Recoupling in solid state NMR using γ prepared states and phase matching

Simplified Preservation of Equivalent Pathways Spectroscopy

Inspired by the recently proposed transverse mixing optimal control pulses (TROP) approach for improving signal in multidimensional magic-angle spinning (MAS) NMR experiments, we present simplified preservation of equivalent pathways spectroscopy (SPEPS). It transfers both transverse components of magnetization that occur during indirect evolutions, theoretically enabling a √2 improvement in sensitivity for each such dimension. We compare SPEPS transfer with TROP and cross-polarization (CP) using membrane protein and fibril samples at MAS of 55 and 100 kHz. In three-dimensional (3D) (H)CANH spectra, SPEPS outperformed TROP and CP by factors of on average 1.16 and 1.69, respectively, for the membrane protein, but only a marginal improvement of 1.09 was observed for the fibril. These differences are discussed, making note of the longer transfer time used for CP, 14 ms, as compared with 2.9 and 3.6 ms for SPEPS and TROP, respectively. Using SPEPS for two transfers in the 3D (H)CANCO experiment resulted in an even larger benefit in signal intensity, with an average improvement of 1.82 as compared with CP. This results in multifold time savings, in particular considering the weaker peaks that are observed to benefit the most from SPEPS.

Sensitivity-Enhanced Multidimensional Solid-State NMR Spectroscopy by Optimal-Control-Based Transverse Mixing Sequences

Recently, proton-detected magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR) spectroscopy has become an attractive tool to study the structure and dynamics of insoluble proteins at atomic resolution. The sensitivity of the employed multidimensional experiments can be systematically improved when both transversal components of the magnetization are transferred simultaneously after an evolution period. The method of preservation of equivalent pathways has been explored in solution-state NMR; however, it does not find widespread application due to relaxation issues connected with increased molecular size. We present here for the first time heteronuclear transverse mixing sequences for correlation experiments at moderate and fast MAS frequencies. Optimal control allows to boost the signal-to-noise ratio (SNR) beyond the expected factor of 2 for each indirect dimension. In addition to the carbon-detected sensitivity-enhanced 2D NCA experiment, we present a novel proton-detected, doubly sensitivity-enhanced 3D hCANH pulse sequence for which we observe a 3-fold improvement in SNR compared to the conventional experimental implementation. The sensitivity gain turned out to be essential to unambiguously characterize a minor fibril polymorph of a human lambda-III immunoglobulin light chain protein that escaped detection so far.

Two pulse recoupling

The paper describes a family of novel recoupling pulse sequences in magic angle spinning (MAS) solid state NMR, called two pulse recoupling. These pulse sequences can be employed for both homonuclear and heteronuclear recoupling experiments and are robust to dispersion in chemical shifts and rf-inhomogeneity. The homonuclear pulse sequence consists of a building block (π)_ϕ(π)_-ϕ where ϕ=π4n, and n is number of blocks in a rotor period. The recoupling block is made robust to rf-inhomogeneity by extending it to (π)_ϕ(π)_-ϕ(π)_π+ϕ(π)_π-ϕ. The heteronuclear recoupling pulse sequence consists of a building block [Formula: see text] and [Formula: see text] on channel I and S, where ϕ₁=3π8n,ϕ₂=π8n and n is number of blocks in a rotor period. The recoupling block is made robust to rf-inhomogeneity by extending it to [Formula: see text] and [Formula: see text] on two channels respectively. The recoupling pulse sequences mix the z magnetization. Experimental quantification of this method is shown for¹³C_α-¹³CO homonuclear recoupling in a sample of Glycine and ¹⁵N-¹³C_α heteronuclear recoupling in Alanine. Application of this method is demonstrated on a sample of tripeptide N-formyl-[U-¹³C,¹⁵N]-Met-Leu-Phe-OH (MLF). Compared to R-sequences (Levitt, 2002), these sequences are more robust to rf-inhomogeneity and give better sensitivity, as shown in Fig. 3.

Four pulse recoupling

The paper describes a family of novel recoupling pulse sequences in magic angle spinning (MAS) solid state NMR, called four pulse recoupling. These pulse sequences can be employed for both homonuclear and heteronuclear recoupling experiments and are robust to dispersion in chemical shifts and rf-inhomogeneity. The homonuclear pulse sequence consists of a building block π2_0°3π2_ϕ°π2_180°+ϕ°3π2_180° where ϕ=πnϕ°=180°n, and n is number of blocks in a two rotor period. The heteronuclear recoupling pulse sequence consists of a building block [Formula: see text] and [Formula: see text] on channel I and S, where ϕ₁=3π2n,ϕ₂=π2n and n is number of blocks in a two rotor period. The recoupling pulse sequences mix the y magnetization. We show that four pulse recoupling is more broadband compared to three pulse recoupling [1]. Experimental quantification of this method is shown for ¹³C_α-¹³CO, homonuclear recoupling in a sample of Glycine and ¹⁵N-¹³C_α, heteronuclear recoupling in Alanine. Application of this method is demonstrated on a sample of tripeptide N-formyl-[U-¹³C,¹⁵N]-Met-Leu-Phe-OH (MLF).

Band-selective heteronuclear dipolar recoupling with dual back-to-back pulses in rotating solids

We propose a robust band-selective heteronuclear ¹⁵N-¹³C recoupling method using dual back-to-back (BABA) pulses (DBP). It contains four 90° pulses in each rotor period and corresponding phase cycling on each channel (¹³C and ¹⁵N). DBP aims at rapid band-selective heteronuclear magnetization transfer between ¹⁵N and ¹³Cα/¹³C', whose efficiency is close to that of the well-known SPECIFIC CP in membrane proteins with relatively short relaxation time in rotating frame (T_1ρ). Compared to SPECIFIC CP, DBP is very simple to set up and highly robust to RF variations. Thus, it can reduce the efforts in experimental optimization, especially for low-sensitive samples, and is very suitable for long-time or quantitative experiments. The efficacy of DBP is demonstrated by the E. coli diacylglycerol kinase (DAGK) proteoliposome. We anticipate that DBP would be useful for (segments of) membrane proteins that undergo the μs-ms timescale motions in magic-angle spinning (MAS) solid-state NMR.

Recoupling pulse sequences with constant phase increments

The paper studies a family of recoupling pulse sequences in magic angle spinning (MAS) solid state NMR, that are characterized by constant phase increments at regular intervals. These pulse sequences can be employed for both homonuclear and heteronuclear recoupling experiments and are robust to dispersion in chemical shifts and rf-inhomogeneity. The homonuclear pulse sequence consists of a building block [Formula: see text] , where ϕ(p)=p(n-1)πn, where n is number of blocks in a rotor period and p=0,1,2,…. The pulse sequence repeats itself every rotor period when n is odd and every two rotor period when n is even. The heteronuclear recoupling pulse sequence consists of a building block [Formula: see text] and [Formula: see text] on channel I and S, where ϕ1(p)=p(2n-3)π2n,ϕ2(p)=p(2n-1)π2n and n is number of blocks in a rotor period. The recoupling pulse sequences mix the z magnetization. Experimental quantification of this method is shown for (13)Cα-(13)CO, homonuclear recoupling in a sample of Glycine and (15)N-(13)Cα, heteronuclear recoupling in Alanine. Application of this method is demonstrated on a sample of tripeptide N-formyl-[U-(13)C,(15)N]- Met-Leu-Phe-OH (MLF).

Three pulse recoupling and phase jump matching

The paper describes a family of novel recoupling pulse sequences, called three pulse recoupling. These pulse sequences can be employed for both homonuclear and heteronuclear recoupling experiments and are robust to dispersion in chemical shifts and rf-inhomogeneity. These recoupling pulse sequences can be used in design of two-dimensional solid state NMR experiments that use powdered dephased antiphase coherence (γ preparation) to encode chemical shifts in the indirect dimension. Both components of this chemical shift encoded gamma-prepared states can be refocused into inphase coherence by a recoupling element. This helps to achieve sensitivity enhancement in 2D NMR experiments by quadrature detection.

Adiabatic Rotor-Echo-Short-Pulse-Irradiation mediated cross-polarization

We present a new dipolar recoupling method for efficient and robust heteronuclear polarization transfer in solid-state NMR under magic-angle-spinning (MAS) conditions. The method combines the recent (RESPIRATION)CP method with a modulation of the amplitude of the rotor-synchronized pulses at one of the involved rf channels through the recoupling condition. In this manner, it is possible to achieve high transfer efficiencies while maintaining robustness towards rf-field inhomogeneities and resonance offsets. The performance of the so-called adiabatic-(RESPIRATION)CP experiment is demonstrated numerically and experimentally using uniformly (13)C,(15)N-labeled samples of alanine and ubiquitin. In particular for cases with relatively high rf inhomogeneity, the scheme offers advantages over the commonly used dipolar recoupling pulse sequences.

Dual-band selective double cross polarization for heteronuclear polarization transfer between dilute spins in solid-state MAS NMR

A sinusoidal modulation scheme is described for selective heteronuclear polarization transfer between two dilute spins in double cross polarization magic-angle-spinning nuclear magnetic resonance spectroscopy. During the second N→C cross polarization, the (13)C RF amplitude is modulated sinusoidally while the (15)N RF amplitude is tangent. This modulation induces an effective spin-lock field in two selective frequency bands in either side of the (13)C RF carrier frequency, allowing for simultaneous polarization transfers from (15)N to (13)C in those two selective frequency bands. It is shown by experiments and simulations that this sinusoidal modulation allows one to selectively polarize from (15)N to its covalently bonded (13)Cα and (13)C' carbons in neighboring peptide planes simultaneously, which is useful for establishing the backbone connectivity between two sequential residues in protein structural elucidation. The selectivity and efficiency were experimentally demonstrated on a uniformly (13)C,(15)N-labeled β1 immunoglobulin binding domain of protein G (GB1).