BIRD sequence

The amino acid sequences of the two hemoglobins differ by only four substitutions, of which only one is unique among the bird sequences determined so far (65, 66). This is H2a Pro (greylag goose) - Ala (bar-... 

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. 

Fig. iL Pulse sequences for pseudo triple-resonance experiments with passive selection of satellites (a) z-filtered 2D-"X, H( "Y) INEPT. (b) -BIRD H,"X( "Y) HMQC. (c) H,"X( "Y) HMQC experiment with biselective excitation pulse (bis) biselective excitation may be achieved with DANTE or P-BIRD sequences, binomial pulses, or shaped pulses. (d) H, X( "Y) HEED-INEPT." ... 

A1) which may dictate fewer ti increments be collected than would otherwise be desirable, so reducing the digital resolution of the fi dimension. These problems have been addressed in some detail and ID sequences presented for parameter optimisation [66], For routine use, it has been suggested [59] that optimising delays for "Jch = 10 Hz should provide most responses of interest, particularly when using the BIRD sequence within the A2 refocusing delay. This corresponds to delays of 50 ms (1/2"Jch) and 33 ms (1/3"Jch) for Ai and A2 respectively. 

Because of the conceivable imperfections of pulses in contrast to the original reference [5.23] a composite 180° pulse was proposed in the INEPT unit. However upon closer examination it is apparent that this four-pulse combination constitutes a selective BIRD filter. The effect of a simultaneous hard and selective pulse cannot be calculated explicitly by NMR-SIM but because both pulse lengths are short relative to the delays in the BIRD sequence, the selective BIRD filter can be represented by a sequential 180° and selective 180° pulse. 

The effects of the J cross-talk are already apparent with residual C and CH2 signals appearing in the CH and CH3 only subspectrum and vice versa. These residual signals may be substantially reduced by using the SEMUT-GL sequence which incorporates a BIRD sequence to scale the coupling evolution to the same extent irrespective of the lJ(C, H) values. 

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