RF-driven and proton-driven NMR polarization transfer for investigating local order

Solid-State NMR Dipolar and Chemical Shift Anisotropy Recoupling Techniques for Structural and Dynamical Studies in Biological Systems

With the development of NMR methodology and technology during the past decades, solid-state NMR (ssNMR) has become a particularly important tool for investigating structure and dynamics at atomic scale in biological systems, where the recoupling techniques play pivotal roles in modern high-resolution MAS NMR. In this review, following a brief introduction on the basic theory of recoupling in ssNMR, we highlight the recent advances in dipolar and chemical shift anisotropy recoupling methods, as well as their applications in structural determination and dynamical characterization at multiple time scales (i.e., fast-, intermediate-, and slow-motion). The performances of these prevalent recoupling techniques are compared and discussed in multiple aspects, together with the representative applications in biomolecules. Given the recent emerging advances in NMR technology, new challenges for recoupling methodology development and potential opportunities for biological systems are also discussed.

Structure of Silk I (Bombyx mori Silk Fibroin before Spinning) -Type II β-Turn, Not α-Helix

Recently, considerable attention has been paid to Bombyx mori silk fibroin by a range of scientists from polymer chemists to biomaterial researchers because it has excellent physical properties, such as strength, toughness, and biocompatibility. These appealing physical properties originate from the silk fibroin structure, and therefore, structural determinations of silk fibroin before (silk I) and after (silk II) spinning are a key to make wider applications of silk. There are discrepancies about the silk I structural model, i.e., one is type II β-turn structure determined using many solid-state and solution NMR spectroscopies together with selectively stable isotope-labeled model peptides, but another is α-helix or partially α-helix structure speculated using IR and Raman methods. In this review, firstly, the process that led to type II β-turn structure by the authors was introduced in detail. Then the problems in speculating silk I structure by IR and Raman methods were pointed out together with the problem in the assignment of the amide I band in the spectra. It has been emphasized that the conformational analyses of proteins and peptides from IR and Raman studies are not straightforward and should be very careful when the proteins contain β-turn structure using many experimental data by Vass et al. In conclusion, the author emphasized here that silk I structure should be type II β-turn, not α-helix.

Effective Hamiltonian and 1H-14N cross polarization/double cross polarization at fast MAS

In this work we investigate in detail the underlying spin-dynamics associated with ¹H-¹⁴N double CP experiments under fast MAS, recently demonstrated by Carnevale et al. We employ matrix logarithm and Floquet theory to compute numerically the effective Hamiltonian associated to the time-dependent problem. Certain common features related to construction of effective Hamiltonians by both approaches are discussed. The main observations related to ¹H-¹⁴N CPMAS/double CP transfer are: (a) various spin terms of the effective Hamiltonian strongly depend on the crystallite orientation; (b) significant CP transfer occurs only when the magnitudes of the effective¹H and ¹⁴N RF strengths are comparable, and simultaneously all pure ¹⁴N terms in the effective Hamiltonian are small, except for the longitudinal and the RF terms; (c) the sign of ¹⁴N CPMAS signal follows the sign of ¹⁴N effective RF strength; (d) sign of the double CP signal is largely independent of crystallite orientation. We predict and verify matching conditions employing multiples of the spinning frequency or involving different ¹⁴N RF strengths. We provide an analytical proof for (d). The proof also provides an estimate for the ratio of ¹H-¹⁴N and ¹⁴N-¹H transfer amplitudes which is further substantiated through simulations. In addition, we find that double CP signals include contributions from several single-quantum coherences present after the first CP process. The uneven contribution from different coherences leads to a reversal of signal at very short contact times, a feature noted experimentally by Carnevale et al. The connection between CPMAS transfer and efficient spin-lock is discussed and illustrated. The factors affecting second-order quadrupolar lineshapes in double CP experiment are examined. With a linear ramp of ¹H RF amplitude we have observed that significant CP transfer occurs for more crystallite orientations resulting in improved sensitivity.

A master-equation approach to the description of proton-driven spin diffusion from crystal geometry using simulated zero-quantum lineshapes

Measurements of proton-driven carbon-13 spin diffusion (PDSD) by NMR spectroscopy are a central component of structural analyses of biomolecules in the solid-state. However, the quantitative link between experimental PDSD data and structural information is difficult to make. Here we observe that a master-equation approach can be used to model full PDSD dynamics accurately in polycrystalline (13)C-labelled L-histidine·HCl·H(2)O under magic-angle spinning. In the master-equation approach, PDSD rates and effective dipolar couplings are related by a function of the carbon-carbon zero-quantum lineshapes; we find that numerical simulations of the zero-quantum lineshapes are sufficiently accurate so as to allow the calculation of PDSD rates that are in good agreement with the measured rates, directly from crystal geometry and with no adjustable parameters. Finally, using carbon-carbon internuclear distances we illustrate the potential of the master-equation approach for structural studies. Generalisation of these results to proton-driven carbon-13 spin diffusion in more complex molecular systems is readily envisaged.

Two-dimensional homonuclear chemical shift correlation established by the cross-relaxation driven spin diffusion in solids

A new type of spin diffusion, cross-relaxation driven spin diffusion (CRDSD), is investigated using (15)N NMR on a N-acetyl-L-valyl-L-leucine (NAVL) single crystal under stationary condition. A two-dimensional (2D) pulse sequence that correlates the chemical shifts of (15)N nuclei, with a radio-frequency spin lock on the (15)N channel during the mixing time, is used to observe CRDSD. Experimental results obtained using CRDSD, rf-driven spin diffusion, and proton driven spin diffusion approaches on the NAVL single crystal are compared. Our experimental results suggest that the (15)N spin diffusion rate can be enhanced by about 1000 times using CRDSD than by the normal proton driven spin diffusion. Interestingly, the required spin-locking rf field strength for CRDSD is much lower than that used for the rf-driven spin diffusion experiments. The cross-peak patterns observed in 2D (15)N-(15)N correlation spectra using CRDSD and RFDSD are very different as they arise from different spin-spin interactions. A detailed theory describing CRDSD and RFDSD processes is also presented using a thermodynamic model. The speedy spin diffusion process rendered by the CRDSD approach will be useful to assign resonances from a uniformly (15)N or (13)C labeled proteins and peptides, particularly in aligned samples.

The stuffed framework structure of SrP2N4: challenges to synthesis and crystal structure determination

SrP2N4 was obtained by high-pressure high-temperature synthesis utilizing the multianvil technique (5 GPa, 1400 degrees C) starting from mixtures of phosphorus(V) nitride and strontium azide. SrP2N4 turned out to be isotypic with BaGa(2)O(4) and is closely related to KGeAlO(4). The crystal structure (SrP2N4, a=17.1029(8), c=8.10318(5) A, space group P6(3) (no. 173), V=2052.70(2) A3, Z=24, R(F2)=0.0633) was solved from synchrotron powder diffraction data by applying a combination of direct methods, Patterson syntheses, and difference Fourier maps adding the unit cell information derived from electron diffraction and symmetry information obtained from 31P solid-state NMR spectroscopy. The structure of SrP2N4 was refined by the Rietveld method by utilizing both neutron and synchrotron X-ray powder diffraction data, and has been corroborated additionally by 31P solid-state NMR spectroscopy by employing through-bond connectivities and distance relations.

A DOQSY approach for the elucidation of torsion angle distributions in biopolymers: application to silk

Silk from the wild silkworm Samia cynthia ricini with a molecular mass of about 300kDa consists of alternating repeats of nano-crystalline poly-(Ala) and non-crystalline glycine-rich domains. The backbone torsion angles between pairs of these two amino acids is determined by DOQSY solid-state NMR spectroscopy: the alanine-rich domains are predominantly in a beta-sheet conformation, whereas the glycine-rich domains are found to be partially in an extended beta-sheet conformation and partially in an approximately 3(1)-helical conformation. In the cast film from liquid silk significantly different secondary structures were found: the alanine-rich domains are alpha-helical conformation, whereas the results for glycine-labelled sample are explained by a random-coil state. A detailed error analysis of the technique is presented.

Microscopic Description of the Polyamorphic Phases of Triphenyl Phosphite by Means of Multidimensional Solid-State NMR Spectroscopy

The structural properties of a second, apparently amorphous phase (aII) of the molecular glass former triphenyl phosphite were studied by means of multidimensional solid-state NMR spectroscopy and X-ray diffraction. Phase aII was prepared by annealing the supercooled liquid in the temperature range 210 K <or= T(a) <or= 230 K. In addition to 1D (1)H and (31)P spectra and spin-lattice relaxation data, we used (31)P radio-frequency-driven spin-diffusion exchange spectroscopy to analyze the arrangement of neighboring TPP molecules on both a local and intermediate scale. For the first time, our results give a detailed microscopic description of phase aII. For T(a) > 223 K a nano- or microcrystalline material is formed, whereas for T(a) < 223 K phase aII is homogeneous and disordered. Our data strongly suggest that some of the TPP molecules in phase aII tend toward a parallel alignment. The regions, where the molecules preferentially align appear to be spatially separated and consist of only a few molecules. Whereas the mean cubic expansion of an individual region does not change within the experimental error, the percentage of correlated molecules increases with rising T(a). Based on our results, phase aII can consistently be described as a second liquid, where a part of the molecules exhibit structural correlations. The transformation of the supercooled liquid into phase aII is therefore considered as a liquid-liquid phase transition.

Analysis of dipolar-coupling-mediated coherence transfer in a homonuclear two spin-12 solid-state system

Homonuclear dipolar-mediated coherence transfer (DCT), a through-space transfer of magnetization between like spins, can yield otherwise difficult-to-obtain structural information for macromolecules by measuring the internuclear distances between two sites of interest. The behavior of a spin-12 system under DCT is analyzed in detail by computing the time development of the density matrix using the product operator formalism. The effect of coherence transfer (CT) via the homonuclear isotropic scalar coupling on DCT is examined. Analytical and computational results that yield useful information on the frequencies, first-maxima, and first-zero of CT for a uniaxially oriented or a single-crystal solid-state system are presented. The results predict that the evolution of the spin angular momentum operators under the homonuclear dipolar coupling Hamiltonian leads to "cylindrical mixing" unlike "isotropic mixing" due to the strong scalar coupling Hamiltonian. These results will find relevance in both the design of RF pulse sequences for the structural studies of uniaxially oriented biological solids and the interpretation of solution NMR results from proteins embedded in partially oriented bicelles. Copyright 1999 Academic Press.

Trans content in atactic polystyrene estimated by double-quantum solid-state NMR

The conformational statistics of atactic polystyrene have been investigated by solid-state NMR, on a sample labeled with 25% 13CH2 groups. Double-quantum pulse sequences are used to select statistically formed 13C-13C spin pairs. An experiment correlating the 13C-13C internuclear vector with the 13C chemical-shift anisotropy determines the conformational statistics of a single backbone torsion angle. Double-quantum experiments correlating the 13CH2 chemical-shift anisotropies of two adjacent segments provide information on two consecutive torsion angles. The analysis of the spectra yields a trans content of 68% (+/-10%). This represents the first direct experimental estimate of the trans/gauche ratio in atactic polystyrene. The trans content found here agrees well with rotational-isomeric-state models but is inconsistent with results from more elaborate atomistic models that take packing effects into account.

Radiofrequency-Driven and Slow-Magic-Angle-Sample-Spinning Polarization-Transfer Techniques: A Comparative Study

The rate constant of radiofrequency-driven (RF-driven) polarization transfer and that of polarization transfer under slow-magic-angle sample spinning (S-MAS) are compared using a model system, polycrystalline alpha-alpha'-13C2-phthalic acid. While the rate constant under RF irradiation in static samples strongly depends on the orientation of the internuclear vector, the rate constant under S-MAS is hardly sensitive to that orientation and, thus, depends almost exclusively on the internuclear distance. Consequently, polarization-transfer rate constants obtained under S-MAS can be interpreted more simply when used to study local order in polycrystalline or amorphous solids. Copyright 1998 Academic Press.