Solvent suppression in liquid state NMR with selective intermolecular zero-quantum coherences

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Abstract

We compared several variants of 2D selective intermolecular zero-quantum pulse sequences which use selective pulses and selective refocusing modules in different positions within the sequence to achieve suppression of the solvent signal. We show that a theoretical two-fold gain in cross peak intensity in the selective spectra, compared to the non-selective sequence, is only valid for certain parameters and is additionally influenced by faster signal build-up and better solvent suppression. We show experimentally that for low concentration solutes application of a single selective mixing pulse suppresses solvent signal efficiently. Further selective refocusing modules can actually reduce this efficiency.

Introduction

Long range dipolar interactions are usually neglected in the treatment of nuclear magnetic resonance (NMR) experiments, because they are averaged to zero in an isotropic environment. If spatial isotropy is broken by strong deviation from a spherical sample geometry, or application of a magnetic field gradient, long range dipolar couplings can lead to significant signal intensities [1], [2]. The origin of these signals can be explained either by adding the effect of the spin polarization itself to the magnetic field in the Bloch equations, or via a formalism describing the observed effects through intermolecular multiple-quantum coherences (iMQC) [3], [4]. One very useful feature of the signals originating from iMQC is that it is possible to obtain relatively narrow line-shapes in inhomogeneous fields. These narrow line-shapes arise because the effects of evolution under field inhomogeneities during the t1-period are locally refocused by the action of the local form of the dipolar field during the t2-period. This was first demonstrated with a dedicated pulse sequence termed HOMOgeneity ENhancement in Intermolecular ZEro-quantum Detection (HOMOGENIZED, see Fig. 1a) [5]. Applications demonstrating the potential of this method have been shown for high-resolution NMR spectroscopy with a strongly-drifting 25 T magnet [6] and for the detection of highly dilute metabolites in in vivo NMR spectroscopy [7]. More recently a similar pulse sequence termed Intermolecular Dipolar interaction Enhanced All Lines (IDEAL), also yielding relatively narrow lineshapes in inhomogeneous fields but employing intermolecular double-quantum (iDQC) instead of zero-quantum coherences (iZQC), has been proposed [8]. Improved versions of the HOMOGENIZED sequence using a selective mixing pulse and additional solvent suppression modules have been proposed by us [9] and independently by Chen et al. [10] (see Fig. 1b and c). This new sequence termed SEL-HOMOGENIZED (SELective HOMOGENIZED) in [10] introduces substantial improvements, which may be key for application of the method to samples with large inhomogeneities, such as porous materials or organs in vivo.

Here we give equations derived from a classical treatment of the dipolar field that allow comparison of signal evolution in HOMOGENIZED and SEL-HOMOGENIZED sequences, and use the product operator formalism to explain the twofold increase in cross peak intensity of the latter sequence. We show applications to solutes at low concentrations, and discuss methods for improved water suppression. For theoretical discussion we will assume that local field variations are negligible.

Section snippets

Theory

The origin of detectable signal in iMQC experiments can be explained by adding a dipolar term to the static magnetic field B0[1]. This term is, for historical reasons, often called the dipolar demagnetizing field and takes into account the polarization of the spins in the sample itself. Equations describing evolution of the detectable signal have been derived in various publications. We will consider here a solution of a dilute spin species S with Larmor frequency ΩS dissolved in a solvent,

Experimental

Experiments were performed on a Bruker DRX400 spectrometer with a 5 mm broadband X-observe probehead on solutions of 1 mM creatine and 3 mM alanine in 90% H2O/10% D2O. To avoid radiation damping effects the coherence selection gradient (CSG) was split into two sections, as suggested in [5], and the first part was applied immediately after the first pulse. The effective strength of the sine shaped CSG was 14 G/cm applied for 1 ms. Water suppression was performed using (GZ−180(selective) −

Results and discussion

Fig. 2 shows HOMOGENIZED spectra with β = 45° (a) and β = 90° (b). Fig. 2c shows a S90 spectrum which was recorded with the pulse sequence shown in Fig. 1b. In Fig. 2b, there are two peaks observed at (ΩSΩI, ΩS) and (ΩIΩS, ΩS) with nearly the same intensity. This agrees with Eqs. (1), (2), and (17) and (18) in [11], but contradicts Eq. (3) in [10]. The latter Letter suggests a dependence of the signal intensity proportional to sin2β, which would give rise to no signal in the HOMOGENIZED spectra

Acknowledgements

We would like to thank the referee of this Letter for correcting an error in the original version, Andrew Webb for helpful comments on the manuscript, Matthias Grüne for making spectrometer time available, and the Deutsche Forschungsgemeinschaft for financial support (Fa474/1).

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