An experimental and theoretical investigation of the chemical shielding tensors of (13)C(alpha) of alanine, valine, and leucine residues in solid peptides and in proteins in solution

We have carried out a solid-state magic-angle sample-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopic investigation of the (13)C(alpha) chemical shielding tensors of alanine, valine, and leucine residues in a series of crystalline peptides of known structure. For alanine and leucine, which are not branched at the beta-carbon, the experimental chemical shift anisotropy (CSA) spans (Omega) are large, about 30 ppm, independent of whether the residues adopt helical or sheet geometries, and are in generally good accord with Omega values calculated by using ab initio Hartree-Fock quantum chemical methods. The experimental Omegas for valine C(alpha) in two peptides (in sheet geometries) are also large and in good agreement with theoretical predictions. In contrast, the "CSAs" (Deltasigma) obtained from solution NMR data for alanine, valine, and leucine residues in proteins show major differences, with helical residues having Deltasigma values of approximately 6 ppm while sheet residues have Deltasigma approximately 27 ppm. The origins of these differences are shown to be due to the different definitions of the CSA. When defined in terms of the solution NMR CSA, the solid-state results also show small helical but large sheet CSA values. These results are of interest since they lead to the idea that only the beta-branched amino acids threonine, valine, and isoleucine can have small (static) tensor spans, Omega (in helical geometries), and that the small helical "CSAs" seen in solution NMR are overwhelmingly dominated by changes in tensor orientation, from sheet to helix. These results have important implications for solid-state NMR structural studies which utilize the CSA span, Omega, to differentiate between helical and sheet residues. Specifically, there will be only a small degree of spectral editing possible in solid proteins since the spans, Omega, for the dominant nonbranched amino acids are quite similar. Editing on the basis of Omega will, however, be very effective for many Thr, Val, and Ileu residues, which frequently have small ( approximately 15-20 ppm) helical CSA (Omega) spans.     

Residue-specific 13C' CSA tensor principal components for ubiquitin: correlation between tensor components and hydrogen bonding

The residue-specific 13C' CSA tensor principal components, sigma(11), sigma(22), sigma(33), and the tensor orientation defined by the rotation angles beta and gamma have been determined by solution NMR for uniformly labeled ubiquitin partially aligned in four different media. Spurious chemical shift deviations due to solvent effects were corrected with an offset calculated by linear regression of the residual dipolar couplings and chemical shifts at increasing alignment strengths. Analysis of this effect revealed no obvious correlation to solvent exposure. Data obtained in solution from a protein offer a better sampling of 13C' CSA for different amino acid types in a complex heterogeneous environment, thereby allowing for the evaluation of structural variables that would be challenging to achieve by other methods. The 13C' CSA principal components cluster about the average values previously determined, and experimental correlations observed between sigma(11), sigma(22) tensorial components and C'O...H(N) hydrogen bonding are discussed. The inverse association of sigma(11) and sigma(22) exemplify the calculated and solid-state NMR observed effect on the tensor components by hydrogen bonding. We also show that 13C' CSA tensors are sensitive to hydrogen-bond length but not hydrogen-bond angle. This differentiation was previously unavailable. Similarly, hydrogen bonding to the conjugated NH of the same peptide plane has no detectable effect. Importantly, the observed weak correlations signify the presence of confounding influences such as nearest-neighbor effects, side-chain conformation, electrostatics, and other long-range factors to the 13C' CSA tensor. These analyses hold future potential for exploration provided that more accurate data from a larger number of proteins and alignments become available.     

Determinations of 15N chemical shift anisotropy magnitudes in a uniformly 15N,13C-labeled microcrystalline protein by three-dimensional magic-angle spinning nuclear magnetic resonance spectroscopy

Amide 15N chemical shift anisotropy (CSA) tensors provide quantitative insight into protein structure and dynamics. Experimental determinations of 15N CSA tensors in biologically relevant molecules have typically been performed by NMR relaxation studies in solution, goniometric analysis of single-crystal spectra, or slow magic-angle spinning (MAS) NMR experiments of microcrystalline samples. Here we present measurements of 15N CSA tensor magnitudes in a protein of known structure by three-dimensional MAS solid-state NMR. Isotropic 15N, 13C alpha, and 13C' chemical shifts in two dimensions resolve site-specific backbone amide recoupled CSA line shapes in the third dimension. Application of the experiments to the 56-residue beta1 immunoglobulin binding domain of protein G (GB1) enabled 91 independent determinations of 15N tensors at 51 of the 55 backbone amide sites, for which 15N-13C alpha and/or 15N-13C' cross-peaks were resolved in the two-dimensional experiment. For 37 15N signals, both intra- and interresidue correlations were resolved, enabling direct comparison of two experimental data sets to enhance measurement precision. Systematic variations between beta-sheet and alpha-helix residues are observed; the average value for the anisotropy parameter, delta (delta = delta(zz) - delta(iso)), for alpha-helical residues is 6 ppm greater than that for the beta-sheet residues. The results show a variation in delta of 15N amide backbone sites between -77 and -115 ppm, with an average value of -103.5 ppm. Some sites (e.g., G41) display smaller anisotropy due to backbone dynamics. In contrast, we observe an unusually large 15N tensor for K50, a residue that has an atypical, positive value for the backbone phi torsion angle. To our knowledge, this is the most complete experimental analysis of 15N CSA magnitude to date in a solid protein. The availability of previous high-resolution crystal and solution NMR structures, as well as detailed solid-state NMR studies, will enhance the value of these measurements as a benchmark for the development of ab initio calculations of amide 15N shielding tensor magnitudes.     

Performance of density functional models to reproduce observed (13)C(alpha) chemical shifts of proteins in solution

The purpose of this work is to test several density functional models (namely, OPBE, O3LYP, OPW91, BPW91, OB98, BPBE, B971, OLYP, PBE1PBE, and B3LYP) to determine their accuracy and speed for computing (13)C(alpha) chemical shifts in proteins. The test is applied to 10 NMR-derived conformations of the 76-residue alpha/beta protein ubiquitin (protein data bank id 1D3Z). With each functional, the (13)C(alpha) shielding was computed for 760 amino acid residues by using a combination of approaches that includes, but is not limited to, treating each amino acid X in the sequence as a terminally blocked tripeptide with the sequence Ac-GXG-NMe in the conformation of the regularized experimental protein structure. As computation of the (13)C(alpha) chemical shifts, not their shielding, is the main goal of this work, a computation of the (13)C(alpha) shielding of the reference, namely, tetramethylsilane, is investigated here and an effective and a computed tetramethylsilane shielding value for each of the functionals is provided. Despite observed small differences among all functionals tested, the results indicate that four of them, namely, OPBE, OPW91, OB98, and OLYP, provide the most accurate functionals with which to reproduce observed (13)C(alpha) chemical shifts of proteins in solution, and are among the faster ones. This study also provides evidence for the applicability of these functionals to proteins of any size or class, and for the validation of our previous results and conclusions, obtained from calculations with the slower B3LYP functional.     

Calculations of the contribution of ring currents to the chemical shielding anisotropy

Ring currents can exert a large effect upon the chemical shielding of NMR resonances. The analytical expression developed by Waugh and Fessenden (Waugh, J. S.; Fessenden, R. W. J. Am. Chem. Soc. 1957, 79, 846) and Johnson and Bovey (Johnson, C. E.; Bovey, F. A. J. Chem. Phys. 1957, 29, 1012) only quantifies the contribution of ring currents to the isotropic component of the shielding tensor. In the work described here an additional analytical expression is developed so that the contribution of ring currents to the full shielding tensor can be calculated, allowing an estimate of the influence of ring currents upon the chemical shielding anisotropy (CSA, Deltasigma). To test that this pair of analytical expressions can provide a reasonable estimate of the contribution of ring currents to the full shielding tensor a series of density functional calculations (DFT) were carried out. A shielding tensor in a model compound was calculated in two distinct ways. For the first series, DFT shielding calculations of the model compound were carried out in the presence of a benzene ring. For the second series a ring current contribution to the shielding tensor was calculated via the new expressions, and this was added to the result of a DFT shielding calculation which used in place of benzene the nonaromatic analogue 1,3 cyclohexadiene. The two series of results proved to be in excellent agreement. The pair of analytical expressions are used to calculate ring current contributions to the CSA (Deltasigma) of 1H(N) backbone amide resonances in a structure of the second type 2 module from the protein fibronectin. Significant CSA variations are predicted in particular for the 1H(N) of G42 which is most likely involved in a N-H...tpi aromatic hydrogen bond.     

Carbon chemical shift tensor components in quinolines and quinoline N-oxides

Chemical shift calculations are carried out for the quinoline carbons in 1,8-bis(2-isopropyl-4-quinolyl)naphthalene, 2-isopropylquinoline, amodiaquine, chloroquine, and quinine and the N-oxide of each compound. Ab initio calculations of the isotropic shielding values are in agreement with experimental chemical shifts. The calculations indicate that changes to the principal components of the shielding tensor upon N-oxidation are similar for each compound. Carbons 2, 4, 8, and 10 are largely shielded in each case as the nitrogen is oxidized. For C2, C4, and C10, this shielding is due to a large change in sigma11 and/or sigma22, indicating a change in pi-electron density. For C8, the large shielding change is due mainly to a change in sigma33, indicating a change in sigma-electron density. Upon examination and comparison of the calculated 13C shielding tensor components in the antimalarial drugs versus those in unsubstituted quinolines, it is found that amodiaquine and chloroquine have increased pi-electron density in the ring containing the amino side chain and quinine has increased pi-electron density in the opposite ring, containing the methoxy substituent.     

113Cd Shielding Tensors of Monomeric Cadmium Compounds Containing Nitrogen Donor Atoms. 3. CP/MAS Studies on Five-Coordinate Cadmium Complexes Having N(3)X(2) (X = H, N, O, and S) Donor Atoms

The principal elements of the (113)Cd shielding tensor for a set of five- coordinate compounds having mixed donor atoms coordinating to the cadmium were determined via CP/MAS NMR experiments. The first complex, [HB(3,5-Me(2)pz)(3)]CdBH(4) (where pz = pyrazolyl), has a CdN(3)H(2) inner coordination sphere. The isotropic chemical shift in the solid state is 355.1 ppm, and its chemical shift anisotropy (CSA, Deltasigma) is -596 ppm with an asymmetry parameter (eta) of 0.64. The second complex, [HB(3,5-Me(2)pz)(3)]Cd[H(2)B(pz)(2)], has five nitrogen donor atoms bonded to the cadmium. This N(5) or N(3)N(2) compound was the only material of this study to manifest dipolar splitting of the cadmium resonance from the quadrupolar (14)N. The isotropic chemical shift, CSA, and the value of eta for this material were therefore determined at higher field where the dipolar splitting was less than the linewidth, yielding values of 226.6 ppm, -247 ppm, and 0.32, respectively. A second N(5) material, [HB(3-Phpz)(3)]Cd[H(2)B(3,5-Me(2)pz)(2)], was also investigated and has an isotropic shift of 190.2 ppm, a CSA of 254 ppm, and an eta of 0.86. Also studied was [HB(3-Phpz)(3)]Cd[(Bu(t)CO)(2)CH], which has an CdN(3)O(2) inner core. The isotropic chemical shift of this complex is 173.6 ppm, and the values of Deltasigma and eta were determined to be -258 ppm and 0.38, respectively. The final compound, [HB(3,5-Me(2)pz)(3)]Cd[S(2)CNEt(2)], with N(3)S(2) donor atoms, has an isotropic shift of 275.8 ppm, an eta of 0.51, and a CSA of +375 ppm. Utilizing previous assignments, the most shielded tensor element was determined to be oriented normal to the plane of the tridentate ligand. The shielding tensor information is used to speculate on the coordination geometry of the CdN(3)O(2) inner core complex.     

Secondary structures of peptides and proteins via NMR chemical-shielding anisotropy (CSA) parameters

Complete nuclear magnetic resonance (NMR) chemical-shielding tensors, sigma, have been computed at different levels of density-functional theory (DFT), within the gauge-including atomic orbital (GIAO) formalism, for the atoms of the peptide model For-L-Ala-NH2 as a function of the backbone dihedral angles phi and psi by employing a dense grid of 10 degrees. A complete set of rigorously orthogonal symmetric tensor invariants, {sigma iso, rho, tau}, is introduced, where sigma iso is the usual isotropic chemical shielding, while the newly introduced rho and tau parameters describe the magnitude and the orientation/shape of the chemical-shielding anisotropy (CSA), respectively. The set {sigma iso, rho, tau} is unaffected by unitary transformations of the symmetric part of the shielding tensor. The mathematically and physically motivated {rho, tau} anisotropy pair is easily connected to more traditional shielding anisotropy measures, like span (Omega) and skew (kappa). The effectiveness of the different partitions of the CSA information in predicting conformations of peptides and proteins has been tested throughout the Ramachandran space by generating theoretical NMR anisotropy surfaces for our For-L-Ala-NH2 model. The CSA surfaces, including Omega(phi, psi), kappa(phi, psi), rho(phi, psi), and tau(phi, psi) are highly structured. Individually, none of these surfaces is able to distinguish unequivocally between the alpha-helix and beta-strand secondary structural types of proteins. However, two- and three-dimensional correlated plots, including Omega versus kappa, rho versus tau, and sigma iso versus rho versus tau, especially for 13Calpha, have considerable promise in distinguishing among all four of the major secondary structural elements.     

15N Chemical shielding in glycyl tripeptides: measurement by solid-state NMR and correlation with X-ray structure

15N chemical shielding parameters are reported for central glycyl residues in crystallographically characterized tripeptides with alpha-helix, beta-strand, polyglycine II (3(1)-helix), and extended structures. Accurate values of the shielding components (2-5 ppm) are determined from MAS and stationary spectra of peptides containing [2-(13)C,(15)N]Gly. Two dipolar couplings, (1)H-(15)N and (13)C(alpha)-(15)N, are used to examine (15)N shielding tensor orientations in the molecular frame and the results indicate that the delta(11), delta(33) plane of the shielding tensor is not coincident with the peptide plane. The observed isotropic shifts, which vary over a range of 13 ppm, depend on hydrogen bonding (direct and indirect) and local conformation. Tensor spans, delta(span) = delta(11) - delta(33), and their deviations from axial symmetry, delta(dev) = delta(22) - delta(33), vary over a larger range and are grouped according to 2 degrees structure. Augmented by previously reported (13)C(alpha) shielding parameters, a prediction scheme for the 2 degrees structure of glycyl residues in proteins based on shielding parameters is proposed.     

Determination of calpha chemical shift tensor orientation in peptides by dipolar-modulated chemical shift recoupling NMR spectroscopy

We present a new method for determining the orientation of chemical shift tensors in polycrystalline solids with site resolution and demonstrate its application to the determination of the Calpha chemical shift tensor orientation in a model peptide with beta-sheet torsion angles. The tensor orientation is obtained under magic angle spinning by modulating a recoupled chemical shift anisotropy (CSA) pattern with various dipolar couplings. These dipolar-modulated chemical shift patterns constitute the indirect dimension of a 2D spectrum and are resolved according to the isotropic chemical shifts of different sites in the direct dimension. These dipolar-modulated CSA spectra are equivalent to the projection of a 2D static separated-local-field spectrum onto its chemical shift dimension, except that its dipolar dimension is multiplied with a modulation function. Both (13)C-(1)H and (13)C-(15)N dipolar couplings can modulate the CSA spectra of the Calpha site in an amino acid and yield the relative orientations of the chemical shift principal axes to the C-H and C-N bonds. We demonstrate the C-H and C-N modulated CSA experiments on methylmalonic acid and N-tBoc-glycine, respectively. The MAS results agree well with the results of the 2D separated-local-field spectra, thus confirming the validity of this MAS dipolar-modulation approach. Using this technique, we measured the Val Calpha tensor orientation in N-acetylvaline, which has beta-sheet torsion angles. The sigma(11) axis is oriented at 158 degrees (or 22 degrees) from the C-H bond, while the sigma(22) axis is tilted by 144 degrees (or 36 degrees) from the C-N bond. Both the orientations and the magnitude of this chemical shift tensor are in excellent agreement with quantum chemical calculations.     

Solid-state (13)C NMR chemical shift anisotropy tensors of polypeptides.

Carbon-13 chemical shift anisotropy (CSA) tensors for various carbon sites of polypeptides, and for carbon sites in alpha-helical and beta-sheet conformations of poly-L-alanine, and polyglycine, are presented. The carbonyl (13)C CSA tensors were determined from one-dimensional CPMAS spectra obtained at a slow spinning speed, whereas the CSA tensors of C(alpha) and other carbons in side chains of peptides were determined using 2D PASS experiments on powder samples. The results suggest that the spans of (13)Carbonyl CSA tensors of alanine and glycine residues in various peptides are similar, even though the magnitude of individual components of the CSA tensor and the isotropic chemical shift are different. In addition, the delta(22) element is the only component of the (13)Carbonyl CSA tensor that significantly depends on the CO.HN hydrogen-bond length. Solid-state NMR experimental results also suggest that (13)Carbonyl and (13)C(alpha) CSA tensors are similar for alpha-helical and beta-sheet conformations of poly-L-alanine, which is in agreement with the reported quantum chemical calculation studies and previous solid-state NMR experimental studies on other systems. On the other hand, the (13)C(alpha) CSA tensor of the first alanine residue is entirely different from that of the second or later alanine residues of the peptide. While no clear trends in terms of the span and the anisotropic parameter were predicted for (13)C(beta) CSA tensors of alanine, they mainly depend on the conformation and dynamics of the side chain as well as on the packing interactions in the solid state of peptides.     

Solid-state NMR and quantum chemical investigations of 13Calpha shielding tensor magnitudes and orientations in peptides: determining phi and psi torsion angles

We report the experimental determination of the (13)C(alpha) chemical shift tensors of Ala, Leu, Val, Phe, and Met in a number of polycrystalline peptides with known X-ray or de novo solid-state NMR structures. The 700 Hz dipolar coupling between (13)C(alpha) and its directly bonded (14)N permits extraction of both the magnitude and the orientation of the shielding tensor with respect to the C(alpha)-N bond vector. The chemical shift anisotropy (CSA) is recoupled under magic-angle spinning using the SUPER technique (Liu et al., J. Magn. Reson. 2002, 155, 15-28) to yield quasi-static chemical shift powder patterns. The tensor orientation is extracted from the (13)C-(14)N dipolar modulation of the powder line shapes. The magnitudes and orientations of the experimental (13)C(alpha) chemical shift tensors are found to be in good accord with those predicted from quantum chemical calculations. Using these principal values and orientations, supplemented with previously measured tensor orientations from (13)C-(15)N and (13)C-(1)H dipolar experiments, we are able to predict the (phi, psi, chi(1)) angles of Ala and Val within 5.8 degrees of the crystallographic values. This opens up a route to accurate determination of torsion angles in proteins based on shielding tensor magnitude and orientation information using labeled compounds, as well as the structure elucidation of noncrystalline organic compounds using natural abundance (13)C NMR techniques.     

Investigation of the neighboring residue effects on protein chemical shifts

In this study, we report nearest neighbor residue effects statistically determined from a chemical shift database. For an amino acid sequence XYZ, we define two correction factors, Delta((X)Y)n,s and Delta(Y(Z))n,s, representing the effects on Y's chemical shifts from the preceding residue (X) and the following residue (Z), respectively, where X, Y, and Z are any of the 20 naturally occurring amino acids, n stands for (1)H(N), (15)N, (1)H(alpha), (13)C(alpha), (13)C(beta), and (13)C' nuclei, and s represents the three secondary structural types beta-strand, random coil, and alpha-helix. A total of approximately 14400 Delta((X)Y)n,s and Delta(Y(Z))n,s, representing nearly all combinations of X, Y, Z, n, and s, have been quantitatively determined. Our approach overcomes the limits of earlier experimental methods using short model peptides, and the resulting correction factors have important applications such as chemical shift prediction for the folded proteins. More importantly, we have found, for the first time, a linear correlation between the Delta((X)Y)n,s (n = (15)N) and the (13)C(alpha) chemical shifts of the preceding residue X. Since (13)C(alpha) chemical shifts of the 20 amino acids, which span a wide range of 40-70 ppm, are largely dominated by one property, the electron density of the side chain, the correlation indicates that the same property is responsible for the effect on the following residue. The influence of the secondary structure on both the chemical shifts and the nearest neighbor residue effect are also investigated.     

Limited variations in 15N CSA magnitudes and orientations in ubiquitin are revealed by joint analysis of longitudinal and transverse NMR relaxation

The site-specific magnitudes and orientations of the chemical shift tensors have been estimated for 70 backbone (15)N-nuclei in human ubiquitin from the field dependence of dynamic independent ratios between relaxation rates, both longitudinal and transverse, measured at 9.4, 11.7, 14.1, and 18.8 T. The results were jointly analyzed with previously published relaxation data [Fushman; Tjandra; Cowburn. J.Am. Chem. Soc. 1998, 120, 10947-10952] [Kover; Batta. J. Mag. Reson. 2001, 150, 137-146]. The effective magnitudes of the anisotropies distribute around 169 ppm with a variability of 5 ppm. The orientation factors, reflecting the orientation of the CSA relative to the NH bond, distribute around -0.80 with a variability of 0.04, which corresponds to an angle between the symmetry axis of an assumed axially symmetric shielding tensor and the NH bond of 21.4 degrees, and a variability of 2.3 degrees. Correlations with the isotropic (15)N-chemical shifts are observed. Variations in the shielding anisotropies add uncertainty to the obtained order parameters proportional to the square of the magnetic field, when data are analyzed using an assumed invariant CSA tensor for all sites. Around 3% additional uncertainty in the order parameters for 800 MHz data is expected. The optimal TROSY field for amide nitrogen TROSY is estimated, with only marginal variations due to site-to-site variations. Variations in the shielding tensors add uncertainty to the exchange terms calculated from cross-correlation rates. An approach for estimating the exchange terms is suggested, where the uncertainty due to CSA-variations is minimized.     

Glycyl C(alpha) chemical shielding in tripeptides: measurement by solid-state NMR and correlation with X-ray structure and theory

We report here (13)C(alpha) chemical shielding parameters for central Gly residues in tripeptides adopting alpha-helix, beta-strand, polyglycine II, and fully extended 2 degrees structures. To assess experimental uncertainties in the shielding parameters and the effects of (14)N-(13)C(alpha) or (15)N-(13)C(alpha) dipolar coupling, stationary and magic angle spinning (MAS) spectra with and without (15)N decoupling were obtained from natural abundance and double-labeled samples containing [2-(13)C, (15)N]Gly. We find that accurate (<1 ppm uncertainty) shielding parameters are measured with good sensitivity and resolution in (15)N decoupled 1D or 2D MAS spectra of double-labeled samples. Compared to variations of isotropic shifts with peptide angles, those of (13)C(alpha) shielding anisotropy and asymmetry are greater. Trends relating shielding parameters to the 2 degrees structure are apparent, and the correlation of the experimental values with unscaled ab initio shielding calculations has an rms error of 3 ppm. Using the experimental data and the ab initio shielding values, the empirical trends relating the 2 degrees structure to shielding are extended to the larger range of torsion angles found in proteins.     

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.     

Structural and solvent effects on the 13C and 15N NMR chemical shifts of indoloquinoline alkaloids: experimental and DFT study

Indoloquinoline alkaloids represent an important class of antimalarial, antibacterial and antiviral compounds. They have been shown to bind to DNA via intercalation preferentially at GC-rich sequences containing nonalternating CC sites. The stability of complexes formed with biological macromolecules depends on noncovalent binding. In the present study, the ability of indoloquinolines to form intermolecular interactions with solvents was investigated by using NMR spectroscopy and density functional theory (DFT) (B3LYP/6-31G**) calculations. NMR data measured for indoloquinoline bases and the corresponding hydrochlorides are discussed in relation to the structure. DFT calculations of shielding constants in vacuo and in solution allowed the investigation of the influence of the environment on the NMR parameters. Calculations incorporating solvent effects indicated significant changes in the anisotropy of the electron distribution, reflected in the span of the chemical shielding tensor (Omega = sigma11 - sigma33). Solvent effects on the span of the 13C and 15N shielding tensor depended on the type of atom and the data indicated a significant influence of solute-solvent interactions.     

Determination of the (17)O Quadrupolar Coupling Constant and of the (13)C Chemical Shielding Tensor Anisotropy of the CO Groups of Pentane-2,4-dione and beta-Diketonate Complexes in Solution. NMR Relaxation Study

13C NMR relaxation times T(1) of the carbonyl groups of pentane-2,4-dione and beta-diketonate complexes Al(acac)(3) and Zr(acac)(4) (acac: pentanedionate anion) were measured for various magnetic field strengths, allowing a determination of the contribution of the chemical shift anisotropy mechanism to the total relaxation. NOE and T(1) measurements for the (13)C nucleus of the central methine carbon furnished the correlation time tau(c) for the reorientation of theses species. The chemical shift tensor anisotropy Deltasigma could be deduced and compared to the values obtained in the solid state. The quadrupolar coupling constant (QCC) of the (17)O nucleus could also be determined by measuring the line width of the (17)O NMR signal and using the tau(c) value. QCC values for the complexes are in the same range as for the pentane-2,4-dione molecule, indicating similar electronic distribution and symmetry around the oxygen atom of these different species. Deltasigma for the complexes are close together, and the values obtained in solution are approximately those obtained in the solid state. They are close to the value reported in the literature for tetraacetylethane, which can be considered as a dimer of a beta-diketone, but slight differences are observed for the individual components of the chemical shielding tensor.     

Experimental and computational characterization of the 17O quadrupole coupling and magnetic shielding tensors for p-nitrobenzaldehyde and formaldehyde

We have used solid-state 17O NMR experiments to determine the 17O quadrupole coupling (QC) tensor and chemical shift (CS) tensor for the carbonyl oxygen in p-nitro-[1-(17)O]benzaldehyde. Analyses of solid-state 17O NMR spectra obtained at 11.75 and 21.15 T under both magic-angle spinning (MAS) and stationary conditions yield the magnitude and relative orientation of these two tensors: CQ = 10.7 +/- 0.2 MHz, etaQ = 0.45 +/- 0.10, delta11 = 1050 +/- 10, delta22 = 620 +/- 10, delta33 = -35 +/- 10, alpha = 90 +/- 10, beta = 90 +/- 2, gamma = 90 +/- 10 degrees. The principal component of the 17O CS tensor with the most shielding, delta33, is perpendicular to the H-C=O plane, and the tensor component with the least shielding, delta11, lies along the C=O bond. For the 17O QC tensor, the largest (chi(zz)) and smallest (chi(xx)) components are both in the H-C=O plane being perpendicular and parallel to the C=O bond, respectively. This study represents the first time that these two fundamental 17O NMR tensors have been simultaneously determined for the carbonyl oxygen of an aldehyde functional group by solid-state 17O NMR. The reported experimental solid-state 17O NMR results provide the first set of reliable data to allow evaluation of the effect of electron correlation on individual CS tensor components. We found that the electron correlation effect exhibits significant influence on 17O chemical shielding in directions within the H-C=O plane. We have also carefully re-examined the existing experimental data on the 17O spin-rotation tensor for formaldehyde and proposed a new set of best "experimental" 17O chemical shielding tensor components: sigma11 = -1139 +/- 80, sigma22 = -533 +/- 80, sigma33 = 431 +/- 5, and sigma(iso) = -414 +/- 60 ppm. Using this new set of data, we have evaluated the accuracy of quantum chemical calculations of the 17O CS tensors for formaldehyde at the Hartree-Fock (HF), density-functional theory (DFT), M?ller-Plesset second-order perturbation (MP2), and coupled-cluster singles and doubles (CCSD) levels of theory. The conclusion is that, while results from HF and DFT tend to underestimate the electron correlation effect, the MP2 method overestimates its contribution. The CCSD results are in good agreement with the experimental data.