Polarization transfer via J coupling by semiselective excitation applied to the high gyromagnetic ratio nucleus

Abstract

A novel sequence is proposed for selectively transferring polarization from a given nucleus A to a nucleus X which is J coupled to A. It consists of applying at the A resonance a semiselective pulse of amplitude γ AH 2 = J 2 and of duration t such that γ AH 2t = π 2 1 2 . This pulse is followed by a nonselective π 2 pulse on A, phase shifted by 90°. A nonselective π 2 pulse on X allows observation of the enhanced X resonances. In practice, instead of the semiselective pulse of low amplitude and long duration, a train of small flip angle pulses (the DANTE sequence of Morris and Freeman) is employed. It is devised to provide the maximum polarization transfer and calculations are given which allow determination of its optimum characteristics. As for other polarization transfer schemes, decoupling of A immediately after the observation pulse is inadequate. The refocusing sequence of Burum and Ernst {( π 2 )(X) − τ − [π(X), π(A)] − τ − Acquisition} was used, which leads to enhanced decoupled spectra. This sequence is further discussed and a procedure is presented for eliminating out-of-phase or unwanted X signals. The proposed polarization transfer sequence is selective regarding both chemical shifts and J coupling constants. This was quantitatively evaluated by means of a complete density matrix calculation. The efficiency and selectivity of the method are thus well characterized: chemical shift selectivity is of the order of 0.15 J if one is able to detect a 10% intensity variation. By taking advantage of this selectivity, correlations can be obtained between A and X chemical shifts. A tripeptide (glutathione) was chosen for providing an example of such a correlation established between proton and carbon-13 chemical shifts.