PELDOR at S- and X-band frequencies and the separation of exchange coupling from dipolar coupling

A pulsed electron double resonance (PELDOR) setup working at S-band frequencies is introduced and its performance compared with an X-band setup. Furthermore, to verify experimentally that it is possible to disentangle the dipolar coupling nu(Dip) from the exchange coupling J by PELDOR we synthesized and investigated four bisnitroxide radicals. They exhibit in pairs the same distances r(AB) between the nitroxide moieties but only one of each pair possesses a non-zero J. The experimental values for r(AB) match the ones from molecular modeling very well for the molecules without exchange coupling. For one bisnitroxide it was possible to separate nu(Dip) from J and to ascertain the magnitude and sign of J to +11 MHz (antiferromagnetic spin-spin coupling).     

Anomalous spin excitation spectrum of the Heisenberg model in a magnetic field

Making the assumption that high-energy fermions exist in the two dimensional spin- 1/2 Heisenberg antiferromagnet, we present predictions based on the pi-flux ansatz for the dynamic structure factor when the antiferromagnet is subject to a uniform magnetic field. The main result is the presence of gapped excitations in a momentum region near (pi,pi) with energy lower than that at (pi,pi). This is qualitatively different from spin-wave theory predictions and may be tested by experiments or by quantum Monte Carlo.     

Spreading-invariant sequences and processes on bounded index sets

We say that a random sequence is spreadable if all subsequences of equal length have the same distribution. For infinite sequences the notion is equivalent to exchangeability but for finite sequences it is more general. The present paper is devoted to a systematic study of finite spreadable sequences and of processes on [0, 1] with spreadable increments. In particular, we show how many basic results in the exchangeable case—notably the predictable sampling theorem, the Wald-type identities, and various martingale and weak convergence results—admit extensions to a spreadable setting. We also identify some additional conditions that ensure the exchangeability of a spreadable sequence or process. Received: 9 November 1999 / Revised version: 16 March 2000 / Published online: 18 September 2000     

SLIM: A Short-Linked,Highly Redox-Stable Trityl Label for High-Sensitivity In-Cell EPR Distance Measurements

The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar EPR spectroscopy (PDS) in conjunction with site-directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in-cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide-functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm , and shows high stability against reduction. Using this label, the guanine-nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.     

Post‐synthetic Spin‐Labeling of RNA through Click Chemistry for PELDOR Measurements

Site‐directed spin labeling of RNA based on click chemistry is used in combination with pulsed electron‐electron double resonance (PELDOR) to benchmark a nitroxide spin label, called here d? . We compare this approach with another established method that employs the rigid spin label Çm for RNA labeling. By using CD spectroscopy, thermal denaturation measurements, CW‐EPR as well as PELDOR we analyzed and compared the influence of d? and Çm on a self‐complementary RNA duplex. Our results demonstrate that the conformational diversity of d? is significantly reduced near the freezing temperature of a phosphate buffer, resulting in strongly orientation‐selective PELDOR time traces of the d? ‐labeled RNA duplex.     

Line strength and collisional broadening coefficients of H2O at 2.7 μm for natural gas quality assurance applications

We employed tunable diode laser absorption spectroscopy to measure the line strength, the methane (CH₄), ethane (C₂H₆) and the propane (C₃H₈) broadening coefficients for the 523–422 H₂O transition at 3619.61 cm^?1. Water amount fractions generated by a stable and accurate humidity transfer standard, traceable to the SI units via the German national humidity standard, were used to calibrate the spectroscopic line strength measurements. We focus on the traceability of the measured line data to the SI and on uncertainty assessments following the guidelines of the Guide to the Expression of Uncertainty in Measurement. We determined the line strength to be (8.42 ± 0.07)×10^?20 cm^?1/(cm^?2 molecule) corresponding to a relative uncertainty of ±0.8%. To the best of our knowledge, we report the first methane, ethane and propane broadening coefficients of (8.037 ± 0.056)×10^?5 cm^?1/hPa, (9.077 ± 0.064)×10^?5 cm^?1/hPa and (10.469 ± 0.073)×10^?5 cm^?1/hPa for the 523–422 H₂O transition at 3619.61 cm^?1, respectively. The relative combined uncertainties of the stated CH₄, C₂H₆ and C₃H₈ broadening coefficients are in the ±0.7% range.     

Trends of Pesticides in Norwegian Streams and Rivers (1996-2000)

In Norway twelve streams and medium size rivers have been monitored for pesticides in a four to six years period. During these years the central authorities have accomplished efforts to minimize the risk for pesticides entering the water bodies. Trend analyses have been done on the years 1996-2000 to gain information on whether there have been reductions in the retrieval of the pesticides: (1) Frequency of pesticides detection; (2) Sum concentration of all individual pesticides in each sample; (3) Environmental risk by weighing the concentration of each pesticide against the environmental maximum residue limits (MRL). As a whole, developments in streams and rivers show both positive and negative trends regarding the different parameters studied. The tendency is that the different parameters show the same development within the stream. The situation in these streams has not changed much during this 5 years period, but there are indications towards a slight positive development. Trend analyses might therefore be useful together with careful interpretation.     

Routine analyses of crude oil fractions by principal component modelling of gas chromatographic profiles

The naphtha fraction of 13 crude North Sea oils was analyzed by high-resolution gas chromatography. Principal component modelling shows that prior normalization to percentages is unnecessary if logarithmic data are used. When the variation in absolute amounts is large (e.g., as a result of manual injection or complicated work-up of samples), the first principal component accounts for differences in chromatographic profiles. In this case, normalization does not change the relations between samples as is observed on the dominant principal components.