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A simple method for assigning long-range carbon-proton couplings through selective proton decoupling
JOURNAL
OF MAGNETIC
RESONANCE
S&496-499
(1984)
A Simple Method for Assigning Long-RangeCarbon-Proton Couplings through SelectiveProton Decoupling G.COMMENGES Laboratoire
de Chimie
de Coordination,
CNRS,
Toulouse,
France
AND
R. C. RAO*? Sanoji-Recherche,
Toulouse,
France
Received December 30, 1983
The study of long-range 13C-‘H couplings is important for the unambiguous analysis of 13C NMR spectra (1, 2). The assignment of these couplings is usually performed using selective proton decoupling. It has been shown recently (3-5) that heteronuclear J-resolved 2-D spectroscopy may offer many advantages in this area; however, the long accumulation time needed to get the few hundred data sets required by a single experiment inhibits a more widespread use of this technique, especially when only small amounts of compounds are available or with less soluble substances. Therefore one must turn toward more classical methods to get the desired information. Selective proton decoupling is generally preferred to arduous and expensive ’ 3C or *H enrichments. It requires a weak irradiation field and this can induce selective ‘H population transfer, resulting in substantial intensity changes of the 13C NMR signals. Although these intensity changes can be useful for the determination of the relative signs of the coupling constants in the case of comparatively simple molecules, they may, however, render difficult the interpretation of spectra of more complex molecules. One way of overcoming this problem (6) makes use of a second decoupling frequency source connected to the input of the proton decoupler amplifier and combines selective proton decoupling with noise decoupling. Under these conditions one obtains selectively decoupled i3C spectra without intensity changes and with the advantage of retaining the NOE. * To whom correspondence should be addressed. t Present address: Laboratoire de Pharmacologic et de Toxicologic Fondamentales, CNRS, 205. route de Narbonne, Toulouse, France.
0022-2364184 $3.00 Copy+& 0 1984 by Academic Press. Inc. All rights of reproduction in any form reserved.
496
NOTES
497
‘II DECOUPLER
‘%
TRANSMITTER
RECEIVER
FIG. I. Sequence used for selective proton decoupling. BB = broadband decoupling; CW = continuouswave decoupling; AT = acquisition time.
We show here that very similar results can be obtained when using the pulse programmer possibilities offered by most of the modem NMR spectrometers, hence dispensing with the need of an expensive second synthesizer. The decoupler mode is first set to proton noise decoupling (high-power, frequency at the center of the proton region) for a time r1 to allow for the build up of the NOE; it is then set to low-power continuous-wave decoupling with the frequency at the particular proton to be irradiated. The FlD is acquired after a delay TV, depending on the switching characteristics of the offset generator (Fig. 1). The delay is typically a few tens of milliseconds and the power identical to that used in the corresponding classical selective irradiation experiment. At the end of the acquisition period the decoupler mode is set back to proton noise decoupling and the whole process is then repeated the number of times necessary to get a reasonable signal-to-noise ratio. Figure 2A shows the proton-coupled 13C NMR spectrum of a mixture of LX-Dglucopyranose (I) and P-D-glucopyranose (II) in solution in D20, obtained under gated decoupling conditions. The interpulse delay was 2.046 s.
498
FIG.
NOTES
2.62.9 MHz proton-coupled 13CNMR spectra of a mixture of U-D and -A &ty©ranose
in 40.
The signal of C-lp appears as a quartet at 98.4 ppm and exhibits one large ‘JCH coupling equal to 160.9 Hz and one small *JCH coupling to H-2, equal to 5.5 Hz; C-la, gives a doublet centered at 94.6 ppm showing only one large ‘JCH coupling equal to 168.3 Hz (7). Figure 2B shows the 13C spectrum obtained under continuous selective irradiation of H-28. The inter-pulse delay was 2.046 s. C-28 now appears as a singlet and C-16 as a doublet. Figure 2C shows the 13C spectrum obtained using our microprogram to selectively irradiate H-28. The interpulse delay was 2.091 s. Selective decoupling is thus achieved with enhanced signal intensities, though full NOE is clearly not achieved, intensity changes are much reduced whereas the time needed to record the spectrum has been only slightly increased. The spectra were obtained at 62.9 MHz on a Bruker WM250 NMR spectrometer at normal probe temperature (25°C) using a 10 mm o.d. sample tube. Spectral widths were 15,000 Hz; spectra were acquired with quadrature detection into 16K data points
NOTES
499
of memory, and the pulse angle was 66”. Chemical shifts are referred to external sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS). ACKNOWLEDGMENT The measurements were carried out on the Bruker WM250 spectrometer of the GRMP, 205, route de Narbonne, Toulouse, France. REFERENCES 1. K. G. R. PACHLER, P. S. STEYN, R. ULEGGAAR, AND P. L. WESSELS, J. Chem. Sot. Perkin Trans. 1, 1182, (1976). 2. R. PACHTER AND P. L. WESSELS, Org. Magn. Reson. 14, 374 (1980). 3. A. BAX AND R. FREEMAN, J. Am. Chem. Sot. 104, 1099 (1982). 4. A. BAX, J. Magn. Reson. 52, 330 (1983). 5. V. RVTAR AND T. C. WONG, J. Magn. Reson. 53, 495 (1983). 6. K. BOCK AND C. PEDERSEN, J. Magn. Reson. 25, 227 (1977). 7. N. CYR, G. K. HAMER, AND A. S. PERLIN, Can. J. Chem. 56,297 (1978).
OF MAGNETIC
RESONANCE
S&496-499
(1984)
A Simple Method for Assigning Long-RangeCarbon-Proton Couplings through SelectiveProton Decoupling G.COMMENGES Laboratoire
de Chimie
de Coordination,
CNRS,
Toulouse,
France
AND
R. C. RAO*? Sanoji-Recherche,
Toulouse,
France
Received December 30, 1983
The study of long-range 13C-‘H couplings is important for the unambiguous analysis of 13C NMR spectra (1, 2). The assignment of these couplings is usually performed using selective proton decoupling. It has been shown recently (3-5) that heteronuclear J-resolved 2-D spectroscopy may offer many advantages in this area; however, the long accumulation time needed to get the few hundred data sets required by a single experiment inhibits a more widespread use of this technique, especially when only small amounts of compounds are available or with less soluble substances. Therefore one must turn toward more classical methods to get the desired information. Selective proton decoupling is generally preferred to arduous and expensive ’ 3C or *H enrichments. It requires a weak irradiation field and this can induce selective ‘H population transfer, resulting in substantial intensity changes of the 13C NMR signals. Although these intensity changes can be useful for the determination of the relative signs of the coupling constants in the case of comparatively simple molecules, they may, however, render difficult the interpretation of spectra of more complex molecules. One way of overcoming this problem (6) makes use of a second decoupling frequency source connected to the input of the proton decoupler amplifier and combines selective proton decoupling with noise decoupling. Under these conditions one obtains selectively decoupled i3C spectra without intensity changes and with the advantage of retaining the NOE. * To whom correspondence should be addressed. t Present address: Laboratoire de Pharmacologic et de Toxicologic Fondamentales, CNRS, 205. route de Narbonne, Toulouse, France.
0022-2364184 $3.00 Copy+& 0 1984 by Academic Press. Inc. All rights of reproduction in any form reserved.
496
NOTES
497
‘II DECOUPLER
‘%
TRANSMITTER
RECEIVER
FIG. I. Sequence used for selective proton decoupling. BB = broadband decoupling; CW = continuouswave decoupling; AT = acquisition time.
We show here that very similar results can be obtained when using the pulse programmer possibilities offered by most of the modem NMR spectrometers, hence dispensing with the need of an expensive second synthesizer. The decoupler mode is first set to proton noise decoupling (high-power, frequency at the center of the proton region) for a time r1 to allow for the build up of the NOE; it is then set to low-power continuous-wave decoupling with the frequency at the particular proton to be irradiated. The FlD is acquired after a delay TV, depending on the switching characteristics of the offset generator (Fig. 1). The delay is typically a few tens of milliseconds and the power identical to that used in the corresponding classical selective irradiation experiment. At the end of the acquisition period the decoupler mode is set back to proton noise decoupling and the whole process is then repeated the number of times necessary to get a reasonable signal-to-noise ratio. Figure 2A shows the proton-coupled 13C NMR spectrum of a mixture of LX-Dglucopyranose (I) and P-D-glucopyranose (II) in solution in D20, obtained under gated decoupling conditions. The interpulse delay was 2.046 s.
498
FIG.
NOTES
2.62.9 MHz proton-coupled 13CNMR spectra of a mixture of U-D and -A &ty©ranose
in 40.
The signal of C-lp appears as a quartet at 98.4 ppm and exhibits one large ‘JCH coupling equal to 160.9 Hz and one small *JCH coupling to H-2, equal to 5.5 Hz; C-la, gives a doublet centered at 94.6 ppm showing only one large ‘JCH coupling equal to 168.3 Hz (7). Figure 2B shows the 13C spectrum obtained under continuous selective irradiation of H-28. The inter-pulse delay was 2.046 s. C-28 now appears as a singlet and C-16 as a doublet. Figure 2C shows the 13C spectrum obtained using our microprogram to selectively irradiate H-28. The interpulse delay was 2.091 s. Selective decoupling is thus achieved with enhanced signal intensities, though full NOE is clearly not achieved, intensity changes are much reduced whereas the time needed to record the spectrum has been only slightly increased. The spectra were obtained at 62.9 MHz on a Bruker WM250 NMR spectrometer at normal probe temperature (25°C) using a 10 mm o.d. sample tube. Spectral widths were 15,000 Hz; spectra were acquired with quadrature detection into 16K data points
NOTES
499
of memory, and the pulse angle was 66”. Chemical shifts are referred to external sodium 2,2-dimethyl-2-silapentane-5-sulfonate (DSS). ACKNOWLEDGMENT The measurements were carried out on the Bruker WM250 spectrometer of the GRMP, 205, route de Narbonne, Toulouse, France. REFERENCES 1. K. G. R. PACHLER, P. S. STEYN, R. ULEGGAAR, AND P. L. WESSELS, J. Chem. Sot. Perkin Trans. 1, 1182, (1976). 2. R. PACHTER AND P. L. WESSELS, Org. Magn. Reson. 14, 374 (1980). 3. A. BAX AND R. FREEMAN, J. Am. Chem. Sot. 104, 1099 (1982). 4. A. BAX, J. Magn. Reson. 52, 330 (1983). 5. V. RVTAR AND T. C. WONG, J. Magn. Reson. 53, 495 (1983). 6. K. BOCK AND C. PEDERSEN, J. Magn. Reson. 25, 227 (1977). 7. N. CYR, G. K. HAMER, AND A. S. PERLIN, Can. J. Chem. 56,297 (1978).