Convection-compensating diffusion experiments with phase-sensitive double-quantum filtering

Abstract

We present a design scheme for phase-sensitive, convection-compensating diffusion experiments with gradient-selected homonuclear double-quantum filtering. The scheme consists of three blocks: a 1/2J evolution period during which antiphase single-quantum coherences are created; a period of double-quantum evolution; and another 1/2J period, during which antiphase single-quantum coherences are converted back into an in-phase state. A single coherence transfer pathway is selected using an asymmetric set of gradient pulses, and both diffusion sensitization and convection compensation are built into the gradient coherence transfer pathway selection. Double-quantum filtering can be used either for solvent suppression or spectral editing, and we demonstrate examples of both applications. The new experiment performs well in the absence of a field-frequency lock and does not require magnitude Fourier transformation. The proposed scheme may offer advantages in diffusion measurements of spectrally crowded systems, particularly small molecules solubilized in colloidal solutions or bound to macromolecules.

PFG NMR diffusion experiments for complex systems

“…In this section, we briefly review the principles of MQ filtering, especially gradient-selected filtering, and demonstrate why its gradient-selected implementation is perfectly suited for PFG diffusion experiments. We also consider an example of its application to diffusion measurements-a recently proposed phase-sensitive, convection-compensating DQF diffusion experiment (DQDiff) (33).…”

“…To illustrate how this approach can be applied in practice, consider the experiment that we have called DQDiff: it is a phase-sensitive, convection-compensated DQF diffusion experiment (33). Its pulse sequence is shown in Fig.…”

“…The sets of gradient amplitudes suitable for a given CTP are not unique; the "best" set must be determined through a screening process based on the uniqueness of the selected CTP; its suitability for convection compensation; and resistance to coherence leaks due to imperfectly set RF pulses and delay lengths. This computationally voluminous process is described in detail in the original article (33); the gradient values (8:Ϫ8:Ϫ7:7:8: Ϫ4) appear to yield the best practical results. The diffusion attenuation for the selected CTP can be found using the approach described in the previous section and Appendix A.…”

Acquisition of pure-phase diffusion spectra using oscillating-gradient spin echo

“…1). The quantity q is the wave vector of the diffusion-sensitive magnetization helix [3,[14][15][16] qðtÞ ¼…”

“…3), a small molecule with multiple homonuclear scalar couplings [15,19], in two test systems using double-echo PGSE [18] and single-echo OGSE. The measurements were made in the presence of thermal convection.…”