Oxygen-17 NMR spectroscopy: Basic principles and applications (Part I)
Introduction
The oxygen atom is probably the most chemically and biologically important element on earth which contains ∼50% by weight of oxygen. Oxygen forms compounds with all elements except for a few noble gases and metals. The structure and dynamics, therefore, of oxygen-containing compounds is of great significance. Compared to 1H, 13C, 15N, 31P and 19F NMR, however, 17O NMR has received little attention [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Between the first observation of a 17O nuclear induction signal in 1951 [11] and the first comprehensive review article of all aspects of 17O NMR in 1981 [4] there have been only about 200 publications dealing with 17O NMR. This limited interest is not surprising since of the three naturally occurring oxygen isotopes (16O, 17O and 18O), only 17O possesses a nuclear spin (I = 5/2). Table 1 lists some useful data of the 17O nucleus. It has a moderate electrical quadrupole moment (Qe = −2.63 × 10−30 e m2), a very small magnetogyric ratio (γ = −3.688 × 10−7rad. T−1 s−1), a low natural abundance (0.037%) and an extremely low absolute sensitivity compared to that of 1H (∼1.1 × 10−5). The 17O isotope is therefore one of the more difficult nuclei to observe by NMR spectroscopy. It is however of great interest to use a nucleus, such as oxygen, that is located at strategic molecular sites and is directly involved in inter- and intra-molecular interactions. The 17O NMR parameters, i.e., isotropic shielding, principal elements of the 17O shielding and electric field gradient tensors and transverse and longitudinal relaxation times can be considered as excellent means for probing structure, bonding and dynamics of oxygen containing compounds. Further, recent advances in instrumentation, the extremely large chemical shift scale (which aids in, several cases, in the resolution of quadrupole broadened resonances) and the availability of 17O enriched compounds have alleviated some of the experimental difficulties; thus, an increased use of the 17O NMR technique can be foreseen [1], [2], [3], [4], [5], [6], [7], [8], [9], [10].
Several reviews of the subject are available but, in general, either only selected aspects have been taken into account or very brief accounts of a wide range of applications. A summary of earlier results on 17O, up to about 1962, can be found in the textbook by Emsley et al. [14]. Silver and Luz published a comprehensive review article in 1966 [15]. 17O NMR has been discussed briefly in two reviews concerned with heteronuclei [16], [17]. Deverill [18] has summarized 17O NMR investigations of water and aqueous alkali salt solutions in a review of multinuclear NMR studies of inorganic electrolyte solutions. Silver [19] has reviewed the use of 17O NMR in studying reaction mechanisms in organic and inorganic chemistry. Achlama-Chmelnick and Fiat [20] have summarized theoretical background and applications of 17O magnetic resonance in paramagnetic systems; Klemperer has reviewed applications of 17O NMR spectroscopy as a structural probe [2], [5]; Cohn [21] and Tsai and Bruzik [22] have covered some specific applications of the isotopic oxygen effects on 31P NMR as probes of enzymatic reactions; Fiat [23] and Pearson and Oldfield [24] have summarized biophysical applications of 17O NMR; Boykin and Baumstark [25] have covered specific applications of 17O NMR in investigating torsion angle effects; Gerothanassis has reviewed the application of 17O NMR to hemoproteins and synthetic model compounds [26] and mentioned results from 17O in a review on multinuclear and multidimensional NMR methodology for studying individual water molecules bound to peptides and proteins in solution [27]; Lynch [28] has mentioned results from 17O in a review on water relaxation in heterogeneous and biological systems and Denisov and Halle [29] reviewed 17O and 2H NMR dispersion studies of protein hydration.
Rodger and Sheppard [1] presented a comprehensive review article with particular emphasis to a wide range of applications up to mid-1977; Amour and Fiat [3] reviewed the literature up to 1980 with particular emphasis on those concepts that are most essential to the understanding of 17O magnetic resonance; Kintzinger [30] reviewed the literature up to 1983 and Gerothanassis has presented brief accounts of a wide range of applications up to 1995 [7] and 1998 [8]. In 1981, Kintzinger [4] published a very comprehensive monograph which covered all aspects of 17O NMR, including experimental considerations, theoretical background of characteristic parameters and a wide range of applications. In 1991, Boykin [6] edited a monograph on 17O NMR in Organic Chemistry with emphasis to both principles and in depth wide range of applications. Both monographs turned out to be the standard reference textbooks in the field of 17O NMR for more than a decade. Berger et al. [10] presented a comprehensive review article with particular emphasis to a wide range of applications up to 1995.
In this review (presented in two parts, Part I and Part II) we have attempted, as in Kintzinger’s [4] and Boykin’s [6] seminal monographs, to treat both the experimental aspects and the theoretical background that is essential to the understanding of 17O NMR and a wide range of applications with particular emphasis on those that most clearly demonstrate the unique potential of this nucleus. A detailed and systematic survey of the literature was carried out with the help of Chemical Abstracts, The Web of Science, and EBSCO’s Academic Premier Database (particularly using the heading “Nuclear magnetic resonance – oxygen 17” and “oxygen properties, atomic isotope of mass 17, NMR”) up to and including 2007. Emphasis will be placed on the presentation of work that has clearly demonstrated the benefits of employing 17O NMR in studies of chemical and biochemical systems. Hence several older, yet classic 17O NMR papers will be discussed. Finally, an attempt is made to define unexplored areas where 17O NMR may provide structural and dynamic information which is difficult or impossible to obtain using other techniques.
Section snippets
Historical perspective
Alder and Hu [11] observed the first nuclear induction signals of 17O from H2O, 2H2O, methanol, ethanol and acetic acid, using a spectrometer operating at a frequency of 5.8 MHz for 17O. The sign of the magnetic moment of 17O was observed to be opposite to that of 2H, i.e., to be negative. Furthermore, it was observed that the relaxation time of 17O in pure water is about 100 times shorter that of protons. It was suggested that the relaxation is due to the interaction with the electric fields of
17O enrichment: synthetic methodologies
The stringent requirements in studies of compounds at natural abundance are the high concentrations (>0.1 M) and the extensive signal averaging needed for successful detection [55]. A 17O NMR investigation of aminoacids in 17O-depleted water at elevated temperatures resulted in relatively sharp resonances with sufficient signal-to-noise (S/N) ratio. However, at room temperature and despite fast pulsing and the application of a sensitivity enhancement function, the spectra were very poor. Fig. 3A
Use of high magnetic field strengths
The high magnetic field strengths available from superconducting magnets have significantly improved the sensitivity of the 17O NMR experiment and the increased chemical shift dispersion has made it possible to study more complicated chemical and biochemical systems [104]. Fig. 5 illustrates the 17O NMR spectrum of a saturated solution of trisodium trimetaphosphate in 17O depleted H2O using a low field instrument (100 MHz for 1H, 13.56 MHz for 17O). The absorption of the bridge oxygen is hardly
Experimental shielding ranges
The resonance frequency, ν, of a nucleus in a given molecule compared to the frequency of a reference compound, νr, allows the definition of the chemical shift δPositive values of δ, thus, indicate deshielding or high frequency shift. A general pattern of 17O shielding of diamagnetic compounds, which cover a total range of over 2500 ppm, is exhibited in Fig. 31. Chemical shift data in this Review are reported relative to H2O or 1,4-dioxane (deshielding positive) (see Section 4.2) or
Part II (Sections 6–12)
Part II of this review will be published in Volume 57 of Progress in NMR Spectroscopy and contains seven sections dealing with: Indirect spin–spin coupling constants ([615], [616], [617], [618], [619], [620], [621], [622], [623], [624], [625], [626], [627], [628], [629], [630], [631], [632], [633], [634], [635], [636], [637], [638], [639], [640], [641], [642], [643], [644], [645], [646], [647], [648], [649], [650]); Nuclear quadrupole coupling constants ([651], [652], [653], [654], [655], [656]
Symbols and abbreviations
- ηas
asymmetry of the chemical shift tensor
- η
asymmetry of the electric field gradient tensor
- AEE
average excitation energy
- CFP
charge field perturbation
- CPMG
Carr–Purcell–Meiboom–Gill
- CBF
cerebral blood flow
- CSA
chemical shift anisotropy
- CSI
chemical shift imaging
- CNDO/S
complete neglect of differential overlap for spectroscopy
- CI
configuration interaction
- τc
correlation time for molecular tumbling
- CCSD
coupled cluster singlets and doublets
- CDFT
coupled DFT
- CHF
coupled Hartree–Fock
- Pc
critical pressure
- Tcr
critical
Acknowledgements
I wish to thank Dr. J. Feeney (Division of Molecular Structure, National Institute for Medical Research, Mill Hill, London, UK) for inviting me to submit this article to Progress in NMR Spectroscopy. This work would not have been possible without the fruitful collaboration, continuing advice and encouragement of Prof. N. Sheppard, FRS (School of Chemical Sciences, University of East Anglia, UK), Dr. J. Lauterwein (Organisch-Chemisches Institut der Universitat, Munster, Germany), Dr. M. Marraud
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