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New in The Bruker User Library: Real-time Pure Shift HSQC Experiment

A new contribution to our online user library (The Resoance Exchange) has just been made available by Peter Kiraly, Mathias Nilsson and Gareth A. Morris from the NMR Methodology Group at the University of Manchester.

The ideal HSQC experiment should provide a single correlation per C-H pair, but instead conventional experiments show multiplet structure in the proton dimension because of homonuclear J-coupling interactions. This is often hidden by the use of short acquisition times to prevent damage caused by high power heteronuclear decoupling. Since adiabatic decoupling on modern instruments now enables the use of longer acquisition time that provide higher resolution, the demand for broadband homodecoupled HSQC experiments, that can increase resolution and sensitivity simultaneously, has increased. We provide here a user-friendly implementation of real-time pure shift HSQC, based on a multiplicity-edited and gradient-enhanced HSQC sequence with rectangular carbon inversion pulses and WURST adiabatic decoupling. The pulse program code supports the use of WaveMaker (included in TopSpin 3.5pl6 and above) for setting the relevant decoupling parameters automatically. Chunk-to-chunk MLEV-16 phase sequencing and variable CTP gradient amplitudes are implemented to improve the quality of homodecoupling.

Figure 1 Projections (a/b) and part of 2D HSQC spectra (c/d) of a mixture of 80 mM D-glucose, 75 mM D-trehalose, 56 mM raffinose, and 30 mM -cyclodextrin in D2O. Conventional (black) and real-time pure shift (red) acquisition methods were used. The conventional 2D spectrum is displaced along the indirect dimension for clarity.

The pulse sequence program provided here is equivalent to that used in reference [3], except that the dropping of early points is not implemented as described there, allowing normal FT processing to be used without the need for a post-processing au program. The first implementation of the real-time pure shift HSQC experiment [1] did not use gradient pulses in the real-time loop. Optional gradient pulses are included in the real-time loop, as in reference [2] to allow applications in the presence of very strong solvent signal, as is typical for proteins. In most chemistry applications this is not recommended, because it increases the duration of the J-refocusing element and causes broadening of the pure shift signals [see examples in reference 3].

The Bruker online User Library, the Resonance Exchange is an online platform that allows Bruker users to share their latest developments. The User Library can be accessed from the following site: (https://www.bruker.com/service/information-communication/nmr-pulse-program-lib/bruker-user-library.html). The readers are encouraged to share their own latest developments with the vast community of the Bruker users via the Resonance Exchange.

References:

[1] L. Paudel et al, Angew.Chem.Int.Ed. 2013, Vol. 125, 11830-11833.

[2] P. Kiraly et al, J.Biomol.NMR. 2015, Vol. 62, 43-52.

[3] P. Kiraly et al, Magn. Reson. Chem., 2017, accepted, DOI: 10.1002/mrc.4704