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(125)Te NMR spectra and spin-lattice relaxation times, T1, have been measured for several GeTe-based materials with Te excess. The spectra show inhomogeneous broadening by several thousand ppm and a systematic variation in T1 relaxation time with resonance frequency. The quadratic dependence of the spin-lattice relaxation rate, 1/T1, on the Knight shift in the Korringa relation is found to be valid over a wide range of Knight shifts. This result confirms that T1 relaxation in GeTe-based materials is mostly dominated by hyperfine interaction between nuclei and free charge carriers. In GeTe with 2.5% excess of Te, about 15% of the material exhibits a Knight shift of ≥4500ppm and a T1 of only 0.3ms, indicating a high hole concentration that could correspond to close to 50% vacancies on the Ge sublattice in this component. Our findings provide a basis for determining the charge carrier concentration and its distribution in complex thermoelectric and phase-change tellurides, which should lead to a better understanding of electronic and thermal transport properties as well as chemical bonding in these materials.

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Complex tellurides, such as doped PbTe, GeTe, and their alloys, are among the best thermoelectric materials. Knowledge of the change in (125)Te NMR chemical shift due to bonding to dopant or "solute" atoms is useful for determination of phase composition, peak assignment, and analysis of local bonding. We have measured the (125)Te NMR chemical shifts in PbTe-based alloys, Pb1-xGexTe and Pb1-xSnxTe, which have a rocksalt-like structure, and analyzed their trends. For low x, several peaks are resolved in the 22-kHz MAS (125)Te NMR spectra. A simple linear trend in chemical shifts with the number of Pb neighbors is observed. No evidence of a proposed ferroelectric displacement of Ge atoms in a cubic PbTe matrix is detected at low Ge concentrations. The observed chemical shift trends are compared with the results of DFT calculations, which confirm the linear dependence on the composition of the first-neighbor shell. The data enable determination of the composition of various phases in multiphase telluride materials. They also provide estimates of the (125)Te chemical shifts of GeTe and SnTe (+970 and +400±150 ppm, respectively, from PbTe), which are otherwise difficult to access due to Knight shifts of many hundreds of ppm in neat GeTe and SnTe.

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We introduce a Python-based program that utilizes the large database of (13)C and (15)N chemical shifts in the Biological Magnetic Resonance Bank to rapidly predict the amino acid type and secondary structure from correlated chemical shifts. The program, called PACSYlite Unified Query (PLUQ), is designed to help assign peaks obtained from 2D (13)C-(13)C, (15)N-(13)C, or 3D (15)N-(13)C-(13)C magic-angle-spinning correlation spectra. We show secondary-structure specific 2D (13)C-(13)C correlation maps of all twenty amino acids, constructed from a chemical shift database of 262,209 residues. The maps reveal interesting conformation-dependent chemical shift distributions and facilitate searching of correlation peaks during amino-acid type assignment. Based on these correlations, PLUQ outputs the most likely amino acid types and the associated secondary structures from inputs of experimental chemical shifts. We test the assignment accuracy using four high-quality protein structures. Based on only the Cα and Cß chemical shifts, the highest-ranked PLUQ assignments were 40-60 % correct in both the amino-acid type and the secondary structure. For three input chemical shifts (CO-Cα-Cß or N-Cα-Cß), the first-ranked assignments were correct for 60 % of the residues, while within the top three predictions, the correct assignments were found for 80 % of the residues. PLUQ and the chemical shift maps are expected to be useful at the first stage of sequential assignment, for combination with automated sequential assignment programs, and for highly disordered proteins for which secondary structure analysis is the main goal of structure determination.

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Several techniques for spectral editing of 2D (13)C-(13)C correlation NMR of proteins are introduced. They greatly reduce the spectral overlap for five common amino acid types, thus simplifying spectral assignment and conformational analysis. The carboxyl (COO) signals of glutamate and aspartate are selected by suppressing the overlapping amide N-CO peaks through (13)C-(15)N dipolar dephasing. The sidechain methine (CH) signals of valine, lecuine, and isoleucine are separated from the overlapping methylene (CH(2)) signals of long-chain amino acids using a multiple-quantum dipolar transfer technique. Both the COO and CH selection methods take advantage of improved dipolar dephasing by asymmetric rotational-echo double resonance (REDOR), where every other π-pulse is shifted from the center of a rotor period t(r) by about 0.15 t(r). This asymmetry produces a deeper minimum in the REDOR dephasing curve and enables complete suppression of the undesired signals of immobile segments. Residual signals of mobile sidechains are positively identified by dynamics editing using recoupled (13)C-(1)H dipolar dephasing. In all three experiments, the signals of carbons within a three-bond distance from the selected carbons are detected in the second spectral dimension via (13)C spin exchange. The efficiencies of these spectral editing techniques range from 60 % for the COO and dynamic selection experiments to 25 % for the CH selection experiment, and are demonstrated on well-characterized model proteins GB1 and ubiquitin.

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Large-scale soil application of biochar may enhance soil fertility, increasing crop production for the growing human population, while also sequestering atmospheric carbon. But reaching these beneficial outcomes requires an understanding of the relationships among biochar's structure, stability, and contribution to soil fertility. Using quantitative (13)C nuclear magnetic resonance (NMR) spectroscopy, we show that Terra Preta soils (fertile anthropogenic dark earths in Amazonia that were enriched with char >800 years ago) consist predominantly of char residues composed of ~6 fused aromatic rings substituted by COO(-) groups that significantly increase the soils' cation-exchange capacity and thus the retention of plant nutrients. We also show that highly productive, grassland-derived soils in the U.S. (Mollisols) contain char (generated by presettlement fires) that is structurally comparable to char in the Terra Preta soils and much more abundant than previously thought (~40-50% of organic C). Our findings indicate that these oxidized char residues represent a particularly stable, abundant, and fertility-enhancing form of soil organic matter.

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For obtaining sideband-free spectra of high-Z spin-1/2 nuclei with large (>1000 ppm) chemical-shift anisotropies and broad isotropic-shift dispersion, we recently identified Gan's modified five-pulse magic-angle turning (MAT) experiment as the best available broadband pulse sequence, and adapted it to fast magic-angle spinning. Here, we discuss technical aspects such as pulse timings that compensate for off-resonance effects and are suitable for large CSAs over a range of 1.8γB(1); methods to minimize the duration of z-periods by cyclic decrementation; shearing without digitization artifacts, by sharing between channels (points); and maximizing the sensitivity by echo-matched full-Gaussian filtering. The method is demonstrated on a model sample of mixed amino acids and its large bandwidth is highlighted by comparison with the multiple-π-pulse PASS technique. Applications to various tellurides are shown; these include GeTe, Sb(2)Te(3) and Ag(0.53)Pb(18)Sb(1.2)Te(20), with spectra spanning up to 190 kHz, at 22 kHz MAS. We have also determined the (125)Te chemical shift anisotropies from the intensities of the spinning sidebands resolved by isotropic-shift separation.

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Nanocrystals of apatitic calcium phosphate impart the organic-inorganic nanocomposite in bone with favorable mechanical properties. So far, the factors preventing crystal growth beyond the favorable thickness of ca. 3 nm have not been identified. Here we show that the apatite surfaces are studded with strongly bound citrate molecules, whose signals have been identified unambiguously by multinuclear magnetic resonance (NMR) analysis. NMR reveals that bound citrate accounts for 5.5 wt% of the organic matter in bone and covers apatite at a density of about 1 molecule per (2 nm)(2), with its three carboxylate groups at distances of 0.3 to 0.45 nm from the apatite surface. Bound citrate is highly conserved, being found in fish, avian, and mammalian bone, which indicates its critical role in interfering with crystal thickening and stabilizing the apatite nanocrystals in bone.

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In experiments on S-L heteronuclear spin systems with evolution of the S-spin magnetization under the influence of a quadrupolar nucleus (L-spin), effects of longitudinal quadrupolar (T(1Q)) relaxation of the L-spin coherence on the sub-millisecond time scale have been documented and explored, and methods for minimizing their effect have been demonstrated. The longitudinal relaxation results in heteronuclear dephasing even in the reference signal S(0) of S[L] REDOR, REAPDOR, RIDER, or SPIDER experiments, due to T(1Q)-relaxation of the transiently generated S(y)L(z) coherence, reducing or even eliminating the observable dephasing DeltaS. Pulse sequences for measuring an improved reference signal S(00) with minimal heteronuclear recoupling but the same number of pulses as for S(0) and S have been demonstrated. From the observed intensity DeltaS(0)=S(00)-S(0) and the SPIDER signal DeltaS/S(0), T(1Q) can be estimated. Accelerated decays analogous to the dipolar S(0) curves will occur in T(2) measurements for J-coupled S-L spin pairs. Even in the absence of recoupling pulses, fast T(1Q) relaxation of the unobserved nucleus shortens the transverse relaxation time T(2S,MAS) of the observed nucleus, in particular at low spinning frequencies, due to unavoidable heteronuclear dipolar evolution during a rotation period. The observed spinning-frequency dependence of T(2S,MAS) matches the theoretical prediction and may be used to estimate T(1Q). The effects are demonstrated on several (13)C[(14)N] spin systems, including an arginine derivative, the natural N-acetylated polysaccharide chitin, and a model peptide, (POG)(10).

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Organic matter from the Murchison meteorite shows pronounced spinning sidebands of the (1)H MAS NMR spectrum and exhibits a large bulk magnetization of 0.75emicro/g extrapolated to 94kOe at 300K. By comparison with data of diamagnetic polystyrene and laponite clay mixed with ferrimagnetic gamma-Fe(2)O(3) nano-particles, we show that the spinning sidebands arise from a combination of dipolar couplings of a given (1)H to magnetic particles, seen in a backscattered-electron image, and to other protons. Signal loss and significant broadening of protonated-carbon peaks in (13)C MAS NMR spectra of polystyrene with Fe(2)O(3) nano-particles is demonstrated, and implications for (13)C NMR spectroscopy of Murchison meteorite are discussed.

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A new nuclear magnetic resonance approach for characterizing the thickness of phosphate, silicate, carbonate, and other nanoparticles in organic-inorganic nanocomposites is presented. The particle thickness is probed using the strongly distant-dependent dipolar couplings between the abundant protons in the organic phase and X nuclei (31P, 29Si, 13C, 27Al, 23Na, etc.) in the inorganic phase. This approach requires pulse sequences with heteronuclear dephasing only by the polymer or surface protons that experience strong homonuclear interactions, but not by dispersed OH or water protons in the inorganic phase, which have long transverse relaxation times T2,H. This goal is achieved by heteronuclear recoupling with dephasing by strong homonuclear interactions of protons (HARDSHIP). The pulse sequence alternates heteronuclear recoupling for approximately 0.15 ms with periods of homonuclear dipolar dephasing that are flanked by canceling 90 degrees pulses. The heteronuclear evolution of the long-T2,H protons is refocused within two recoupling periods, so that 1H spin diffusion cannot significantly dephase these coherences. For the short-T2,H protons of a relatively immobile organic matrix, the heteronuclear dephasing rate depends simply on the heteronuclear second moment. Homonuclear interactions do not affect the dephasing, even though no homonuclear decoupling is applied, because long-range 1H-X dipolar couplings approximately commute with short-range 1H-1H couplings, and heteronuclear recoupling periods are relatively short. This is shown in a detailed analysis based on interaction representations. The algorithm for simulating the dephasing data is described. The new method is demonstrated on a clay-polymer nanocomposite, diamond nanocrystals with protonated surfaces, and the bioapatite-collagen nanocomposite in bone, as well as pure clay and hydroxyapatite. The diameters of the nanoparticles in these materials range between 1 and 5 nm. Simulations show that spherical particles of up to 10 nm diameter can be characterized quite easily.

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The mobility and domain structure of various standard humic substances have been investigated by 1H and 1H-13C solid-state nuclear magnetic resonance (NMR) experiments. In four dry humic acids, a fulvic acid, a natural organic matter sample, and a whole peat sample, segments that undergo fast, large-amplitude motions account for <9% of the sample. This disproves a previous suggestion, based on 1H NMR data, that flexible domains, presumably carbohydrates, make up >40% of various humic acids; these putative mobile domains were also linked to dual-mode sorption. In particular, neither the polar alkyl (carbohydrate) nor the aromatic components show any fast, large-amplitude mobility. A small fraction of mobile nonpolar alkyl segments identified by us before is the only component undergoing large-amplitude motions, apart from absorbed water that we observe in humic acids exposed to ambient air. 1H-13C wide-line separation NMR shows that, contrary to previous suggestions, the dipolar couplings in the aromatic regions are smaller than in the polar alkyl segments, most likely due to differences in local 1H densities. Series of 1H-13C heteronuclear correlation experiments with 1H spin diffusion reveal close proximity of aromatic and polar alkyl segments in several humic acids, precluding carbohydrate domains on a scale of > 1-nm diameter. In the standard peat humic acid, nonpolar aromatic segments also do not form sorption domains of significant size, while nonpolar aliphatic domains, which we had previously shown to correlate with sorption capacity, have been confirmed.

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Proton spin diffusion is widely used to determine domain sizes in heterogeneous organic solids. For an accurate analysis, spin diffusion coefficients are required. However, in most cases they are not directly measured, but instead derived from model systems. The effects of magic-angle spinning (MAS), mobility, or spin-lock fields on spin-diffusion coefficients have also been difficult to quantify. In this work, direct measurement of local (1)H spin-diffusion coefficients in any rigid polymer is achieved in experiments with heteronuclear dephasing of the (1)H magnetization, a mixing time for (1)H spin diffusion, and (13)C detection after cross-polarization. In the presence of (1)H homonuclear decoupling and (13)C 180 degrees-pulse recoupling, each (13)C spin dephases a significant number (3-20) of protons, depending on the dephasing time. For (13)C and other sufficiently dilute heteronuclei, the dephasing of the protons is described by simple spin-pair REDOR curves. As a result, every (13)C nucleus will "burn" a spherical hole of known diameter and profile into the proton magnetization distribution. (1)H spin diffusion into the hole during the mixing time can be monitored and simulated accurately for every resolved (13)C site, with the spin-diffusion coefficient as the only significant unknown parameter. By varying the dephasing time, holes with diameters of 0.4-0.8 nm can be burned into the proton magnetization profile and thus the dependence of the local spin-diffusion coefficients on the proton density or partial mobility can be explored. The effects of transverse or magic-angle spin-lock fields on spin diffusion can be quantified conveniently by this method. Analytical and numerical fits yield short-range spin-diffusion coefficients of 0.2-0.5 nm(2)/ms on the 0.5-nm scale, which is smaller than the value of 0.8 nm(2)/ms for organic solids previously measured on the 10-nm scale.

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One-dimensional (1D) exchange NMR experiments can elucidate the geometry, time scale, memory, and heterogeneity of slow molecular motions (1 ms-1 s) in solids. The one-dimensional version of pure-exchange (PUREX) solid-state exchange NMR, which is applied to static samples and uses the chemical shift anisotropy as a probe for molecular motion, is particularly promising and convenient in applications where site resolution is not a problem, i.e., in systems with few chemical sites. In this work, some important aspects of the 1D PUREX experiment applied to systems with complex molecular motions are analyzed. The influence of intermediate-regime (10 micros-1 ms) motions and of the distribution of reorientation angles on the pure-exchange intensity are discussed, together with a simple method for estimating the activation energy of motions occurring with a single correlation time. In addition, it is demonstrated that detailed information on the motional geometry can be obtained from 1D PUREX spectral line shapes. Experiments on a molecular crystal, dimethyl sulfone, confirm the analysis quantitatively. In two amorphous polymers, atactic polypropylene (aPP) and polyisobutylene (PIB), which differ only by one methyl group in the repeat unit, the height of the normalized exchange intensity clearly reveals a striking difference in the width of the distribution of correlation times slightly above the glass transition. The aPP shows the broad distribution and Williams-Landel-Ferry temperature dependence of correlation times typical of polymers and other "fragile" glass formers. In contrast, the dynamics in PIB occur essentially with a single correlation time and exhibits Arrhenius behavior, which is more typical of "strong" glass formers; this is somewhat surprising given the weak intermolecular forces in PIB.

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A robust new solid-state nuclear magnetic resonance (NMR) method for selecting CH2 signals in magic-angle spinning (MAS) 13C NMR spectra is presented. Heteronuclear dipolar evolution for a duration of 0.043 ms, under MREV-8 homonuclear proton decoupling, converts 13C magnetization of CH2 groups into two- and three-spin coherences. The CH2 selection in the SIJ (C H H) spin system is based on the three-spin coherence S(x)I(z)J(z), which is distinguished from 13C magnetization (S(x)) by a 1H 0 degrees/90 degrees pulse consisting of two 45 degrees pulses. The two-spin coherences of the type S(y)I(z) are removed by a 13C 90 degrees x-pulse. The three-spin coherence is reconverted into magnetization during the remainder of the rotation period, still under MREV-8 decoupling. The required elimination of 13C chemical-shift precession is achieved by a prefocusing 180 degrees pulse bracketed by two rotation periods. The selection of the desired three-spin coherence has an efficiency of 13% theoretically and of 8% experimentally relative to the standard CP/MAS spectrum. However, long-range couplings also produce some three-spin coherences of methine (CH) carbons. Therefore, the length of the 13C pulse flipping the two-spin coherences is increased by 12% to slightly invert the CH signals arising from two-spin coherences and thus cancel the signal from long-range three-spin coherences. The signal intensity in this cleaner spectrum is 6% relative to the regular CP/TOSS spectrum. The only residual signal is from methyl groups, which are suppressed at least sixfold relative to the CH2 peaks. The experiment is demonstrated on cholesteryl acetate and applied to two humic acids.

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The influence of an antimicrobial peptide, protegrin-1 (PG-1), on the curvature and lateral diffusion coefficient (D(L)) of phosphocholine bilayers is investigated using one- (1D) and two-dimensional (2D) (31)P exchange NMR. The experiments utilize the fact that lipid lateral diffusion over the curved surface of vesicles changes the molecular orientation and thus the (31)P chemical shift anisotropy. This reorientation is manifested in 2D spectra as off-diagonal intensities and in 1D stimulated-echo experiments as reduced echo heights. The 2D spectra give information on the reorientation-angle distribution while the decay of the stimulated-echo intensity, which closely tracks the second-order correlation function in our experiments, yields the correlation times of the reorientation. The relationships among the 2D exchange spectra, stimulated-echo intensities, the correlation function, and reorientation-angle distributions are analyzed in detail. In the absence of PG-1, both dilaurylphosphotidylcholine (DLPC) and palmitoyloleoylphosphatidylcholine (POPC) vesicles show biexponential decays of the stimulated-echo intensities to equilibrium values of 0.20-0.25, suggesting that the curvature of the lipid vesicles has a bimodal distribution. The addition of PG-1 to DLPC vesicles increased the decay time constants, indicating that D(L) decreases due to peptide binding. In contrast, the addition of PG-1 to POPC vesicles decreased the decay constants by three to fivefold, indicating that the POPC vesicles are fragmented into smaller vesicles. On the basis of the changes in D(L) and the decay constants, we estimate that the radius of the POPC vesicles decreases by threefold due to PG-1 binding. Simulations of the 2D exchange spectra yielded quantitative reorientation-angle distributions that are consistent with the bimodal distributions of the vesicle curvature and the effects of the peptide on the two types of lipid bilayers. Thus, (31)P exchange NMR provides useful insights into the membrane morphological changes induced by this antimicrobial peptide.

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An improved approach for accurately determining the aromatic carbon fraction (fa) and nonprotonated aromatic carbon fraction (faN) in natural organic matter by solid-state 13C NMR is described. Quantitative peak areas are obtained from direct polarization 13C nuclear magnetic resonance (NMR) under high-speed magic angle spinning (MAS). The problem of overlap between aromatic and alkyl carbon resonances around 90-120 ppm in 13C NMR spectra is solved by a 13C chemical shift anisotropy (CSA) filter technique. After correction for residual spinning sidebands, an accurate value of the aromaticity fa is obtained. To obtain a quantitative faN fraction, dipolar dephasing was adapted for high-speed MAS 13C NMR; the separation of the signals of nonprotonated alkyl and aromatic carbons was achieved by CSA filtering plus dipolar dephasing. The method is demonstrated on a peat humic acid, yielding fa = 45 +/- 2% and faN = (0.64 +/- 0.07) x 45%.

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A very simple method for reducing probehead background signal in one-pulse 1H nuclear magnetic resonance (NMR) spectra is presented. Two one-pulse spectra are recorded, the first with pulse length tp1, the second with an L-times longer pulse, e.g. with L = 2. The second spectrum scaled by 1/L is subtracted from the first. Since the weak pulses experienced by spins outside the coil are in the linear regime, the background from outside the coil is effectively subtracted out. The background suppression efficiency is approximately 1.5b2, where b is the ratio of the B1 field inside the coil relative to that outside the coil. Experimentally, background suppression by at least a factor of 10 was achieved. Examples of background suppression in 1H wideline as well as 1H fast magic-angle spinning (MAS) one-pulse spectra of clay and polymer samples are shown.

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Selection of alkyl-carbon and suppression of aromatic-carbon 13C nuclear magnetic resonance (NMR) signals has been achieved by exploiting the symmetry-based, systematic difference in their 13C chemical-shift anisotropies (CSAs). Simple three- or five-pulse CSA-recoupling sequences with "gamma-integral" cleanly suppress the signals of all sp2- and sp-hybridized carbons. The chemical-shift-anisotropy-based dephasing is particularly useful for distinguishing the signals of di-oxygenated alkyl (O-C-O) carbons, found for instance as anomeric carbons in carbohydrates, from bands of aromatic carbons with similar 13C isotropic chemical shifts. The alkyl signals are detected with an efficiency of > 60%, with little differential dephasing. Combined with C-H dipolar dephasing, the CSA filter can identify ketal (unprotonated O-C-O) carbons unambiguously for the first time. Conversely, after short cross polarization and the CSA filter, O-CH-O (acetal) carbon signals are observed selectively. The methods are demonstrated on various model compounds and applied to a humic acid.

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Previous research has shown that long-term intensive cropping of irrigated lowland rice has led to significant grain-yield declines in field trials. The yield decline was attributed to decreased availability of soil nitrogen, which is held mostly in the soil organic matter. By advanced solid-state NMR spectroscopy, we have detected significant amounts of amide nitrogen directly bonded to aromatic rings in a humic acid fraction extracted from a continually submerged, triple-cropped rice soil. Because nitrogen bonded to aromatics is not readily plant-available, this observation can explain the yield decline. Quantitative (13)C NMR combined with advanced spectral editing showed that this humic acid is rich in lignin derivatives (>45% of all carbon), whereas the corresponding humic acid fraction extracted from an aerobic, single-cropped rice soil contains less lignin and less nitrogen bonded to aromatics.

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We extracted the acid-soluble portion of municipal biosolids, fractionated it by both molecular weight (MW) and hydrophobicity, and used various solid-state nuclear magnetic resonance (NMR) methods and diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy to characterize the fractions. Spectroscopic characterization of the MW components of the biosolids-derived organic matter fractions revealed the presence of functionally distinct groups of compounds. Quantitative 13C NMR, CH spectral editing, and several two-dimensional NMR experiments show that the high-MW hydrophilic fraction in particular is structurally simple, consisting predominantly of N-acetylated polysaccharides, perhaps derived from bacterial peptidoglycans. In the high-MW hydrophobic fraction, aromatic compounds were present in addition to the N-acetylated polysaccharides. Infrared spectroscopy confirmed that hydrophilic fractions were dominated by carbohydrates and indicated that the lower-MW fractions lacked amide moieties. Complementary interpretations of the DRIFT and NMR spectra improved our knowledge of the components separated by this fractionation scheme, allowing better characterization of biosolids organic matter. Moreover, fractionation based on both MW and hydrophobicity may prove useful in detailed characterization of the structure of biosolids-derived organic matter and other similarly heterogeneous natural organic matter in soils and sediments.

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