Inverse proton editing in 1D and 2D spectra by a combination of the SHIPPED and COSY techniques

Abstract

Inverse NMR spectroscopy leads to increased sensitivity in X, H correlation spectroscopy, since the nucleus with the higher gyromagnetic ratio ( ‘H ) is used both for proton polarization to start the pulse sequence and for detection ( 1, 2). Recently a new pulse sequence has been developed: SHIPPED or selective heteronuclear inverse polarization transfer proton editing (3). The pulse sequence is shown in Fig. la. This inverse pulse sequence allows DEPT editing (4-6) of the ‘H NMR spectrum. One possible application of the SHIPPED experiment is the acquisition of oneand two-dimensional spectra, which contain only signals from XH groups. In order to decouple the X nucleus with the GARPl sequence ( 7), it is necessary to extend the original ID SHIPPED sequence by a second delay A’ = A to refocus the direct heteronuclear coupling. In the last two delays A’ two 180” pulses were inserted to refocus the chemical shifts in order to avoid phase problems (Fig. 1 b) . An interesting idea is to combine a 2D SHIPPED sequence with a COSY transfer. A 2D version of the SHIPPED experiment (Fig. la) replaces the 90” excitation pulse in the H,H COSY.90. This would lead to a 3D experiment, but to avoid this, the second evolution time is converted into a fixed delay AZ. This new variant creates an inverse H, X correlation with a H-relayed transfer (Fig. lc). During ti only the chemical shift of the X nucleus evolves, whereas after the SHIPPED part of the experiment XH magnetization is selected. During A2 the transverse magnetization of the methine proton I, evolves under homonuclear JHH coupling to the relayed proton I,, which is not bound to 13C. The 90” mixing pulse at the end of the sequence transfers the magnetization from spin I, to spin I,. During t2 the transverse magnetization of spin I1 evolves under heteronuclear I JXH coupling. The spectrum obtained shows the doublet signals of the methine protons, which are separated in 1L; by ‘JXH. The signals of the relayed protons show up as singlets. Figure 2 shows a section of a H-relayed SHIPPED-COSY spectrum from the olefinic region of strychnine. The second experiment is a combination of the refocused 1 D SHIPPED and COSY experiments (Fig. Id). After the SHIPPED part of the experiment only transverse magnetization of the methine protons, which evolves during tl under homonuclear JHH and heteronuclear ’ Jxn coupling, is selected. Because of the mixing pulse at the end of tl , polarization is transferred from spin 1, to spin I*, which is not directly bound