A methyl 1H double quantum CPMG experiment to study protein conformational exchange

Protein conformational changes play crucial roles in enabling function. The Carr-Purcell-Meiboom-Gill (CPMG) experiment forms the basis for studying such dynamics when they involve the interconversion between highly populated and sparsely formed states, the latter having lifetimes ranging from ~ 0.5 to ~ 5 ms. Among the suite of experiments that have been developed are those that exploit methyl group probes by recording methyl ¹H single quantum (Tugarinov and Kay in J Am Chem Soc 129:9514-9521, 2007) and triple quantum (Yuwen et al. in Angew Chem Int Ed Engl 55:11490-11494, 2016) relaxation dispersion profiles. Here we build upon these by developing a third experiment in which methyl ¹H double quantum coherences evolve during a CPMG relaxation element. By fitting single, double, and triple quantum datasets, akin to recording the single quantum dataset at static magnetic fields of B_o, 2B_o and 3B_o, we show that accurate exchange values can be obtained even in cases where exchange rates exceed 10,000 s^-1. The utility of the double quantum experiment is demonstrated with a pair of cavity mutants of T4 lysozyme (T4L) with ground and excited states interchanged and with exchange rates differing by fourfold (~ 900 s^-1 and ~ 3600 s^-1), as well as with a fast-folding domain where the unfolded state lifetime is ~ 80 µs.

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.

Ultrafast acquisition of 1H-1H dipolar correlation experiments in spinning elastomers

We show that two widely used 2D solid-state NMR (ssNMR) pulse sequences can be implemented in an ultrafast (UF) manner, and yield 2D spectra of elastomers in a single scan, under magic-angle spinning. UF 2D ssNMR provides an acceleration of one to several orders of magnitude for classic experiments.

Preparing and preserving the double quantum coherence in NV- centers in Diamond at low fields

We present and demonstrate a simple idea to excite and preserve the double-quantum-coherence (DQC) in the ground state of the electron spin of the Nitrogen-vacancy (NV) color center in diamond. We measure the coherence time of the DQC and compare it to the single quantum coherence time, both, in a Ramsey fringe experiment and under a Hahn echo sequence. We also demonstrate a robust pulse sequence based on the DANTE pulse sequence for selectively isolating the signal from the electron transitions conditional on the state of the always-present Nitrogen spin.