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Improvements in Radiation-Damping Control in High-Resolution NMR
JOURNAL OF MAGNETIC RESONANCE,
Series B 109, 218–222 (1995)
COMMUNICATIONS Improvements in Radiation-Damping Control in High-Resolution NMR DANIEL ABERGEL, CLAIRE CARLOTTI, ALAIN LOUIS-JOSEPH,
AND
JEAN-YVES LALLEMAND
Groupe de RMN, Laboratoire DCSO, Ecole Polytechnique, 91128 Palaiseau, France Received May 1, 1995; revised July 25, 1995
Recently, several authors have investigated various approaches in order to suppress the effects of radiation damping in high-field spectrometers (1, 2). In our laboratory, we have implemented a method for which the principle is based on the canceling of the radiation-damping field by introduction of the appropriate correcting signal through the decoupling network of the spectrometer. This radiation-damping field is known to be proportional to the magnetization of the abundant spins of the solvent, and phase shifted by 907 with respect to the solvent magnetization. Our goal was thus to create a magnetization-dependent field in the sample, which exactly compensates the existing radiation-damping field. The method consists in the following operations. First, the total signal induced in the probehead coil is detected. It is then transposed to low frequency by demodulation with the carrier frequency of the spectrometer, and subsequently filtered so as to retain only a small bandwidth about the solvent frequency. Finally, the output signal is fed back into the probe via the decoupling channel of the spectrometer after transposition back to the correct frequency. This has been proven to be an efficient scheme for radiation-damping suppression. A first description of the method has been presented elsewhere (3). However, some improvements are still necessary in order to achieve the important goal of using the above method when recording spectra of biomolecules in water. First, a crucial point for practical use is the description of criteria that should be used to make sure that the proper correction is reached. Accurate phase and gain adjustment are indeed necessary in order to effectively cancel the radiation-damping field in the sample. Second, the approach developed is also capable of successfully generating an average compensating field when used in a time-shared mode during acquisition. This could be helpful in some cases in order to minimize both transmitter–receiver artifacts and the possibility of spontaneous oscillation of the system. New applications of the feedback system will also be introduced, which allow appreciable improvement in the dynamics of NMR spectra. A reverse use of our strategy can be conveniently made in order to amplify radiation damping. 1064-1866/95 $12.00 Copyright q 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.
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This will be shown to improve water suppression and consequently to provide better signal dynamics in certain pulse sequences of biomolecular NMR. These different points will be investigated in this Communication. As mentioned above, when a proper correction to the radiation-damping field is wanted, one must make sure that the compensating field is equal and opposite to the former. Indeed, the dramatic improvement of the FID does not necessarily ensure an exact correction, since a phase error, for instance, could be compensated by a larger gain and lead to an apparent correction of radiation damping. Although this cannot be simply investigated in a direct manner, it is nonetheless possible to use indirect experimental but easily accessible criteria to calibrate the radiation-damping control system. We will describe the principles of the method in a simple way and leave out straightforward but cumbersome calculations, which are of little interest for a good understanding of the problem. The method of the determination of the proper phase and gain corrections to be applied simply relies on measuring the linewidth of the solvent peak. For small flip angles, the linewidth T of the water peak, corresponding to an apparent relaxation time, is related to the inhomogeneous linewidth T* 2 and to the characteristic time for radiation damping Trd by the simple relation (4, 5) 1 T
1 1 / , * T2 Trd
so when the proper compensating field is generated in the sample, the linewidth should be the same as the inhomogeneous linewidth. Consider first the case of the phase of the compensating field. Obviously, when it is parallel to the radiation-damping field ( f Å p /2 is the angle between the transverse magnetization and the radiation-damping field), the action will be that of amplifying the radiation-damping field action, that is, forcing the magnetization back to its equilibrium position. On the contrary, when the compensating field points in the opposite direction ( f Å 0 p /2), the radiation-damping ef-
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FIG. 1. The orientations of the various vectors with respect to the water transverse magnetization are illustrated in this figure. The radiation-damping field HRD is phase shifted by f Å p /2. The compensating field Hcomp is antiparallel to HRD ( f Å 0 p /2). See text for details.
fect on the magnetization will be lowest. Finally, when the compensating field is parallel or anti-parallel to the magnetization ( f Å 0, p ), the torque it exerts on the water magneti-
219
zation will be zero, and the effect of the radiation-damping field remains unchanged (Fig. 1). The linewidth profile obtained by numerical simulations shows that, as expected, the linewidth is maximum for the proper phase correction, whereas it is minimum for an opposite phase correction (Fig. 2). Consequently, an experimental determination of the proper compensating field proceeds as follows. The radiation-damping correcting field intensity is set to a fixed starting value. The linewidth of the solvent (the water, as is usually true in biomolecular NMR) is then measured for phases varying from 07 up to 3607. The desired phase correction is obtained for the minimum of the solvent linewidth. It is then quite easy to determine the appropriate correcting field intensity by gain adjustment of the amplifiers once the phase of the compensating system has been adjusted. Linewidth measurements at the adequate phase correction are to be made for different gain values. The correction for which the measured linewidth is the same as the inhomogeneous linewidth is assumed to be the correct one. The natural linewidth can be measured following the procedure of Gue´ron and Leroy ( 4 ) , or simply by setting the probe grossly out of correct tuning. Experimental results are shown in Fig. 3. An illustrative and simple application of the radiationdamping controlling system can be demonstrated on a simple jump-and-return (JR) sequence (6). This selective composite pulse sequence is known to provide good water suppres-
FIG. 2. Simulations showing the solvent linewidth dependence as a function of the phase of the applied compensating field. The results correspond to an FID obtained after a small flip angle. Each curve corresponds to a fixed amplitude value of the compensating field. Note that the minimum of the linewidth corresponds for all curves to a correcting field pointing opposite to the radiation-damping field (radiation-damping attenuation), whereas the maximum of the linewidth corresponds to the correcting field parallel to the radiation-damping field (radiation-damping amplification). The compensating field has no effect on the linewidth when it is parallel or antiparallel to the magnetization. The horizontal line corresponds to the natural linewidth T *2 (see text). The linewidths are given in arbitrary units.
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FIG. 3. Experimental results corresponding to the simulations of Fig. 2. The curves are presented for intensities of the correction field in ascending order from (a) to (c), respectively. Note the variations of the extrema, and the fixed point corresponding to a p phase of the field. The absence of the symmetry in the curves is due to the nonlinear voltage dependence of the phase in the electronic phase shifter.
sion. The first 907 pulse flips all magnetization into the xy plane. During the JR delay, the spin magnetizations dephase according to their chemical shift. The second 907 JR pulse rotates back all magnetization into the zy plane just before acquisition. Note that the water magnetization is flipped back to the z axis and, consequently, is not detected during acquisition. Also, there is no radiation damping in this case. However, the efficiency of the pulse is conditioned by the fact that the water magnetization has completely returned to its equilibrium direction. Obviously, any transverse component of the water magnetization at the beginning of the acquisition period will strongly alter the pulse efficiency. Moreover, this
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sequence provides bad water suppression in situations where the water magnetization points toward the 0z direction at the beginning of the detection period. In this case, the water magnetization indeed generates a characteristic ‘‘maser pulse’’ at the end of the sequence, that is, during the acquisition period. This results in a severe degradation of the spectrum. With the use of a system which neutralizes radiation damping during the whole experiment, it is possible to use a JR read pulse in any case, for instance, after a 1807 hard preparation pulse. This leads to an ‘‘upside down’’ JR. This experiment performed on a 3 mM sample of BPTI (10% D2O–907% H2O) shows that efficient water suppression is indeed achieved when radiation damping is canceled during acquisition. On the contrary, in the reference case (radiation damping present), the return of the water magnetization to the /z axis during acquisition strongly alters the detection of the signal of the protein. In the latter case, the receiver gain was four times lower than in the former. The results are reported in Fig. 4. Another interesting point was to investigate a way, similar to homodecoupling, in which the correcting field would be applied in a time-sharing mode. Indeed, instead of generating a continuous correction which induces the radiation-damping compensating field in the sample, it is possible to use a timeshared correction. This method makes use of most of the standard decoupling channel circuit. However, the compensating field is generated only during the intervals of time in which the receiver has been gated off. This amounts to creating an average radiation-damping correcting field, the timesharing rate being much higher than both the radiation-damping characteristic time and the precession frequency of the solvent magnetization in the rotating frame. However, the homodecoupling scheme described here implies that the electronic system should be able to work in a pulsed way. Figure 5 shows the feedback corrections of the FIDs obtained on a water sample by this method. As mentioned above, we will now emphasize new possible uses of a radiation-damping control system. It has been shown that the effect of the system on radiation damping is a function of both the intensity and the phase of the compensating field. When it is set parallel to the radiation-damping field, an amplification of the phenomenon is obtained. It thus provokes a faster return of the solvent magnetization to its equilibrium direction, resulting in an apparent T * 1 relaxation of the water resonance. This effect can be taken advantage of in various contexts. Let us consider, for instance, the action of radiation damping in a NOESY-JR experiment, as a function of the mixing time tm (see Ref. (7) for a recent study of the question). When tm (Fig. 6) is long enough for the water magnetization to have returned to its equilibrium direction, the water signal will be efficiently suppressed by the JR read pulse. On the other hand, for shorter mixing times, the state of the magnetization will vary: (i) from one transient to the next, according to the phase program, and (ii) from one t1 increment to the next. This causes a sometimes
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FIG. 4. The ‘‘upside-down’’ jump–return sequence consists of a preliminary inversion hard pulse, followed by a regular JR sequence. (a) Spectrum of BPTI 3 mM with radiation-damping field present. The water magnetization returns to its equilibrium direction during acquisition, which prevents recording of a spectrum with acceptable dynamics. (b) Radiation damping is suppressed during acquisition; the water magnetization remains on the z axis, and the jump–return pulse provides efficient water suppression. In (b), the receiver gain was four times larger than that in (a). Acquisition time (186 ms) and noise level are the same in both spectra.
important loss of efficiency in the JR read pulse, and, consequently, it results in a degradation of the dynamics of the spectrum.
FIG. 5. Illustration of a time-shared mode correction of the radiationdamping field during acquisition. FIDs with radiation damping obtained after 907 and 1807 excitation pulses are shown in (a, c), respectively. (b, d) FIDs recorded after 907 and 1807 excitation, respectively, with radiation damping canceled. Note that the FIDs obtained are similar to those in Ref. (3).
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As a solution to (i), it is possible to force the return of the magnetization to /z by using the radiation damping control system. In these circumstances, the water magnetization is oriented along /z at the beginning of the read pulse of each transient, which always allows good water suppression. This is illustrated in Fig. 6. The spectra corresponding to the first increment of a regular NOESY-JR and to the first increment of a NOESY-JR in which radiation damping has been amplified are shown, respectively. In the reference NOESY-JR experiment (Figs. 6a, 6b), there is a persistent transverse component of the water magnetization at the beginning of acquisition, due to a relatively short mixing time. Consequently, the receiver gain was kept to a low value. The same experiment was performed with the correction field inverted and amplified with respect to the radiationdamping field (Figs. 6c, 6d). In this case, it is possible to bring the water magnetization back to the /z direction at the end of the mixing time, and, consequently, the JR pulse sequence is more efficient. In this second experiment, a
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FIG. 6. Effect of radiation damping in a NOESY pulse sequence (e) when the mixing time tm is short compared with the delay needed for a complete return of the water magnetization to equilibrium. In this case, poor water suppression is obtained (see a, b). This can be efficiently circumvented by proper amplification of the radiation-damping field, making the water magnetization return to the equilibrium direction at the end of the 100 ms mixing time, before acquisition starts (see c, d). In this case, the receiver gain was four times larger than it was in the reference experiment.
higher receiver gain was used, resulting in a nice improvement of the spectrum in terms of water suppression and of signal dynamics. In conclusion, we point out that the various improvements detailed in this paper open the way to a better practical use of radiation-damping control in the context of highresolution NMR in liquids. Note also that this radiationdamping control system is likely to reduce the water linewidth in nonphysical dimensions of multidimensional NMR experiments, and to provide better spectra near the water region (1). Besides, the elimination of radiation damping is also likely to improve the water trace in the f1 dimension of 2D experiments, which we believe is probably altered by nonlinearities introduced by the radiation damping. To some extent, this effect could be responsible for ‘‘T 1 noise’’ at the water frequency. Finally, let us mention that the elimination of radiation damping in NMR experiments with proteins opens new original ways to manipulate the water magnetiza-
m4235$7288
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tion. This allows one to control the apparent water relaxation time and very conveniently gives access to measurements on the behavior of the exchangeable protons of biomolecules with water (8). New results in this field will be presented later. REFERENCES 1. C. Anklin, M. Rindlisbacher, G. Otting, and F. H. Laukien, J. Magn. Reson. B 106, 199 (1995). 2. P. Broekaert and J. Jeener, J. Magn. Reson. A 113, 60 (1995). 3. A. Louis-Joseph, D. Abergel, and J.-Y. Lallemand, J. Biol. NMR 5, 212 (1995). 4. M. Gue´ron and J.-L. Leroy, J. Magn. Reson. 85, 209 (1989). 5. S. Bloom, J. Appl. Phys. 28, 800 (1957). 6. P. Plateau and M. Gue´ron, J. Am. Chem. Soc. 104, 7310 (1982). 7. J. Stonehouse, G. L. Shaw, and J. Keeler, J. Biol. NMR 4, 799 (1994). 8. G. Otting, oral communication at the EENC, Oulu, 1994.
AP: Mag Res
Series B 109, 218–222 (1995)
COMMUNICATIONS Improvements in Radiation-Damping Control in High-Resolution NMR DANIEL ABERGEL, CLAIRE CARLOTTI, ALAIN LOUIS-JOSEPH,
AND
JEAN-YVES LALLEMAND
Groupe de RMN, Laboratoire DCSO, Ecole Polytechnique, 91128 Palaiseau, France Received May 1, 1995; revised July 25, 1995
Recently, several authors have investigated various approaches in order to suppress the effects of radiation damping in high-field spectrometers (1, 2). In our laboratory, we have implemented a method for which the principle is based on the canceling of the radiation-damping field by introduction of the appropriate correcting signal through the decoupling network of the spectrometer. This radiation-damping field is known to be proportional to the magnetization of the abundant spins of the solvent, and phase shifted by 907 with respect to the solvent magnetization. Our goal was thus to create a magnetization-dependent field in the sample, which exactly compensates the existing radiation-damping field. The method consists in the following operations. First, the total signal induced in the probehead coil is detected. It is then transposed to low frequency by demodulation with the carrier frequency of the spectrometer, and subsequently filtered so as to retain only a small bandwidth about the solvent frequency. Finally, the output signal is fed back into the probe via the decoupling channel of the spectrometer after transposition back to the correct frequency. This has been proven to be an efficient scheme for radiation-damping suppression. A first description of the method has been presented elsewhere (3). However, some improvements are still necessary in order to achieve the important goal of using the above method when recording spectra of biomolecules in water. First, a crucial point for practical use is the description of criteria that should be used to make sure that the proper correction is reached. Accurate phase and gain adjustment are indeed necessary in order to effectively cancel the radiation-damping field in the sample. Second, the approach developed is also capable of successfully generating an average compensating field when used in a time-shared mode during acquisition. This could be helpful in some cases in order to minimize both transmitter–receiver artifacts and the possibility of spontaneous oscillation of the system. New applications of the feedback system will also be introduced, which allow appreciable improvement in the dynamics of NMR spectra. A reverse use of our strategy can be conveniently made in order to amplify radiation damping. 1064-1866/95 $12.00 Copyright q 1995 by Academic Press, Inc. All rights of reproduction in any form reserved.
m4235$7288
10-10-95 11:11:22
This will be shown to improve water suppression and consequently to provide better signal dynamics in certain pulse sequences of biomolecular NMR. These different points will be investigated in this Communication. As mentioned above, when a proper correction to the radiation-damping field is wanted, one must make sure that the compensating field is equal and opposite to the former. Indeed, the dramatic improvement of the FID does not necessarily ensure an exact correction, since a phase error, for instance, could be compensated by a larger gain and lead to an apparent correction of radiation damping. Although this cannot be simply investigated in a direct manner, it is nonetheless possible to use indirect experimental but easily accessible criteria to calibrate the radiation-damping control system. We will describe the principles of the method in a simple way and leave out straightforward but cumbersome calculations, which are of little interest for a good understanding of the problem. The method of the determination of the proper phase and gain corrections to be applied simply relies on measuring the linewidth of the solvent peak. For small flip angles, the linewidth T of the water peak, corresponding to an apparent relaxation time, is related to the inhomogeneous linewidth T* 2 and to the characteristic time for radiation damping Trd by the simple relation (4, 5) 1 T
1 1 / , * T2 Trd
so when the proper compensating field is generated in the sample, the linewidth should be the same as the inhomogeneous linewidth. Consider first the case of the phase of the compensating field. Obviously, when it is parallel to the radiation-damping field ( f Å p /2 is the angle between the transverse magnetization and the radiation-damping field), the action will be that of amplifying the radiation-damping field action, that is, forcing the magnetization back to its equilibrium position. On the contrary, when the compensating field points in the opposite direction ( f Å 0 p /2), the radiation-damping ef-
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FIG. 1. The orientations of the various vectors with respect to the water transverse magnetization are illustrated in this figure. The radiation-damping field HRD is phase shifted by f Å p /2. The compensating field Hcomp is antiparallel to HRD ( f Å 0 p /2). See text for details.
fect on the magnetization will be lowest. Finally, when the compensating field is parallel or anti-parallel to the magnetization ( f Å 0, p ), the torque it exerts on the water magneti-
219
zation will be zero, and the effect of the radiation-damping field remains unchanged (Fig. 1). The linewidth profile obtained by numerical simulations shows that, as expected, the linewidth is maximum for the proper phase correction, whereas it is minimum for an opposite phase correction (Fig. 2). Consequently, an experimental determination of the proper compensating field proceeds as follows. The radiation-damping correcting field intensity is set to a fixed starting value. The linewidth of the solvent (the water, as is usually true in biomolecular NMR) is then measured for phases varying from 07 up to 3607. The desired phase correction is obtained for the minimum of the solvent linewidth. It is then quite easy to determine the appropriate correcting field intensity by gain adjustment of the amplifiers once the phase of the compensating system has been adjusted. Linewidth measurements at the adequate phase correction are to be made for different gain values. The correction for which the measured linewidth is the same as the inhomogeneous linewidth is assumed to be the correct one. The natural linewidth can be measured following the procedure of Gue´ron and Leroy ( 4 ) , or simply by setting the probe grossly out of correct tuning. Experimental results are shown in Fig. 3. An illustrative and simple application of the radiationdamping controlling system can be demonstrated on a simple jump-and-return (JR) sequence (6). This selective composite pulse sequence is known to provide good water suppres-
FIG. 2. Simulations showing the solvent linewidth dependence as a function of the phase of the applied compensating field. The results correspond to an FID obtained after a small flip angle. Each curve corresponds to a fixed amplitude value of the compensating field. Note that the minimum of the linewidth corresponds for all curves to a correcting field pointing opposite to the radiation-damping field (radiation-damping attenuation), whereas the maximum of the linewidth corresponds to the correcting field parallel to the radiation-damping field (radiation-damping amplification). The compensating field has no effect on the linewidth when it is parallel or antiparallel to the magnetization. The horizontal line corresponds to the natural linewidth T *2 (see text). The linewidths are given in arbitrary units.
m4235$7288
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FIG. 3. Experimental results corresponding to the simulations of Fig. 2. The curves are presented for intensities of the correction field in ascending order from (a) to (c), respectively. Note the variations of the extrema, and the fixed point corresponding to a p phase of the field. The absence of the symmetry in the curves is due to the nonlinear voltage dependence of the phase in the electronic phase shifter.
sion. The first 907 pulse flips all magnetization into the xy plane. During the JR delay, the spin magnetizations dephase according to their chemical shift. The second 907 JR pulse rotates back all magnetization into the zy plane just before acquisition. Note that the water magnetization is flipped back to the z axis and, consequently, is not detected during acquisition. Also, there is no radiation damping in this case. However, the efficiency of the pulse is conditioned by the fact that the water magnetization has completely returned to its equilibrium direction. Obviously, any transverse component of the water magnetization at the beginning of the acquisition period will strongly alter the pulse efficiency. Moreover, this
m4235$7288
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sequence provides bad water suppression in situations where the water magnetization points toward the 0z direction at the beginning of the detection period. In this case, the water magnetization indeed generates a characteristic ‘‘maser pulse’’ at the end of the sequence, that is, during the acquisition period. This results in a severe degradation of the spectrum. With the use of a system which neutralizes radiation damping during the whole experiment, it is possible to use a JR read pulse in any case, for instance, after a 1807 hard preparation pulse. This leads to an ‘‘upside down’’ JR. This experiment performed on a 3 mM sample of BPTI (10% D2O–907% H2O) shows that efficient water suppression is indeed achieved when radiation damping is canceled during acquisition. On the contrary, in the reference case (radiation damping present), the return of the water magnetization to the /z axis during acquisition strongly alters the detection of the signal of the protein. In the latter case, the receiver gain was four times lower than in the former. The results are reported in Fig. 4. Another interesting point was to investigate a way, similar to homodecoupling, in which the correcting field would be applied in a time-sharing mode. Indeed, instead of generating a continuous correction which induces the radiation-damping compensating field in the sample, it is possible to use a timeshared correction. This method makes use of most of the standard decoupling channel circuit. However, the compensating field is generated only during the intervals of time in which the receiver has been gated off. This amounts to creating an average radiation-damping correcting field, the timesharing rate being much higher than both the radiation-damping characteristic time and the precession frequency of the solvent magnetization in the rotating frame. However, the homodecoupling scheme described here implies that the electronic system should be able to work in a pulsed way. Figure 5 shows the feedback corrections of the FIDs obtained on a water sample by this method. As mentioned above, we will now emphasize new possible uses of a radiation-damping control system. It has been shown that the effect of the system on radiation damping is a function of both the intensity and the phase of the compensating field. When it is set parallel to the radiation-damping field, an amplification of the phenomenon is obtained. It thus provokes a faster return of the solvent magnetization to its equilibrium direction, resulting in an apparent T * 1 relaxation of the water resonance. This effect can be taken advantage of in various contexts. Let us consider, for instance, the action of radiation damping in a NOESY-JR experiment, as a function of the mixing time tm (see Ref. (7) for a recent study of the question). When tm (Fig. 6) is long enough for the water magnetization to have returned to its equilibrium direction, the water signal will be efficiently suppressed by the JR read pulse. On the other hand, for shorter mixing times, the state of the magnetization will vary: (i) from one transient to the next, according to the phase program, and (ii) from one t1 increment to the next. This causes a sometimes
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221
FIG. 4. The ‘‘upside-down’’ jump–return sequence consists of a preliminary inversion hard pulse, followed by a regular JR sequence. (a) Spectrum of BPTI 3 mM with radiation-damping field present. The water magnetization returns to its equilibrium direction during acquisition, which prevents recording of a spectrum with acceptable dynamics. (b) Radiation damping is suppressed during acquisition; the water magnetization remains on the z axis, and the jump–return pulse provides efficient water suppression. In (b), the receiver gain was four times larger than that in (a). Acquisition time (186 ms) and noise level are the same in both spectra.
important loss of efficiency in the JR read pulse, and, consequently, it results in a degradation of the dynamics of the spectrum.
FIG. 5. Illustration of a time-shared mode correction of the radiationdamping field during acquisition. FIDs with radiation damping obtained after 907 and 1807 excitation pulses are shown in (a, c), respectively. (b, d) FIDs recorded after 907 and 1807 excitation, respectively, with radiation damping canceled. Note that the FIDs obtained are similar to those in Ref. (3).
m4235$7288
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magal
As a solution to (i), it is possible to force the return of the magnetization to /z by using the radiation damping control system. In these circumstances, the water magnetization is oriented along /z at the beginning of the read pulse of each transient, which always allows good water suppression. This is illustrated in Fig. 6. The spectra corresponding to the first increment of a regular NOESY-JR and to the first increment of a NOESY-JR in which radiation damping has been amplified are shown, respectively. In the reference NOESY-JR experiment (Figs. 6a, 6b), there is a persistent transverse component of the water magnetization at the beginning of acquisition, due to a relatively short mixing time. Consequently, the receiver gain was kept to a low value. The same experiment was performed with the correction field inverted and amplified with respect to the radiationdamping field (Figs. 6c, 6d). In this case, it is possible to bring the water magnetization back to the /z direction at the end of the mixing time, and, consequently, the JR pulse sequence is more efficient. In this second experiment, a
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COMMUNICATIONS
FIG. 6. Effect of radiation damping in a NOESY pulse sequence (e) when the mixing time tm is short compared with the delay needed for a complete return of the water magnetization to equilibrium. In this case, poor water suppression is obtained (see a, b). This can be efficiently circumvented by proper amplification of the radiation-damping field, making the water magnetization return to the equilibrium direction at the end of the 100 ms mixing time, before acquisition starts (see c, d). In this case, the receiver gain was four times larger than it was in the reference experiment.
higher receiver gain was used, resulting in a nice improvement of the spectrum in terms of water suppression and of signal dynamics. In conclusion, we point out that the various improvements detailed in this paper open the way to a better practical use of radiation-damping control in the context of highresolution NMR in liquids. Note also that this radiationdamping control system is likely to reduce the water linewidth in nonphysical dimensions of multidimensional NMR experiments, and to provide better spectra near the water region (1). Besides, the elimination of radiation damping is also likely to improve the water trace in the f1 dimension of 2D experiments, which we believe is probably altered by nonlinearities introduced by the radiation damping. To some extent, this effect could be responsible for ‘‘T 1 noise’’ at the water frequency. Finally, let us mention that the elimination of radiation damping in NMR experiments with proteins opens new original ways to manipulate the water magnetiza-
m4235$7288
10-10-95 11:11:22
magal
tion. This allows one to control the apparent water relaxation time and very conveniently gives access to measurements on the behavior of the exchangeable protons of biomolecules with water (8). New results in this field will be presented later. REFERENCES 1. C. Anklin, M. Rindlisbacher, G. Otting, and F. H. Laukien, J. Magn. Reson. B 106, 199 (1995). 2. P. Broekaert and J. Jeener, J. Magn. Reson. A 113, 60 (1995). 3. A. Louis-Joseph, D. Abergel, and J.-Y. Lallemand, J. Biol. NMR 5, 212 (1995). 4. M. Gue´ron and J.-L. Leroy, J. Magn. Reson. 85, 209 (1989). 5. S. Bloom, J. Appl. Phys. 28, 800 (1957). 6. P. Plateau and M. Gue´ron, J. Am. Chem. Soc. 104, 7310 (1982). 7. J. Stonehouse, G. L. Shaw, and J. Keeler, J. Biol. NMR 4, 799 (1994). 8. G. Otting, oral communication at the EENC, Oulu, 1994.
AP: Mag Res