Subhradip Paul

Supercycled homonuclear dipolar decoupling sequences in solid-state NMR

We compare the performance of the windowed phase-modulated Lee-Goldburg (wPMLG) and the windowed decoupling using mind boggling optimisation (wDUMBO) sequences at various magic-angle spinning rates and nutation frequencies of the pulses. Additionally, we introduce a supercycled version of wDUMBO and compare its efficiency with that of the non-supercycled implementation of wDUMBO. The efficiency of the supercycled version of wPMLG, denoted wPMLG-S2, is compared with a new supercycled version of wPMLG that we notate as wPMLG-S3. The interaction between the supercycled homonuclear dipolar decoupling sequences and the sample rotation is analysed using symmetry-based selection rules.

Heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance under ultra-high magic-angle spinning

We compare in this communication several heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance experiments under a magic-angle spinning frequency of 60 kHz. The decoupling radiofrequency field amplitudes considered are 190 and 10 kHz. No substantial difference was found among the sequences considered here in performance barring the difference in the optimisation protocol of the various schemes, an aspect that favours the use of swept-frequency two pulse phase modulation (SW(f)-TPPM).

Simultaneous acquisition of 13Cα–15N and 1H–15N–15N sequential correlations in proteins: application of dual receivers in 3D HNN

We describe here, adaptation of the HNN pulse sequence for multiple nuclei detection using two independent receivers by utilizing the detectable 13Cα transverse magnetization which was otherwise dephased out in the conventional HNN experiment. It enables acquisition of 2D 13Cα–15N sequential correlations along with the standard 3D 15N–15N–1H correlations, which provides directionality to sequential walk in HNN, on one hand, and enhances the speed of backbone assignment, on the other. We foresee that the implementation of dual direct detection opens up new avenues for a wide variety of modifications that would further enhance the value and applications of the experiment, and enable derivation of hitherto impossible information.

BEST-HNN and 2D-(HN)NH experiments for rapid backbone assignment in proteins

HNN has proven to be an extremely valuable experiment for rapid and unambiguous backbone (H(N), (15)N) assignment in ((13)C, (15)N) labeled proteins. However, low sensitivity of the experiment is often a limiting factor, especially when the transverse relaxation times (T(2)) are short. We show here that BEST modification Schanda et al. (2006) [2] increases the sensitivity per unit time by more than a factor of 2.0 and thus substantially increases the speed of data collection; good 3D data can be collected in 8-10h. Next, we present a simple method for amino-acid type identification based on simple 2D versions of the HNN experiment, labeled here as 2D-(HN)NH. Each of these experiments which produce anchor points for Gly, Ala, Ser/Thr residues, can be recorded in less than an hour. These enable rapid data acquisition, rapid analysis, and consequently rapid assignment of backbone (H(N), (15)N) resonances. The 2D-(HN)NH experiment does not involve aliphatic/aromatic protons and hence can be applied to deuterated protein samples as well, which is an additional advantage. The experiments have been demonstrated with human ubiquitin (76 aa) and acetic-acid denatured HIV-1 protease (99 aa), as representatives of folded and unfolded protein systems, respectively.

A Structural Model for a Self-Assembled Nanotube Provides Insight into Its Exciton Dynamics

The design and synthesis of functional self-assembled nanostructures is frequently an empirical process fraught with critical knowledge gaps about atomic-level structure in these noncovalent systems. Here, we report a structural model for a semiconductor nanotube formed via the self-assembly of naphthalenediimide-lysine (NDI-Lys) building blocks determined using experimental 13C–13C and 13C–15N distance restraints from solid-state nuclear magnetic resonance supplemented by electron microscopy and X-ray powder diffraction data. The structural model reveals a two-dimensional-crystal-like architecture of stacked monolayer rings each containing ∼50 NDI-Lys molecules, with significant π-stacking interactions occurring both within the confines of the ring and along the long axis of the tube. Excited-state delocalization and energy transfer are simulated for the nanotube based on time-dependent density functional theory and an incoherent hopping model. Remarkably, these calculations reveal efficient energy migration from the excitonic bright state, which is in agreement with the rapid energy transfer within NDI-Lys nanotubes observed previously using fluorescence spectroscopy.

Efficient heteronuclear decoupling in MAS solid-state NMR using non-rotor-synchronized rCW irradiation.

We present new non-rotor-synchronized variants of the recently introduced refocused continuous wave (rCW) heteronuclear decoupling method significantly improving the performance relative to the original rotor-synchronized variants. Under non-rotor-synchronized conditions the rCW decoupling sequences provide more efficient decoupling, are easier to setup, and prove more robust towards experimental parameters such as radio frequency (rf) field amplitude and spinning frequency. This is demonstrated through numerical simulations substantiated with experimental results under different sample spinning and rf field amplitude conditions for powder samples of U-(13)C-glycine and U-(13)C-L-histidine·HCl·H2O.

On the choice of heteronuclear dipolar decoupling scheme in solid-state NMR.

We present here a comparison of different heteronuclear dipolar decoupling sequences at the moderate magic-angle spinning (MAS) frequency (ν(r)) of 30 kHz. The radio-frequency (RF) amplitude (ν₁) ranges from the low power (ν₁2ν(r)) and includes the rotary resonance conditions (ν₁=nν(r)) where n=1, 2. For decoupling at the rotary resonance condition, we recently introduced a modification of TPPM, namely high-phase TPPM, whose properties will be discussed here. Finally, based on earlier published and current experimental results we suggest the optimal sequence for heteronuclear dipolar decoupling at any RF amplitude and MAS frequencies up to 35 kHz.

Efficient heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance at rotary resonance conditions.

We introduce here a heteronuclear dipolar decoupling scheme in solid-state nuclear magnetic resonance that performs efficiently at the rotary resonance conditions, where otherwise dipolar couplings are re-introduced. Results are shown proving the efficiency of this scheme at two magnetic fields under magic-angle spinning frequencies of 30 and 20 kHz.

r TPPM: towards improving solid-state NMR two-pulse phase-modulation heteronuclear dipolar decoupling sequence by refocusing.

We present here a simple refocused modification, r TPPM, of the Two-Pulse Phase-Modulation (TPPM) heteronuclear decoupling method, which improves decoupling and makes the sequence much more robust with respect to essential experimental parameters. The modified sequence is compared with the established TPPM sequence and a variety of other decoupling sequences at low to moderate magic-angle spinning frequencies. Simulations are shown to compare TPPM and r TPPM with respect to various experimental parameters. The observations from simulations are corroborated with experimental findings at two spinning frequencies on U-(13)C-glycine and U-(13)C-L-histidine.HCl.H2O.

1H Homonuclear dipolar decoupling using symmetry-based pulse sequences at ultra fast magic-angle spinning frequencies

We demonstrate here the application of symmetry-based pulse sequences for homonuclear dipolar decoupling in solid-state NMR at magic-angle spinning (MAS) frequencies up to 65 kHz using moderate radiofrequency (RF) amplitudes. Theoretical arguments favouring the requirement of low RF amplitudes at high MAS frequencies are given for these sequences. A comparison with wPMLGmmxx¯ is given at 65 kHz of MAS frequency to emphasise that the symmetry-based pulse sequences have a lower RF amplitude requirement at high MAS frequencies.