermal compressibility can be measured. The results indicate a significant and systematic dependence of the solutions' compressibility on both molecular mass and concentration of PEG, unlike the solutions' osmotic pressures and activity of the water. This implies that the structure of water in the vicinity of PEG is considerably altered relative to the bulk state even though the activity coefficient of water remains nearly invariant in this range. Graphs of S ( q ) for 3% w/w to 17% w/w solutions each show a gradual rise from the low- q side to a broad plateau, which indicates weak intermediate-range correlations between oligomers that are probably associated with soft, repulsive, solvent-mediated PEG–PEG interactions. Since both the water and PEG change structures from their neat forms, any quantitative assignment of changes in partial volumes must necessarily be arbitrary. However, the linear change in compressibility with PEG concentration below ∼7% w/v can be said to indicate a composite solution, which parallels the behavior of composite solids.

Small-angle neutron scattering (SANS) provides a unique method to probe soft matter in the 10–100 nm length scale in solutions. In order to determine the shape and size of biological macromolecular structures correctly with SANS, a background-subtracted, undistorted scattering curve must be measured, and the required accuracy and precision is especially needed at the short-length-scale limit. A true scattering curve is also needed to discern whether intermolecular interactions are present, which also are probed in the SANS experiment. This article shows how to detect intermolecular interactions so that subsequent structure modeling can be performed using only data that do not contain such contributions. It is also shown how control of many factors can lead to an accurate baseline, or background, correction for scattering from proteins, especially to account for proton incoherent scattering. Failure to make this background correction properly from proteins, polymers, nucleic acids and lipids can result in incorrect values for the calculated shapes and sizes of the molecules as well as the derived magnitudes of the intermolecular interactions.

We introduce a two-dimensional method for mass spectrometry in solution that is based on the interaction between a nanometer-scale pore and analytes. As an example, poly(ethylene glycol) molecules that enter a single α-hemolysin pore cause distinct mass-dependent conductance states with characteristic mean residence times. The conductance-based mass spectrum clearly resolves the repeat unit of ethylene glycol, and the mean residence time increases monotonically with the poly(ethylene glycol) mass. This technique could prove useful for the real-time characterization of molecules in solution.

Quality data collection for macromolecular cryocrystallography requires suppressing the formation of crystalline or microcrystalline ice that may result from flash-freezing crystals. Described here is the use of lithium formate, lithium chloride and other highly soluble salts for forming ice-ring-free aqueous glasses upon cooling from ambient temperature to 100 K. These cryosalts are a new class of cryoprotectants that are shown to be effective with a variety of commonly used crystallization solutions and with proteins crystallized under different conditions. The influence of cryosalts on crystal mosaicity and diffraction resolution is comparable with or superior to traditional organic cryoprotectants.

A truncated transmission line probe (TLP) has been utilized to excite and detect time domain responses after pulsed excitation in electron paramagnetic resonance (EPR) spectroscopic experiments in the frequency range 200-400 MHz. The TLP device is a modified short-circuited coaxial line, which allows the irradiation of the sample by the traveling wave B1 fields in the frequency range of kilohertz to 30 GHz. In EPR studies at 300 MHz carrier frequency, with 10 W incident power, a 45 degrees pulse is 45 ns in duration. This corresponds to a 0.9-G B1 field. Using the TLP, time-domain responses from the solid N-methyl pyridinium tetra-cyanoquinodimethane (TCNQ) were collected at 200, 250, 300, and 350 MHz, with the range limited by the amplifiers. In addition two tubes containing TCNQ placed side-by-side vertically along the axis of the probe were used to collect time domain responses in the presence of magnetic field gradients to test the feasibility of two-dimensional imaging using a TLP. The magnetic field gradient was steered in the xz plane and 36 projections were collected at 5 degrees intervals. Using filtered back-projection image reconstruction, the two-dimensional spatial image in the xz plane was obtained at good resolution. Copyright 1998 Academic Press.

The adsorption of fibrinogen (Fb) and bovine serum albumin onto polycrystalline Au coated with HS(CH2)3O(CH2CH2O)5CH3 was determined by surface plasmon resonance from bare Au (0% coverage) to the complete ( approximately 100% coverage) self-assembled monolayer (SAM). Both proteins exhibit similar adsorption curves with common onset ( approximately 60% coverage) and range ( approximately 60% to 80% coverage) of minimal protein adsorption. Reflection-absorption infrared spectroscopic data show that widespread order is not present in the films over this range of coverage, indicating loosely packed, bound oligomers that are uniformly distributed and fully screen the underlying substrate. On the basis of our data, we propose a mechanism of protein rejection by oligo(ethylene oxide) (OEO)-modified surfaces in terms of changes in free energy (DeltaGsystem; system = protein + surface) due to oligomer conformational constriction over an area greater than the contact area. Minimal protein adsorption corresponds to the maximum DeltaGsystem for a given compression. This controlled study of protein adsorption provides insights into the molecular level understanding of protein adsorption unavailable from previous polymer and comparative SAM studies.

The low-resolution three-dimensional structure of purified native beef heart mitochondrial cytochrome c oxidase (COX) in asolectin unilamellar liposomes has been measured by small-angle neutron scattering under the conditions where the protein remains fully functional. From a neutron scattering perspective, the use of mixed-lipid liposomes provided for a more homogeneous matrix than can be achieved using a single lipid. As a result, the measurements were able to be performed under conditions where the liposome scattering was essentially eliminated (contrast-matched conditions). The protein structure in the membrane was modeled as a simple parallelepiped with side lengths of (59 × 70 × 120) A with uncertainties, respectively, (11, 12, 20 A). The molecular mass calculated for a typical protein with this volume is estimated to be (410 ± 124) kDa, which indicates the mass of a COX dimer. The longest dimension has some uncertainty due to intermolecular scattering contributing to the data. Nevertheless, that length was estimated using an average protein density and the known dimer molecular mass. Using the same cross sectional dimensions for the structure, the length is estimated to be 120 A. However, the measured scattering curve of the dimer in the liposome differs significantly from that calculated from the X-ray structure of the dimer in a crystal of mixed micelles (PDB 3AG1). The calculated SANS scattering from the crystal structure was fit with a parallelepiped, measuring (59 × 101 × 129) A with fitting uncertainties, respectively, (2, 3, 3 A). Our results suggest that COX is a functional dimer when reconstituted into mixed-lipid liposomes.

In-situ spectroscopic ellipsometry (SE) was utilized to examine the formation of the self-assembled monolayers (SAMs) of the water-soluble oligo(ethylene oxide) [OEO] disulfide [S(CH 2 CH 2 O) 6 CH 3 ] 2 {[S(EO) 6 ] 2 } and two analogous thiols – HS(CH 2 CH 2 O) 6 CH 3 {(EO) 6 } and HS(CH 2 ) 3 O(CH 2 CH 2 O) 5 CH 3 {C 3 (EO) 5 } – on Au from aqueous solutions. Kinetic data for all compounds follow simple Langmuirian models with the disulfide reaching a self-limiting final state ( d = 1.2 nm) more rapidly than the full coverage final states of the thiol analogs ( d = 2.0 nm). The in-situ ellipsometric thicknesses of all compounds were found to be nearly identical to earlier ex-situ ellipsometric measurements suggesting similar surface coverages and structural models in air and under water. Exposure to bovine serum albumin (BSA) shows the self-limiting ( d = 1.2 nm) [S(EO) 6 ] 2 SAMs to be the most highly protein resistant surfaces relative to bare Au and completely-formed SAMs of the two analogous thiols and octadecanethiol (ODT). When challenged with up to near physiological levels of BSA (2.5 mg/mL), protein adsorption on the final state [S(EO) 6 ] 2 SAM was only 3% of that which adsorbed to the bare Au and ODT SAMs.

Measurement in chemical analysis requires at its base validation of analytical methods. Validation shows that the measurement of content is correct for a specific type of material within some limit of uncertainty, where the uncertainty is due to random variation after any bias of the measurement is removed. Analyses involve sampling, sample preparation, use of standards, and an assay method that is applied to measure the content. Statistical evaluation of the results in each step can combined to evaluate the overall validty of the measurement. Keywords: sampling; sample preparation; assay; validation; uncertainty; bias; sensitivity; detection limit

Self-assembled monolayers (SAMs) of the disulfide [S(CH2CH2O)6CH3]2 ([S(EO)6]2) on Au from 95% ethanol and from 100% water are described. Spectroscopic ellipsometry and reflection-absorption infrared spectroscopy indicate that the [S(EO)6]2 films are similar to the disordered films of HS(CH2CH2O)6CH3 ((EO)6) and HS(CH2)3O(CH2CH2O)5CH3 (C3EO5) at their protein adsorption minima. The [S(EO)6]2 SAMs exhibit constant film thickness (d) of 1.2 +/- 0.2 nm over long immersion times (up to 20 days) and do not attain the highly ordered, 7/2 helical structure of the (EO)6 and C3EO5 SAMs (d = 2.0 nm). Exposure of these self-limiting [S(EO)6]2 SAMs to bovine serum albumin show high resistance to protein adsorption.

Resonators for in vivo EPR are composed of capacitors and inductors, which may be properties of the structures of the resonator itself. The capacitive and inductive parts are identified for the five general types used for general and localized CW-EPR and FT-EPR. Sources of noise unique to in vivo resonators are also addressed.

At frequencies below 1 GHz, resonant sections of coaxial lines have long been used in CW-Electron Paramagnetic Resonance (EPR) with the sample placed at the position of maximum B 1 at a short circuited end. Here, we show that because of the excellent separation of the B and E fields, the shield of the line can be removed in the region of the truncated end without greatly perturbing the RF properties of the line. The open region of the shield provides an aperture for local imaging in MRI. The B 1 fields can be shaped by contouring the inner conductor and outer shield, and the image aperture is controlled by the shape of the shield cutout. The shield opening can range from a narrow longitudinal slit up to a full 360° section that has only a few conducting strips of the shield remaining. Imaging with probes having shield diameters from 2 mm to 10 cm have been demonstrated. For imaging the useful depth is limited to approximately three to four times the probe's outer radius. Alternately, a relatively sharp cutoff at only a mm depth can be obtained by controlling the region of the shield removed, the RF power applied, and the probe diameter. The probes described here can be resonant or nonresonant. Because of the inherent broad bandwidth of the nonresonant truncated line probes, they have the potential for use in FT-EPR and FT-EPR imaging as well as other applications that require minimizing dead times.

The construction and imaging characteristics of flat, truncated line probes (FTLPs) are described here. These probes illustrate a novel design of local probes for magnetic resonance imaging, with four major differences from conventional loop surface probes: (1) The B1 fields are directed perpendicular to the usual loop probes' direction. (2) The RF fringe electric fields are inherently shielded, which allows reduced loading from electrically lossy samples. (3) The homogeneity across the plane of the probe can be adjusted locally. And (4) when not used with tuning and matching circuits, a probe's local impedance can be set to match the RF line impedance. The probes are, in essence, a single loop significantly flattened with the outside conductor (away from the imaged object) wider than the inside conductor (against the imaged object). The probes are shaped so as to provide a homogeneous signal across the plane area of the probe. The signal intensity drops off faster than a loop of the same size. With a minimally loading phantom, along the midline normal to the surface, the S/N at the surface region is approximately 20% greater than a commercial probe (Phillips R2) of the same area dimensions, while at 5 cm depth, the S/N is lower. However, when used for imaging a body--again along the midline normal to the probe--the S/N at 5 cm depth is equal, and rises to approximately twice that of the Philips R2 probe at the surface.

New ferromagnetic resonance experiments under degenerate resonance conditions are reported for FeNiPB metallic glass ribbons in annealed and polished samples. An interpretation of these experimental results in terms of established theories in Ferromagnetic and Spin Wave Resonance Spectroscopy (FSWR) is proposed. The spin-wave analysis is used to interpret the effects of magnetic anisotropy on the lineshapes of metallic glasses before and after annealing. The conclusion is reached that a model of dipolar-coupled regions which assume a common resonance frequency band is appropriate in ferromagnetic metallic glasses. The role of surface pits scattering, microheterogeneities in chemical composition, clustering processes and long-range magnetic ordering in these systems is discussed in relation to the changes of glass properties by annealing. We also conclude that FSWR techniques are more sensitive than differential scanning calorimetry, X-ray scattering and static, magnetic techniques which are currently used to study structural relaxation in glasses.

The design principles for a broadband (up to 10 GHz) electron‐paramagnetic‐resonance (EPR) spectrometer are presented. The upper frequency limit is determined by the losses in the coaxial cables. The sample is inserted along the central conductor at the end of a short‐circuited coaxial line that has an air dielectric. An adequate H1 field is generated near the surface of the central conductor and is proportional to the rf current therein. The short circuit of the cable serves to separate the E and H microwave fields with the sample predominately in the H‐field portion. The instrument is currently operated in the cw mode in absorption with direct detection. The changes in the standing wave due to the resonance absorption are monitored through an E‐field probe set at the first E node away from the truncation. A similar geometry is expected to be usable for broadband Fourier transform (FT) EPR spectrometry through the addition of appropriate pulse‐generation equipment and signal‐capture hardware. Also, obtai...

The sodium-current voltage-clamp data of Haydon and Kimura obtained on squid nerves treated with n-pentane (J. Physiol. 312 (1981) 57) are fitted with a previously described model (K.A. Rubinson, J. Physiol. 281 (1978) 14P; Biophys. Chem. 15 (1982) 245). The apparently complex action of the perturbant can be interpreted as due to a shift in shielding of the applied potential jumps, a change in channel conductivity, and an increase in the rate constant of channel shutoff. The shift in shielding due to n-pentane is found to be quantitatively the same for variables describing both kinetic and equilibrium quantities, which are independent. The transmembrane sodium potential remains unchanged, however.

Von der Titelverbindung, dem Cu-haltigen Analogen von Vitamin B 12, deren Ligand aus Chromatium vinosum extrahiert wurde, werden CD- und magnetische CD-Spektren, EPR-Spektren in gefrorener Losung und Elektron-Kern-Doppelresonanz-Spektren mitgeteilt.

The consistency of an electrodiffusion kinetics to describe the time-dependent opening of sodium channels of nerve suggests that motions over relativel long distances (on the atomic scale) are involved in the equilibrium as well. As a result, it is expected that a relatively large fraction of possible macromolecular conformations are unreactive. An equilibrium constant between locally reactive forms and the unreactive conformations is introduced. The consequences of this formalism is investigated in a square well potential, a harmonic potential, and a system consisting of two harmonic potentials with different spatial extents. The limits of knowledge from Nernstian behavior are shown. As an alternative to the Nernstian analysis, the experimental data of the sodium channel's quasi-equilibrium - the probability of the channel's being open as a function of voltage - can be described as resulting from motion caused by an electric field on a charge which is confined by a harmonic potential. A force constant is found from large-displacement vibrations are overdamped and, hence, spectroscopically unobservable.) From the force constant, an approximate value of the Young's modulus can be calculated. The modulus' value falls in the range for rubber. As for rubbers, the restoring force is, then, expected to be mostly entropic rather than enthalpic in origin. Using the appropriate theory for linear chains of rubber and the Young's modulus, the approximate length of the chain causing the rubber-like force is calculated. The result is found to be near the length suggested for the hydrophilic chains that connect transmembrane sections of the sodium channel.

In low-absorbance assays, the necessity for eliminating ultraviolet and visible absorbing impurities in buffering agents and detergents is discussed. The method proposed here involves electrochemical reduction of the interfering material. An example of the elimination of an artifact in an enzyme assay is presented.

Ferromagnetic resonance (FMR) and scanning electron microscope observations of surface effects in Metglas 2826 ribbons are presented. These effects are considered to be due to spin-wave scattering by surface pits. The considerable reduction in surface effects by a selected surface treatment allowed, for the first time, the FMR investigation of annealing effects and magnetic anisotropy. These observations are interpreted in terms of magnetic dipolar ordering and are related to recent experiments on structural relaxation in metallic glasses.

Voltage clamp; Na + current; Dielectric relaxation: Charge mobility: Intramemhrune diffusion A model is presented which explains the Na + currents of voltage-clamped nerve as resulting from a heterogeneous initiation of a sequential kinetic process. This is in analogy with the heterogeneity of the kinetics of other dielectric relaxations. The results suggest that: (1) The kinetic processes responsible for the voltage response occur within the membrane rather than at the surface; (2) The heterogeneity is due to simultaneous thermal diffusion and electric field-induced charge migration; (3) The slow turnoff upon prolonged depolarization is a voltage-independent, thermally controlled process: (4) The fast turnoff upon instantaneous repolarizalion is the reverse of the turning-on process. All the kinetic parameters depend on the transmembrane potential in accord with the possible behavior expected from activated-stale theory. The diffusion coefficient of the charged species in the membrane as found from the data agrees with that found by photobleaching experiments on general proteins in membranes. The charge on the molecule responsible for the heterogeneous ‘gating’ can be calculated unambiguously from the data.

An electronic emission spectrum has been obtained from a carrier identified as the diatomic molecule TiN. The system, which occurs in the red, has been analyzed as 2Πr → (X)2Σ, and it appears to be the analogue of the orange–red systems of the isoelectronic molecules CaF and ScO.The lower (ground?) state has B0″ = 0.6211 cm−1 corresponding to r0″ = 1.583 Å, whereas the excited 2Πr state has B0′ = 0.6103 cm−1. The system has a well-defined homogeneous perturbation in the excited state, showing the presence of a second, unobserved, 2Π term.