Line narrowing induced by selective population transfer

JOURNAL

OF MAGNETIC

RESONANCE

(1979)

36,443-446

Line Narrowing Induced by Selective Popmkttion Tramtier The enhancement of the sensitivity of low gyromagnetic ratio nuclei in solution has been accomplished by several different methods (l-10). One of the methods which has shown significant promise is selective population transfer (SPT) which entails the selective irradiation of a single proton transition to effect an inversion in the populations of the energy levels connected by the irradiated transition. This inversion results in an increase in the signal of low gyromagnetic ratio nuclei scalar-coupled to the protons (5-9). The SPT method has recently been extended to induce population transfer without selective irradiation of a proton resonance (10). The details of the SPT experiment have been presented elsewhere (1,2, 5-9). During the course of a preliminary investigation of the use of SPT to enhance the signal of low gyromagnetic ratio nuclei, we observed that SPT also induces a significant line narrowing of the observed signal. Some typical experimental results are shown in Fig. 1. The spectrum in Fig. 1A is the normal “P Fourier transform spectrum of trimethyl phosphate obtained in an inhomogeneous magnetic field. The spectrum in Fig. 1B is of the same sample, but the phosphorus observation pulse was preceded by a selective 180” pulse applied exactly at the frequency of the high-field proton transition. The SPT spectrum (Fig. 1B) appears to be composed of a broad component with many sharp features. The SPT difference spectrum, of the SPT (Fig. 1B) minus the normal spectrum (Fig. lA), is shown in Fig. 1C. For comparison, the normal proton-decoupled spectrum is shown in Fig. 1D. It is seen that the narrow components of the SPT difference spectrum, linewidths of about 2 Hz, are much narrower than the proton-decoupled linewidth, about 20 Hz. A typical SPT difference spectrum is shown in Fig. 2 in an expanded form to illustrate several features. The SPT difference spectrum does not look like the usual symmetric difference spectrum obtained when the sample is in a very homogeneous field. Examination of the SPT difference spectrum reveals that it can be decomposed into two components, one of which is the SPT spectrum expected from irradiation of the high-field proton transition under conditions of good homogeneity and the other to the SPT spectrum expected from irradiation of the low-field proton transition in a homogeneous field. It is useful to note that the two components are shifted relative to one another by about 4 Hz, which is 0.1 ppm in this experiment. This shift between the two components is instructive in pointing out the origin of the line narrowing. The line narrowing can be explained by considering the effect of irradiation on the nuclear populations of an inhomogeneous sample. The centers of the two proton transitions are separated by about 10 Hz (0.1 ppm), which is the proton-phosphorus coupling constant JPH. With the inhomogeneous fields used to observe line narrowing in these experiments, there is considerable overlap of the proton transitions as the proton linewidths are about 80 Hz. The selective pulse acts on the two proton transitions to invert the population levels of those molecules which are in a region of 443

0022-2364/79/120443-04SO2.OWI Copyright @ 1979 by Academtc Press, Inc. All rights of reproduction in any form reserved Printed in Great Britain

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FIG. 1. (A) Normal “P Fourier transform spectrum of trimethyl phosphate obtained in an inhomogeneous magnetic field. The 40.5-MHz 31P spectrum, sweepwidth of 200 Hz, was obtained using an XL-100-15 spectrometer with Nicolet Fourier transform accessories. The spectrum was the sum of four accumulations after the sample had been equilibrated with eight pulses. A l-Hz exponential line broadening was applied to the spectrum. The sample was at about 20” in a 12-mm tube, and the concentration was about 5% trimethyl phosphate in *HaO. (B) 31P SPT spectrum of trimethyl phosphate. Conditions are as in Fig. 1A, except that a pulse of 0.5 set was applied to the center of the high-field proton transition before the phosphorus observation pulse. The strength of the proton pulse had been calibrated to 180”. (C) ‘iP SPT difference spectrum which is the spectrum in Fig. 1B minus the spectrum of Fig. 1A. (D) Proton-decoupled 3’P spectrum of trimethyl phosphate. Conditions are as in Fig. 1A.

the inhomogeneous field satisfying rHHo = 2nYirr. Since both transitions have significant absorption at the irradiation frequency Virr there will be two 31P components of the SPT difference spectrum; irradiation of one of the transitions has an effect equal and opposite to that of irradiation of the other. The two components do not cancel out since (a) their intensities will generally not be equal, and (b) they are H

FIG.

IOHt

2. A typical 31P SPT difference spectrum of trimethyl phosphate.

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shifted relative to one another by about 4 Hz (0.1 ppm). The shift between the two 31Pcomponents upon irradiation at proton frequency Yirrarises from the fact that the 31P nuclei influenced by inversion of the upfield proton transition will resonate upfield Jr~(yr/~n) from the 31P nuclei influenced by inversion of the downfield proton transition. The difference in intensities of the two 31P components is due to the different nuclear populations present at the two magnetic field strengths. Thus, line-narrowing results from magnetization transfer occurring only in small regions of the magnetic field and the two components of the SPT difference spectrum arise from the difference in local magnetic field experienced by the two proton transitions. It is interesting to note that the linewidths in the SPT difference method are not controlled by magnetic field homogeneity, but rather are governed by the length of the selective proton r pulse. The hypothesis for the line-narrowing mechanism was tested by observing the SPT difference spectrum for a variety of different proton irradiation frequencies. The postulated mechanism predicts that a lOO-Hz shift of the proton irradiation frequency would shift the SPT difference spectrum by about 40 Hz as nuclei from a different region of the magnetic field are being monitored. The experimental results in Fig. 3 confirm this prediction of the mechanism. Thus, the SPT procedure provides a convenient monitor of the distribution of molecules in an inhomogeneous sample by using the proton frequency to selectively enhance the 31P signal from a selected smell region of the sample. The SPT-induced line narrowing may find some utility in examining the coupling constants of nuclei in an inhomogeneous sample whose natural linewidths are narrow ‘H

0 Hz

100 FIG. 3. 31P SPT difference spectra of trimethyl phosphate obtained for different proton irrctdintian frequencies. The frequencies given are upfield from the high-field proton transition. Other conditions are as in Fig. 1A.

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but are not directly observable. A possible application is the observation of the coupling constants of cellular phosphates in vivo. The method might also find some use in zeugmatography. Instead of applying a single proton frequency, as was done in the experiments discussed here, a number of frequencies could be irradiated simultaneously to induce population transfer. This procedure would allow the distribution of the molecules of interest in a magnetic field gradient to be mapped out in a single experiment. Line narrowing is observed for the 13C of dioxane (AX2 spin system) and methyl iodide (AX3 spin system), as well as the 31P of trimethyl phosphate (AX9 spin system). A type of line narrowing similar to that of the SPT experiment has been observed in heteronuclear two-dimensional NMR (Bodenhausen and Bolton, unpublished) and may have the same origin. Use of the SPT method to enhance sensitivity of low gyromagnetic ratio nuclei which are scalar-coupled to protons is well known though little exploited. Since the method can also be used to obtain narrow linewidths from inhomogeneous samples, the method may become more popular especially with decoupling to obtain protonenhanced, proton-decoupled spectra. As a cautionary note, the line-narrowing effect must be taken into consideration even in relatively homogeneous fields when comparing the signal intensity obtained by the SPT method with that of other procedures. ACKNOWLEDGMENTS Financial support was provided by Grant GM 25018 from the National Institutes of Health and by Grant RR 00892 from the Division of Research Resources, National Institutes of Health, for maintenance of the UCSF Magnetic Resonance Laboratory. T.L.J. is the recipient of a Research Career Development Award (AM 00291) from the National Institutes of Health.

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H.J.JACOBSENANDH.BILDSBE, J. Magn.Reson.26,183(1977). S.SBRENSEN,R.S.HANSEN,AND H.J.JACOBSEN, J. Magn.Reson. 14,243 (1974). A. A.CHALMERS,K.G.R.PACHLER,AND P.L.WESSELS, Org. Mag.Reson.6,443 C.L.MAGNE,D.W.ALDERMAN,ANDD.M.GRANT, J. Chem.Phys.63,2514(1975). T.BUNDGAARD AND H.J.JAKOBSEN, J. Magn.Reson. 18,209(1975). 10. G. A.MORRISANDR.FREEMAN, J. Am.Chem.Soc. 101,760 (1979).

(1974).

PHILIP H. BOLTON THOMAS L. JAMES

Department of Pharmaceutical Chemistry School of Pharmacy University of California San Francisco, California 94143 Received July 19, 1979