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Berlin Brandenburg

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  • 1
    Language: English
    In: The journal of physical chemistry. B, 29 October 2015, Vol.119(43), pp.13619-29
    Description: A new technique is developed that allows one to carry out the signal amplification by reversible exchange (SABRE) experiments at high magnetic field. SABRE is a hyperpolarization method, which utilizes transfer of spin order from para-hydrogen to the spins of a substrate in transient iridium complexes. Previously, it has been thought that such a transfer of spin order is only efficient at low magnetic fields, notably, at level anti-crossing (LAC) regions. Here it is demonstrated that LAC conditions can also be fulfilled at high fields under the action of a RF field. The high-field RF-SABRE experiment can be implemented using commercially available nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) machines and does not require technically demanding field-cycling. The achievable NMR enhancements are around 100 for several substrates as compared to their NMR signals at thermal equilibrium conditions at 4.7 T. The frequency dependence of RF-SABRE is comprised of well pronounced peaks and dips, whose position and amplitude are conditioned solely by the magnetic resonance parameters such as chemical shifts and scalar coupling of the spin system involved in the polarization transfer and by the amplitude of the RF field. Thus, the proposed method can serve as a new sensitive tool for probing transient complexes. Simulations of the dependence of magnetization transfer (i.e., NMR signal amplifications) on the frequency and amplitude of the RF field are in good agreement with the developed theoretical approach. Furthermore, the method enables continuous re-hyperpolarization of the SABRE substrate over a long period of time, giving a straightforward way to repetitive NMR experiments.
    Keywords: Amplitude ; Magnetic-Field ; Exchange ; Chemical-Shift ; Thermal-Equilibrium ; Iridium-Complex ; Spinning-System ; Hydrogen ; Magnetisation-Transfer ; Amplitude ; Magnetisches Feld ; Austausch ; Chemische Verschiebung ; Thermisches Gleichgewicht ; Iridiumkomplex ; Spinnsystem ; Wasserstoff ; Magnetisierungstransfer ; Chemistry;
    ISSN: 15206106
    E-ISSN: 1520-5207
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  • 2
    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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  • 3
    In: Physical Chemistry Chemical Physics, 2015, Vol.18(1), pp.89-93
    Description: We demonstrate an analytical model for the description of the signal amplification by reversible exchange (SABRE) process. The model relies on a combined analysis of chemical kinetics and the evolution of the nuclear spin system during the hyperpolarization process. The presented model for the first time provides rationale for deciding which system parameters ( i.e. J -couplings, relaxation rates, reaction rate constants) have to be optimized in order to achieve higher signal enhancement for a substrate of interest in SABRE experiments.
    Keywords: Amplification ; Mathematical Models ; Exchange ; Signal Processing ; Physical Chemistry ; Rate Constants ; Evolution ; Mathematical Analysis ; Miscellaneous Sciences (So) ; Chemical and Electrochemical Properties (MD) ; Chemical and Electrochemical Properties (Ep) ; Chemical and Electrochemical Properties (Ed) ; Chemical and Electrochemical Properties (EC);
    ISSN: 1463-9076
    E-ISSN: 1463-9084
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  • 4
    Language: English
    In: The Journal of Chemical Physics, 28 December 2013, Vol.139(24)
    Description: Coherent polarization transfer effects in a coupled spin network have been studied over a wide field range. The transfer mechanism is based on exciting zero-quantum coherences between the nuclear spin states by means of non-adiabatic field jump from high to low magnetic field. Subsequent evolution of these coherences enables conversion of spin order in the system, which is monitored after field jump back to high field. Such processes are most efficient when the spin system passes through an avoided level crossing during the field variation. The polarization transfer effects have been demonstrated for N-acetyl histidine, which has five scalar coupled protons; the initial spin order has been prepared by applying RF-pulses at high magnetic field. The observed oscillatory transfer kinetics is taken as a clear indication of a coherent mechanism; level crossing effects have also been demonstrated. The experimental data are in very good agreement with the theoretical model of coherent polarization transfer. The method suggested is also valid for other types of initial polarization in the spin system, most notably, for spin hyperpolarization.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
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  • 5
    Language: English
    In: The Journal of Chemical Physics, 21 October 2014, Vol.141(15)
    Description: Nuclear Magnetic Relaxation Dispersion (NMRD) of protons was studied in the pentapeptide Met-enkephalin and the amino acids, which constitute it. Experiments were run by using high-resolution Nuclear Magnetic Resonance (NMR) in combination with fast field-cycling, thus enabling measuring NMRD curves for all individual protons. As in earlier works, Papers I–III, pronounced effects of intramolecular scalar spin-spin interactions, J-couplings, on spin relaxation were found. Notably, at low fields J-couplings tend to equalize the apparent relaxation rates within networks of coupled protons. In Met-enkephalin, in contrast to the free amino acids, there is a sharp increase in the proton T 1 -relaxation times at high fields due to the changes in the regime of molecular motion. The experimental data are in good agreement with theory. From modelling the relaxation experiments we were able to determine motional correlation times of different residues in Met-enkephalin with atomic resolution. This allows us to draw conclusions about preferential conformation of the pentapeptide in solution, which is also in agreement with data from two-dimensional NMR experiments (rotating frame Overhauser effect spectroscopy). Altogether, our study demonstrates that high-resolution NMR studies of magnetic field-dependent relaxation allow one to probe molecular mobility in biomolecules with atomic resolution.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
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  • 6
    Language: English
    In: Chemical Physics Letters, 16 September 2016, Vol.661, pp.77-82
    Description: We describe the performance of methods for enhancing NMR (Nuclear Magnetic Resonance) signals of “insensitive”, but important NMR nuclei, which are based on the SABRE (Signal Amplification By Reversible Exchange) technique, i.e., on spin order transfer from hydrogen (H molecule in its nuclear singlet spin state) to a substrate in a transient organometallic complex. Here such transfer is performed at high magnetic fields by INEPT-type NMR pulse sequences, modified for SABRE. Signal enhancements up to three orders of magnitude are obtained for N nuclei; the possibility of sensitive detection of 2D-NMR H- N spectra of SABRE complexes and substrates is demonstrated.
    Keywords: Chemistry
    ISSN: 0009-2614
    E-ISSN: 1873-4448
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  • 7
    Language: English
    In: Infrared Physics and Technology, Sept, 2013, Vol.60, p.306(6)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.infrared.2013.05.014 Byline: A.V. Pravdivtsev, M.N. Akram Abstract: acents Stray light effect in IR camera is modelled using non-sequential ray tracing method. acents Energy scattering into multiple diffraction orders is taken into account. acents The influence of heated areas of mounts on the stray light signature is discussed. acents The non-sequential ray trace was used to calculate the Narcissus effect. acents The method is useful to identity critical areas in precision IR imaging systems. Article History: Received 12 March 2013
    ISSN: 1350-4495
    Source: Cengage Learning, Inc.
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  • 8
    Language: English
    In: The journal of physical chemistry. A, 15 November 2018, Vol.122(45), pp.8948-8956
    Description: We revisited only para-hydrogen spectroscopy (OPSY) for the analysis of para-hydrogen-enhanced NMR spectra at high magnetic fields. We found that the sign of the gradients and interpulse delays are pivotal for the performance of the sequence: the variant of double-quantum filter OPSY, where the second time interval is twice as long as the first one (OPSYd-12) converts the antiphase spectrum to in-phase and efficiently suppresses the background signal in a single scan better than the other variants. OPSYd-12 strongly facilitates the analysis of para-hydrogen-derived NMR spectra in homogeneous and inhomogeneous magnetic fields. Furthermore, the net magnetization produced is essential for subsequent applications such as imaging, e.g., in a reaction chamber or in vivo.
    ISSN: 10895639
    E-ISSN: 1520-5215
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  • 9
    Language: English
    In: The Journal of Chemical Physics, 07 September 2012, Vol.137(9)
    Description: Effects of spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of protons were studied in a situation where spin ½ hetero-nuclei are present in the molecule. As in earlier works [K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, J. Chem. Phys. 129 , 234513 ( 2008 ) 10.1063/1.3040272 ; S. E. Korchak, K. L. Ivanov, A. V. Yurkovskaya, and H.-M. Vieth, J. Chem. Phys. 133 , 194502 ( 2010 ) 10.1063/1.3495988 ] , spin-spin interactions have a pronounced effect on the relaxivity tending to equalize the longitudinal relaxation times once the spins become strongly coupled at a sufficiently low magnetic field. In addition, we have found influence of 19 F nuclei on the proton NMRD, although in the whole field range, studied protons and fluorine spins were only weakly coupled. In particular, pronounced features in the proton NMRD were found; but each feature was predominantly observed only for particular spin states of the hetero-nuclei. The features are explained theoretically; it is shown that hetero-nuclei can affect the proton NMRD even in the limit of weak coupling when (i) protons are coupled strongly and (ii) have spin-spin interactions of different strengths with the hetero-nuclei. We also show that by choosing the proper magnetic field strength, one can selectively transfer proton spin magnetization between spectral components of choice.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
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  • 10
    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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