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Berlin Brandenburg

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  • 1
    In: Physical Chemistry Chemical Physics, 2014, Vol.16(16), pp.7584-7594
    Description: Magnetic field dependence of long-lived spin states (LLSs) of the -CH 2 protons of aromatic amino acids was studied. LLSs are spin states, which are immune to dipolar relaxation, thus having lifetimes far exceeding the longitudinal relaxation times; the simplest example of an LLS is given by the singlet state of two coupled spins. LLSs were created by means of the photo-chemically induced dynamic nuclear polarization technique. The systems studied were amino acids, histidine and tyrosine, with different isotopomers. For labeled amino acids with the -CH and aromatic protons substituted by deuterium at low fields the LLS lifetime, T LLS , for the -CH 2 protons was more than 40 times longer than the T 1 -relaxation time. Upon increasing the number of protons the ratio T LLS / T 1 was reduced; however, even in the fully protonated amino acids it was about 10; that is, the long-lived mode was still preserved in the system. In addition, the effect of paramagnetic impurities on spin relaxation was studied; field dependencies of T 1 and T LLS were measured. LLSs were also formed in tyrosine-containing dyads; a T LLS / T 1 ratio of 7 was found, usable for extending the spin polarization lifetime in such systems.
    Keywords: Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 2
    In: Physical Chemistry Chemical Physics, 2013, Vol.15(35), pp.14660-14669
    Description: Spin hyperpolarization can be coherently transferred to other nuclei in field-cycling NMR experiments. At low magnetic fields spin polarization is redistributed in a strongly coupled network of spins. Polarization transfer is most efficient at fields where level anti-crossings (LACs) occur for the nuclear spin-states. A further condition is that field switching to the LAC positions is non-adiabatic in order to convert the starting population differences into spin coherences that cause time-dependent mixing of states. The power of this method has been demonstrated by studying transfer of photo-Chemically Induced Dynamic Nuclear Polarization (photo-CIDNP) in N -acetyl-tryptophan. We have investigated the magnetic field dependence and time dependence of coherent CIDNP transfer and directly assessed nuclear spin LACs by studying polarization transfer at specific field positions. The proposed approach based on LACs is not limited to CIDNP but is advantageous for enhancing NMR signals by spin order transfer from any type of hyper-polarized nuclei.
    Keywords: Magnetic Fields ; Coherence ; Dynamic Tests ; Polarization ; Nuclei ; Nuclear Magnetic Resonance ; Nuclear Spin ; Time Dependence ; Miscellaneous Sciences (So);
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 3
    In: Physical Chemistry Chemical Physics, 2010, Vol.12(7), pp.1444-1452
    Description: A wedge-shaped amphiphilic molecule, 3,4,5-tridodecyloxycinnamic acid, was used as a model system to explore the role of different constitutive elements of the chemical structure in the formation of 2D and 3D self-assemblies. The polar heads forming cyclic hydrogen-bonded dimers determine the two largest unit cell parameters, which depend only slightly on the dimensionality of the system ( i.e. 2D versus 3D) and on the sample thermal history. By contrast, the structure of the alkyl side chains is very sensitive to the phase transformations, and is likely to be responsible for the rich polymorphic behaviour of the studied compound. Thus, in the monotropic SmC phase, the alkyl chains form a liquid-crystalline sub-lattice with hexagonal symmetry that can further crystallize either in a triclinic sub-cell (metastable crystalline phase) or in an orthorhombic sub-cell (stable crystalline phase). In 2D, at the interface with the graphite surface the molecular orientation is guided by the epitaxy. Although the largest lattice parameters are close to those in the bulk, the alkyl chains adopt a particular alternating orientation. In one molecule of the dimer, two alkyl chains have their molecular planes parallel to the substrate while the third chain is perpendicular to it, and the other way around for the other molecule of the dimer. To our knowledge, such alternating orientation of the alkyl chains in the monolayer is reported for the first time.
    Keywords: Kunststoff ; Selbstorganisation ; Werkstoffgefüge ; Modellierung ; Chemische Struktur ; Zweidimensionale Darstellung ; Dreidimensionale Abbildung ; Morphologie ; Dimer ; Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 4
    In: Physical Chemistry Chemical Physics, 2003, Vol.5(16), pp.3470-3480
    Description: The magnetic field dependence of chemically induced dynamic nuclear polarization (CIDNP) formed in the photoreaction of excited 2,2′-dipyridyl and tyrosine is measured in the field range 0–7 T and compared with numerical simulations. The experiments employ field cycling between a variable field of polarization and a fixed field of detection. The simulations of CIDNP are based on the low-viscosity approximation in the Green function theory of geminate reactions. The effects of adiabatic field change and of duration of the RF excitation pulse are taken into account. Good agreement between theory and experiment in the whole field range is found for both net and multiplet polarization as observed in three coupled spin pairs (CH 2 protons in β -position of tyrosine, H2,3 and H5,6 protons of its ring). The wide applicability of the theoretical model and the experimental technique make them useful for the characterization of shortlived reaction intermediates and are suited to CIDNP studies of protein surface structure and folding process of protein.
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 5
    In: Physical Chemistry Chemical Physics, 2014, Vol.16(45), pp.24672-24675
    Description: A new technique is proposed to carry out Signal Amplification By Reversible Exchange (SABRE) experiments at high magnetic fields. SABRE is a method, which utilizes spin order transfer from para-hydrogen to the spins of a substrate in transient complexes using suitable catalysts. Such a transfer of spin order is efficient at low magnetic fields, notably, in the Level Anti-Crossing (LAC) regions. Here it is demonstrated that LAC conditions can also be fulfilled at high fields in the rotating reference frame under the action of an RF-field. Spin mixing at LACs allows one to polarize substrates at high fields as well; the achievable NMR enhancements are around 360 for the ortho-protons of partially deuterated pyridine used as a substrate and around 700 for H 2 and substrate in the active complex with the catalyst. High-field SABRE effects have also been found for several other molecules containing a nitrogen atom in the aromatic ring.
    Keywords: Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 6
    In: Physical Chemistry Chemical Physics, 2011, Vol.13(14), pp.6619-6627
    Description: Chemically Induced Dynamic Nuclear Polarization (CIDNP) of the diamagnetic products of radical reactions is exploited for the purpose of determination of the hyperfine coupling constants (HFCCs) of the radical intermediates. A simple proportionality relation between geminate CIDNP of a nucleus and its HFCC at the radical stage is established. The applicability range of this relation is determined: the relation is fulfilled in the case of a large difference in g -factor between the radicals involved and for the situation where the number of magnetic nuclei in the system is sufficiently large. The validity of the relation was confirmed by CIDNP experiments on radical pairs with precisely known HFCCs. Using the proportionality relation we were able to measure the HFCCs in various short-lived radicals of the amino acids histidine and tryptophan and of the S-N-centered cyclic radical of methionine derived from the methionine–glycine dipeptide in aqueous solution.
    Keywords: Free Radicals -- Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 7
    Language: English
    In: Physical chemistry chemical physics : PCCP, 14 May 2012, Vol.14(18), pp.6459-68
    Description: A theoretical approach is proposed to describe Overhauser-type Dynamic Nuclear Polarization (DNP) for pulsed EPR pumping by application of a train of short pulses with a duration on the nanosecond time scale. We obtained an elegant general expression for the NMR enhancement provided by the DNP effect. The expression for the enhancement is similar to that known for cw-pumping except for the saturation factor, which is re-defined as the deviation of the electron spin magnetization from its equilibrium value averaged over the cycle of the pulse sequence. It is shown that one can achieve the maximal theoretically allowed NMR enhancement for pulsed pumping even when the duty cycle of pumping is low. This becomes possible because coherent motion of the electron spins in the B(1)-field is exploited, a key feature of the pulsed DNP experiment also enabling optimization of the achievable NMR enhancement. The dependence of the effect on the duty cycle, pulse duration and electron spin relaxation times has been studied in detail. Once the lines in the EPR spectrum are inhomogeneously broadened, higher DNP effects are expected in the pulsed pumping mode than in the cw-mode for the same total power of microwave irradiation. The theoretical results are in good agreement with experimental data obtained for the pumping frequencies of 300 MHz and 1.4 GHz.
    Keywords: Coherence ; Short Pulses ; Electron Spin ; Deviation ; Pumping ; Nuclear Magnetic Resonance ; Trains ; Nanostructure ; Miscellaneous Sciences (So);
    ISSN: 14639076
    E-ISSN: 1463-9084
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  • 8
    In: Physical Chemistry Chemical Physics, 2014, Vol.16(35), pp.18707-18719
    Description: A method of transferring hyperpolarization among scalar-coupled nuclear spins is proposed, which is based on spin mixing at energy Level Anti-Crossing (LAC) regions. To fulfill LAC conditions a resonant RF-field was applied with properly set frequency and amplitude. In this situation LACs occur between the nuclear spin levels in the rotating doubly tilted reference frame. The validity of the approach is demonstrated by taking as an example the transfer of para -hydrogen induced polarization in a symmetric molecule, whose coupled spin network can be modeled as a four-spin AAMM-system with two pairs of isochronous spins. For this spin system LAC positions have been identified; rules for the sign of spin polarization have been established. The dependence of the polarization transfer efficiency on the RF-field parameters and on the time profile of switching off the RF-field has been studied in detail; experimental results are in excellent agreement with the theory developed. In general, exploiting LACs in the rotating doubly tilted frame is a powerful tool for manipulating hyperpolarization in multispin systems.
    Keywords: Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 9
    In: Physical Chemistry Chemical Physics, 2013, Vol.15(34), pp.14248-14255
    Description: We propose a technique for transferring the multiplet spin polarization (CIDNP or PHIP, or one created by any other method), which is the mutual entanglement of spins, into net hyper-polarization with respect to the direction of a high magnetic field by slowly (adiabatically) switching-off a strong external RF-field with a specially selected frequency. The net hyper-polarized molecules can then be used in NMR spectroscopy or imaging for strong signal enhancement.
    Keywords: NMR Spectroscopy ; Magnetic Fields ; Switching ; Physical Chemistry ; Entanglement ; Polarization ; Imaging ; Adiabatic Flow ; Miscellaneous Sciences (So);
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 10
    In: Physical Chemistry Chemical Physics, 2018, Vol.20(18), pp.12396-12405
    Description: An experimental method is described allowing fast field-cycling Nuclear Magnetic Resonance (NMR) experiments over a wide range of magnetic fields from 5 nT to 10 T. The method makes use of a hybrid technique: the high field range is covered by positioning the sample in the inhomogeneous stray field of the NMR spectrometer magnet. For fields below 2 mT a magnetic shield is mounted on top of the spectrometer; inside the shield the magnetic field is controlled by a specially designed coil system. This combination allows us to measure T 1 -relaxation times and nuclear Overhauser effect parameters over the full range in a routine way. For coupled proton-carbon spin systems relaxation with a common T 1 is found at low fields, where the spins are strongly coupled. In some cases, experiments at ultralow fields provide access to heteronuclear long-lived spin states. Efficient coherent polarization transfer is seen for proton-carbon spin systems at ultralow fields as follows from the observation of quantum oscillations in the polarization evolution. Applications to analysis and the manipulation of heteronuclear spin systems are discussed.
    Keywords: Magnetic-Field ; Oscillation ; Spinning-System ; Inhomogeneous-Field ; Experimental-Method ; Hybrid-Technique ; Carbon ; NMR-Spectrometers ; Magnetic-Shielding ; Magnetic-Shields ; Spectrometers ; Relaxion-Time ; Magnetic-Resonance ; Magnetisches Feld ; Oszillation ; Spinnsystem ; Inhomogenes Feld ; Experimentalmethode ; Hybridtechnik ; Kohlenstoff ; NMR-Spektrometer ; Magnetische Abschirmung ; Magnetschild ; Spektrometer ; Relaxationszeit ; Magnetische Resonanz ; Chemistry;
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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