33 research outputs found
Singlet state encoded magnetic resonance (SISTEM) spectroscopy
Magnetic resonance spectroscopy (MRS) allows the analysis of biochemical
processes non invasively and in vivo. Still, its application in clinical
diagnostics is rare. Routine MRS is limited to spatial, chemical and temporal
resolutions of cubic centimetres, mM and minutes. In fact, the signal of many
metabolites is strong enough for detection, but the resonances significantly
overlap, exacerbating identification and quantification. In addition, the
signals of water and lipids are much stronger and dominate the entire spectrum.
To suppress the background and isolate selected signals, usually, relaxation
times, J-coupling and chemical shifts are used. Here, we propose methods to
isolate the signals of selected molecular groups within endogenous metabolites
by using long-lived spin states (LLS). We exemplify the method by preparing the
LLSs of coupled protons in the endogenous molecules N-acetyl-L-aspartic acid
(NAA). First, we store polarization in long-lived, double spin states and then
apply saturation pulses and double quantum filters to suppress background
signals. We show that LLS can be used to selectively prepare and measure the
signals of chosen metabolites or drugs in the presence of water, inhomogeneous
field and highly concentrated fatty solutions. The pH measurement presented
here is one of the possible applications.Comment: 15 pages, 5 figures and supporting material
Coherent transfer of nuclear spin polarization in field-cycling NMR experiments
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
High resolution NMR study of T1 magnetic relaxation dispersion. IV. Proton relaxation in amino acids and Met-enkephalin pentapeptide
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 T1-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
Spin mixing at level anti-crossings in the rotating frame makes high-field SABRE feasible
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 H2 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
Exploiting Level Anti-Crossings (LACs) in the rotating frame for transferring spin hyperpolarization
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 AA′MM′-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
Magnetic field dependent long-lived spin states in amino acids and dipeptides
Magnetic field dependence of long-lived spin states (LLSs) of the β-CH2
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 α-CH and aromatic protons substituted by
deuterium at low fields the LLS lifetime, TLLS, for the β-CH2 protons was more
than 40 times longer than the T1-relaxation time. Upon increasing the number
of protons the ratio TLLS/T1 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 T1 and TLLS were measured.
LLSs were also formed in tyrosine-containing dyads; a TLLS/T1 ratio of
[similar]7 was found, usable for extending the spin polarization lifetime in
such systems
Analysis of Nutation Patterns in Fourier-Transform NMR of Non-Thermally Polarized Multispin Systems
The complex spin order of hyperpolarized multispin systems giving rise to
anomalous NMR spectral patterns that vary with the RF excitation angle is
analyzed by decomposing its nutation behavior in a superposition of Fourier
harmonics. The product operator formalism is applied to calculating the
spectral contributions of the various mutual alignments of scalar coupled
spins. Two cases are treated, namely systems exhibiting only differences in
population of their spin states and systems showing in addition zero-quantum
coherences between states, a situation often seen at hyperpolarization. After
deriving the general solution a number of representative examples are
discussed in detail. The theoretical treatment is applied to analyzing the
spin order observed in a hyperpolarized two-spin system that is prepared in
the singlet state by para-hydrogen induced polarization
Influence of spin 1/2 hetero-nuclei on spin relaxation and polarization transfer among strongly coupled protons
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 19F
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