Heteronuclear zero quantum spectroscopy as a conformational probe

Abstract

Investigations of the conformations of molecules in liquids are often aided by the information present in the proton NMR spectrum of the sample. The proton chemical shifts and proton-proton couplings often contain sufficient information for determination of the structural features of interest. However, in many cases the proton spectrum is obscured by the signals from solvent or an enzyme or by overlap of signals from the molecule being studied. One approach to overcoming this problem is to use heteronuclear two-dimensional NMR to detect indirectly the proton spectral information (1, 2). The heteronuclear two-dimensional NMR method can be used to obtain proton spectra of sufficient quality to answer conformational questions (35). The technique has recently been extended to detection of protons which share a coupling partner with the heteronucleus but are not directly coupled to the heteronucleus (6, 7). Another approach to spying on protons is to use heteronuclear multiple quantum spectroscopy (8-10). Since indirect detection of protons is of use in examining the conformations and structures of molecules where conventional approaches fail, it was of interest to begin an investigation of the merits of multiple quantum spectroscopy relative to heteronuclear two-dimensional NMR. Zero quantum spectroscopy, in particular, appears to be quite promising. The linewidths obtained in the crucial domain containing the proton information are only weakly dependent on the field homogeneity. A more advantageous feature is that the proton spectra obtained via heteronuclear zero quantum spectroscopy correspond to the heteronuclear decoupled proton spectrum when only one proton is coupled to the heteronucleus. The pulse sequence used for the generation and detection of heteronuclear zero quantum coherence is shown in Fig. 1. The phases of the pulses are cycled so that the phosphorus-31 signals are modulated only by the zero quantum coherence as a function of tl. Unlike the situation for homonuclear zero quantum spectroscopy, phase cycling can discriminate between zero quantum coherence and longitudinal magnetization. The phase cycling modulates the phase of the detected signal, allowing “quadrature” detection to be used in both frequency domains. The frequency of the heteronuclear zero quantum signal, in the absence of protonproton coupling, is the difference between the proton Larmor frequency and the proton transmitter frequency minus the difference between the phosphorus-31 Lar-