Efficient Access to Elusive 1D 13C NMR Spectra through Highly Resolved 1H, 13C-Long-Range Correlation Spectroscopy.

A method for obtaining 1D 13C NMR spectra from natural products or metabolites using proton detection is described. The approach delivers singlets for every 13C signal without conducting any broadband 1H decoupling (CPD) and is based on calculating 13C projections from constant-time HMBC and conventional HSQC experiments, recorded at high digital resolution and processed to pure phases. Paramount to the proposed method is the implication of nonuniform sampling and echo processing. The echo processing produces phase-sensitive 2D CT-HMBC spectra with narrow 13C signal line shapes. Two simple HMBC pulse sequences are utilized with the suppression of homo- and heteronuclear couplings. Due to the removal of the 1H multiplet structure in F1 (no tilt at higher digital resolution), 13C singlets arise. An overall increase in 13C signal-to-noise (SINO) for all types of carbon multiplicities is observed, making the proposed technique superior compared to direct 13C excitation. For otherwise difficult-to-measure quaternary carbon atoms, a SINO enhancement of up to 6 and 12 depending on F1 resolution (3 and 6 Hz/point) is reported. Echo/anti-Echo signal detection cleans up the spectrum. Nonuniform sampling (NUS) lays the groundwork to significantly reduce the total acquisition time. Final 1D 13C projections are obtained by combining the 13C projection from CT HMBC and conventional HSQC. This orthogonal concept of combining the 13C projections from different spectra inherently minimizes the risk of missing 13C cross-peaks by inappropriate setting of long-range nJHC coupling delays and the shortcoming of T2 relaxations. The advantages and some limitations of the concept are discussed.

Boosting the resolution of multidimensional NMR spectra by complete removal of proton spin multiplicities.

Over decades multidimensional NMR spectroscopy has become an indispensable tool for structure elucidation of natural products, peptides and medium sized to large proteins. Heteronuclear single quantum coherence (HSQC) spectroscopy is one of the work horses in that field often used to map structural connectivity between protons and carbons or other hetero nuclei. In overcrowded HSQC spectra, proton multiplet structures of cross peaks set a limit to the power of resolution and make a straightforward assignment difficult. In this work, we provide a solution to improve these penalties by completely removing the proton spin multiplet structure of HSQC cross peaks. Previously reported sideband artefacts are diminished leading to HSQC spectra with singlet responses for all types of proton multiplicities. For sideband suppression, the idea of restricted random delay (RRD) in chunk interrupted data acquisition is introduced and exemplified. The problem of irreducible residual doublet splitting of diastereotopic CH2 groups is simply solved by using a phase sensitive JRES approach in conjunction with echo processing and real time broadband homodecoupling (BBHD) HSQC, applied as a 3D experiment. Advantages and limitations of the method is presented and discussed.

Mirror symmetric broadband homodecoupled perfect echo spectroscopy.

A new experiment for recording phase sensitive ω1-broadband homodecoupled TOCSY spectra is presented. The method is an extension of the already existing perfect echo (PE) filter, proposed to sample t1 chemical shift under sustained homodecoupling. The modification is made by attaching a time reversed perfect echo filter to a regular perfect echo scheme. Thus it becomes possible to acquire for longer t1 acquisition times without compromising the quality of homodecoupling. The mirror symmetric double perfect echo is implemented into the evolution period of a TOCSY experiment. A spin lock pulse purges undesired dispersive antiphase components at the end of the central t1 evolution period. Pure absorptive lineshapes with reduced proton spin multiplicities are obtained. The approach can be used in conjunction with real or constant time chemical shift evolution. In case of compounds with reduced T2 relaxation time, the real time approach is advisable, where the echo delays are an extension of the t1 evolution period. In this way, an unnecessary loss due to T2 relaxation is avoided. Using the pulse sequence in constant time mode at high t1max values gives ω1-homodecoupled TOCSY spectra without a significant dependence of the transfer amplitude on J. All experiments were carried out using non uniform sampling to decrease the measurement time. Experimental setup, advantages and limitations are discussed.

Small angle double quantum spectroscopy (SAQS NMR).

A new experiment for recording double quantum spectra is introduced. The 2D DQ NMR experiment yields phase sensitive spectra with double quantum frequencies in F1. The appearance of remote peaks is vastly suppressed by using a small flip angle double quantum excitation and reconversion. Pulse sequences and phase sensitive processing are discussed. The complexity of the SQ antiphase magnetization given in larger proton spin networks could be reduced by using the option of band selective decoupling during the preparation period. In addition, an ACCORDION element is applied by incrementing the J evolution delay in concert with the t1 period. With this the excitation of double quantum coherence over a wider range of J values is achieved. A broadband homodecoupled version of the DQ experiment is proposed, where correlation peaks with singlet response at F2 chemical shifts and double quantum frequencies in F1 are obtained. We call this experiment Small Angle double Quantum Spectroscopy SAQS NMR.