Benzoxazine oligomers: evidence for a helical structure from solid-state NMR spectroscopy and DFT-based dynamics and chemical shift calculations

A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MAS (1)H NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and (1)H chemical-shift calculations. Also supporting these proposed structures are homonuclear (1)H[bond](1)H double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative (15)N[bond](1)H distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.     

Hydrogen bonding and chemical shift assignments in carbazole functionalized isocyanides from solid-state NMR and first-principles calculations

Carbazole functionalized polyisocyanides are known to exhibit excellent electronic properties (E. Schwartz, et al., Chemistry of Materials, 2010, 22, 2597). The functionalities and properties of such materials crucially depend on the organization and stability of the polymer structure. We combine solid-state Nuclear Magnetic Resonance (NMR) experiments with first-principles calculations of isotropic chemical shifts, within the recently developed converse approach, to rationalize the origin of isotropic chemical shifts in the crystalline monomer l-isocyanoalanine 2-(9H-carbazol-9-yl) ethyl amide (monomer 1) and thereby gain insight into the structural organization of its polymer (polymer 2). The use of state-of-the-art solid-state NMR experiments combined with Density Functional Theory (DFT) based calculations allows an unambiguous assignment of all proton and carbon resonances of the monomer. We were able to identify the structure stabilising interactions in the crystal and understand the influence of the molecular packing in the crystal structure on the chemical shift data observed in the NMR spectra. Here the Nuclear Independent Chemical Shift (NICS) approach allows discriminating between 'physical' interactions amongst neighboring molecules such as ring-current effects and 'chemical' interactions such as hydrogen bonding. This analysis reveals that the isocyanide monomer is stabilized by multiple hydrogen bonds such as a bifurcated hydrogen bond involving -N-H, -C-H and O=C- moieties and Ar-H···C≡N- hydrogen bonding (Ar = aromatic group). Based on the geometrical arrangement it is postulated that the carbazole units are involved in the weak σ-π interactions giving rise to a Herringbone packing of the molecules. The chemical shift analysis of the polymer spectra readily establishes the existence of N-H···O=C hydrogen bonds despite the limited resolution exhibited by the polymer spectra. It is also elucidated that the relative arrangement of the carbazole units in the polymer differs significantly from that of the monomer.     

Supramolecular assembly of dendritic polymers elucidated by 1H and 13C solid-state MAS NMR spectroscopy

Advanced solid-state NMR methods under fast magic-angle spinning (MAS) are used to study the structure and dynamics of large supramolecular systems, which consist of a polymer backbone with dendritic side groups and self-assemble into a columnar structure. The NMR experiments are performed on as-synthesized samples, i.e., no isotopic enrichment is required. The analysis of (1)H NMR chemical-shift effects as well as dipolar (1)H-(1)H or (1)H-(13)C couplings provide site-specific insight into the local structure and the segmental dynamics, in particular, of phenyl rings and -CH(2)O- linking units within the dendrons. Relative changes of (1)H chemical shifts (of up to -3 ppm) serve as distance constraints and allow protons to be positioned relative to aromatic rings. Together with dipolar spinning sideband patterns, pi-pi packing phenomena and local order parameters (showing variations between 30% and 100%) are selectively and precisely determined, enabling the identification of the dendron cores as the structure-directing moieties within the supramolecular architecture. The study is carried out over a representative selection of systems which reflect characteristic differences, such as different polymer backbones, sizes of dendritic side groups, or length and flexibility of linking units. While the polymer backbone is found to have virtually no effect on the overall structure and properties, the systems are sensitively affected by changing the generation or the linkage of the dendrons. The results help to understand the self-assembly process of dendritic moieties and aid the chemical design of self-organizing molecular structures.     

Solution and solid-state effects on NMR chemical shifts in sesquiterpene lactones: NMR, X-ray, and theoretical methods

Selected guaianolide type sesquiterpene lactones were studied combining solution and solid-state NMR spectroscopy with theoretical calculations of the chemical shifts in both environments and with the X-ray data. The experimental (1)H and (13)C chemical shifts in solution were successfully reproduced by theoretical calculations (with the GIAO method and DFT B3LYP 6-31++G**) after geometry optimization (DFT B3LYP 6-31 G**) in vacuum. The GIPAW method was used for calculations of solid-state (13)C chemical shifts. The studied cases involved two polymorphs of helenalin, two pseudopolymorphs of 6α-hydroxydihydro-aromaticin and two cases of multiple asymmetric units in crystals: one in which the symmetry-independent molecules were connected by a series of hydrogen bonds (geigerinin) and the other in which the symmetry-independent molecules, deprived of any specific intermolecular interactions, differed in the conformation of the side chain (badkhysin). Geometrically different molecules present in the crystal lattices could be easily distinguished in the solid-state NMR spectra. Moreover, the experimental differences in the (13)C chemical shifts corresponding to nuclei in different polymorphs or in geometrically different molecules were nicely reproduced with the GIPAW calculations.     

Spectroscopic study on chemical structure of plasma-polymerized hexamethyldisiloxane

A plasma-polymerized material was produced from hexamethyldisiloxane vapor by a glow discharge polymerization technique. Spectroscopic interpretation of the chemical structure of the polymerized hexamethyldisiloxane was studied by spectroscopic means such as IR, XPS, and NMR. The plasma polymer was barely soluble in the usual organic solvents, although it contained a small amount of the monomer and its oligomers. The IR spectrum indicated that the polymer consisted of Si? CH₃, Si? O, Si? CH₂, and Si? H groups. The surface of the polymer was found to retain structural units similar to the monomer from the XPS measurement. On the other hand, the ¹³C and ²⁹Si high-resolution, solid-state NMR measurements revealed that the plasma polymer was highly crosslinked with a variety of conformations and a number of O atoms surrounding a Si atom. Results from the XPS and NMR spectra suggested that the bulk of the polymer was more oxidized than the surface layer; Si atom was preferentially oxidized. A hypothetical chemical structure was proposed for the polymerized hexamethyl-disiloxane.     

Protein structure determination by high-resolution solid-state NMR spectroscopy: application to microcrystalline ubiquitin

High-resolution solid-state NMR spectroscopy has become a promising method for the determination of three-dimensional protein structures for systems which are difficult to crystallize or exhibit low solubility. Here we describe the structure determination of microcrystalline ubiquitin using 2D (13)C-(13)C correlation spectroscopy under magic angle spinning conditions. High-resolution (13)C spectra have been acquired from hydrated microcrystals of site-directed (13)C-enriched ubiquitin. Inter-residue carbon-carbon distance constraints defining the global protein structure have been evaluated from 'dipolar-assisted rotational resonance' experiments recorded at various mixing times. Additional constraints on the backbone torsion angles have been derived from chemical shift analysis. Using both distance and dihedral angle constraints, the structure of microcrystalline ubiquitin has been refined to a root-mean-square deviation of about 1 A. The structure determination strategies for solid samples described herein are likely to be generally applicable to many proteins that cannot be studied by X-ray crystallography or solution NMR spectroscopy.     

Spatial structure of peptides determined by residual dipolar couplings analysis

The gated decoupled (13)C NMR spectra of a dipeptide (Glu-Trp) and a tetrapeptide (NAc-Ser-Phe-Val-Gly-OMe) were recorded in D(2)O and in a lyotropic alignment medium (pentaethylene glycol monododecyl ether/n-hexanol). The residual dipolar couplings were extracted as the differences between the observed couplings for the magnetic nuclei dissolved in the latter and former media. Using a computational optimization, the spatial structures of the compounds were calculated starting from their respective low energy conformations obtained on a semiempirical basis. The uniformity of each conformation was confirmed by the solid-state (13)C NMR spectra of powder samples. Differences between the starting structures and final ones, optimized when employing residual dipolar couplings, are discussed.     

Effective Hamiltonians by optimal control: solid-state NMR double-quantum planar and isotropic dipolar recoupling

We report the use of optimal control algorithms for tailoring the effective Hamiltonians in nuclear magnetic resonance (NMR) spectroscopy through sophisticated radio-frequency (rf) pulse irradiation. Specifically, we address dipolar recoupling in solid-state NMR of powder samples for which case pulse sequences offering evolution under planar double-quantum and isotropic mixing dipolar coupling Hamiltonians are designed. The pulse sequences are constructed numerically to cope with a range of experimental conditions such as inhomogeneous rf fields, spread of chemical shifts, the intrinsic orientation dependencies of powder samples, and sample spinning. While the vast majority of previous dipolar recoupling sequences are operating through planar double-or zero-quantum effective Hamiltonians, we present here not only improved variants of such experiments but also for the first time homonuclear isotropic mixing sequences which transfers all I(x), I(y), and I(z) polarizations from one spin to the same operators on another spin simultaneously and with equal efficiency. This property may be exploited to increase the signal-to-noise ratio of two-dimensional experiments by a factor of square root 2 compared to conventional solid-state methods otherwise showing the same efficiency. The sequences are tested numerically and experimentally for a powder of (13)C(alpha),(13)C(beta)-L-alanine and demonstrate substantial sensitivity gains over previous dipolar recoupling experiments.     

Solid-state NMR study of the post-fluorination of (C2.5F)n fluorine-GIC

The conversion of (C2.5F)n fluorine-graphite intercalation compounds (GIC) into covalent graphite fluoride during a post-treatment in pure F2 gas is studied by solid-state NMR. First, a careful characterization of the starting material is performed; in particular, for the first time for fluorinated carbons, two-dimensional 19F--> 13C cross-polarization wide-line separation (CP-WISE) experiments were carried out. This completes the classical NMR data such as 19F and 13C chemical shifts, quantitative 13C solid echo, and C-F bond length measurements, which were estimated by dipolar recoupling using inverse CP MAS. The data of the raw (C2.5F)n and of the samples post-fluorinated at 350, 450, and 550 degrees C were compared to investigate the C-F bonding as a function of the treatment temperature. The C-F bonding is discussed taking into account a hyperconjugation of the C-F bonds with neighboring unfluorinated carbon atoms.     

High-resolution characterization of liquid-crystalline [60]fullerenes using solid-state nuclear magnetic resonance spectroscopy

Liquid-crystalline materials containing fullerenes are valuable in the development of supramolecular switches and in solar cell technology. In this study, we characterize the liquid-crystalline and dynamic properties of fullerene-containing thermotropic compounds using solid-state natural abundance (13)C NMR experiments under stationary and magic angle spinning sample conditions. Chemical shifts spectra were measured in isotropic, liquid-crystalline nematic and smectic A and crystalline phases using one-dimensional (13)C experiments, while two-dimensional separated local-field experiments were used to measure the (1)H- (13)C dipolar couplings in mesophases. Chemical shift and dipolar coupling parameters were used to characterize the structure and dynamics of the liquid-crystalline dyads. NMR data of fullerene-containing thermotropic liquid crystals are compared to that of basic mesogenic unit and mesomorphic promoter compounds. Our NMR results suggest that the fullerene-ferrocene dyads form highly dynamic liquid-crystalline phases in which molecules rotate fast around the symmetry axis on the characteristic NMR time scale of approximately 10 (-4) s.     

Ionothermal synthesis, crystal structure and solid-state NMR spectroscopy of a new organically templated gallium oxalatophosphate: (H2TMPD)0.5[Ga3(C2O4)0.5(PO4)3] (TMPD=N,N,N',N'-tetramethyl-1,3-propanediamine)

A new organically templated gallium oxalatophosphate, (C7H20N2)0.5[Ga3(C2O4)0.5(PO4)3], has been synthesized by using a low-melting-point eutectic mixture of choline chloride and oxalic acid as a solvent and characterized by single-crystal X-ray diffraction, thermogravimetric analysis and solid-state NMR spectroscopy. It is the first example of ionothermal synthesis of organically templated metal oxalatophosphate. The structure contains double 6-ring units of the composition Ga6(PO4)6 which are connected by oxalate ligands and P-O-Ga bonds to form a 3-D framework. The charge-compensating organic ammonium cations which are disordered over two positions are located at the intersections of two types of 8-ring channels. 1H MAS and 13C CPMAS NMR studies confirm the presence of N,N,N',N'-tetramethyl-1,3-propanediammonium cation. The 71Ga and 31P MAS NMR spectra are also consistent with the crystal structure analysis results.     

Triple oscillating field technique for accurate distance measurements by solid-state NMR

We present a new concept for homonuclear dipolar recoupling in magic-angle-spinning (MAS) solid-state NMR experiments which avoids the problem of dipolar truncation. This is accomplished through the introduction of a new NMR pulse sequence design principle: the triple oscillating field technique. We demonstrate this technique as an efficient means to accomplish broadband dipolar recoupling of homonuclear spins, while decoupling heteronuclear dipolar couplings and anisotropic chemicals shifts and retaining influence from isotropic chemical shifts. In this manner, it is possible to synthesize Ising interaction (2IzSz) Hamiltonians in homonuclear spin networks and thereby avoid dipolar truncation--a serious problem essentially all previous homonuclear dipolar recoupling experiments suffer from. Combination of this recoupling concept with rotor assisted dipolar refocusing enables easy readout of internuclear distances through comparison with analytical Fresnel curves. This forms the basis for a new class of solid-state NMR experiments with potential for structure analysis of uniformly 13C labeled proteins through accurate measurement of 13C-13C internuclear distances. The concept is demonstrated experimentally by measurement of C alpha-C', C beta-C', and C gamma-C' internuclear distances in powder samples of the amino acids L-alanine and L-threonine.     

A high-resolution solid-state NMR approach for the structural studies of bicelles

Bicelles are increasingly being used as membrane mimicking systems in NMR experiments to investigate the structure of membrane proteins. In this study, we demonstrate the effectiveness of a 2D solid-state NMR approach that can be used to measure the structural constraints, such as heteronuclear dipolar couplings between 1H, 13C, and 31P nuclei, in bicelles without the need for isotopic enrichment. This method does not require a high radio frequency power unlike the presently used rotating-frame separated-local-field (SLF) techniques, such as PISEMA. In addition, multiple dipolar couplings can be measured accurately, and the presence of a strong dipolar coupling does not suppress the weak couplings. High-resolution spectra obtained from magnetically aligned DMPC:DHPC bicelles even in the presence of peptides suggest that this approach will be useful in understanding lipid-protein interactions that play a vital role in shaping up the function of membrane proteins.     

Solid-state NMR characterization and determination of the orientational order of a nematogen

Thermotropic liquid crystalline compounds are of considerable importance due to their potential applications as advanced functional materials. A mesogen consisting of a terminal dimethylamino group, which can act as a charge-transfer donor, is particularly valuable for its light emission and nonlinear optical properties. In this study, we report the solid-state NMR investigation of the nematic behavior of one such novel mesogen (4-(dodecyloxy)benzoic acid 4-[((4-(dimethylamino)phenyl)imino)methyl]phenyl ester). Static and MAS experiments were performed on nematic and crystalline phases of the compound to measure (13)C chemical shift, (13)C-(1)H dipolar coupling, and (1)H chemical shift values. 2D chemical shift correlation of (1)H and (13)C nuclei confirmed the (13)C chemical shift values determined from 1D CPMAS experiments. The appearance of more peaks in both CPMAS and (13)C-(1)H HETCOR spectra of a crystalline solid suggests the heterogeneous orientations of phenyl rings of the mesogenic core. Variable-temperature experiments infer the motional averaging of these orientations before melting. The (1)H-(13)C dipolar coupling values, measured by 2D PITANSEMA experiments, were used to determine the orientational order of the mesogenic core at various temperatures. The influence of the linking unit and terminal substituents on the order parameter values of the mesogenic core is discussed.     

Insight into the structure of silver cyanide from (13)C and (15)N solid-state NMR spectroscopy

The structure of silver cyanide has been investigated by solid-state multinuclear magnetic resonance spectroscopy. Carbon-13 and nitrogen-15 NMR spectra of magic-angle-spinning (MAS) and stationary powder samples of isotopically enriched Ag(13)CN, Ag(13)C(15)N, and AgC(15)N have been acquired at the external applied magnetic field strengths 4.7, 7.05, and 9.4 T. Axially symmetric carbon and nitrogen chemical shift (CS) tensors provide evidence for linearity of the polymeric (-Ag-CN-)(n)() chains. A two-site model is required to successfully simulate the (13)C MAS NMR line shape, which is dominated by indirect nuclear spin-spin coupling between (109/107)Ag and (13)C nuclei. In combination with relativistic zeroth-order regular approximation density functional theory (ZORA-DFT) calculations on model AgCN fragments, the (13)C MAS NMR results show that 30 +/- 10% of the silver sites are disordered, that is, either -NC-Ag-CN- or -CN-Ag-NC-, and 70 +/- 10% of the silver sites are ordered, that is, -NC-Ag-NC-. Effective dipolar coupling data extracted from (13)C NMR spectra of stationary samples allow an upper limit of 1.194 A to be placed on the carbon-nitrogen internuclear distance. After incorporation of the effects of anisotropic indirect nuclear spin-spin coupling and motional averaging on the NMR-derived distance, a corrected value of r(CN) = 1.16 +/- 0.03 A is obtained. This work provides an example of the type of information which may be obtained from solid-state NMR studies of disordered materials and how such information may complement that available from diffraction studies.     

A new approach in 1D and 2D 13C high-resolution solid-state NMR spectroscopy of paramagnetic organometallic complexes by very fast magic-angle spinning

Novel 1D and multidimensional solid-state NMR (SSNMR) methods using very fast magic-angle spinning (VFMAS) (spinning speed > 20 kHz) for performing 13C high-resolution SSNMR of paramagnetic organometallic complexes are discussed. VFMAS removes a majority of 13C-1H and 1H-1H dipolar couplings, which are often difficult to remove by RF pulse techniques in paramagnetic complexes because of large paramagnetic shifts. In the first systematic approach using the unique feature of VFMAS for paramagnetic complexes, we demonstrate a means of obtaining well-resolved 1D and multidimensional 13C SSNMR spectra, sensitivity enhancements via cross polarization, and signal assignments, and applications of dipolar recoupling methods for nonlabeled paramagnetic organometallic complexes of moderate paramagnetic shifts ( approximately 800 ppm). Experimental results for powder samples of small nonlabeled coordination complexes at 1H frequencies of 400.2-400.3 MHz show that highly resolved 13C SSNMR spectra can be obtained under VFMAS, without requirements of 1H decoupling. Sensitivity enhancement in 13C SSNMR via cross polarization from 1H spins was demonstrated with an amplitude-sweep high-power CP sequence using strong RF fields ( approximately 100 kHz) available in the VFMAS probe. 13C CPMAS spectra of nonlabeled Cu(II)(dl-alanine)2.(H2O) and V(III)(acetylacetonate)3 (V(acac)3) show that it is possible to obtain high-resolution spectra for a small quantity ( approximately 15 mg) of nonlabeled paramagnetic organometal complexes within a few minutes under VFMAS. Experiments on Cu(II)(dl-alanine)2.(H2O) demonstrated that 1H-13C dipolar recoupling for paramagnetic organometal complexes can be performed under VFMAS by application of rotor-synchronous pi-pulses to 1H and 13C spins. The results also showed that signal assignments for 13CH, 13CH3, and 13CO groups in paramagnetic complexes are possible on the basis of the amount of 13C-1H dipolar dephasing induced by dipolar recoupling. Furthermore, the experimental 2D 13C/1H chemical-shift correlation NMR spectrum obtained for nonlabeled V(acac)3 exhibits well-resolved lines, which overlap in 1D 13C and 1H spectra. Signals for different chemical groups in the 2D spectrum are distinguished by the 13C-1H dipolar dephasing method combined with the 2D 13C/1H correlation NMR. The assignments offer information on the existence of nonequivalent ligands in the coordination complex in solids, without requiring a single-crystal sample.     

On the origin of the color in the solid state. Crystal structure and optical and magnetic properties of 4-cyanopyridinium hydrogensquarate monohydrate

The novel hydrogensquarate salt of 4-cyanopyridine was synthesized, and its structure and properties were elucidated in detail spectroscopically, thermally, and structurally, using single-crystal X-ray diffraction, linear-polarized solid-state IR spectroscopy, UV spectroscopy, TGA, DSC, DTA, and positive and negative ESI MS as well as 1H and 13C NMR methods. Quantum chemical methods were used to calculate the electronic structure, vibrational data, and electronic spectra. 4-Cyanopyridinium hydrogensquarate monohydrate crystallizes in the space group P and exhibits a layered structure with molecules linked by intermolecular NH...O(HSq-) (2.651 A) and HOH...O(HSq-) (2.792 and 2.563 A) hydrogen bonds with participation of cations, anions, and the solvent molecule. The formation of stable layers of the type (2HSq-.2H2O)n and the observation of a red color in the solid state is discussed. The optical and magnetic properties were elucidated in comparison to the data for neutral 4-cyanopyridine as well as its four known salts.     

Separate-local-field NMR spectroscopy on half-integer quadrupolar nuclei

New approaches to the characterization of resonances in the solid-state NMR spectroscopy of half-integer quadrupolar nuclei are explored, on the basis of the acquisition of heteronuclear separate-local-field spectra on rotating solids. In their two-dimensional version, these experiments correlate for each chemical site a second-order quadrupolar MAS powder pattern with the dipolar MAS sideband pattern to nearby heteronuclei. As 3D NMR sequences, such 2D anisotropic correlation spectra become separated for inequivalent chemical sites along a third, isotropic dimension. Extending in such manner separate-local-field NMR approaches to quadrupoles facilitates the assignment of inequivalent resonances to specific structural environments, and provides new tools for the investigation of dynamics in solids. Details about these 2D and 3D NMR experiments are given, and their application is illustrated with 1H-23Na recoupling experiments on mononucleotides possessing multiple bound cations.     

Structural integration of tellurium oxide into mixed-network-former glasses: connectivity distribution in the system NaPO(3)-TeO(2).

Sodium phosphate tellurite glasses in the system (NaPO(3))(x)(TeO(2))(1-) (x) were prepared and structurally characterized by thermal analysis, vibrational spectroscopy, X-ray photoelectron spectroscopy (XPS) and a variety of complementary solid-state nuclear magnetic resonance (NMR) techniques. Unlike the situation in other mixed-network-former glasses, the interaction between the two network formers tellurium oxide and phosphorus oxide produces no new structural units, and no sharing of the network modifier Na(2)O takes place. The glass structure can be regarded as a network of interlinked metaphosphate-type P(2) tetrahedral and TeO(4/2) antiprismatic units. The combined interpretation of the O 1s XPS data and the (31)P solid-state NMR spectra presents clear quantitative evidence for a nonstatistical connectivity distribution. Rather, the formation of homoatomic P--O--P and Te--O--Te linkages is favored over mixed P--O--Te connectivities. As a consequence of this chemical segregation effect, the spatial sodium distribution is not random, as also indicated by a detailed analysis of (31)P/(23)Na rotational echo double-resonance (REDOR) experiments.     

Structure determination of a membrane protein with two trans-membrane helices in aligned phospholipid bicelles by solid-state NMR spectroscopy

The structure of the membrane protein MerFt was determined in magnetically aligned phospholipid bicelles by solid-state NMR spectroscopy. With two trans-membrane helices and a 10-residue inter-helical loop, this truncated construct of the mercury transport membrane protein MerF has sufficient structural complexity to demonstrate the feasibility of determining the structures of polytopic membrane proteins in their native phospholipid bilayer environment under physiological conditions. PISEMA, SAMMY, and other double-resonance experiments were applied to uniformly and selectively (15)N-labeled samples to resolve and assign the backbone amide resonances and to measure the associated (15)N chemical shift and (1)H-(15)N heteronuclear dipolar coupling frequencies as orientation constraints for structure calculations. (1)H/(13)C/(15)N triple-resonance experiments were applied to selectively (13)C'- and (15)N-labeled samples to complete the resonance assignments, especially for residues in the nonhelical regions of the protein. A single resonance is observed for each labeled site in one- and two-dimensional spectra. Therefore, each residue has a unique conformation, and all protein molecules in the sample have the same three-dimensional structure and are oriented identically in planar phospholipid bilayers. Combined with the absence of significant intensity near the isotropic resonance frequency, this demonstrates that the entire protein, including the loop and terminal regions, has a well-defined, stable structure in phospholipid bilayers.