Creating Long-Lived Spin States at Variable ... - ACS Publications

Letter pubs.acs.org/JPCL

Creating Long-Lived Spin States at Variable Magnetic Field by Means of Photochemically Induced Dynamic Nuclear Polarization Alexey S. Kiryutin,†,‡ Sergey E. Korchak,†,§ Konstantin L. Ivanov,‡,∥ Alexandra V. Yurkovskaya,‡,∥ and Hans-Martin Vieth*,† †

Institute of Experimental Physics, Freie Universität Berlin, Arnimallee14, D-14195 Berlin, Germany International Tomography Center, Institutskaya 3a, 630090, Novosibirsk, Russia § Physikalisch-Technische Bundesanstalt, Abbestr. 2-12, D-10587 Berlin, Germany ∥ Novosibirsk State University, Pirogova 2, 630090, Novosibirsk, Russia ‡

S Supporting Information *

ABSTRACT: We have shown that long-lived spin states (LLS) can be selectively populated by photogenerated chemically induced dynamic nuclear polarization (CIDNP) over a wide range of magnetic fields. Relaxation times of LLS of the βCH2 protons in N-acetyl histidine and partially deuterated histidine have been measured. Our experiments demonstrate that CIDNP enables creating LLS in the amino acid in a field range of up to a few Tesla and that their lifetimes can be 45 times longer than T1. The advantage of the method is thus two-fold: it allows one to accumulate high levels of spin hyperpolarization and to preserve them for periods of time far exceeding T1. Therefore, photo-CIDNP is a technique suitable for creating long-lived spin order in biologically relevant molecules.

SECTION: Spectroscopy, Photochemistry, and Excited States

L

Previously, it has been shown that appropriate strategies for strong nonequilibrium population of the slowly relaxing spin modes can be provided by means of para-hydrogen induced polarization (PHIP)5,7,8 or by dynamic nuclear polarization (DNP) in combination with subsequent NMR pulse sequences,3 which convert longitudinal spin order into the long-lived mode. In this Letter, we propose to populate LLSs by the technique of photochemically induced dynamic nuclear polarization (photo-CIDNP) because it has several advantages as compared with other techniques. CIDNP often arises in the course of photoreactions with radical pair intermediates and reveals itself in anomalous phases and intensities of the NMR lines,9−11 providing NMR enhancements up to several orders of magnitude. The origin of CIDNP is selectivity of the radical pair recombination with respect to the nuclear spin state caused by magnetic hyperfine interactions of nuclei with electrons in the radicals. An advantage of photo-CIDNP is that one can precisely control the time scheme of the experiment by application of short laser pulses that initiate the photochemical reaction. CIDNP is promising in the context of the long-lived spin modes because it provides their selective population. At low polarization fields Bp (where hyperfine interaction in the transient radicals is predominant), CIDNP populates the

ack of sensitivity is frequently the limiting factor in NMR spectroscopy and imaging. One of the possible ways of tackling this problem is using strongly nonthermally polarized (also termed hyperpolarized) spins. A key issue in using spin hyperpolarization (HP) is preserving it for as long as possible because relaxation to the thermal equilibrium imposes a limited time window for utilizing HP. Typically, the longitudinal relaxation times (T1) of protons in liquids are only a few seconds or even shorter, thus considerably reducing the HP application range. However, there is an alternative approach,1−4 which can remedy the situation. It is based on storing polarization not as longitudinal spin order but as another type of spin alignment. Typically, the main relaxation source is intramolecular dipole−dipole interaction, characterized by a particular symmetry. If the eigen-states of the spin system have the same symmetry, then certain states become immune to the dipolar interaction, that is, become long-lived.1−6 This is the case, for instance, for systems of two coupled spins 1/2 at low field: one of the eigen-states is a singlet state, which cannot be mixed with any of the three other triplet states. As a consequence, using long-lived spin states (LLSs) potentially allows one to go far beyond the T1 limit and considerably extend the observation time window. This is attractive for many NMR and MRI applications. Furthermore, the process of generating HP can be extended over a longer time before it runs into saturation and thus will result in higher stationary levels of polarization. © 2012 American Chemical Society

Received: April 24, 2012 Accepted: June 18, 2012 Published: June 18, 2012 1814

dx.doi.org/10.1021/jz3005046 | J. Phys. Chem. Lett. 2012, 3, 1814−1819

The Journal of Physical Chemistry Letters

Letter

Figure 1. Experimental protocols for studying CIDNP-induced long-lived spin states (LLS) at variable magnetic field: relaxation of CIDNP (a) and accumulation of CIDNP (b). Field switching time τfv is chosen that it is much shorter than relaxation but long enough that field variation is adiabatic.

condition of selective state populations with respect to their total spin, PS ≠ (PT+ + PT0 + PT−)/3, is fulfilled at any magnetic field up to ∼2 T. As long as the triplet relaxation is much faster than that of the LLS, all three triplet states will be rapidly equilibrated and have the same population PT (neglecting their small Boltzmann polarization), which is still different from PS. Therefore, (i) at the beginning of the field switch (see Figure 1) we have PS ≠ PT+ ≈ PT0 ≈ PT− and (ii) the LLS yield, that is, the difference (PS − PT), is large at any field. Hence, CIDNP is a technique suitable to populate selectively the LLS in a wide field range. To analyze the relaxation behavior, we studied the dependence of the CIDNP intensities as a function of τp and τ (see Figure 1) and extracted the fast and slow components for each line. Details of how CIDNP reveals itself in the NMR spectral patterns are given in the SI. A typical spectrum of the low-field CIDNP is given in Figure 2 together with the NMR spectrum taken after thermal polarization at 7 T. Signal enhancements, ε, with respect to the thermally polarized molecules at 7 T provided by CIDNP are on the order of 100, reaching even 200 for certain NMR lines. These values are in agreement with the

nuclear spin eigen-states of the reaction products according to their total spin momentum only: in the case of a two-spin 1/2 system, all three triplet states, |T+⟩, |T0⟩, and |T−⟩, have the same population, PT, which, in general, is different from that of the singlet state PS.12−14 In this context, the magnitude of (PS − PT) with PT defined as PT = (PT+ + PT0 + PT−)/3

(1)

is of primary importance because (i) it describes the level of HP and (ii) this type of polarization is long-lived. Here we will show that photo-CIDNP is a suitable technique for populating and manipulating LLSs. Experiments are performed for the amino acids N-acetyl-L -histidine (N-His) and partially deuterated histidine (DL-[α,2,4-D3] histidine) (His-D3) at variable magnetic field by means of the fast field-cycling NMR setup previously described.15 In the cases under study, the singlet state of the two β-CH2 protons is under-populated; that is, PS < PT.12−14,16 Here we will demonstrate that in both N-His and His-D3 an LLS exists in a fairly wide field range and that it can be populated by use of CIDNP. Field variation gives an additional degree of freedom for optimization of the polarization and relaxation process. This goes beyond previous studies, as they were limited to the zero-field case or to the high-field case under spin locking conditions with the only exception of ref 17. At zero field, CIDNP directly populates (or depopulates) the LLS, as it is an eigen-state of the Hamiltonian. As the field increases, the picture becomes more complicated for two reasons: (i) the spins cross from the regime of strong to weak coupling, where the singlet state is no longer a spin eigen-state, and (ii) the population pattern provided by CIDNP also changes. The first factor is determined by the “mixing angle” of the Zeeman states, θ, which is defined as tan 2θ = J/δν. (Here δν is the difference in Zeeman interaction of the spins with the external magnetic field B and J is the spin−spin coupling constant.) Because δν is proportional to B, the value |θ| = π/4 corresponds to B = 0, whereas θ = 0 represents the high-field case. For the β-CH2 protons of the amino acids, the low-field region (i.e., J ≫ δν and θ ≈ π/4) extends to at least 1 T. Therefore, the LLS character of one state in such a two-spin system is expected to be seen in a broad field range. As far as the second factor is concerned, as we found by numerical simulations of CIDNP (see Supporting Information, SI), the

Figure 2. CIDNP enhancement of β-CH2 protons of His-D3. Singlescan CIDNP spectrum taken at B0 = 7 T with the preparation field Bp = 1.5 mT (red) compared with a 100 times vertically enlarged NMR spectrum taken at 7 T (blue). CIDNP was measured after 500 laser pulses applied during 10 s. τfv was 270 ms. For detection, a π/4 rf-pulse was used. Enhancement factors, ε, are given for individual NMR lines. 1815

dx.doi.org/10.1021/jz3005046 | J. Phys. Chem. Lett. 2012, 3, 1814−1819

The Journal of Physical Chemistry Letters

Letter

Figure 3. Relaxation kinetics of CIDNP in His-D3 and N-His. Left: relaxation experiment (protocol shown in Figure 1a), His-D3 at Bp = 0.1 mT, π/4 detection pulse. Solid curve is simulation with parameters: TLLS = 62 s, contribution of fast component was negligible at this field. Right: accumulation experiment (protocol shown in Figure 1b), N-His at Bp = 0.1 mT, π/4 pulse. Solid curve is simulation with parameters: TLLS = 6 s, contribution of fast component was negligible at this field. Symbols β1, β2, β3, and β4 of different color denote individual lines in the NMR spectrum of the β-CH2 protons; the same color coding is used for the relaxation kinetics. Inset shows the results of a standard inversion−recovery experiment for His-D3 at 7 T giving T1 = 1.35 s.

theoretical description given in the SI. Therefore, the CIDNP technique provides huge NMR enhancements that can be stored in the LLS. The LLSs are therefore useful not only for preserving polarization but also for accumulating high HP levels. Although spins polarized this way carry only small longitudinal polarization, one can observe very strongly enhanced NMR signals. If preferable, pulse sequences can be used to convert the spin order into net polarization. We compared experiments on His-D3 having protons only in the β-CH2 positions with N-His. In the first case, there is a twospin 1/2 system with properties, as discussed above, whereas in the second case, at least three coupled spins have to be considered (if the weaker couplings between the β-CH2 protons and the ring protons are neglected). It is known that LLSs can exist in coupled three- and four-spin systems5,6,18 at B = 0; however, it is not yet studied in detail in what field range they can be found. Theoretical treatment of such a problem can only be done numerically because the Hamiltonian of three coupled spins at arbitrary field cannot be solved analytically. Typical time evolution curves for both kinds of experiment are given in Figure 3 for a magnetic field of 0.1 mT. In each case, the kinetics can be fitted by the simple formula I = A1 exp( −τ /TLLS) + A 2 exp(−τ /Tshort) + A 0

Here the fast relaxation mode always has a characteristic time constant Tshort (fast mode) that is almost independent of Bp as we are in the fast motional limit, whereas TLLS (slow mode of the LLS) was studied as a function of magnetic field. In eq 2, Tshort describes the mixture of fast-relaxing modes:17 triplet relaxation with characteristic time TT and residual T1 relaxation. In our measurements, we could not separate TT and T1 and therefore used a common relaxation time Tshort to take both processes into account. The offset A0 describes accumulation of Boltzmann polarization during field variation, τfv. In the experiments with accumulation of CIDNP (protocol Figure 1b), the determination of Tshort was problematic because the contribution to CIDNP rising with short time constant is saturated at a much lower level than that having a long rise time. The values of TLLS were the same for both experimental protocols. The weight, A1, of the slow component was predominant, meaning that CIDNP is indeed an appropriate method for observing the LLS. Prolongation of the sustaining interval for HP, that is, the ratio TLLS/T1, achieved at low field (below 0.1 T) reaches the value of 12 for N-His and the outstanding value of 45 for HisD3. This value is considerably larger than the highest TLLS/T1 previously reported19 that was equal to 37. Our study also shows that the field range, in which TLLS ≫ T1 is fulfilled,

(2) 1816

dx.doi.org/10.1021/jz3005046 | J. Phys. Chem. Lett. 2012, 3, 1814−1819

The Journal of Physical Chemistry Letters

Letter

extends to ∼1 T: at this field, we obtained for His-D3 TLLS/T1 = 20. The method is applicable not only to two-spin 1/2 systems but also to larger spins systems: although the prolongation of relaxation time is not as impressive as for two-spin 1/2, considerable values of ∼7.5 are reached for the βCH2 protons of N-His at 1 T. In general, the relaxivity of the LLS due to the dipolar couplings at arbitrary magnetic field can be described by the following expression RLLS,d =

20 + 2 cos 4θ −

2(81 − 80 cos 4θ + cos 8θ) 60

R1d

Summing up the work, we emphasize again that photoCIDNP in reversible reactions with radical pairs as intermediates is an efficient technique for creating HP of long-lived states and for utilizing them in various NMR applications such as probing long-lasting dynamic processes or enhancing contrast in MRI where slow relaxation is an important issue. As further advantages of the method, we mention that the preparation of polarization is easy to control because it is done via light irradiation. This is not the case, for instance, for PHIP. The experiment can be repeated many times with the same sample due to high reversibility of the photoreactions used. Although, with respect to the absolute NMR enhancement, our method (at least for radical pairs in aqueous solution at room temperature) cannot compete with PHIP7,8 and dissolution DNP,3,17,23 which give the highest HP reported, it is advantageous in many respects. In particular, the method is applicable to samples under physiological conditions, that is, aqueous solutions at room temperature. In contrast with the dissolution DNP freezing of the sample and cooling it to cryogenic temperatures, which is a severe limitation for many biologically relevant systems, is not required. At the same time, CIDNP is to a much larger extent limited to specific chemistry, which can be both a weakness and an advantage of the technique. The method of producing HP is simple and fast: enhancement of 100−200 (also obtained independently in a recent work24) can be produced in only tens of seconds. It is worth noting that 200 is not the theoretical maximum of what one can get from CIDNP: there are systems (i.e., cyclic biradicals)25 where it is about an order of magnitude higher. The method does not require the presence of stable paramagnetic compounds in the system, which can be a disadvantage as (a) they are often toxic and (b) they result in a reduction21 of TLLS unless free radicals are scavenged26 by a reducing agent. In the CIDNP case, the spin system acquires a very high polarization level, whereas there is no reduction of the TLLS/T1 ratio that is even larger than the highest value9 previously reported. This is because the transient radicals are present only during a short time and disappear on the microsecond time scale having no time to affect T1 and TLLS. In contrast with DNP, the method can be relatively easily adapted to polarize the spin system at variable field strength. Use of variable magnetic fields can also be employed for NMR relaxometry of LLS to study slow processes obscured by fast dipolar relaxation.27,28 By choosing systems with proper NMR parameters (chemical shifts and J-couplings), one can extend the field range for creating LLSs up to high fields of several Tesla.29 The chemical compounds used are biologically relevant or biologically compatible molecules, which potentially broadens the range of bio-NMR applications of the technique.24,30 Our studies14,16,31 of the CIDNP field dependence for the CIDNPactive amino acids histidine, tyrosine, and tryptophan and small peptides containing these residues revealed close similarity in CIDNP pattern under field variation for protons in the βposition of the peptide backbone; therefore, we anticipate similar behavior for their LLSs. Our studies of LLS in other biologically relevant compounds confirm the wider applicability of the CIDNP technique to populating LLSs and will be published in a separate publication. Moreover, CIDNP is by no means limited to this specific class of compounds but can also be used, for instance, for populating LLSs in CH2 groups of cyclic ketones.32,33

(3)

Here R1d is the T1-relaxation rate for the dipolar mechanism. This formula has been obtained (see SI) using the rates of the relaxation transition in the two-spin system from ref 20 as an eigenvalue of the relaxation matrix. The relaxation rate RLLS,d is strongly field-dependent (due to the presence of θ in the formula), and at B = 0 (i.e., θ = π/4) it goes to zero (infinite relaxation time, i.e., full LLS character). In the high field limit RLLS,d = R1d/3, hence some character of a LLS is still present in the system. The field dependence of 1/TLLS of His-D3 is shown in Figure 4. At fields up to 0.1 T, TLLS is field-independent, but at higher

Figure 4. Field dependence of 1/TLLS for the β-CH2 protons in HisD3 (red symbols) and in N-His (black symbols). TLLS was determined by fitting the relaxation kinetics obtained for protocol (a) using eq 2. Red solid line shows simulation for His-D3 by expression RLLS,d + R0; black solid line is shown only as a guide for the eye.

fields it decreases: at 7 T, the ratio TLLS/T1 has dropped to 4.5, indicating that some LLS character still survives at high field. The experimentally observed field dependence of 1/TLLS can be well-simulated by the expression RLLS,d + R0, where R0 stands for relaxation contributions caused by mechanisms other than dipolar. In such a simplified description, we neglected the fact21 that such contributions may affect T1 and TLLS differently. It is important to note that here R0 ≈ 0.017 s−1 is the only fitting parameter, whereas RLLS,d is known at any field because R1d + R0 was measured independently and θ is fully determined by the known NMR parameters. The theoretical expression fits the experimental points well, which is an indication of correct interpretation of the data. The field dependence of the LLS in N-His cannot be modeled with the simple expression RLLS,d + R0 obtained for the two-spin system because there are more coupled proton spins in the molecule. Coupling to the α-CH proton also results in a sharp feature in the field dependence of 1/TLLS at ∼0.3 T, which is caused by a level anticrossing22 in the three-spin system formed by the α-CH and β-CH2 protons. 1817

dx.doi.org/10.1021/jz3005046 | J. Phys. Chem. Lett. 2012, 3, 1814−1819

The Journal of Physical Chemistry Letters

■

Letter

(2) Carravetta, M.; Johannessen, O. G.; Levitt, M. H. Beyond the T1 Limit: Singlet Nuclear Spin States in Low Magnetic Fields. Phys. Rev. Lett. 2004, 92, 153003/1−153003/4. (3) Vasos, P. R.; Comment, A.; Sarkar, R.; Ahuja, P.; Jannin, S.; Ansermet, J. P.; Konter, J. A.; Hautle, P.; Van Den Brandt, B.; Bodenhausen, G. Long-Lived States to Sustain Hyperpolarized Magnetization. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 18469−18473. (4) Sarkar, R.; Ahuja, P.; Moskau, D.; Vasos, P. R.; Bodenhausen, G. Extending the Scope of Singlet-State Spectroscopy. ChemPhysChem 2007, 8, 2652−2656. (5) Vinogradov, E.; Grant, A. K. Hyperpolarized Long-Lived States in Solution NMR: Three-Spin Case Study in Low Field. J. Magn. Reson. 2008, 194, 46−57. (6) Grant, A. K.; Vinogradov, E. Long-Lived States in Solution NMR: Theoretical Examples in Three- and Four-Spin Systems. J. Magn. Reson. 2008, 193, 177−190. (7) Bouguet-Bonnet, S.; Reineri, F.; Canet, D. Effect of the Static Magnetic Field Strength on Parahydrogen Induced Polarization NMR Spectra. J. Chem. Phys. 2009, 130, 234507/1−234507/9. (8) Jonischkeit, T.; Bommerich, U.; Stadler, J.; Woelk, K.; Niessen, H. G.; Bargon, J. Generating Long-Lasting 1H and 13C Hyperpolarization in Small Molecules with Parahydrogen-Induced Polarization. J. Chem. Phys. 2006, 124, 201109/1−201109/5. (9) Salikhov, K. M.; Molin, Y. N.; Sagdeev, R. Z.; Buchachenko, A. L. Spin Polarization and Magnetic Effects in Chemical Reactions; Elsevier: Amsterdam, 1984. (10) Morozova, O. B.; Yurkovskaya, A. V. Intramolecular Electron Transfer in the Photooxidized Peptides Tyrosine−Histidine and Histidine−Tyrosine: A Time-Resolved CIDNP Study. Angew. Chem., Int. Ed. 2010, 49, 7996−7999. (11) Wirmer, J.; Kuhn, T.; Schwalbe, H. Millisecond Time Resolved Photo-CIDNP NMR Reveals a Non-Native Folding Intermediate on the Ion-Induced Refolding Pathway of Bovine α-Lactalbumin. Angew. Chem., Int. Ed. 2001, 40, 4248−4251. (12) Kaptein, R.; Den Hollander, J. A. Chemically Induced Dynamic Nuclear Polarization. X. Magnetic Field Dependence. J. Am. Chem. Soc. 1972, 94, 6269−6280. (13) Tarasov, V. F.; Shkrob, I. A. Low-Field CIDNP in Intramicellar Radical Disproportionation. Violation of Equivalency in J-Coupled Nuclear Spin Systems. J. Magn. Reson., Ser. A 1994, 109, 65−73. (14) Ivanov, K. L.; Vieth, H.-M.; Miesel, K.; Yurkovskaya, A. V.; Sagdeev, R. Z. Investigation of the Magnetic Field Dependence of CIDNP in Multi-Nuclear Radical Pairs. Part II. Photoreaction of Tyrosine and Comparison of Model Calculation with Experimental Data. Phys. Chem. Chem. Phys. 2003, 5, 3470−3480. (15) Grosse, S.; Yurkovskaya, A. V.; Lopez, J.; Vieth, H.-M. Field Dependence of Chemically Induced Dynamic Nuclear Polarization (CIDNP) in the Photoreaction of N-Acetyl-Histidine with 2,2′Dipyridyl in Aqueous Solution. J. Phys. Chem. A 2001, 105, 6311− 6319. (16) Ivanov, K. L.; Lukzen, N. N.; Vieth, H. M.; Grosse, S.; Yurkovskaya, A. V.; Sagdeev, R. Z. Investigation of the Magnetic Field Dependence of CIDNP in Multinuclear Radical Pairs. 1. Photoreaction of Histidine and Comparison of Model Calculation with Experimental Data. Mol. Phys. 2002, 100, 1197−1208. (17) Bornet, A.; Jannin, S.; Bodenhausen, G. Three-Field NMR to Preserve Hyperpolarized Proton Magnetization as Long-lived States in Moderate Magnetic Fields. Chem. Phys. Lett. 2011, 512, 151−154. (18) Karabanov, A. A.; Bretschneider, C.; Kö ckenberger, W. Symmetries of the Master Equation and Long-Lived States of Nuclear Spins. J. Chem. Phys. 2009, 131, 204105/1−204105/10. (19) Sarkar, R.; Ahuja, P.; Vasos, P. R.; Bodenhausen, G. Measurement of Slow Diffusion Coefficients of Molecules with Arbitrary Scalar Couplings via Long-Lived Spin States. ChemPhysChem 2008, 9, 2414−2419. (20) Freeman, R.; Wittekoek, S.; Ernst, R. R. High-Resolution NMR Study of Relaxation Mechanisms in a Two-Spin System. J. Chem. Phys. 1970, 52, 1529−1544.

EXPERIMENTAL METHODS Experiments were done according to the two protocols shown in Figure 1. In the first protocol (Figure 1a), CIDNP was formed at the variable magnetic field Bp after a short period, τp, of pulsed laser irradiation of the sample with λ = 308 nm and pulse repetition rate up to 100 Hz; the light intensity absorbed by the sample was ∼20 mJ/pulse. To achieve the maximal NMR enhancement for His, we adjusted the experimental parameters (irradiation power, repetition rate, and concentration of dye and His) in such a way that every amino acid molecule passed through at least one photoreaction cycle during the irradiation period, τp, that was kept shorter than the CIDNP relaxation time. Polarization was produced in the reversible chemical reaction of hydrogen atom transfer from the amino acid to the triplet photoexcited dye 2,2′-dipyridyl (DP).34 After irradiation, a waiting period τ of variable duration was introduced, during which the system relaxes back to thermal equilibrium. During the subsequent time interval τfv < 0.27 s, the magnetic field was rapidly switched to the detection level B0 = 7 T, where the high-resolution NMR spectrum was recorded. By repetitive measurements with stepwise varying Bp, we obtained the field dependence of relaxation for different relaxation modes. In some of the cases, the LLS preparation stage was slightly modified with the purpose to increase the signal-to-noise ratio: CIDNP was produced at B = 0; then, the field was rapidly switched to the intermediate field of interest, where the spins relaxed freely and the relaxation times Tshort and TLLS were measured. In the second protocol (Figure 1b), no waiting period was introduced; instead, the duration of the preparation period, τp, was varied. In the first type of experiment we were thus measuring the relaxation kinetics (NMR signals as a function of delay τ), whereas in the second one we accumulated varying levels of different spin orders depending on their relaxation times. In both cases, we performed biexponential fits of the kinetics and determined the contributions of the fast and the slowly relaxing modes.

■

ASSOCIATED CONTENT

S Supporting Information *

Theoretical description of LLS population by means of CIDNP and its relaxation at variable magnetic field. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +49(30)838-55062. Fax: +49(30)838-56081. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS K.L.I., A.S.K., and A.V.Y. acknowledge support from the Alexander von Humboldt Foundation. The research was financially supported by the Russian Fund for Basic Research (project no. 11-03-00296a), the program P-220 of the Russian Government (grant no. 11.G34.31.0045), and the Program of the Division of Chemistry and Material Science RAS (project 5.1.1).

■

REFERENCES

(1) Carravetta, M.; Levitt, M. H. Long-Lived Nuclear Spin States in High-Field Solution NMR. J. Am. Chem. Soc. 2004, 126, 6228−6229. 1818

dx.doi.org/10.1021/jz3005046 | J. Phys. Chem. Lett. 2012, 3, 1814−1819

The Journal of Physical Chemistry Letters

Letter

(21) Tayler, M. C. D.; Levitt, M. H. Paramagnetic Relaxation of Nuclear Singlet States. Phys. Chem. Chem. Phys. 2011, 13, 9128−9130. (22) Miesel, K.; Ivanov, K. L.; Yurkovskaya, A. V.; Vieth, H. M. Coherence Transfer During Field-Cycling NMR Experiments. Chem. Phys. Lett. 2006, 425, 71−76. (23) Ahuja, P.; Sarkar, R.; Jannin, S.; Vasos, P. R.; Bodenhausen, G. Proton Hyperpolarisation Preserved in Long-Lived States. Chem. Commun. 2010, 46, 8192−8194. (24) Lee, J. H.; Sekhar, A.; Cavagnero, S. 1H-Detected 13C PhotoCIDNP as a Sensitivity Enhancement Tool in Solution NMR. J. Am. Chem. Soc. 2011, 133, 8062−8065. (25) Morozova, O. B.; Tsentalovich, Y. P.; Yurkovskaya, A. V.; Sagdeev, R. Z. 1H and 13C Nuclear Polarization in Consecutive Biradicals during the Photolysis of 2,2,12,12-Tetramethylcyclododecanone. J. Phys. Chem. A 1997, 101, 399−406. (26) Miéville, P.; Ahuja, P.; Sarkar, R.; Jannin, S.; Vasos, P. R.; Gerber-Lemaire, S.; Mishkovsky, M.; Comment, A.; Gruetter, R.; Ouari, O.; Tordo, P.; Bodenhausen, G. Scavenging Free Radicals To Preserve Enhancement and Extend Relaxation Times in NMR using Dynamic Nuclear Polarization. Angew. Chem., Int. Ed. 2010, 49, 6182− 6185. (27) Ahuja, P.; Sarkar, R.; Vasos, P. R.; Bodenhausen, G. Molecular Properties Determined from the Relaxation of Long-Lived Spin States. J. Chem. Phys. 2007, 127, 134112/1−134112/6. (28) Bornet, A.; Ahuja, P.; Sarkar, R.; Fernandes, L.; Hadji, S.; Lee, S. Y.; Haririnia, A.; Fushman, D.; Bodenhausen, G.; Vasos, P. R. LongLived States to Monitor Protein Unfolding by Proton NMR. ChemPhysChem 2011, 12, 2729−2734. (29) Tayler, M. C. D.; Levitt, M. H. Singlet Nuclear Magnetic Resonance of Nearly-Equivalent Spins. Phys. Chem. Chem. Phys. 2011, 13, 5556−5560. (30) Morozova, O. B.; Hore, P. J.; Bychkova, V. E.; Sagdeev, R. Z.; Yurkovskaya, A. V. Time-Resolved CIDNP Study of Non-Native States of Bovine and Human α-Lactalbumins. J. Phys. Chem. B 2005, 109, 5912−5918. (31) Morozova, O. B.; Korchak, S. E.; Vieth, H.-M.; Yurkovskaya, A. V. Photo-CIDNP Study of Transient Radicals of Met-Gly and Gly-Met Peptides in Aqueous Solution at Variable pH. J. Phys. Chem. B 2009, 112, 12859−12862. (32) Yurkovskaya, A.; Grosse, S.; Dvinskikh, S.; Morozova, O.; Vieth, H.-M. Spin and Molecular Dynamics of Biradicals as Studied by Low Field Nuclear Polarization at Variable Temperature. J. Phys. Chem. A 1999, 103, 980−988. (33) De Kanter, F. J. J.; Sagdeev, R. Z.; Kaptein, R. Magnetic Field Dependent Carbon-13 and Proton CIDNP From Biradicals. The Role of the Hyperfine Coupling Constant. Chem. Phys. Lett. 1978, 58, 334− 339. (34) Tsentalovich, Y. P.; Morozova, O. B.; Yurkovskaya, A. V.; Hore, P. J.; Sagdeev, R. Z. Time-Resolved CIDNP and Laser Flash Photolysis Study of the Photoreactions of N-Acetyl-Histidine with 2,2′-Dipyridyl in Aqueous Solution. J. Phys. Chem. A 2000, 104, 6912−6916.

1819

dx.doi.org/10.1021/jz3005046 | J. Phys. Chem. Lett. 2012, 3, 1814−1819