The Long and...
REPORT slice—first in one spatial dimension a n d t h e n in an orthogonal dimen sion—followed by 2D FT spectros copy. This technique, initially called Fourier zeugmatography, h a s been widely adopted for medical imaging.
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Offshoots of 2D spectroscopy
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2D experiment t h a t traces the car bon backbone of t h e molecule one link at a time. This technique (21) relies on the presence of two directly adjacent 13 C spins and is therefore of low sensitivity. It excites 13 C double quantum coherence during the evo lution period and reconverts it into observable magnetization during de tection. T h u s not only is each 1 3 C spin pair labeled in an unambiguous manner, but the intense and u n d e s i r a b l e s i g n a l s from i s o l a t e d 1 3 C spins are suppressed. The result is a surprisingly faithful representation of the carbon-carbon connectivity in which chain branching and ring for mation are clearly identified.
Magnetic resonance imaging The concept of 2D FT spectroscopy had an important impact in a quite different field. In 1973 magnetic res onance methods were applied to de rive spatial images of living objects by exciting proton resonance in in tense magnetic field gradients (22). The intent was to investigate anom alies in the physiology (e.g., tumors) by obtaining maps of proton density within the body. Later, these ideas were expanded to cover angiography, organs in motion such as the beating heart, tissue differentiation through
relaxation effects, and in vivo spec troscopy. The operative word for this t e c h n i q u e is n o n i n v a s i v e , a n a p proach that differs from the classical reliance on the scalpel or irradiation of the body with X - r a y s . Even for magnetic resonance imaging, the t e r m noninvasive is actually a eu phemism because the patient is ex posed to an i n t e n s e static field, rf fields, and rapidly pulsed field gradi ents during the investigation. Usually the experiment starts w i t h selection of t h e NMR s i g n a l from a particular "slice" through the patient. The goal is to obtain a 2D m a p of proton density within t h a t slice. The observable quantity is the projection of the spin density onto the direction defined by the applied field gradient. If several m e a s u r e ments are t a k e n with the gradients applied in different directions, t h e projected densities contain enough information to reconstruct a 2D pro ton density map. This data process ing scheme, called projection-recon struction, is inspired by earlier work in X-ray tomography. Ernst showed that there is a much more efficient method (23) in which the time dependence of the NMR sig nal is followed while gradients are applied in the plane of the selected
752 A • ANALYTICAL CHEMISTRY, VOL. 65, NO. 17, SEPTEMBER 1, 1993
Two-dimensional spectroscopy be came the catalyst for a host of new NMR techniques. Previously, highresolution spectroscopists working with liquid samples had given little t h o u g h t to t h e d e s i g n of complex pulse sequences, although such se quences were widely used in solidstate NMR spectroscopy. Now "spin choreography" became an essential art. For example, polarization t r a n s fer techniques for enhancing the sen sitivity of rare spins (24) and meth ods for editing 13 C spectra according to the n u m b e r of a t t a c h e d protons (25) derive directly from ideas used in 2D heteronuclear correlation ex p e r i m e n t s . F o r c e d to l e a r n n e w tricks of spin gymnastics in 2D exper iments, the high-resolution spectroscopist began to apply them in many other situations. Scientific curiosities, such as multiple-quantum coherence, suddenly became serious business. One development that would prove i m p o r t a n t for c o n v e n t i o n a l h i g h resolution work stemmed from an idea used in heteronuclear 2D COSY. In this technique, designed to relate proton and 13 C chemical shifts, it is convenient to remove the effects of CH coupling during the evolution pe riod so t h a t CH splittings are sup pressed in t h e / ! frequency dimen s i o n . T h i s c a n be a c h i e v e d by introducing a 180° pulse at the mid point of the evolution period so that two diverging 1 3 C magnetization vec tors are brought back to a focus. At t h a t time the accepted method for broadband decoupling was noise irradiation but, in some applications at high field, difficulties were experi enced with excessive rf power depo sition in the sample. A more efficient scheme was sought. One possibility was to adapt the refocusing idea, ap p l y i n g a r a p i d s e q u e n c e of 180° pulses to the protons and confining the 18 C sampling to the focus points. U n f o r t u n a t e l y , t h e u s e of t h i s technique restricts the sampling rate too severely for practical 1 3 C spec troscopy. However, by using compos ite refocusing pulses, made up of the 90° (*), 180° (y), 90° (*) sequence, the s a m e r e f o c u s i n g effect c a n be achieved for much lower rf power (26). Then the pulses can be applied continuously, but to avoid cumula tive errors they must be made part
of a p h a s e - a l t e r n a t i n g cycle or su percycle (26). The 13 C free induction signal may be sampled at any de sired r a t e , not necessarily in syn chronism with the proton pulses. The use of this technique led to the devel opment of the widely used broadband decoupling sequences MLEV-64 and WALTZ-16 (27). Some 2D spectra can be projected in such a way as to create a new ID spectrum. One of the most useful ex a m p l e s derives from a 2D proton / - s p e c t r u m obtained by exciting a modulated spin-echo during the evo lution period (28). By using the sym m e t r y properties of the 2D m u l t i plets, high-resolution proton spectra can be recorded without any s p i n spin splittings (29). More generally, 2D experiments can often be simpli fied into a ID version by the substi tution of a selective rf pulse (30, 3D or can be extended into three (32) or four (33) frequency dimensions by tacking together two or more build ing blocks (e.g., COSY or NOESY). Revolutions are usually irrevers ible. This is certainly the case for FT spectroscopy, and the slow-passage method—once universally accepted as the norm—now seems to afford no advantage whatsoever. Multidimen sional spectroscopy is common today, but it could hardly have developed without the FT concept. We owe a g r e a t debt to those who imagined this entirely new approach to NMR.
(15) Freeman, R.; Hill, H.D.W. /. Chem. (32) Greisinger, C ; Sorensen, O. W.; Phys. 1970, 53, 4103. Ernst, R. R. /. Magn. Reson. 1989, 84, 14. (16) Kaptein, R.; Freeman, R.; Hill, (33) Kay, L. E.; Clore, G. M.; Bax, Α.; H.D.W. Chem. Phys. Lett. 1974, 26, 104. Gronenborn, A. M. Science 1990, 249, (17) Jeener, J. Presented at Ampere In 411. ternational Summer School, Basko Polje, Yugoslavia, 1971. (18) Aue, W. P.; Bartholdi, E.; Ernst, R. R. /. Chem. Phys. 1976, 64, 2229. (19) Ernst, R. R.; Bodenhausen, G.; Wokaun, A. Principles of Nuclear Magnetic Res onance in One and Two Dimensions; Oxford University Press: Oxford, 1987. (20) Wuthrich, K. NMR Spectra of Proteins and Nucleic Acids; John Wiley & Sons: New York, 1986. (21) Bax, Α.; Freeman, R.; Frenkiel, T. A. /. Am. Chem. Soc. 1981, 103, 2102. (22) Lauterbur, P. C. Nature 1973, 242, 190. (23) Kumar, Α.; Welti, D.; Ernst, R. R. Ray Freeman received his doctorate from Naturwissenschaften 1975, 62, 34. Oxford University with hL· thesis on NMR (24) Morris, G. Α.; Freeman, R. J. Am. Chem. Soc. 1979, 101, 760. spectroscopy of Co, Ga, and 77. After post (25) Doddrell, D. M.; Pegg, D. T.; Bendoctoral research in Paris, he studied dou dall, M. R. /. Magn. Reson. 1982, 48, 323. ble-resonance techniques at the National (26) Levitt, M. H.; Freeman, R. /. Magn. Physical Laboratory in Teddington, En Reson. 1979, 33, 473; 1981, 43, 502. gland. In 1961 he joined Varian Associ (27) Shaka, A. J.; Keeler, J.; Freeman, R. /. Magn. Reson. 1983, 53, 313. ates, where he developed new physical (28) Aue, W. P.; Karhan, J.; Ernst, R. R. techniques in high-resolution NMR spec / Chem. Phys. 1976, 64, 4226. troscopy and constructed the first commer (29) Xu, P.; Wu, X. L.; Freeman, R. /. cial FT spectrometer. He lectured in phys Am. Chem. Soc. 1991, 113, 3596. (30) Kessler, H.; Oschkinat, H.; Griesical chemistry at Oxford University for 14 inger, C.; Bermel, W. /. Magn. Reson. years and has held the John Humphrey 1986, 70, 106. Plummer chair of magnetic resonance at (31) Kessler, H.; Mronga, S.; Gemmecker, Cambridge University since 1987. G. Magn. Reson. Chem. 1991, 29, 527.
...short of it.
The author is indebted to Weston Anderson, Richard Ernst, and Howard Hill for information about early FT experiments and for carefully reading the manuscript. Figures 3 and 4 are re produced with the kind permission of Weston Anderson.
References (1) Arnold, J. T.; Dharmatti, S. S.; Pack ard, M. E.J. Chem. Phys. 1961, 19, 507. (2) Arnold, J. T. Phys. Rev. 1956,102, 136. (3) Anderson, W. A. Phys. Rev. 1966, 102, 151. (4) Bloch, F. Phys. Rev. 1956, 102, 104. (5) Hahn, E. L. Phys. Rev. 1950, 80, 580. (6) Varian, R. H. U.S. Patent 3 287 629, 1966. (7) Lowe, I. J.; Norberg, R, E. Phys. Rev. 1957,107,46. (8) Fellgett, P., Ph.D. Thesis, Cambridge University, 1951. (9) Ernst, R. R.; Anderson, W. A. Rev. Sci. Instrum. 1966, 37, 93. (10) Anderson, W. Α.; Ernst, R. U.S. Patent 3 475 680, 1969. (11) Anderson, W. A. Radiology Today 1992 9 1 (12) Coo'ley- J· W.; Tukey, J. W. Mathe matical Computation 1965, 19, 297. (13) Freeman, R.; Wittekoek, S. /. Magn. Reson. 1969, 1, 238. (14) Void, R. L.; Waugh, J. S.; Klein, M. P.; Phelps, D. E. / Chem. Phys. 1968, 43, 3831.
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