Assignment of the nonexchanging protons of the alpha-spectrin SH3 domain by two- and three-dimensional 1H-13C solid-state magic-angle spinning NMR and comparison of solution and solid-state proton chemical shifts

Protein Motional Details Revealed by Complementary Structural Biology Techniques

Proteins depend on defined molecular plasticity for their functionality. How to comprehensively capture dynamics correctly is of ubiquitous biological importance. Approaches commonly used to probe protein dynamics include model-free elucidation of site-specific motion by NMR relaxation, molecular dynamics (MD)-based approaches, and capturing the substates within a dynamic ensemble by recent eNOE-based multiple-structure approaches. Even though MD is sometimes combined with ensemble-averaged NMR restraints, these approaches have largely been developed and used individually. Owing to the different underlying concepts and practical requirements, it has remained unclear how they compare, and how they cross-validate and complement each other. Here, we extract and compare the differential information contents of MD simulations, NMR relaxation measurements, and eNOE-based multi-state structures for the SH3 domain of chicken α-spectrin. The data show that a validated, consistent, and detailed picture is feasible both for timescales and actual conformational states sampled in the dynamic ensemble. This includes the biologically important side-chain plasticity, for which experimentally cross-validated assessment is a significant challenge.

Sensitivity and resolution of proton detected spectra of a deuterated protein at 40 and 60 kHz magic-angle-spinning

The use of small rotors capable of very fast magic-angle spinning (MAS) in conjunction with proton dilution by perdeuteration and partial reprotonation at exchangeable sites has enabled the acquisition of resolved, proton detected, solid-state NMR spectra on samples of biological macromolecules. The ability to detect the high-gamma protons, instead of carbons or nitrogens, increases sensitivity. In order to achieve sufficient resolution of the amide proton signals, rotors must be spun at the maximum rate possible given their size and the proton back-exchange percentage tuned. Here we investigate the optimal proton back-exchange ratio for triply labeled SH3 at 40 kHz MAS. We find that spectra acquired on 60 % back-exchanged samples in 1.9 mm rotors have similar resolution at 40 kHz MAS as spectra of 100 % back-exchanged samples in 1.3 mm rotors spinning at 60 kHz MAS, and for (H)NH 2D and (H)CNH 3D spectra, show 10-20 % higher sensitivity. For 100 % back-exchanged samples, the sensitivity in 1.9 mm rotors is superior by a factor of 1.9 in (H)NH and 1.8 in (H)CNH spectra but at lower resolution. For (H)C(C)NH experiments with a carbon-carbon mixing period, this sensitivity gain is lost due to shorter relaxation times and less efficient transfer steps. We present a detailed study on the sensitivity of these types of experiments for both types of rotors, which should enable experimentalists to make an informed decision about which type of rotor is best for specific applications.

Assignment strategies for aliphatic protons in the solid-state in randomly protonated proteins

Biological solid-state nuclear magnetic resonance spectroscopy developed rapidly in the past two decades and emerged as an important tool for structural biology. Resonance assignment is an essential prerequisite for structure determination and the characterization of motional properties of a molecule. Experiments, which rely on carbon or nitrogen detection, suffer, however, from low sensitivity. Recently, we introduced the RAP (Reduced Adjoining Protonation) labeling scheme, which allows to detect backbone and sidechain protons with high sensitivity and resolution. We present here a (1)H-detected 3D (H)CCH experiment for assignment of backbone and sidechain proton resonances. Resolution is significantly improved by employing simultaneous (13)CO and (13)Cβ J-decoupling during evolution of the (13)Cα chemical shift. In total, ~90% of the (1)Hα-(13)Cα backbone resonances of chicken α-spectrin SH3 could be assigned.

Sum frequency generation and solid-state NMR study of the structure, orientation, and dynamics of polystyrene-adsorbed peptides

The power of combining sum frequency generation (SFG) vibrational spectroscopy and solid-state nuclear magnetic resonance (ssNMR) spectroscopy to quantify, with site specificity and atomic resolution, the orientation and dynamics of side chains in synthetic model peptides adsorbed onto polystyrene (PS) surfaces is demonstrated in this study. Although isotopic labeling has long been used in ssNMR studies to site-specifically probe the structure and dynamics of biomolecules, the potential of SFG to probe side chain orientation in isotopically labeled surface-adsorbed peptides and proteins remains largely unexplored. The 14 amino acid leucine-lysine peptide studied in this work is known to form an alpha-helical secondary structure at liquid-solid interfaces. Selective, individual deuteration of the isopropyl group in each leucine residue was used to probe the orientation and dynamics of each individual leucine side chain of LKalpha14 adsorbed onto PS. The selective isotopic labeling methods allowed SFG analysis to determine the orientations of individual side chains in adsorbed peptides. Side chain dynamics were obtained by fitting the deuterium ssNMR line shape to specific motional models. Through the combined use of SFG and ssNMR, the dynamic trends observed for individual side chains by ssNMR have been correlated with side chain orientation relative to the PS surface as determined by SFG. This combination provides a more complete and quantitative picture of the structure, orientation, and dynamics of these surface-adsorbed peptides than could be obtained if either technique were used separately.

Determination of protein structures in the solid state from NMR chemical shifts

Solid-state NMR spectroscopy does not require proteins to form crystalline or soluble samples and can thus be applied under a variety of conditions, including precipitates, gels, and microcrystals. It has recently been shown that NMR chemical shifts can be used to determine the structures of the native states of proteins in solution. By considering the cases of two proteins, GB1 and SH3, we provide an initial demonstration here that this type of approach can be extended to the use of solid-state NMR chemical shifts to obtain protein structures in the solid state without the need for measuring interatomic distances.

Practical aspects of Lee-Goldburg based CRAMPS techniques for high-resolution 1H NMR spectroscopy in solids: implementation and applications

Elucidating the local environment of the hydrogen atoms is an important problem in materials science. Because (1)H spectra in solid-state nuclear magnetic resonance (NMR) suffer from low resolution due to homogeneous broadening, even under magic-angle spinning (MAS), information of chemical interest may only be obtained using certain high-resolution (1)H MAS techniques. (1)H Lee-Goldburg (LG) CRAMPS (Combined Rotation And Multiple-Pulse Spectroscopy) methods are particularly well suited for studying inorganic-organic hybrid materials, rich in (1)H nuclei. However, setting up CRAMPS experiments is time-consuming and not entirely trivial, facts that have discouraged their widespread use by materials scientists. To change this status quo, here we describe and discuss some important aspects of the experimental implementation of CRAMPS techniques based on LG decoupling schemes, such as FSLG (Frequency Switched), and windowed and windowless PMLG (Phase Modulated). In particular, we discuss the influence on the quality of the (1)H NMR spectra of the different parameters at play, for example LG (Lee-Goldburg) pulses, radio-frequency (rf) phase, frequency switching, and pulse imperfections, using glycine and adamantane as model compounds. The efficiency and robustness of the different LG-decoupling schemes is then illustrated on the following materials: organo-phosphorus ligand, N-(phosphonomethyl)iminodiacetic acid [H(4)pmida] [I], and inorganic-organic hybrid materials (C(4)H(12)N(2))[Ge(2)(pmida)(2)OH(2)] x 4H(2)O [II] and (C(2)H(5)NH(3))[Ti(H(1.5)PO(4))(PO(4))](2) x H(2)O [III].

Multidimensional solid state NMR of anisotropic interactions in peptides and proteins

Accurate determinations of chemical shift anisotropy (CSA) tensors are valuable for NMR of biological systems. In this review we describe recent developments in CSA measurement techniques and applications, particularly in the context of peptides and proteins. These techniques include goniometeric measurements of single crystals, slow magic-angle spinning studies of powder samples, and CSA recoupling under moderate to fast MAS. Experimental CSA data can be analyzed by comparison with ab initio calculations for structure determination and refinement. This approach has particularly high potential for aliphatic (13)C analysis, especially Calpha tensors which are directly related to structure. Carbonyl and (15)N CSA tensors demonstrate a more complex dependence upon hydrogen bonding and electrostatics, in addition to conformational dependence. The improved understanding of these tensors and the ability to measure them quantitatively provide additional opportunities for structure determination, as well as insights into dynamics.

Solid-state NMR study and assignments of the KcsA potassium ion channel of S. lividans

The extraordinary efficiency and selectivity of potassium channels have made them ideal systems for biophysical and functional studies of ion conduction. We carried out solid-state NMR studies of the selectivity filter region of the protein. Partial site-specific assignments of the NMR signals were obtained based on high field multidimensional solid-state NMR spectra of uniformly (13)C, (15)N enriched KcsA potassium channel from Streptomyces lividans. Both backbone and sidechain atoms were assigned for residues V76-D80 and P83-L90, in and near the selectivity filter region of the protein; this region exhibits good dispersion and useful chemical shift fingerprints. This study will enable structure, dynamic and mechanistic studies of ion conduction by NMR.

Proton-detected solid-state NMR spectroscopy of fully protonated proteins at 40 kHz magic-angle spinning

Remarkable progress in solid-state NMR has enabled complete structure determination of uniformly labeled proteins in the size range of 5-10 kDa. Expanding these applications to larger or mass-limited systems requires further improvements in spectral sensitivity, for which inverse detection of 13C and 15N signals with 1H is one promising approach. Proton detection has previously been demonstrated to offer sensitivity benefits in the limit of sparse protonation or with approximately 30 kHz magic-angle spinning (MAS). Here we focus on experimental schemes for proteins with approximately 100% protonation. Full protonation simplifies sample preparation and permits more complete chemical shift information to be obtained from a single sample. We demonstrate experimental schemes using the fully protonated, uniformly 13C,15N-labeled protein GB1 at 40 kHz MAS rate with 1.6-mm rotors. At 500 MHz proton frequency, 1-ppm proton line widths were observed (500 +/- 150 Hz), and the sensitivity was enhanced by 3 and 4 times, respectively, versus direct 13C and 15N detection. The enhanced sensitivity enabled a family of 3D experiments for spectral assignment to be performed in a time-efficient manner with less than a micromole of protein. CANH, CONH, and NCAH 3D spectra provided sufficient resolution and sensitivity to make full backbone and partial side-chain proton assignments. At 750 MHz proton frequency and 40 kHz MAS rate, proton line widths improve further in an absolute sense (360 +/- 115 Hz). Sensitivity and resolution increase in a better than linear manner with increasing magnetic field, resulting in 14 times greater sensitivity for 1H detection relative to that of 15N detection.

High resolution 1H detected 1H,13C correlation spectra in MAS solid-state NMR using deuterated proteins with selective 1H,2H isotopic labeling of methyl groups

MAS solid-state NMR experiments applied to biological solids are still hampered by low sensitivity and resolution. In this work, we employ a deuteration scheme in which individual methyl groups are selectively protonated. This labeling scheme allows the acquisition of proton carbon correlation spectra with a resolution comparable to that in solution-state NMR experiments. We observe an increase in resolution by a factor of 10-15 compared to standard heteronuclear correlation experiments using PMLG for 1H,1H dipolar decoupling in the indirect dimension. At the same time, the full sensitivity of the proton-based experiment is retained. In comparison to the heteronuclear detected version of the experiment, a gain in sensitivity of a factor of approximately 4.7 is achieved.

Combined solid state and solution NMR studies of alpha,epsilon-15N labeled bovine rhodopsin

Rhodopsin is the visual pigment of the vertebrate rod photoreceptor cell and is the only member of the G protein coupled receptor family for which a crystal structure is available. Towards the study of dynamics in rhodopsin, we report NMR-spectroscopic investigations of alpha,epsilon-15N-tryptophan labeled rhodopsin in detergent micelles and reconstituted in phospholipids. Using a combination of solid state 13C,15N-REDOR and HETCOR experiments of all possible 13C'(i-1) carbonyl/15N(i)-tryptophan isotope labeled amide pairs, and H/D exchange 1H,15N-HSQC experiments conducted in solution, we assigned chemical shifts to all five rhodopsin tryptophan backbone 15N nuclei and partially to their bound protons. 1H,15N chemical shift assignment was achieved for indole side chains of Trp35(1.30) and Trp175(4.65). 15N chemical shifts were found to be similar when comparing those obtained in the native like reconstituted lipid environment and those obtained in detergent micelles for all tryptophans except Trp175(4.65) at the membrane interface. The results suggest that the integrated solution and solid state NMR approach presented provides highly complementary information in the study of structure and dynamics of large membrane proteins like rhodopsin.

Characterization of microporous aluminophosphate IST-1 using (1)H Lee-Goldburg techniques

The presence of two independent methylamine species in microporous aluminophosphate IST-1 (|(CH(3)NH(2))(4)(CH(3)NH(+)(3))(4)(OH(-))(4)|[Al(12)P(12)O(48)]) has been shown previously by synchrotron powder X-ray diffraction. One of these species, [N(1)-C(1)], links to a six-coordinated framework Al-atom [Al(1)], while the other methylamine [N(2)-C(2)] is protonated and hydrogen-bonded to three O-atoms [O(1), O(2) and O(12)]. We revisit the structure of IST-1 and report the complete assignment of the (1)H NMR spectra by combining X-ray data and high-resolution heteronuclear/homonuclear solid-state NMR techniques based on frequency-switched Lee-Goldburg homonuclear decoupling and (31)P-(31)P homonuclear recoupling. Careful analysis of the 2D (1)H-X homonuclear correlation (X=(1)H) and 2D heteronuclear correlation (X=(13)C, (31)P and (27)Al) spectra allowed the distinction of both methylamine species and the assignment of all (31)P and (13)C resonances. For the first time at a relatively high (9.4 T) magnetic field, symmetric doublet patterns have been observed in the (13)C spectra, caused by the influence of the (14)N second-order quadrupolar interaction.

X-ray Diffraction and Solid-State NMR Studies of a Germanium Binuclear Complex

A compound formulated as (C4H12N2)[Ge2(pmida)2(OH)2] x 4 H2O (where pmida(4-) = N-(phosphonomethyl)iminodiacetate and C4H12N2(2+) = piperazinedium cation), containing the anionic [Ge2(pmida)2(OH)2]2- complex, has been synthesised by the hydrothermal approach and its structure determined by single-crystal X-ray diffraction analysis. Several high-resolution solid-state magic-angle spinning (MAS) NMR techniques, in particular two-dimensional 1H-X(13C,31P) heteronuclear correlation (HETCOR) and 1H-1H homonuclear correlation (HOMCOR) experiments incorporating a frequency-switched Lee-Goldburg (FS-LG) decoupling scheme, have been employed for the first time in such a material. Using these tools in tandem affords an excellent general approach to study the structure of other inorganic-organic hybrids. We assigned the NMR resonances with the help of C...H and P...H internuclear distances obtained through systematic statistical analyses of the crystallographic data. The compound was further characterised by powder X-ray diffraction techniques, IR and Raman spectroscopy, and by elemental and thermal analyses (thermogravimetric analysis and differential scanning calorimetry).

Theory of heteronuclear decoupling in solid-state nuclear magnetic resonance using multipole-multimode Floquet theory

A formal theory for heteronuclear decoupling in solid-state magic angle spinning (MAS) nuclear magnetic resonance experiments is presented as a first application of multipole-multimode Floquet theory. The method permits a straightforward construction of the multispin basis and describes the spin dynamics via effective Floquet Hamiltonians obtained using the van Vleck transformation method in the Floquet-Liouville space. As a test case, we consider a model three-spin system (I2S) under asynchronous time modulations (both MAS and rf irradiation) and derive effective Hamiltonians for describing the spin dynamics in the Floquet-Liouville space during heteronuclear decoupling. Furthermore, we describe and evaluate the origin of cross terms between the various anisotropic interactions and illustrate their exact contributions to the spin dynamics. The theory presented herein should be applicable to the design and understanding of pulse sequences for heteronuclear and homonuclear recoupling and decoupling.

3D NMR spectroscopy for resonance assignment and structure elucidation of proteins under MAS: novel pulse schemes and sensitivity considerations

Two types of 3D MAS NMR experiments are introduced, which combine standard (NC,CC) transfer schemes with (1H,1H) mixing to simultaneously detect connectivities and structural constraints of uniformly 15N,13C-labeled proteins with high spectral resolution. The homonuclear CCHHC and CCC experiments are recorded with one double-quantum evolution dimension in order to avoid a cubic diagonal in the spectrum. Depending on the second transfer step, spin systems or proton-proton contacts can be determined with reduced spectral overlap. The heteronuclear NHHCC experiment encodes NH-HC proton-proton interactions, which are indicative for the backbone conformation of the protein. The third dimension facilitates the identification of the amino acid spin system. Experimental results on U-[15N,13C]valine and U-[15N,13C]ubiquitin demonstrate their usefulness for resonance assignments and for the determination of structural constraints. Furthermore, we give a detailed analysis of alternative multidimensional sampling schemes and their effect on sensitivity and resolution.

The alignment, structure and dynamics of membrane-associated polypeptides by solid-state NMR spectroscopy

Solid-state NMR spectroscopy is being developed at a fast pace for the structural investigation of immobilized and non-crystalline biomolecules. These include proteins and peptides associated with phospholipid bilayers. In contrast to solution NMR spectroscopy, where complete or almost complete averaging leads to isotropic values, the anisotropic character of nuclear interactions is apparent in solid-state NMR spectra. In static samples the orientation dependence of chemical shift, dipolar or quadrupolar interactions, therefore, provides angular constraints when the polypeptides have been reconstituted into oriented membranes. Furthermore, solid-state NMR spectroscopy of aligned samples offers distinct advantages in allowing access to dynamic processes such as topological equilibria or rotational diffusion in membrane environments. Alternatively, magic angle sample spinning (MAS) results in highly resolved NMR spectra, provided that the sample is sufficiently homogenous. MAS spinning solid-state NMR spectra allow to measure distances and dihedral angles with high accuracy. The technique has recently been developed to selectively establish through-space and through-bond correlations between nuclei, similar to the approaches well-established in solution-NMR spectroscopy.

Assignments of Carbon NMR Resonances for Microcrystalline Ubiquitin

Solid-state NMR 2D spectroscopy was used to correlate carbon backbone and side-chain chemical shifts for uniformly (13)C,(15)N-enriched microcrystalline ubiquitin. High applied field strengths, 800 MHz for protons, moderate proton decoupling fields, 80-100 kHz, and high magic angle sample spinning frequencies, 20 kHz, were used to narrow the most of the carbon line widths to 0.5-0.8 ppm. Homonuclear magnetization transfer was effected by matching the proton RF field to the spinning frequency, the so-called dipolar-assisted rotational resonance (DARR) (Takegoshi, K.; Nakamura, S.; Terao, T. Chem. Phys. Lett. 2001, 344, 631-637), and a mixing time of 20 ms was used to maximize the intensity of one-bond transfers between carbon atoms. This polarization transfer sequence resulted in roughly 14% transfer efficiencies for directly bonded carbon pairs and 4% transfer efficiencies for carbons separated by a third carbon. With this simple procedure, the majority of the one-bond correlations was observed with moderate transfer efficiencies, and many two-bond correlations were also observed with weaker intensities. Spin systems could be identified for more than half of the amino acid side chains, and site-specific assignments were readily possible via comparison with 400 MHz (15)N-(13)C-(13)C correlation spectroscopy (described separately).

Assignment of the Backbone Resonances for Microcrystalline Ubiquitin

Site-specific assignments for the solid-state NMR spectra of uniformly 13C,15N-enriched ubiquitin are described. The assignments are derived from three three-dimensional 15N-13C-13C correlation spectra collected at 400 MHz on microcrystalline material. A few residues (the loop near Threonine 9 and the C-terminal fragment) were missing and correspond to regions previously reported to be mobile on the basis of X-ray crystallography and solution NMR studies. A few additional sites exhibit shifts that differ from previously reported solution NMR assignments. Nonetheless, these de novo assignments indicate close agreement between the chemical shifts observed in solution and those in microcrystalline or precipitated solids. The methods utilized are likely to be generally applicable for other noncrystalline, nonsoluble proteins.

NMR Spectra of a Microcrystalline Protein at 30 kHz MAS

Proteins are not always available in amounts desirable for solid-state magic-angle spinning (MAS) nuclear-magnetic resonance (NMR) spectroscopy. To maximize the signal-to-noise ratio achievable with small samples, the filling factor must be optimized by using small-diameter MAS rotors. These rotors have the added benefit of allowing higher radio frequency field amplitudes during polarization transfer steps and during decoupling periods as well as allowing higher spinning frequencies. We demonstrate the advantages of relatively fast MAS (30 kHz using a 2.5 mm rotor) compared to MAS at 12 kHz for the 10.4 kDa model protein Crh with 93 residues and show that the signal-to-noise ratio in two-dimensional correlation spectra can be significantly improved by taking advantage of optimized pulse sequences available with rapid MAS.

Preparation of protein nanocrystals and their characterization by solid state NMR

Preparation of proteins in their crystalline state has been found to be important in producing stable therapeutic protein formulations, cross-linked enzyme crystals for application in industrial processes, generating novel porous media for separations, and of course in structure elucidation. Of these applications only X-ray crystallography requires large crystals, defined here as being crystals 100s of microns or greater in size. Smaller crystals have attractive attributes in many instances, and are just as useful in structure determination by solid state NMR (ssNMR) as are large crystals. In this paper we outline a simple set of procedures for preparing nanocrystalline protein samples for ssNMR or other applications and describe the characterization of their crystallinity by ssNMR and X-ray powder diffraction. The approach is demonstrated in application to five different proteins: ubiquitin, lysozyme, ribonuclease A, streptavidin, and cytochrome c. In all instances the nanocrystals produced are found to be highly crystalline as judged by natural abundance 13C ssNMR and optical and electron microscopy. We show for ubiquitin that nanocrystals prepared by rapid batch crystallization yield equivalent 13C ssNMR spectra to those of larger X-ray diffraction quality crystals. Single crystal and powder X-ray diffraction measurements are made to compare the degree of order present in polycrystalline, nanocrystalline, and lyophilized ubiquitin. Solid state 13C NMR is also used to show that ubiquitin nanocrystals are thermally robust, giving no indication of loss of local order after repeated temperature cycling between liquid nitrogen and room temperature. The methods developed are rapid and should scale well from the tenths of milligram to multi-gram scales, and as such should find wide utility in the preparation of protein nanocrystals for applications in catalysis, separations, and especially in sample preparation for structural studies using ssNMR.

Water−Protein Interactions in Microcrystalline Crh Measured by 1H−13C Solid-State NMR Spectroscopy

Using solid-state NMR carbon-proton dipolar correlation spectroscopy, we observed hydrogen exchange on the millisecond time scale between water molecules and protein protons in a solid sample. These interactions are shown to be related to important structural features of the protein such as hydrogen-bonding or salt-bridge networks.

Band-selective recoupling of homonuclear double-quantum dipolar interaction with a generalized composite 0 degrees pulse: application to 13C aliphatic region-selective magnetization transfer in solids

Recoupling of homonuclear double quantum (DQ)-dipolar interactions is a useful technique for the structural analysis of molecules in solids. We have designed a series of elemental 0 degrees pulses for the recoupling sequences with the rf phase rotation about the z-axis, known as CN. The proposed 0 degrees pulses whose total flip angle >/=360 degrees provide spin rotation vectors in the xy-plane. Thus, the residual spin rotation can be canceled by rf phase rotation about the z-axis. An analysis by the coherent averaging theory showed that effective bandwidths of the recoupling sequences are limited not by the reduction in the dipolar scaling factor but by the increase in the residual spin rotation due to offset. A CN sequence with these elemental pulses provides an effective bandwidth of DQ-dipolar recoupling from ca. 0.5nu(R) to 4nu(R) for numerical simulations. Here, nu(R) is the sample spinning frequency. The 0 degrees pulses were applied to band-selective recoupling for the magnetization transfer in uniformly 13C-labeled molecules. Narrow-band recoupling enhances the magnetization transfer between spins within the effective range by decoupling the dipolar interactions between spins one of which is outside the range. The narrow band operation reduces rf field strength, which improves the CH decoupling. Increases in signal intensities by the use of the proposed 0 degrees pulses are experimentally shown for 13C-labeled amino acids.

Structure of a protein determined by solid-state magic-angle-spinning NMR spectroscopy

The determination of a representative set of protein structures is a chief aim in structural genomics. Solid-state NMR may have a crucial role in structural investigations of those proteins that do not easily form crystals or are not accessible to solution NMR, such as amyloid systems or membrane proteins. Here we present a protein structure determined by solid-state magic-angle-spinning (MAS) NMR. Almost complete (13)C and (15)N resonance assignments for a micro-crystalline preparation of the alpha-spectrin Src-homology 3 (SH3) domain formed the basis for the extraction of a set of distance restraints. These restraints were derived from proton-driven spin diffusion (PDSD) spectra of biosynthetically site-directed, labelled samples obtained from bacteria grown using [1,3-(13)C]glycerol or [2-(13)C]glycerol as carbon sources. This allowed the observation of long-range distance correlations up to approximately 7 A. The calculated global fold of the alpha-spectrin SH3 domain is based on 286 inter-residue (13)C-(13)C and six (15)N-(15)N restraints, all self-consistently obtained by solid-state MAS NMR. This MAS NMR procedure should be widely applicable to small membrane proteins that can be expressed in bacteria.

3D TEDOR NMR experiments for the simultaneous measurement of multiple carbon-nitrogen distances in uniformly (13)C,(15)N-labeled solids

We describe three-dimensional magic-angle-spinning NMR experiments for the simultaneous measurement of multiple carbon-nitrogen distances in uniformly (13)C,(15)N-labeled solids. The approaches employ transferred echo double resonance (TEDOR) for (13)C-(15)N coherence transfer and (15)N and (13)C frequency labeling for site-specific resolution, and build on several previous 3D TEDOR techniques. The novel feature of the 3D TEDOR pulse sequences presented here is that they are specifically designed to circumvent the detrimental effects of homonuclear (13)C-(13)C J-couplings on the measurement of weak (13)C-(15)N dipolar couplings. In particular, homonuclear J-couplings lead to two undesirable effects: (i) they generate anti-phase and multiple-quantum (MQ) spin coherences, which lead to spurious cross-peaks and phase-twisted lines in the 2D (15)N-(13)C correlation spectra, and thus degrade the spectral resolution and prohibit the extraction of reliable cross-peak intensities, and (ii) they significantly reduce cross-peak intensities for strongly J-coupled (13)C sites (e.g., CO and C(alpha)). The first experiment employs z-filter periods to suppress the anti-phase and MQ coherences and generates 2D spectra with purely absorptive peaks for all TEDOR mixing times. The second approach uses band-selective (13)C pulses to refocus J-couplings between (13)C spins within the selective pulse bandwidth and (13)C spins outside the bandwidth. The internuclear distances are extracted by using a simple analytical model, which accounts explicitly for multiple spin-spin couplings contributing to cross-peak buildup. The experiments are demonstrated in two U-(13)C,(15)N-labeled peptides, N-acetyl-L-Val-L-Leu (N-ac-VL) and N-formyl-L-Met-L-Leu-L-Phe (N-f-MLF), where 20 and 26 (13)C-(15)N distances up to approximately 5-6 A were measured, respectively. Of the measured distances, 10 in N-ac-VL and 13 in N-f-MLF are greater than 3 A and provide valuable structural constraints.

A study of a moleculartweezer host-guest system by a combination of quantum-chemical calculations and solid-state NMR experiments

A study of a host-guest system consisting of a naphthalene-spaced tweezer with a 1,4 dicyanobenzene guest molecule is presented. The complex is investigated using a combination of quantum-chemical calculations and solid-state NMR experiments. The advantages of such an approach are illustrated. The focus is on the calculation of (1) 1H NMR and (2) 13C NMR chemical shifts for model fragments of the solid-state structure, (3) the analysis of host-guest interactions important for molecular recognition, and (4) the investigation of the process of a guest molecule rotation. For modeling the solid-state structure, up to three host-guest units are considered and the convergence with respect to the size of the solid-state fragment is investigated.