Improving the accuracy of Cu(ii)-nitroxide RIDME in the presence of orientation correlation in water-soluble Cu(ii)-nitroxide rulers.

Ritsch I, Hintz H, Jeschke G, Godt A, Yulikov M. Improving the accuracy of Cu(ii)-nitroxide RIDME in the presence of orientation correlation in water-soluble Cu(ii)-nitroxide rulers. Physical chemistry chemical physics : PCCP. 2019;21(19):9810-9830.Orientation selection is a challenge in distance determination with double electron electron resonance (DEER) spectroscopy of rigid molecules. The problem is reduced when applying the Relaxation-Induced Dipolar Modulation Enhancement (RIDME) experiment. Here we present an in-depth study on nitroxide-detected RIDME in Cu(ii)-nitroxide spin pairs using two Cu(ii)-nitroxide rulers that are both water soluble and have comparable spin-spin distances. They differ in the type of the ligand (TAHA and PyMTA) for the Cu(ii) ion which results in different contributions of exchange coupling. Both rulers feature substantial orientation correlation between the molecular frames of the Cu(ii) complex and the nitroxide. We discuss how the spin-spin couplings can be accurately measured and how they can be correlated to the nitroxide resonance frequencies. In that, we pay particular attention to the suppression of nuclear modulation and of echo crossing artefacts, to background correction, and to orientation averaging. With a nitroxide observer sequence based on chirp pulses, we achieve wideband detection of all nitroxide orientations. Two-dimensional Fourier transformation of data obtained in this manner affords observer-EPR correlated RIDME spectra that enable visual understanding of the orientation correlation. The syntheses of the Cu(ii)-nitroxide rulers are presented. The synthetic route is considered to be of general use for the preparation of [metal ion complex]-nitroxide rulers, including water soluble ones

Physical state of 2-methylbutane-1,2,3,4-tetraol in pure and internally mixed aerosols

Lessmeier J, Dette HP, Godt A, Koop T. Physical state of 2-methylbutane-1,2,3,4-tetraol in pure and internally mixed aerosols. Atmospheric Chemistry and Physics. 2018;18(21):15841-15857.2-Methylbutane-1,2,3,4-tetraol (hereafter named tetraol) is an important oxidation product of isoprene and can be considered as a marker compound for isoprene-derived secondary organic aerosols (SOAs). Little is known about this compound's physical phase state, although some field observations indicate that isoprene-derived secondary organic aerosols in the tropics tend to be in a liquid rather than a solid state. To gain more knowledge about the possible phase states of tetraol and of tetraol-containing SOA particles, we synthesized tetraol as racemates as well as enantiomerically enriched materials. Subsequently the obtained highly viscous dry liquids were investigated calorimetrically by differential scanning calorimetry revealing subambient glass transition temperatures Tg. We also show that only the diastereomeric isomers differ in their Tg values, albeit only by a few kelvin. We derive the phase diagram of water–tetraol mixtures over the whole tropospheric temperature and humidity range from determining glass transition temperatures and ice melting temperatures of aqueous tetraol mixtures. We also investigated how water diffuses into a sample of dry tetraol. We show that upon water uptake two homogeneous liquid domains form that are separated by a sharp, locally constrained concentration gradient. Finally, we measured the glass transition temperatures of mixtures of tetraol and an important oxidation product of α-pinene-derived SOA: 3-methylbutane-1,2,3-tricarboxylic acid (3-MBTCA). Overall, our results imply a liquid-like state of isoprene-derived SOA particles in the lower troposphere at moderate to high relative humidity (RH), but presumably a semisolid or even glassy state at upper tropospheric conditions, particularly at low relative humidity, thus providing experimental support for recent modeling calculations

Intermolecular background decay in RIDME experiments.

Keller K, Qi M, Gmeiner C, et al. Intermolecular background decay in RIDME experiments. Physical chemistry chemical physics : PCCP. 2019;21(16):8228-8245.The relaxation-induced dipolar modulation enhancement (RIDME) technique allows the determination of distances and distance distributions in pairs containing two paramagnetic metal centers, a paramagnetic metal center and an organic radical, and, under some conditions, also in pairs of organic radicals. The strengths of the RIDME technique are its simple setup requirements, and the absence of bandwidth limitations for spin inversion which occurs through relaxation. A strong limitation of the RIDME technique is the background decay, which is often steeper than that in the double electron electron resonance experiment, and the absence of an appropriate description of the intermolecular background signal. Here we address the latter problem and present an analytical calculation of the RIDME background decay in the simple case of two types of randomly distributed spin centers each with total spin S = 1/2. The obtained equations allow the explaination of the key trends in RIDME experiments on frozen chelated metal ion solutions, and singly spin-labeled proteins. At low spin label concentrations, the RIDME background shape is determined by nuclear-driven spectral diffusion processes. This fact opens up a new path for structural characterization of soft matter and biomacromolecules through the determination of the local distribution of protons in the vicinity of the spin-labeled site

Highly oriented surface-growth and covalent dye labeling of mesoporous metal-organic frameworks

Mesoporous amino-functionalized metal-organic framework thin films with the UiO-68 topology were grown in a highly oriented fashion on two different self-assembled monolayers on gold. The oriented MOF films were covalently modified with the fluorescent dye Rhodamine B inside the pore system, as demonstrated with size-selective fluorescence quenching studies. Our study suggests that mesoporous metal-organic frameworks are promising hosts for the covalent attachment of numerous functional moieties in a molecularly designed crystalline environment. © The Royal Society of Chemistry 2012

Two-Dimensional Distance Correlation Maps from Pulsed Triple Electron Resonance (TRIER) on Proteins with Three Paramagnetic Centers

Pribitzer S, Ibanez LF, Gmeiner C, et al. Two-Dimensional Distance Correlation Maps from Pulsed Triple Electron Resonance (TRIER) on Proteins with Three Paramagnetic Centers. APPLIED MAGNETIC RESONANCE. 2018;49(11):1253-1279

High-sensitivity Gd3+-Gd3+ EPR distance measurements that eliminate artefacts seen at short distances

We would like to acknowledge EPSRC (EP/R)13705/1) for current funding on the HiPER project, and the Wellcome Trust for a multi-user equipment grant (099149/Z/12/Z) for upgrades on the Q-band system. We thank the Royal Society for an International Exchanges Grant and The Weizmann-UK Joint Research Program for allowing bilateral travel and research between the University of St Andrews and the Weizmann Institute of Science. JEL thanks the Royal Society for a University Research Fellowship. MJT thanks EPSRC for a CM-CDT studentship (EP/LO15110/1). MQ and AG thank the Deutsche Forschungsgemeinschaft (DFG) for funding within SPP 1601 (GO555/6-2).Gadolinium complexes are attracting increasing attention as spin labels for EPR dipolar distance measurements in biomolecules and particularly for in-cell measurements. It has been shown that flip-flop transitions within the central transition of the high spin Gd3+ ion can introduce artefacts in dipolar distance measurements, particularly when measuring distances less than 3–4 nm. Previous work has shown some reduction of these artefacts through increasing the frequency separation between the two frequencies required for the Double Electron-Electron Resonance (DEER) experiment. Here we use a high power (1 kW), wideband, non-resonant, system operating at 94 GHz to evaluate DEER measurement protocols using two rigid Gd(III)-rulers, consisting of two [GdIII(PyMTA)] complexes, with separations of 2.1 nm and 6.0 nm, respectively. We show that by avoiding the |−1/2⟩ → |1/2⟩ central transition completely, and placing both the pump and the observer pulses on either side of the central transition, we can now observe apparently artefact-free spectra and narrow distance distributions, even for a Gd-Gd distance of 2.1 nm. Importantly we still maintain excellent signal-to-noise ratio and relatively high modulation depths. These results have implications for in-cell EPR measurements at naturally occurring biomolecule concentrations.Publisher PDFPeer reviewe

Synthesis of unsymmetrical 1,4-diarylbutadiynes by Stille coupling

Godt A. Synthesis of unsymmetrical 1,4-diarylbutadiynes by Stille coupling. JOURNAL OF ORGANIC CHEMISTRY. 1997;62(21):7471-7474

Non-rusty [2]catenanes with huge rings and their polymers

Godt A. Non-rusty [2]catenanes with huge rings and their polymers. EUROPEAN JOURNAL OF ORGANIC CHEMISTRY. 2004;2004(8):1639-1654.We describe the design and synthetic realisation of [2]catenanes characterised by having huge rings that can rotate freely and shift laterally within the constraints of concatenation. The synthetic strategy, which is based on using a carbonate group as a covalent template and oxidative dimerization of alkynes to achieve ring formation, is versatile with respect to ring size and the presence of functional groups. The monocyclic constitutional isomer of the catenane is available by a simple exchange of steps in the sequence. EPR spectroscopy revealed that in solution the catenanes adopt all possible co-conformations in equal abundances. The thermotropic liquid crystallinity of the [2]catenanes and their corresponding non-intertwined macrocycles proves intermolecular ordering in the bulk phase. The [2]catenanes were polymerized through ester formation and through acyclic diene metathesis. All of the poly[2] catenanes have rather low degrees of polymerisation ( approximate to 10), possibly because cyclisation occurs. For the preparation of [n]catenanes having n > 3, we propose fusing the rings of [2]catenanes so that the crucial steps of threading and cyclisation need only occur during the synthesis of the smallest catenane

Synthesis and characterization of monodisperse oligo(phenyleneethynylene)s

Ziener U, Godt A. Synthesis and characterization of monodisperse oligo(phenyleneethynylene)s. The Journal of Organic Chemistry. 1997;62(18):6137-6143