Category Archives: CIDNP

Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data

Panov, M., et al., Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data. Appl. Magn. Reson., 2014. 45(9): p. 893-900.

http://dx.doi.org/10.1007/s00723-014-0568-9

In this work we demonstrate that low-field chemically induced dynamic nuclear polarization (CIDNP) is strongly affected by re-distribution of polarization, which is formed in the course of spin evolution in transient radical pairs, in diamagnetic reaction products. This phenomenon is of importance when the spins of the reaction product are coupled strongly meaning that spin–spin interactions between them are comparable to the differences in their Zeeman interactions with the external magnetic field. In this case, polarization transfer relies on a coherent mechanism; as a consequence, spins can acquire significant polarization even when they have no hyperfine coupling to the electron spins in the radical pairs, i.e., cannot be polarized directly by CIDNP. This is demonstrated by taking CIDNP of n-butylamine as an example: in this case only the α-CH2 protons are polarized directly, which is confirmed by high-field CIDNP, whereas the β-CH2, γ-CH2 and δ-CH3 protons get polarized only indirectly due to the transfer of polarization from the α-CH2 protons. These results show that low-field CIDNP data should be interpreted with care to discriminate between the effects of spin evolution in transient radical pairs and in diamagnetic reaction products.

Spectral editing through laser-flash excitation in two-dimensional photo-CIDNP MAS NMR experiments

Sai Sankar Gupta, K.B., et al., Spectral editing through laser-flash excitation in two-dimensional photo-CIDNP MAS NMR experiments. J Magn Reson, 2014. 246C(0): p. 9-17.

http://www.ncbi.nlm.nih.gov/pubmed/25063951

In solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) MAS NMR experiments, strong signal enhancement is observed from molecules forming a spin-correlated radical pair in a rigid matrix. Two-dimensional 13C-13C dipolar-assisted rotational resonance (DARR) photo-CIDNP MAS NMR experiments have been applied to obtain exact chemical shift assignments from those cofactors. Under continuous illumination, the signals are enhanced via three-spin mixing (TSM) and differential decay (DD) and their intensity corresponds to the electron spin density in pz orbitals. In multiple-13C labelled samples, spin diffusion leads to propagation of signal enhancement to all 13C spins. Under steady-state conditions, direct signal assignment is possible due to the uniform signal intensity. The original intensities, however, are inaccessible and the information of the local electron spin density is lost. Upon laser-flash illumination, the signal is enhanced via the classical radical pair mechanism (RPM). The obtained intensities are related to isotropic hyperfine interactions aiso and both enhanced absorptive and emissive lines can be observed due to differences in the sign of the local isotropic hyperfine interaction. Exploiting the mechanism of the polarization, selectivity can be increased by the novel time-resolved two-dimensional dipolar-assisted rotational resonance (DARR) MAS NMR experiment which simplifies the signal assignment compared to complex spectra of the same RCs obtained by continuous illumination. Here we present two-dimensional time-resolved photo-CIDNP MAS NMR experiments providing both directly: signal assignment and spectral editing by sign and strength of aiso. Hence, this experiment provides a direct key to the electronic structure of the correlated radical pair.

The role of level anti-crossings in nuclear spin hyperpolarization

Ivanov, K.L., et al., The role of level anti-crossings in nuclear spin hyperpolarization. Prog. NMR. Spec., 2014. 81(0): p. 1-36.

http://www.sciencedirect.com/science/article/pii/S0079656514000454

Nuclear spin hyperpolarization is an important resource for increasing the sensitivity of NMR spectroscopy and MRI. Signal enhancements can be as large as 3–4 orders of magnitude. In hyperpolarization experiments, it is often desirable to transfer the initial polarization to other nuclei of choice, either protons or insensitive nuclei such as 13C and 15N. This situation arises primarily in Chemically Induced Dynamic Nuclear Polarization (CIDNP), Para-Hydrogen Induced Polarization (PHIP), and the related Signal Amplification By Reversible Exchange (SABRE). Here we review the recent literature on polarization transfer mechanisms, in particular focusing on the role of Level Anti-Crossings (LACs) therein. So-called “spontaneous” polarization transfer may occur both at low and high magnetic fields. In addition, transfer of spin polarization can be accomplished by using especially designed pulse sequences. It is now clear that at low field spontaneous polarization transfer is primarily due to coherent spin-state mixing under strong coupling conditions. However, thus far the important role of LACs in this process has not received much attention. At high magnetic field, polarization may be transferred by cross-relaxation effects. Another promising high-field technique is to generate the strong coupling condition by spin locking using strong radio-frequency fields. Here, an analysis of polarization transfer in terms of LACs in the rotating frame is very useful to predict which spin orders are transferred depending on the strength and frequency of the B1 field. Finally, we will examine the role of strong coupling and LACs in magnetic-field dependent nuclear spin relaxation and the related topic of long-lived spin-states.

Spectral editing through laser-flash excitation in two-dimensional photo-CIDNP MAS NMR experiments

Gupta, K.B.S.S., et al., Spectral editing through laser-flash excitation in two-dimensional photo-CIDNP MAS NMR experiments. J. Magn. Reson., 2014(0).

http://www.sciencedirect.com/science/article/pii/S1090780714001761

In solid-state photochemically induced dynamic nuclear polarization (photo-CIDNP) MAS NMR experiments, strong signal enhancement is observed from molecules forming a spin-correlated radical pair in a rigid matrix. Two-dimensional 13C-13C dipolar-assisted rotational resonance (DARR) photo-CIDNP MAS NMR experiments have been applied to obtain exact chemical shift assignments from those cofactors. Under continuous illumination, the signals are enhanced via three-spin mixing (TSM) and differential decay (DD) and their intensity corresponds to the electron spin density in pz orbitals. In multiple-13C labeled samples, spin diffusion leads to propagation of signal enhancement to all 13C spins. Under steady-state conditions, direct signal assignment is possible due to the uniform signal intensity. The original intensities, however, are inaccessible and the information of the local electron spin density is lost. Upon laser-flash illumination, the signal is enhanced via the classical radical pair mechanism (RPM). The obtained intensities are related to isotropic hyperfine interactions aiso and both enhanced absorptive and emissive lines can be observed due to differences in the sign of the local isotropic hyperfine interaction. Exploiting the mechanism of the polarization, selectivity can be increased by the novel time-resolved two-dimensional dipolar-assisted rotational resonance (DARR) MAS NMR experiment which simplifies the signal assignment compared to complex spectra of the same RCs obtained by continuous illumination. Here we present two-dimensional time-resolved photo-CIDNP MAS NMR experiments providing both directly: signal assignment and spectral editing by sign and strength of aiso. Hence, this experiment provides a direct key to the electronic structure of the correlated radical pair.

Proton polarization in photo-excited aromatic molecule at room temperature enhanced by intense optical source and temperature control

Sakaguchi, S., et al., Proton polarization in photo-excited aromatic molecule at room temperature enhanced by intense optical source and temperature control. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2013(0).

http://www.sciencedirect.com/science/article/pii/S0168583X13008872

Proton polarization at room temperature, produced in a p-terphenyl crystal by using electron population difference in a photo-excited triplet state of pentacene, was enhanced by utilizing an intense laser with an average power of 1.5 W. It was shown that keeping the sample temperature below 300 K is critically important to prevent the rise of the spin–lattice relaxation rate caused by the laser heating. It is also reported that the magnitude of proton polarization strongly depends on the time structure of the laser pulse such as its width and the time interval between them.

Hyperpolarization Methods and Applications in NMR

Köckenberger, W. and J. Matysik, Hyperpolarization Methods and Applications in NMR, in Encyclopedia of Spectroscopy and Spectrometry (Second Edition), L. Editor-in-Chief: John, Editor. 2010, Academic Press: Oxford. p. 963-970.

http://dx.doi.org/10.1016/B978-0-12-374413-5.00054-3

Nuclear magnetic resonance (NMR) is a well known and versatile spectroscopic and analytical technique. Its enormous success is due to its capacity to determine structures of molecules and proteins in solutions and amorphous solids, to measure local mobilities, to provide information on reaction mechanisms, and to construct three-dimensional images. The main constraints of NMR are its limitation to microsecond time resolution and the low sensitivity. The latter problem is caused by the weak magnetic properties of nuclei and a resulting unfavorable Boltzmann distribution occurring under equilibrium conditions. Therefore, several methods have been developed to produce non-Boltzmann nuclear spin states leading to hyperpolarization and enhanced sensitivity. These hyperpolarization methods induce spin-chemical processes that overcome the Boltzmann equilibrium of the nuclear spins. Under these conditions, signals enhanced to a factor of more than 10 000 are observed. Hyperpolarization methods use light, microwaves, or chemical reactions and, therefore, link different forms of spectroscopy.

Hyperpolarization Methods and Applications in NMR

Köckenberger, W. and J. Matysik, Hyperpolarization Methods and Applications in NMR, in Encyclopedia of Spectroscopy and Spectrometry (Second Edition), L. Editor-in-Chief: John, Editor. 2010, Academic Press: Oxford. p. 963-970.

http://dx.doi.org/10.1016/B978-0-12-374413-5.00054-3

Nuclear magnetic resonance (NMR) is a well known and versatile spectroscopic and analytical technique. Its enormous success is due to its capacity to determine structures of molecules and proteins in solutions and amorphous solids, to measure local mobilities, to provide information on reaction mechanisms, and to construct three-dimensional images. The main constraints of NMR are its limitation to microsecond time resolution and the low sensitivity. The latter problem is caused by the weak magnetic properties of nuclei and a resulting unfavorable Boltzmann distribution occurring under equilibrium conditions. Therefore, several methods have been developed to produce non-Boltzmann nuclear spin states leading to hyperpolarization and enhanced sensitivity. These hyperpolarization methods induce spin-chemical processes that overcome the Boltzmann equilibrium of the nuclear spins. Under these conditions, signals enhanced to a factor of more than 10 000 are observed. Hyperpolarization methods use light, microwaves, or chemical reactions and, therefore, link different forms of spectroscopy.

Light-Induced Spin Polarization in Porphyrin-Based Donor–Acceptor Dyads and Triads

van der Est, A. and P. Poddutoori, Light-Induced Spin Polarization in Porphyrin-Based Donor–Acceptor Dyads and Triads. Appl. Magn. Reson., 2013. 44(1-2): p. 301-318.

http://dx.doi.org/10.1007/s00723-012-0420-z

The light-induced spin polarization generated by sequential electron transfer in an axially bound triad based on Al(III) porphyrin (AlPor) is discussed. In the triad, TTF > Ph > py > AlPor > Ph > NDI, the electron acceptor naphthalene diimide (NDI) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) via an appended pyridine (py) on the opposite face of the porphyrin ring. Excitation of the porphyrin at room temperature in solution leads to charge separation between the donor and acceptor. In the liquid crystalline solvent 5CB, a spin-polarized transient electron paramagnetic resonance spectrum of a weakly coupled radical pair is observed and is assigned to the state TTFþNDI. In the absence of the donor, a spectrum of the triplet state of the strongly coupled radical pair AlPorþNDIis obtained. The analysis of the spectra is described using a model developed by Kandrashkin et al. (Appl Magn Reson 15: 417–447, 1998). It is shown that in the triad the spectrum of TTFþNDI shows evidence of the singlet–triplet mixing in the precursor AlPorþNDI. At later time, singlet recombination leads to inversion of the spectrum, from which the singlet back reaction lifetime is estimated as 350 ns. The decay of the inverted spectrum yields a lifetime of 8.3 ls for the triplet back reaction lifetime.

Light-Induced Spin Polarization in Porphyrin-Based Donor–Acceptor Dyads and Triads

van der Est, A. and P. Poddutoori, Light-Induced Spin Polarization in Porphyrin-Based Donor–Acceptor Dyads and Triads. Appl. Magn. Reson., 2013. 44(1-2): p. 301-318.

http://dx.doi.org/10.1007/s00723-012-0420-z

The light-induced spin polarization generated by sequential electron transfer in an axially bound triad based on Al(III) porphyrin (AlPor) is discussed. In the triad, TTF > Ph > py > AlPor > Ph > NDI, the electron acceptor naphthalene diimide (NDI) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) via an appended pyridine (py) on the opposite face of the porphyrin ring. Excitation of the porphyrin at room temperature in solution leads to charge separation between the donor and acceptor. In the liquid crystalline solvent 5CB, a spin-polarized transient electron paramagnetic resonance spectrum of a weakly coupled radical pair is observed and is assigned to the state TTFþNDI. In the absence of the donor, a spectrum of the triplet state of the strongly coupled radical pair AlPorþNDIis obtained. The analysis of the spectra is described using a model developed by Kandrashkin et al. (Appl Magn Reson 15: 417–447, 1998). It is shown that in the triad the spectrum of TTFþNDI shows evidence of the singlet–triplet mixing in the precursor AlPorþNDI. At later time, singlet recombination leads to inversion of the spectrum, from which the singlet back reaction lifetime is estimated as 350 ns. The decay of the inverted spectrum yields a lifetime of 8.3 ls for the triplet back reaction lifetime.

A Novel Tri-Enzyme System in Combination with Laser-Driven NMR Enables Efficient Nuclear Polarization of Biomolecules in Solution

Lee, J.H. and S. Cavagnero, A Novel Tri-Enzyme System in Combination with Laser-Driven NMR Enables Efficient Nuclear Polarization of Biomolecules in Solution. The Journal of Physical Chemistry B, 2013. 117(20): p. 6069-6081.

http://dx.doi.org/10.1021/jp4010168

NMR is an extremely powerful, yet insensitive technique. Many available nuclear polarization methods that address sensitivity are not directly applicable to low-concentration biomolecules in liquids and are often too invasive. Photochemically induced dynamic nuclear polarization (photo-CIDNP) is no exception. It needs high-power laser irradiation, which often leads to sample degradation, and photosensitizer reduction. Here, we introduce a novel tri-enzyme system that significantly overcomes the above challenges, rendering photo-CIDNP a practically applicable technique for NMR sensitivity enhancement in solution. The specificity of the nitrate reductase (NR) enzyme is exploited to selectively in situ reoxidize the reduced photo-CIDNP dye FMNH2. At the same time, the oxygen-scavenging ability of glucose oxidase (GO) and catalase (CAT) is synergistically employed to prevent sample photodegradation. The resulting tri-enzyme system (NR-GO-CAT) enables prolonged sensitivity-enhanced data collection in 1D and 2D heteronuclear NMR, leading to the highest photo-CIDNP sensitivity enhancement (48-fold relative to SE-HSQC) achieved to date for amino acids and polypeptides in solution. NR-GO-CAT extends the concentration limit of photo-CIDNP NMR down to the low micromolar range. In addition, sensitivity (relative to the reference SE-HSQC) is found to be inversely proportional to sample concentration, paving the way for the future analysis of even more diluted samples.

Have a question?

If you have questions about our instrumentation or how we can help you, please contact us.