Subscriber access provided by CORNELL UNIVERSITY LIBRARY
Article
Nuclear Spin Singlet Order Selection by Adiabatically Ramped RF-fields Andrey N. Pravdivtsev, Alexey Sergeevich Kiryutin, Alexandra V. Yurkovskaya, Hans-Martin Vieth, and Konstantin L. Ivanov J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b08879 • Publication Date (Web): 27 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
The Journal of Physical Chemistry B is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Nuclear Spin Singlet Order Selection by Adiabatically Ramped RF-fields Andrey N. Pravdivtsev,a,b† Alexey S. Kiryutin,a,b† Alexandra V. Yurkovskaya,a,b Hans-Martin Vieth,a,c Konstantin L. Ivanova,b* a
International Tomography Center, Siberian Branch of the Russian Academy of Science, Institutskaya 3A, Novosibirsk, 630090 (Russia) b
c
Novosibirsk State University, Pirogova 2, Novosibirsk, 630090 (Russia)
Institut für Experimentalphysik, Freie Universität Berlin, Arnimallee 14, Berlin, 14195 (Germany)
* Corresponding author; e-mail:
[email protected] ABSTRACT
We describe a Nuclear Magnetic Resonance (NMR) method to generate singlet order in spin pairs from longitudinal spin magnetization and to suppress residual background signals. This method can be used for generating and observing long-lived spin states. A Singlet Order Selection filter (SOSfilter) is proposed, which allows us to find signals of the spin pair of interest buried in a crowded NMR spectrum. Likewise, SOS-filtering enables proton NMR measurements in H2O without pulse sequences for solvent suppression. We demonstrate that the method works perfectly for weakly coupled spin pairs, as well as for strongly coupled spin pairs. Furthermore, it can be combined with
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 36
standard NMR pulse sequences: in this way T1- and T2-relaxation times for spin pairs of interest can be measured. The power of the SOS-filter is demonstrated by relaxation studies in biomolecules.
2 ACS Paragon Plus Environment
Page 3 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
I. Introduction Studies of spin relaxation in magnetic resonance provide insights into molecular mobility in a wide range of systems, such as polymers, biomolecules, liquid crystals, etc. Typically, londitudinal, T1, and transverse, T2, relaxation is investigated; however, recently relaxation of singlet spin states has drawn significant attention. Nuclear singlet states are known to have extended lifetimes1-2; for this reason, singlet-state Nuclear Magnetic Resonance (NMR) is a promising method for studying slow processes and preserving non-thermal spin order. For symmetry reasons, spin pairs prepared in their singlet state are immune to in-pair dipolar relaxation (typically, the main source of relaxation in NMR) thus often forming a Long-Lived spin State (LLS). LLSs lifetimes can exceed nuclear T1relaxation times by more than an order of magnitude: for instance, for the β-CH2 protons of partially deuterated aromatic amino acids they can be as long at 45 ⋅ .3-4 Extraordinarily long 1H singletstate lifetimes, , of about 4 min at ambient temperature have been found for the vinyl group of maleic acid dimethyl ester;5-6 for hetero nuclei can be much longer, e.g. in the molecule7; presently, the longest times of about 1 hour are found for designed
13
13
15
N-labeled N2O
C spins in specially
C-labeled molecules8. Such LLSs have found a number of applications; namely, LLSs
can be efficiently utilized for probing slow molecular motions9-10, slow transport11-15 and drugscreening16. An important application of LLSs is preserving nuclear spin hyperpolarization17-20, which provides significant NMR enhancements but has a transient nature: T1-relaxation strongly limits the potential range of applications of hyperpolarization. The focus of this work is developing a robust method for selectively preparing and detecting singlet spin order with efficient suppression of residual background signals. Such a development is of importance for a wide range of applications, for instance, for assessing LLSs in biological molecules having crowded NMR spectra. To this end, we exploit a technique, which we recently proposed21-22; 3 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 4 of 36
in this method magnetization-to-singlet (M2S) and singlet-to-magnetization (S2M) conversion is performed by means of adiabatic switching of an RF-field of a suitable frequency. The idea behind it is the correlation of nuclear spin states in the RF-rotating frame of reference; we have demonstrated that this method provides a conversion efficiency going up to the maximal theoretical value23. Here we use a modification of this method21 with the aim to suppress residual NMR lines in order to obtain a neat, background-free NMR spectrum of a spin pair under investigation. Thus, we come up with a Singlet Order Selection filter (SOS-filter). We also combine the SOS-filter with standard NMR pulse sequences, here with the inversion-recovery sequence and the spin echo sequence, with the aim to analyze the T1 and T2-relaxation times of a spin pair of interest. To demonstrate the power of our method we apply it to biomolecules having spin pairs, namely, CH2-groups. Our results clearly show that the new method enables background-free detection of coupled spin pairs that passed through the SOS-filter by suppression of residual lines in crowded NMR spectra, paving the way to new experiments using SOS-filtering. Further applications of the spin order conversion technique and SOS-filter are discussed at the end of the paper. II. Methods A. Spin order conversion method Let us briefly explain the idea of the Adiabatic-Passage Spin Order Conversion (APSOC) technique, which is used to generate singlet spin order. In APSOC, an RF-field is used with a time-dependent amplitude, ( ), which is increased from zero to a maximal value, . When the RF-field switch is slow (namely, adiabatic), it is sufficient for analyzing its consequences to correlate the spin states in the RF-rotating frame at = 0 and = . Adiabatic correlation simply means that the population of the highest level (in energy) at = 0 goes to the population of the highest level at strong ; likewise, the population of the second highest level in the absence of the RF-field goes to 4 ACS Paragon Plus Environment
Page 5 of 36
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
the population of the second highest level in the presence of the RF-field and so on. When the RFfield is strong and the spin-spin interaction, , is positive (for < 0 the method works essentially in the same way), the second lowest state (in energy) is the singlet state of the spin pair. At the same time, once we work in the rotating frame, the energies of the spin states at = 0 depend on the offset, Δ, of the RF-frequency from the center of the spectrum of the spin pair. Interestingly, for an arbitrary spin pair the off-set can be chosen such that the second lowest state is either the T or T triplet state. In this way, it becomes possible to convert the population of the T (or T state) into the population of the long-lived singlet state. Thus, the M2S-conversion is performed, since the population difference between the T± states and the longitudinal magnetization of a pair of spins are directly proportional to each other. Since adiabatic transitions are reversible, the method for the S2M conversion is straightforward: such a conversion can be performed by an RF-field, which is reduced from to zero in an adiabatic fashion. One should note that once the spin pair goes to its singlet state, it can be “locked” there by applying a sufficiently strong RF-field (spin-locking). Importantly, the APSOC method works for an arbitrary spin pair, no matter whether it is coupled weakly or strongly, as it is shown in a previous publication24 and confirmed below. Here, to keep the description simple, we do not go into detail of the theory behind APSOC, which is described in detail in previous publications. B. Background suppression method Thus, the RF-field switches work as NMR “pulses”, which drive transitions of the kind S ↔ T and S ↔ T and generate triplet-singlet imbalance25. Such pulses selectively convert the population of the T or T states into that of the singlet state (and vice versa). Interestingly, the “phase” of these pulses, that is, selection of T or T , is varied by changing the sign of Δ. Such a representation enables performing pseudo “phase cycles” in order to suppress all background signals21; the spectral 5 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 36
contrast with respect to singet spin order achieved in this method is better than that for alternative techniques10,
26
, resulting not only in a better suppression of residual signals but also in lower
attenuation of the signals of interest. In contrast to other methods, in the SOS-filter experiment the shape of lines stays exactly the same as in the thermal spectrum. Indeed, if we repeat the experiment twice with, say, different “phase” of the RF-on “pulse” (performing the conversion of the kind T → S and T → S, which produces an overpopulated or underpopulated singlet state, respectively) and subtract the resulting spectra the signal of the spin pair under study will be doubled, whereas signals from other spins will vanish (because their magnetization evolves in an almost identical way in both experiments being insensitive to the small variation of the RF-on frequency). Below, we demonstrate that this method provides excellent background suppression, that is, SOS-filtering.
Scheme 1. Optimization of the RF-field off-set, Δ, for APSOC in a two-spin system having four NMR lines. For performing S ↔ T and S ↔ T conversion, the off-set Δ has to be positive and negative (with respect to the center of the spectrum, ν ). For a weakly coupled AX-system (left) one should set |Δ|
