BIRD pulse

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

A BIRD pulse sandwich [33] followed by a delay can be used to suppress the magnetization from C-bound protons [34]. The BIRD pulse sequence is 90° (i7)-r-180°(i7, C )-r-90° (if). With the delay r set to 1/(2J), where J is the one-bond coupling constant, the effect of the sequence is... 

This suppression scheme has been shown to work well together with HMQC experiments of small molecules at natural abundance. Even cleaner spectra are obtained, if the BIRD sequence is combined with HSQC experiments already containing a spin-lock purge pulse. Drawbacks of the BIRD pulse scheme are the fact that the relaxation delay between scans cannot be chosen freely anymore and that complete suppression of all C-bound proton signals is impossible, if they have different relaxation times. Furthermore, the BIRD pulse scheme is not applicable to molecules in the slow motional regime, since negative NOEs between the inverted proton spins and the non-inverted C-bound proton spins would reduce the magnetization of the latter. 

Values of /CH were also determined by two-dimensional /-resolved spectroscopy using an INEPT-type pulse-sequence,29 with a BIRD pulse for the suppression of "/c H and folding in the F, dimension. Rules for the calculation of the correct values of /c H from the reduced splittings observed were given. The values were also obtained from a set of /-modulated, one-dimensional 13C spectra via the... 

In order to determine couplings to nuclei in natural abundance, it is necessary to suppress the signals of protons that are not bonded to a magnetically active heteronucleus. An (iix hetero half-filter that selects for such nuclei in F, via the phase cycle was used for this purpose. Presaturation of the protons bonded to 12C by the BIRD pulse allows a rapid pulse-sequence (2 scans per second). The resulting 2D spectra are TOCSY spectra in which the cross peaks show the desired E. COSY pattern. From the results shown, the only limitation seems to be the resolution obtained, although the authors do not hesitate to use a third heteronuclear frequency domain for improvement. 

FIGURE 9.8 (a) Bilinear rotation decoupling (BIRD) pulse sequence, (b) Behavior of the H magnetization from a proton coupled to 13C. (c) Behavior of the H magnetization from a proton not coupled to 13C. See text for details. 

Some methods take advantage of a difference in a particular property between water and the molecule to be studied. In particular, a macromolecule usually has a shorter value for proton Tt than water and a much lower diffusion coefficient. One of the oldest methods for water signal suppression is WEFT (water elimination Fourier transform), in which an inversion recovery sequence is applied (see Fig. 2.12) with r chosen to be the time that the water signal goes through zero (Tj In 2), just as in the BIRD pulse sequence. Another method makes use of the technique described in Section 9.3 to measure diffusion coefficients. 

In 1986 Bax and Subramanian [10] proposed the utilization of the discriminatory power of the Bilinear Rotational Decoupling [36] (BIRD) pulse cluster to achieve an even better suppression of unwanted H- C signals (Figure 13). 

At the end of the BIRD pulse cluster the magnetization of protons coupled by a one-bond coupling to a nucleus is on the +z axis, while the magnetization of protons not presenting such a coupling is on the — z axis. 

Thus, the BIRD pulse reinforces the discrimination between and H-... 

The FLOCK sequence (so named because it contains three BIRD pulses) of Reynolds is similar to the rest of the pulse sequences that have been discussed in that it is a variahle-evolution-time experiment (/] becomes progressively larger). FLOCK thus avoids the potential absence of C-H correlations. Its pulse sequence is given in Figure 7-19. 

The second BIRD pulse is selective for vectors during A ] and acts as a simple C 180° pulse for Jch vectors, which tend to refocus. Nonetheless, there may be some one-bond polarization transfer. The Jqw vectors, however, move to antiphase for maximum polarization transfer. In addition, the BIRD pulse refocuses magnetic-field inhomogeneities for protons that are indirectly bonded to and refocuses the individual vector components (which themselves are defocusing) prior to polarization transfer. 

The third BIRD pulse, during A2, also is selective for Vch vectors, which are focused from an antiphase orientation. nuclei that are directly bonded to protons behave in either of two ways. On the one hand, the 7ch vectors of the majority of nuclei, which have not undergone any polarization transfer, remain focused, but are eliminated by phase-cycling subtraction. On the other hand, the minority of nuclei that have undergone some polarization transfer remain at antiphase and are not observed. 

In addition to a variable /], FLOCK has four fixed delay times. The first delay time (for relaxation) is a function of the H-, not the X-nucleus, T S because, like HETCOR, FLOCK is a polarization transfer experiment. The second (t, the delay in the BIRD pulses) is a... 

Figure 7-19 The FLOCK pulse sequence. The 180° H pulses in parentheses are BIRD pulses (90°-t-180°-t-90°) with T = (27CH) The relative phases of the three pulses are x, y, -x for the first BIRD pulse and x, x, -x for the. second and third BIRD pulses.

Heteronuclear coupling constants (1,b7c,h) are most commonly measured from heteronuclear 2D experiments. The 3/c H couplings can be easily extracted from /-resolved spectra as well as from f or F2 proton coupled HSQC spectra. The undesired evolution of "/CH during q can be eliminated with use of an appropriate bilinear rotation decoupling (BIRD) pulse, such as BIRDd,x in. /-resolved spectroscopy35 and 111RD in Fi-coupled HSQC.36 Spin-state selective excitation techniques, S3E and S3CT37 38 (spin-state-selective coherence transfer), can also be used for the measurement of... 

The readout of line frequencies is simpler in Fb but potentially less precise due to limited digital resolution. The digital resolution can be increased if long-range, either proton-carbon or proton—proton, interactions are removed by the action of a BIRD pulse.36,175,180 If required, overlap reduction can be achieved by separating the a//3 states into two spectra as demonstrated in S3-CT-HSQC37,147,148 or SPITZE (spin state selective zero overlap)-HSQC.170... 

Application of the -BIRD pulse sequence ret 184 application of z-filtered polarization transfer ret 183. Ar = 2,6- Pr2-C6H3. 

The BIRD pulse [14] is in fact a cluster of pulses (Fig. 6.12) used as a tool in NMR to differentiate spins that possess a heteronuclear coupling from those that do not. The effect of the pulse can vary depending on the phases of the pulses within the cluster, so we concentrate here on the selective inversion described above. For illustrative purposes, proton pulse phases of x, y, X will be considered as this provides a clearer picture with the vector model, although equivalent results are achieved with phases x, x, —x, as in the original publication. The scheme (Fig. 6.14) begins with a proton excitation pulse followed by a spin-echo. Since carbon-12 bound protons have no one-bond... 

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