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Submicrosecond flash-CIDNP was used to study the dependences of the kinetics and amplitude of CIDNP, arising in biradicals produced in the photolysis of cyclic aliphatic ketones on the temperature and viscosity of the medium. It has been found that for biradicals the geminate CIDNP increases with the diffusion coefficient of the medium. The theoretical consideration of geminate recombinaiion kinetics is based on the numerical solution of the stochastic Liouville equation for the Fourier transform of the spin density matrix. The dependences of nuclear polarization on the lifetime of the triplet state of initial ketone, on electronic paramagnetic relaxation times, on scavenging rate, and on exchange interaction have been analyzed and compared with erperimental data. The conditions of quantum beats manifestaiion in CIDNP kinetics have been considered theoretically.

A theoretical description of para-hydrogen-induced polarization (PHIP) is developed, applicable to coupled multi-spin systems that are polarized at an arbitrary magnetic field. Scalar spin-spin interaction is considered to be the leading factor governing PHIP formation and transfer. At low magnetic fields, these interactions make the spins strongly coupled and cause efficient, coherent re-distribution of spin polarization. We describe the effects of strong coupling and field cycling for a three-spin system and compare calculated spectra with the experimental examples available. By using a fast field-cycling device, which shuttles the whole NMR probe, and thereby makes high-resolution NMR detection at high field possible, we studied PHIP patterns for a set of different fields between 0.1 mT and 7 T. PHIP spectra were measured for ethylbenzene as the product of a catalytic reaction between para-hydrogen and styrene. Additionally, the polarizations of ethylbenzene bound to the catalyst, and of the starting styrene molecule were analyzed. This is the first time that the full field dependence of PHIP has been determined experimentally. The spectra obtained are in perfect agreement with the simulations for the CH(2) and CH(3) protons of ethylbenzene and even for its weakly-polarized aromatic protons. Analysis of styrene polarization shows that the time profile of the field variation has pronounced effects on the PHIP pattern. Our study gives evidence that scalar spin-spin interactions determine the PHIP patterns. Possible applications of the theory are discussed.

A theoretical approach to the description of longitudinal (T(1)) relaxation in scalar coupled systems of spin 1/2 nuclei at arbitrary magnetic field is developed, which is based on the Redfield theory. The consideration is addressed to field-cycling relaxometry experiments with high-resolution NMR detection, in which the field dependence of T(1)-relaxation times, the nuclear magnetic relaxation dispersion (NMRD), can be studied for individual spins of the molecule. Our study reveals well-pronounced effects of spin-spin couplings on the NMRD curves. First, coupled spins having completely different high-field T(1) times tend to relax at low field with a common relaxation time. Second, the NMRD curves exhibit sharp features at the fields corresponding to the positions of nuclear spin level anticrossings. Such effects of spin-spin couplings show up not only for individual spins but also for the T(1)-relaxation of the total spin magnetization of the molecule. The influence of spin-spin coupling is of importance as long as the coupling strength J is larger than the inverse T(1)-relaxation times of the spins. Around J x T(1) = 1 there is also a coherent contribution to the relaxation kinetics resulting in an oscillatory component of the kinetic curves. Application of the theory to experimental examples will be described in subsequent publications.

The efficiency of the chemical pathway of DNA repair is studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP) using the model system containing guanosyl base radicals, and tryptophan as the electron donor. Radicals were generated photochemically by pulsed laser irradiation of a solution containing the photosensitizer 2,2'-dipyridyl, guanosine-5'-monophosphate, and N-acetyl tryptophan. Depending on the pH of the aqueous solution, four protonation states of the guanosyl radical are formed via electron or hydrogen atom transfer to the triplet excited dye. The rate constants of electron transfer from the amino acid to the guanosyl radical were determined by quantitative analysis of the CIDNP kinetics, which is very sensitive to the efficiency of radical reactions in the bulk, and rate constants vary from (1.0 +/- 0.3) x 10(9) M(-1) s(-1) for the cation and dication radicals of the nucleotide to (1.2 +/- 0.3) x 10(7) M(-1) s(-1) for the radical in its anionic form. They were found to be higher than the corresponding values for electron transfer in the case of N-acetyl tyrosine as the reducing agent.

CIDNP with microsecond time resolution was used to study the kinetics of nuclear polarization formation during the photolysis of acetone in isopropanol over a wide temperature range. The coincidence of signs of multiplet effects of CIDNP (A/E) for cage and escape reaction products as well as the presence of net polarization (E) changing its sign with decreasing temperature have been revealed. The account of scalar electron-nuclear cross-relaxation has been shown to be necessary for the interpretation of the results obtained.

Relative values of the 1H and 13C isotropic hyperfine couplings in the cationic oxidized tryptophan radical TrpH*+ in aqueous solution are determined. The data are obtained from the photo-CIDNP (chemically induced dynamic nuclear polarization) enhancements observed in the microsecond time-resolved NMR spectra of the diamagnetic products of photochemical reactions in which TrpH*+ is a transient intermediate. The method is validated using the tyrosyl neutral radical Tyr*, whose 1H and 13C hyperfine couplings have previously been determined by electron paramagnetic resonance spectroscopy. Good agreement is found with hyperfine coupling constants for TrpH*+ calculated using density functional theory methods but only if water molecules are explicitly included in the calculation.

The kinetics of the chemically induced dynamic nuclear polarization (CIDNP) produced in reactions of hen lysozyme with photosensitizers have been studied for the native state of the protein at pH 3.8 and for two denatured states. The latter were generated by raising the temperature to 80 degrees C or by combining a temperature rise (to 50 degrees C) with the addition of chemical denaturant (10 M urea). Detailed analysis of the CIDNP time dependence on a microsecond time scale revealed that, in both denatured states, intramolecular electron transfer (IET) from a tyrosine residue to the cation radical of a tryptophan residue (rate constant k(f)) is highly efficient and plays a decisive role in the evolution of the nuclear polarization. To describe the observed CIDNP kinetics with a self-consistent set of parameters, IET in the reverse direction, from a tryptophan residue to a tyrosine residue radical (rate constant k(r)), has also to be taken into account. The IET rate constants determined by analysis of the CIDNP kinetics are, at 80 degrees C: k(f) = 1 x 10(5) s(-1) and k(r) = 1 x 10(4) s(-1); at 50 degrees C in the presence of 10 M urea: k(f) = 7 x 10(4) s(-1), k(r) = 1 x 10(4) s(-1). IET does not appear to influence the CIDNP kinetics of the native state.

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.

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.

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.

A general theoretical approach to coherent transfer of hyperpolarization (HP) among coupled spins is suggested and applied to field-cycling experiments where the polarized spin system is prepared at low magnetic field while observation is performed at high field. The formalism works for an arbitrary mechanism of HP formation and takes into account finite periods of HP preparation, free evolution at the polarization field, and field variation, which was considered in the two limiting regimes of sudden and adiabatic field jump. The polarization transfer is of a coherent nature and proceeds due to strong coupling of spins at low field, where the difference of their Zeeman interaction with the field is less than or comparable to their scalar spin-spin coupling. The case of three nonequivalent coupled spins is studied in detail including the calculation of HP transfer efficiency in the range from zero to high field. Characteristic transfer times are found. New features in the field dependence of polarization are predicted that are due to avoided crossings of the nuclear spin eigenstates of the molecule. The feasibility of HP transfer via spacer spins among spins that have no direct coupling is explored. Another application is the description of para-hydrogen induced polarization in a three-spin system over a wide field range.

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 (CH2 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.

The influence of the surfactants sodium dodecyl sulphate, cetyltrimethyl-ammonium bromide and triton X-100 on the photochemically induced dynamic nuclear polarization (CIDNP) of N-acetyl tyrosine has been investigated. Three photosensitizers were used to generate polarization: thionin, eosin Y and flavin mononucleotide. 600 MHz 1H photo-CIDNP experiments, supported by laser flash photolysis transient absorption measurements, indicate that the neutral triton surfactant has no influence on the nuclear polarization, but that the other two, charged, amphiphiles affect the photochemistry in a variety of ways, depending on the surfactant concentration and the identity of the sensitizer.

The singlet-triplet evolution of alkyl-acyl biradicals under S-T 0 mixing within submicrosecond time range has been studied by the method of flash CIDNP 1 H during the photolysis of cyclic aliphatic ketones. The maximum of nuclear polarization has been detected 160 ns after exciting laser pulse. The invariance of the CIDNP time-dependence curve relative to the isotopic substitution of ketone in the α-position at the polymethylene chain length of 10 and 11 has been demonstrated. The nuclear polarization evolution has been calculated both in terms of formal kinetics and using the kinematic approximation in the theory of geminate recombination for the two-position biradical model.

The development of nuclear spins hyperpolarization, and the search for molecules that can be efficiently hyperpolarized is an active area in nuclear magnetic resonance. In this work we present a detailed study of SABRE SHEATH (signal amplification by reversible exchange in shield enabled alignment transfer to heteronuclei) experiments on 15 N2 -azobenzene. In SABRE SHEATH experiments the nuclear spins of the target are hyperpolarized through transfer of spin polarization from parahydrogen at ultralow fields during a reversible chemical process. Azobenzene exists in two isomers, trans and cis. We show that all nuclear spins in cis-azobenzene can be efficiently hyperpolarized by SABRE at suitable magnetic fields. Enhancement factors (relative to 9.4 T) reach up to 3000 for 15 N spins and up to 30 for the 1 H spins. We compare two approaches to observe either hyperpolarized magnetization of 15 N/1 H spins, or hyperpolarized singlet order of the 15 N spin pair. The results presented here will be useful for further experiments in which hyperpolarized cis-15 N2 -azobenzene is switched by light to trans-15 N2 -azobenzene for storing the produced hyperpolarization in the long-lived spin state of the 15 N pair of trans-15 N2 -azobenzene.

The anomalously large nuclear polarization during photolysis of cyclic ketones due to chemically induced dynamic nuclear polarization was observed. Nuclear polarization in such reactions may be used for creating a polarized target.(AIP)

Upon cooling, the emission maximum position shifts to low field and the width of the 1 H CIDNP field dependencies of the acyl-alkyl biradical products decreases. Below 243 K these effects reverse direction, furthermore, a significant slowing down of the SNP decay is observed. Calculations of the 1 H and 13 C CIDNP field dependencies and the SNP kinetics for the primary biradical have been performed based on the numerical solution of the stochastic Liouville equation for the biradical spin density matrix. They use a model of restricted diffusion for the description of the molecular dynamics and take into account the distance dependence of the exchange interaction. Good quantitative agreement between the calculations and experimental data is found for the whole temperature range when using the appropriate dependence of some of the parameters on temperature. In particular, it was necessary for the quantitative simulation in the low temperature range to introduce a temperature dependence of the biradical reactivity. The model calculations qualitatively confirm that the low-temperature broadening of the CIDNP field dependencies, and the increase of the SNP decay time are caused by the slowing down of the molecular mobility and the decreasing of reactivity. For the products of the secondary biradical, a qualitative interpretation is given showing that in comparison with the products of the primary biradical spin-orbit interaction is of less importance for the biradical spin dynamics, that the influence of the molecular mobility under the temperature variation is more pronounced, that the lifetime is longer, and that the nuclear polarization is more efficiently formed.

The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of 2,2′-dipyridyl (DP) and N-acetyl histidine (HisH) in aqueous solution have been studied using laser flash photolysis and time-resolved CIDNP techniques. The triplet state TDP reacts with protonated HisH 2+ via hydrogen atom transfer with a rate constant k H = 1.2 × 10 8 M -1 s -1, and with deprotonated His - via electron transfer with k e = 7.5 × 10 9 M -1 s -1. No reaction occurs when the histidine imidazole ring is in its neutral state HisH, or when the dipyridyl triplet is protonated, TDPH +. The nuclear spin-lattice relaxation times in the radicals formed in these reactions have been determined from the CIDNP kinetics: T 1 = 44 ± 9 μs for all DP protons, T 1 = 196 ± 25 μs for the β-CH 2 protons of HisH, and T 1 = 16 ± 5 μs for the H-2 and H-4 protons of HisH. Under strongly basic conditions the CIDNP is greatly affected by degenerate electron exchange between the neutral His • radical and His - anion, with rate constant k ex = 1.5 × 10 8 M -1 S -1. © 2000 American Chemical Society.

Photolysis of acetone in the presence of various hydrogen donors and of 3-hydroxy-3-methyl-2-butanone involves the formation of propan-2-olyl radicals which show both electron and nuclear spin polarization. The electron polarization of the radicals leads to additional nuclear polarization of the reaction products. Transfer of electron to nuclear polarization can occur by cross-relaxation and cross-correlation. The latter is described in detail. Experimentally, the mechanisms lead to formation of net nuclear polarization for symmetrical radical pairs as well as an unusual kinetic behavior of multiplet effects

Laser flash photolysis (LFP), low-field chemically induced dynamic nuclear polarization (CIDNP), and time-resolved electron paramagnetic resonance (TREPR) techniques have been used for the comparative study of magnetic field and spin effects in acyl−ketyl and bis(ketyl)biradicals formed during the photolysis of 2-hydroxy-2,12-dimethylcyclododecanone (1-OH) and 2,12-dihydroxy-2,12-dimethylcyclododecanone (2-OH), respectively. The short biradical lifetime, the small magnetic field effect (MFE) on a biradical lifetime, and the low intensity of spin-correlated radical pair (SCRP) polarization observed during the photolysis of 1-OH indicate that the main channel of intersystem crossing in acyl−ketyl biradical is spin−orbit coupling (SOC). For bis(ketyl) biradicals observed during the photolysis of 2-OH, SOC is of minor importance, and both MFE and SCRP polarization are much larger. It is shown that the presence of significant SOC in biradicals can result in an increase of the CIDNP intensity at low magnetic fields. Calculations of biradical evolution, based on the numerical solution of the stochastic Liouville equation (SLE), were carried out by taking into account (1) the molecular dynamics of the polymethylene chain linking the radical centers, (2) the distance-dependent exchange interaction, (3) the chemical reactions of the biradical, (4) the spin relaxation processes, and (5) state-to-state transitions caused by hyperfine interaction and SOC. A common set of parameters was found which allows quantitative description of the field dependence of the biradical lifetime, the CIDNP field dependence, and the TREPR spectra and kinetics.

Some nuclear spin systems support long-lived states, which display greatly extended relaxation times relative to the relaxation time of nuclear spin magnetization. In spin-1/2 pairs, such a long-lived state is given by singlet order, representing the difference of the population of the nuclear singlet state and the mean population of the three triplets. In many cases, the experiments with long-lived singlet order are very time-consuming because of the need to wait for singlet order decay before the experiment can be repeated; otherwise, spin order remaining from a previous measurement may lead to experimental artifacts. Here, we propose techniques for fast and efficient singlet order destruction. These methods exploit coherent singlet-triplet conversion; in some cases, multiple conversion steps are introduced. We demonstrate that singlet order destruction enables a dramatic reduction of the waiting time between consecutive experiments and suggest to use this approach in singlet-state Nuclear Magnetic Resonance (NMR) experiments with nearly equivalent spins.