A solid state 13C NMR, crystallographic, and quantum chemical investigation of chemical shifts and hydrogen bonding in histidine dipeptides

We report the first solid-state NMR, crystallographic, and quantum chemical investigation of the origins of the 13C NMR chemical shifts of the imidazole group in histidine-containing dipeptides. The chemical shift ranges for Cgamma and Cdelta2 seen in eight crystalline dipeptides were very large (12.7-13.8 ppm); the shifts were highly correlated (R2= 0.90) and were dominated by ring tautomer effects and intermolecular interactions. A similar correlation was found in proteins, but only for buried residues. The imidazole 13C NMR chemical shifts were predicted with an overall rms error of 1.6-1.9 ppm over a 26 ppm range, by using quantum chemical methods. Incorporation of hydrogen bond partner molecules was found to be essential in order to reproduce the chemical shifts seen experimentally. Using AIM (atoms in molecules) theory we found that essentially all interactions were of a closed shell nature and the hydrogen bond critical point properties were highly correlated with the N...H...O (average R2= 0.93) and Nepsilon2...H...N (average R2= 0.98) hydrogen bond lengths. For Cepsilon1, the 13C chemical shifts were also highly correlated with each of these properties (at the Nepsilon2 site), indicating the dominance of intermolecular interactions for Cepsilon1. These results open up the way to analyzing 13C NMR chemical shifts, tautomer states (from Cdelta2, Cepsilon1 shifts), and hydrogen bond properties (from Cepsilon1 shifts) of histidine residue in proteins and should be applicable to imidazole-containing drug molecules bound to proteins, as well.     

Solid-state NMR fermi contact and dipolar shifts in organometallic complexes and metalloporphyrins

We have used density functional theory methods to investigate the solid-state "magic-angle" spinning (MAS) NMR and single-crystal NMR/ENDOR spectra of paramagnetic organometallic complexes and metalloporphyrins. The solid-state MAS NMR chemical shifts (including both diamagnetic and hyperfine contributions) are predicted with a slope of 1.007 and an R2 = 0.967, corresponding to a 28 ppm (or 6.3%) error over the entire 442 ppm range. Single-crystal ENDOR hyperfine values, including both isotropic Fermi contact and dipolar couplings, are predicted with a slope of 1.009 and an R2 = 0.998, corresponding to a 0.93 MHz (or 1.2%) error over the entire 78.37 MHz range. In addition, single-crystal NMR shifts (including both hyperfine terms) are predicted with an R2 = 0.961. The ability to compute solid-state MAS NMR and single-crystal NMR/ENDOR data should facilitate the use of these techniques in investigating paramagnetic metal complexes and should be of particular use in studying paramagnetic metalloproteins, where structures are less accurately known.     

A density functional study of the 13C NMR chemical shifts in functionalized single-walled carbon nanotubes

The 13C NMR chemical shifts for functionalized (7,0), (8,0), (9,0), and (10,0) single-walled carbon nanotubes (SWNTs) have been studied computationally using gauge-including projector-augmented plane-wave (GIPAW) density functional theory (DFT). The functional groups NH, NCH3, NCH2OH, and CH2NHCH2 have been considered, and different sites where covalent addition or substitution may occur have been examined. The shifts of the carbons directly attached to the group are sensitive to the bond which has been functionalized and may, therefore, be used to identify whether the group has reacted with a parallel or a diagonal C-C bond. The addition of NH to a parallel bond renders the functionalized carbons formally sp3-hybridized, yielding shifts of around 44 ppm, independent of the SWNT radius. Reaction with a diagonal bond retains the formal sp2 hybridization of the substituted carbons, and their shifts are slightly lower or higher than those of the unsubstituted carbon atoms. The calculated 1H NMR shifts of protons in the functional groups are also dependent upon the SWNT-group interaction. Upon decreasing the degree of functionalization for the systems where the group is added to a parallel bond, the average chemical shift of the unfunctionalized carbons approaches that of the pristine tube. At the same time, the shifts of the functionalized carbons remain independent upon the degree of functionalization. For the SWNTs where N-R attaches to a parallel bond, the average shift of the sp2 carbons was found to be insensitive to the substituent R. Moreover, the shifts of the functionalized sp3 carbons, as well as of the carbons within the group itself, are independent of the SWNT radius. The results indicate that a wealth of knowledge may be obtained from the 13C NMR of functionalized SWNTs.     

Visualization and quantification of the anisotropic effect of C=C double bonds on 1H NMR spectra of highly congested hydrocarbons-indirect estimates of steric strain

The anisotropic effect of the olefinic C=C double bond has been calculated by employing the NICS ( nucleus independent chemical shift) concept and visualized as an anisotropic cone by a through space NMR shielding grid. Sign and size of this spatial effect on (1)H chemical shifts of protons in norbornene, exo- and endo-2-methylnorbornenes, and in three highly congested tetracyclic norbornene analogs have been compared with the experimental (1)H NMR spectra as far as published. (1)H NMR spectra have also been calculated at the HF/6-31G* level of theory to get a full, comparable set of proton chemical shifts. Differences between delta( (1)H)/ppm and the calculated anisotropic effect of the CC double bond are discussed in terms of the steric compression that occurs in the compounds studied.     

A density functional theory investigation of Fe-N-O bonding in heme proteins and model systems

We report the results of a series of density functional theory (DFT) calculations of the M?ssbauer quadrupole splittings and isomer shifts in NO heme model compounds, together with the results of calculations of the M?ssbauer quadrupole splittings, isomer shifts, and electron paramagnetic resonance hyperfine coupling constants in a model Fe(II)(NO)(imidazole) complex as a function of Fe-NO bond length and Fe-N-O bond angle. The results of the M?ssbauer quadrupole splitting and isomer shift calculations on the NO heme model compounds show good accord between theory and experiment, with the largest errors being observed for structures having the largest crystallographic R(1) values. The results of the property surface calculations were then used to calculate Fe-NO bond length and Fe-N-O bond angle probability surfaces (Z-surfaces) for a nitrosyl hemoglobin, using, in addition, an energy filter. The results obtained yielded a most probable Fe-NO bond length (r) of 1.79 A and an Fe-N-O bond angle (beta) of 136 degrees -137 degrees. This bond length is somewhat longer than those observed in most model compounds but may be due, at least in part, to hydrogen bond formation with the distal His residue. Bond elongation was also observed in a geometry optimized Fe(II)(NO)(imidazole) complex hydrogen bonded to an imidazole residue, in which we find r = 1.76-1.78 A and beta = 137 degrees -138 degrees. The computed bond angles are close to the canonical approximately 140 degrees value found in most model systems. Highly bent Fe-N-O bond angles or very long Fe-NO bond lengths seem unlikely to occur in proteins, due to their high energies. We also investigated the molecular orbitals and spin densities in each of the six coordinate systems investigated and found the orbitals and spin densities to be generally similar those described previously for five coordinate systems. Taken together, these results show that M?ssbauer quadrupole splittings and isomer shifts, in addition to electron paramagnetic resonance hyperfine coupling constants, can now be calculated for nitrosyl heme systems with relatively good accuracy and that the results so obtained can be used to determine Fe-N-O geometries in metalloproteins. The Z-surface approach is thus applicable to both diamagnetic (CO) and paramagnetic (NO) heme proteins with in both cases the metal-ligand binding geometries found in the proteins being very close to those seen in model systems.     

Carbon-13 NMR shielding in the twenty common amino acids: comparisons with experimental results in proteins

We have used ab initio quantum chemical techniques to compute the (13)C(alpha) and (13)C(beta) shielding surfaces for the 14 amino acids not previously investigated (R. H. Havlin et al., J. Am. Chem. Soc. 1997, 119, 11951-11958) in their most popular conformations. The spans (Omega = sigma(33) - sigma(11)) of all the tensors reported here are large ( approximately 34 ppm) and there are only very minor differences between helical and sheet residues. This is in contrast to the previous report in which Val, Ile and Thr were reported to have large ( approximately 12 ppm) differences in Omega between helical and sheet geometries. Apparently, only the beta-branched (beta-disubstituted) amino acids have such large CSA span (Omega) differences; however, there are uniformly large differences in the solution-NMR-determined CSA (Deltasigma = sigma(orth) - sigma(par)) between helices and sheets in all amino acids considered. This effect is overwhelmingly due to a change in shielding tensor orientation. With the aid of such shielding tensor orientation information, we computed Deltasigma values for all of the amino acids in calmodulin/M13 and ubiquitin. For ubiquitin, we find only a 2.7 ppm rmsd between theory and experiment for Deltasigma over an approximately 45 ppm range, a 0.96 slope, and an R(2) = 0.94 value when using an average solution NMR structure. We also report C(beta) shielding tensor results for these same amino acids, which reflect the small isotropic chemical shift differences seen experimentally, together with similar C(beta) shielding tensor magnitudes and orientations. In addition, we describe the results of calculations of C(alpha), C(beta), C(gamma)1, C(gamma)2, and C(delta) shifts in the two isoleucine residues in bovine pancreatic trypsin inhibitor and the four isoleucines in a cytochrome c and demonstrate that the side chain chemical shifts are strongly influenced by chi(2) torsion angle effects. There is very good agreement between theory and experiment using either X-ray or average solution NMR structures. Overall, these results show that both C(alpha) backbone chemical shift anisotropy results as well as backbone and side chain (13)C isotropic shifts can now be predicted with good accuracy by using quantum chemical methods, which should facilitate solution structure determination/refinement using such shielding tensor surface information.     

31P NMR chemical shifts in hypervalent oxyphosphoranes and polymeric orthophosphates

We report the first quantum chemical investigation of the solid- and solution-state 31P NMR chemical shifts in models for phosphoryl transfer enzyme reaction intermediates and in polymeric inorganic phosphates. The 31P NMR chemical shifts of five- and six-coordinate oxyphosphoranes containing a variety of substitutions at phosphorus, as well as four-coordinate polymeric orthophosphates and four-coordinate phosphonates, are predicted with a slope of 1.00 and an R2= 0.993 (N = 34), corresponding to a 3.8 ppm (or 2.1%) error over the entire 178.3 ppm experimental chemical shift range, using Hartree-Fock methods. For the oxyphosphoranes, we used either experimental crystallographic structures or, when these were not available, fully geometry optimized molecular structures. For the four-coordinate phosphonates we used X-ray structures together with charge field perturbation, to represent lattice interactions. For the three-dimensional orthophosphates (BPO4, AlPO4, GaPO4 we again used X-ray structures, but for these inorganic systems we employed a self-consistent charge field perturbation approach on large clusters, to deduce peripheral atom charges. For pentaoxyphosphoranes, the solvent effect on 31P NMR chemical shieldings was found to be very small (<0.5 ppm). The 31P NMR chemical shielding tensors in the pentaoxyphosphoranes were in most cases found to be close to axially symmetric and were dominated by changes in the shielding tensor components in the equatorial plane (sigma22 and sigma33). The isotropic shifts were highly correlated (R2= 0.923) with phosphorus natural bonding orbital charges, with the larger charges being associated with shorter axial P-O bond lengths and, hence, more shielding. Overall, these results should facilitate the use of 31P NMR techniques in investigating the structures of more complex systems, such as phosphoryl transfer enzymes, as well as in investigating other, complex oxide structures.     

Binding energies and 19F nuclear magnetic deshielding in paramagnetic halogen-bonded complexes of TEMPO with haloperfluorocarbons

19F NMR measurements and theoretical calculations were performed to study paramagnetic complexes of iodoperfluorocarbons with stable nitroxide radicals. Contrary to what is usually measured for diamagnetic halogen-bonded complexes involving iodoperfluorocarbons, it was found that the formation of complexes with the 2,2,6,6-tetramethyl(piperidin-1-yloxyl) (TEMPO) radical determines downfield shifts in the 19F NMR spectra. The experimental finding was confirmed by calculating nuclear shielding using density functional theory and correcting the isotropic diamagnetic (19)F chemical shift with contact interactions evaluated from the hyperfine coupling tensor. The computational analysis of the interaction between CF3I and TEMPO, by using DFT and MP2 theories, showed that the occurrence of the halogen bond between the interacting partners is associated with a significant charge transfer to CF3I and that the measured downfield shift is due to the occurring spin transfer.     

31P chemical shift of adsorbed trialkylphosphine oxides for acidity characterization of solid acids catalysts

A comprehensive study has been made to predict the adsorption structures and (31)P NMR chemical shifts of various trialkylphosphine oxides (R3PO) probe molecules, viz., trimethylphosphine oxide (TMPO), triethylphosphine oxide (TEPO), tributylphosphine oxide (TBPO), and trioctylphosphine oxide (TOPO), by density functional theory (DFT) calculations based on 8T zeolite cluster models with varied Si-H bond lengths. A linear correlation between the (31)P chemical shifts and proton affinity (PA) was observed for each of the homologous R3PO probe molecules examined. It is found that the differences in (31)P chemical shifts of the R3POH(+) adsorption complexes, when referring to the corresponding chemical shifts in their crystalline phase, may be used not only in identifying Br?nsted acid sites with varied acid strengths but also in correlating the (31)P NMR data obtained from various R3PO probes. Such a chemical shift difference therefore can serve as a quantitative measure during acidity characterization of solid acid catalysts when utilizing (31)P NMR of various adsorbed R3PO, as proposed in our earlier report (Zhao; et al. J. Phys. Chem. B 2002, 106, 4462) and also illustrated herein by using a mesoporous H-MCM-41 aluminosilicate (Si/Al = 25) test adsorbent. It is indicative that, with the exception of (TMPO), variations in the alkyl chain length of the R3PO (R = C(n)H(2n+1); n > or = 2) probe molecules have only negligible effect on the (31)P chemical shifts (within experimental error of ca. 1-2 ppm) either in their crystalline bulk or in their corresponding R3POH(+) adsorption complexes. Consequently, an average offset of 8 +/- 2 ppm was observed for (31)P chemical shifts of adsorbed R3PO with n > or = 2 relative to TMPO (n = 1). Moreover, by taking the value of 86 ppm predicted for TMPO adsorbed on 8T cluster models as a threshold for superacidity (Zheng; et al. J. Phys. Chem. B 2008, 112, 4496), a similar threshold (31)P chemical shift of ca. 92-94 ppm was deduced for TEPO, TBPO, and TOPO.     

(67)Zn NMR chemical shifts and electric field gradients in zinc complexes: a quantum chemical investigation

We have used quantum chemical methods to predict 67Zn NMR chemical shifts as well as quadrupole coupling constants (CQ) in a series of biomimetic and inorganic zinc complexes. The 67Zn chemical shifts are predicted with an R2 = 0.975, corresponding to a 24.3 ppm or 6.7% error over the entire 365 ppm 67Zn chemical shift range. The 67Zn CQ values are predicted with an R2 = 0.991, corresponding to a 1.17 MHz or 3.0% error over the entire 38.75 MHz range. The 67Zn NMR shifts in a series of complexes containing N,O ligands are, in general, highly correlated with the number of oxygen ligands. The ability to compute 67Zn NMR shifts as well as CQ values opens up the possibility of using both of these properties in structure determination or refinement in proteins.     

Computational studies of 13C NMR chemical shifts of saccharides

The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.     

Microenvironmental and conformational structure of triblock copolymers in aqueous solution by 1H and 13C NMR spectroscopy

1H and 13C nuclear magnetic resonance (NMR) spectra of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers in D2O solutions have been systematically investigated. The detailed assignments of various 1H and 13C NMR signals are presented. The hyperfine structure of PO -CH2- protons was clearly assigned, the arising reason of this hyperfine structure was attributed to the influence of the chiral center of -CHCH3- groups and the direct coupling between the PO -CH2- and -CH3 protons. The external standard 2,2-dimethyl-2-silapentane-5-sulfonate sodium salt (DSS) was firstly applied in this system. Accurate chemical shift values referenced to the external standard DSS were obtained. 1H NMR chemical shift of PO -CH2- and -CH3 signals shows a larger decrease in ppm values than that of EO -CH2- signal with the increase of PPO/PEO ratio or temperature indicating that PO segments exist in a more hydrophobic microenvironment. A new resonance signal assigned to the PO -CH2- protons appeared when the temperature is above the CMT, which is attributed to the breakdown of the intra-molecular (C-H)...O hydrogen bond between the PO -CH2- protons and the ester oxygens. The breakdown of this intra-molecular hydrogen bond may result in a decrease of gauche conformers of the PPO chain. The increase of 13C NMR chemical shift of block copolymers validates this conformational change assumption. It can be inferred that the amount of gauche conformers decreases whereas that of trans conformers increases in both PO and EO chains when elevating the PPO/PEO ratio or temperature. The observed 13C NMR chemical shifts of PO segments show a bigger increase than those of EO segments, supporting the formation of a nonpolar microenvironment around PO segments.     

Nuclear magnetic resonance shifts in paramagnetic metalloporphyrins and metalloproteins

We report the first detailed investigation of the (1)H, (13)C, (15)N, and (19)F nuclear magnetic resonance (NMR) spectroscopic shifts in paramagnetic metalloprotein and metalloporphyrin systems. The >3500 ppm range in experimentally observed hyperfine shifts can be well predicted by using density functional theory (DFT) methods. Using spin-unrestricted methods together with large, locally dense basis sets, we obtain very good correlations between experimental and theoretical results: R(2) = 0.941 (N = 37, p < 0.0001) when using the pure BPW91 functional and R(2) = 0.981 (N = 37, p < 0.0001) when using the hybrid functional, B3LYP. The correlations are even better for C(alpha) and C(beta) shifts alone: C(alpha), R(2) = 0.996 (N = 8, p < 0.0001, B3LYP); C(beta), R(2) = 0.995 (N = 8, p < 0.0001, B3LYP), but are worse for C(meso), in part because of the small range in C(meso) shifts. The results of these theoretical calculations also lead to a revision of previous heme and proximal histidine residue (13)C NMR assignments in deoxymyoglobin which are confirmed by new quantitative NMR measurements. Molecular orbital (MO) analyses of the resulting wave functions provide a graphical representation of the spin density distribution in the [Fe(TPP)(CN)(2)](-) (TPP = 5,10,15,20-tetraphenylporphyrinato) system (S = (1)/(2)), where the spin density is shown to be localized primarily in the d(xz) (or d(yz)) orbital, together with an analysis of the frontier MOs in Fe(TPP)Cl (S = (5)/(2)), Mn(TPP)Cl (S = 2), and a deoxymyoglobin model (S = 2). The ability to now begin to predict essentially all heavy atom NMR hyperfine shifts in paramagnetic metalloporphyrins and metalloproteins using quantum chemical methods should open up new areas of research aimed at structure prediction and refinement in paramagnetic systems in much the same way that DFT methods have been used successfully in the past to predict/refine elements of diamagnetic heme protein structures.     

Solution and solid state NMR studies of some selenium analogues of auranofin (an anti-arthritic gold drug)

Three mixed ligand complexes of gold(I) with phosphines and selenones, [Et₃PAuSe=C<]Br as analogues of auranofin (Et₃PAuSR) have been prepared and characterized by elemental analysis, IR and NMR methods. A decrease in the IR frequency of the C=Se mode of selenones upon complexation is indicative of selenone binding to gold(I) via a selenone group. An upfield shift in ¹³C NMR for the C=Se resonance of the selenones and downfield shifts in ³¹P NMR for the R₃P moiety are consistent with the selenium coordination to gold(I). ¹³C solid state NMR shows the chemical shift difference between free and bound selenone to gold(I) for ImSe and DiazSe to be ca 10 and 17?ppm respectively. Large ⁷⁷Se NMR chemical shifts (55?ppm) upon complexation in the solid state for [Et₃PAuDiazSe]Br compared to [Et₃PAuImSe]Br (10?ppm) indicates the former to be more stable and the Au–Se bond to be stronger than in the latter complex.     

Density functional study of the 13C NMR chemical shifts in small-to-medium-diameter infinite single-walled carbon nanotubes

NMR chemical shifts were calculated for semiconducting (n,0) single-walled carbon nanotubes (SWNTs) with n ranging from 7 to 17. Infinite isolated SWNTs were calculated using a gauge-including projector-augmented plane-wave (GIPAW) approach with periodic boundary conditions and density functional theory (DFT). In order to minimize intertube interactions in the GIPAW computations, an intertube distance of 8 A was chosen. For the infinite tubes, we found a chemical shift range of over 20 ppm for the systems considered here. The SWNT family with lambda = mod(n, 3) = 0 has much smaller chemical shifts compared to the other two families with lambda = 1 and lambda = 2. For all three families, the chemical shifts decrease roughly inversely proportional to the tube's diameter. The results were compared to calculations of finite capped SWNT fragments using a gauge-including atomic orbital (GIAO) basis. Direct comparison of the two types of calculations could be made if benzene was used as the internal (computational) reference. The NMR chemical shifts of finite SWNTs were found to converge very slowly, if at all, to the infinite limit, indicating that capping has a strong effect (at least for the (9,0) tubes) on the calculated properties. Our results suggest that (13)C NMR has the potential for becoming a useful tool in characterizing SWNT samples.     

Investigating sorption on iron-oxyhydroxide soil minerals by solid-state NMR spectroscopy: a 6Li MAS NMR study of adsorption and absorption on goethite

High-resolution 2H MAS NMR spectra can be obtained for nanocrystalline particles of goethite (alpha-FeOOH, particle size approximately 4-10 nm) at room temperature, facilitating NMR studies of sorption under environmentally relevant conditions. Li sorption was investigated as a function of pH, the system representing an ideal model system for NMR studies. 6Li resonances with large hyperfine shifts (approximately 145 ppm) were observed above the goethite point of zero charge, providing clear evidence for the presence of Li-O-Fe connectivities, and thus the formation of an inner sphere Li+ complex on the goethite surface. Even larger Li hyperfine shifts (289 ppm) were observed for Li+-exchanged goethite, which contains lithium ions in the tunnels of the goethite structure, confirming the Li assignment of the 145 ppm Li resonance to the surface sites.     

Rapid calculation of protein chemical shifts using bond polarization theory and its application to protein structure refinement

Although difficult to analyze, NMR chemical shifts provide detailed information on protein structure. We have adapted the semi-empirical bond polarization theory (BPT) to protein chemical shift calculation and chemical shift driven protein structure refinement. A new parameterization for BPT amide nitrogen chemical shift calculation has been derived from MP2 ab initio calculations and successfully evaluated using crystalline tripeptides. We computed the chemical shifts of the small globular protein ubiquitin, demonstrating that BPT calculations can match the results obtained at the DFT level of theory at very low computational cost. In addition to the calculation of chemical shift tensors, BPT allows the calculation of chemical shift gradients and consequently chemical shift driven geometry optimizations. We applied chemical shift driven protein structure refinement to the conformational analysis of a set of Trypanosoma brucei (the causative agent of African sleeping sickness) tryparedoxin peroxidase Px III structures. We found that the interaction of Px III with its reaction partner Tpx seems to be governed by conformational selection rather than by induced fit.     

Solid‐State 17O NMR Spectroscopy of Paramagnetic Coordination Compounds

High‐quality solid‐state ¹⁷O (I=5/2) NMR spectra can be successfully obtained for paramagnetic coordination compounds in which oxygen atoms are directly bonded to the paramagnetic metal centers. For complexes containing V^III (S=1), Cu^II (S=1/2), and Mn^III (S=2) metal centers, the ¹⁷O isotropic paramagnetic shifts were found to span a range of more than 10 000 ppm. In several cases, high‐resolution ¹⁷O NMR spectra were recorded under very fast magic‐angle spinning (MAS) conditions at 21.1 T. Quantum‐chemical computations using density functional theory (DFT) qualitatively reproduced the experimental ¹⁷O hyperfine shift tensors.     

Is NMR the tool to characterize the structure of C20 isomers?

We investigate the feasibility of using nuclear magnetic resonance (NMR) chemical shift calculations as a tool to provide structural information for C₂₀ fullerene type molecules. NMR chemical shifts are extremely sensitive to the local chemical environment of an atom, reflecting unambiguously its bond lengths and angles as well as its hybridization. Thus, they can distinguish between the different isomers that are candidates for the ground state of this molecule. We calculate the NMR shifts for several C₂₀ isomers and show that NMR constitutes a potential tool to discriminate and identify experimentally a particular C₂₀ molecular conformation, and also the level of theory which best describes the experimental structure.     

Regression formulas for density functional theory calculated 1H and 13C NMR chemical shifts in toluene-d8

This study aimed at investigating the performance of a series of basis sets, density functional theory (DFT) functionals, and the IEF-PCM solvation model in the accurate calculation of (1)H and (13)C NMR chemical shifts in toluene-d(8). We demonstrated that, on a test set of 37 organic species with various functional moieties, linear scaling significantly improved the calculated shifts and was necessary to obtain more accurate results. Inclusion of a solvation model produced larger deviations from the experimental data as compared to the gas-phase calculations. Moreover, we did not find any evidence that very large basis sets were necessary to reproduce the experimental NMR data. Ultimately, we recommend the use of the BMK functional. For the (1)H shifts the use of the 6-311G(d) basis set gave linearly scaled mean unsigned (MU) and root-mean-square (rms) errors of 0.15 ppm and 0.21 ppm, respectively. For the calculation of the (13)C chemical shifts the 6-31G(d) basis set produced MUE of 1.82 ppm and RMSE of 3.29 ppm.