Kooperativer Bibliotheksverbund

Berlin Brandenburg

and
and

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
Filter
Type of Medium
Language
Year
  • 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
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 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
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 3
    Language: English
    In: The Journal of Chemical Physics, 21 March 2017, Vol.146(11)
    Description: Time resolved measurements of Optical Nuclear Polarization (ONP) have been performed on hyperpolarized triplet states in molecular crystals created by light excitation. Transfer of the initial electron polarization to nuclear spins has been studied in the presence of radiofrequency excitation; the experiments have been performed with different pulse sequences using different doped molecular systems. The experimental results clearly demonstrate the dominant role of coherent mechanisms of spin order transfer, which manifest themselves in well pronounced oscillations. These oscillations are of two types, precessions and nutations, having characteristic frequencies, which are the same for the different molecular systems and the pulse sequences applied. Hence, precessions and nutations constitute a general feature of polarization transfer in ONP experiments. In general, coherent manipulation of spin order transfer creates a powerful resource for improving the performance of the ONP method, which paves the way to strong signal enhancement in nuclear magnetic resonance.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 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
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 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
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 6
    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
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 7
    Language: English
    In: The journal of physical chemistry. B, 08 September 2016, Vol.120(35), pp.9277-86
    Description: Photo-oxidation of seven cyclic dipeptides containing methionine, Met, and/or S-methylcysteine, Cys(Me) by electron transfer from the sulfur atom was studied in aqueous solution by time-resolved and field dependent CIDNP (chemically induced dynamic nuclear polarization). Hyperpolarized high resolution NMR spectral patterns of the starting peptides detected immediately after pulsed laser excitation show signals of all protons that are bound to carbons neighboring the sulfur atom, thus proving the involvement of sulfur-centered cation radicals. The magnetic field dependence of CIDNP shows a pronounced maximum that is determined by the g-factors and hyperfine coupling constants of the transient radical species. From simulation of the experimental data obtained for the magnetic field dependences of CIDNP, three types of radical structures were characterized: (1) a linear sulfur-centered cation radical of the methionine (Met) residue (g = 2.0107 ± 0.0010) for cyclo-(d-Met-l-Met) (trans-configuration), cyclo-(d-Met-l-Cys(Me)) (trans-configuration), and cyclo-(Gly-Met); (2) a cyclic radical (S∴O)(+) (g = 2.0088 ± 0.0010) with a two-center three-electron bond (2c-3e) structure between the sulfur atom of the Cys(Me) residue and the oxygen atom of cyclo-(d-Met-l-Cys(Me)) and cyclo-(Gly-Cys(Me)); (3) a cyclic radical (S∴S)(+) (g = 2.013 ± 0.0020) with a two-center three-electron bond structure between the two sulfur atoms of the peptides cyclo-(l-Met-l-Met), cyclo-(l-Met-l-Cys(Me)), and cyclo-(l-Cys(Me)-l-Cys(Me)). In contrast, no indication of any type of cyclic radicals with a two-center three-electron bond between sulfur and nitrogen atoms was found. In addition, the hyperfine coupling constants (HFCCs) were determined.
    Keywords: Electrons ; Nuclear Magnetic Resonance, Biomolecular ; Dipeptides -- Chemistry ; Sulfides -- Chemistry
    ISSN: 15206106
    E-ISSN: 1520-5207
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 8
    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
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 9
    Language: English
    In: The Journal of Chemical Physics, 21 November 2010, Vol.133(19)
    Description: Effects of scalar spin-spin interactions on the nuclear magnetic relaxation dispersion (NMRD) of coupled multispin systems were analyzed. Taking spin systems of increasing complexity we demonstrated pronounced influence of the intramolecular spin-spin couplings on the NMRD of protons. First, at low magnetic fields where there is strong coupling of spins the apparent relaxation times of the coupled spins become equal. Second, there are new features, which appear at the positions of the nuclear spin level anticrossings. Finally, in coupled spin systems there can be a coherent contribution to the relaxation kinetics present at low magnetic fields. All these peculiarities caused by spin-spin interactions are superimposed on the features in NMRD, which are conditioned by changes of the motional regime. Neglecting the effects of couplings may lead to misinterpretation of the NMRD curves and significant errors in determining the correlation times of molecular motion. Experimental results presented are in good agreement with theoretical calculations.
    Keywords: Articles
    ISSN: 0021-9606
    E-ISSN: 1089-7690
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
  • 10
    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
    Library Location Call Number Volume/Issue/Year Availability
    BibTip Others were also interested in ...
Close ⊗
This website uses cookies and the analysis tool Matomo. Further information can be found on the KOBV privacy pages