Single-crystal studies of peptide prolyl and glycyl 15N shielding tensors

15N shielding tensors were determined for the central peptide groups in GGV, AGG, and APG by single-crystal NMR. We find that the angle between the downfield component (delta11) and the N-H or the N-C(delta) (pro) bonds is in the range of 20-23 degrees and in accord with previous solid-state NMR measurements. However, AGG, unlike APG or GGV, has a distorted peptide plane, and delta11 lies approximately in the plane of N, C(alpha), and H rather than in the peptide plane defined by heavy atoms. Accurate orientations of delta22 and delta33 were determined, and the usual assumption that delta22 is along the peptide normal was found only in APG which has a highly nonaxial tensor. More generally, delta22 and delta33 are rotated about the delta11 axis (36 degrees in GGV). These results are compared with DFT calculations to gain a structural understanding of the effects of intermolecular interactions on shielding tensor principal components and orientations. Trimeric clusters containing H-bonded neighbors predict the orientations of the principal components within 2-3 degrees, but calculated principal components are less quantitative. Possible reasons for this disagreement are explored.     

Direct observation of photoinduced bent nitrosyl excited-state complexes

Ground-state structures with side-on nitrosyl (eta (2)-NO) and isonitrosyl (ON) ligands have been observed in a variety of transition-metal complexes. In contrast, excited-state structures with bent-NO ligands have been proposed for years but never directly observed. Here, we use picosecond time-resolved infrared spectroscopy and density functional theory (DFT) modeling to study the photochemistry of Co(CO) 3(NO), a model transition-metal-NO compound. Surprisingly, we have observed no evidence for ON and eta (2)-NO structural isomers, but we have observed two bent-NO complexes. DFT modeling of the ground- and excited-state potentials indicates that the bent-NO complexes correspond to triplet excited states. Photolysis of Co(CO) 3(NO) with a 400-nm pump pulse leads to population of a manifold of excited states which decay to form an excited-state triplet bent-NO complex within 1 ps. This structure relaxes to the ground triplet state in ca. 350 ps to form a second bent-NO structure.     

NMR shielding tensors from auxiliary density functional theory

The working equations for the calculation of NMR shielding tensors in the framework of auxiliary density functional theory are derived. It is shown that in this approach the numerical integration over gauge-including atomic orbitals can be avoided without the loss of accuracy. New integral recurrence relations for the required analytic electric-field-type integrals are derived. The computational performance of the resulting formalism permits shielding tensor calculations of systems with more than 1000 atoms and 15,000 basis functions.     

Probing platinum azido complexes by 14N and 15N NMR spectroscopy

Metal azido complexes are of general interest due to their high energetic properties, and platinum azido complexes in particular because of their potential as photoactivatable anticancer prodrugs. However, azido ligands are difficult to probe by NMR spectroscopy due to the quadrupolar nature of (14)N and the lack of scalar (1)H coupling to enhance the sensitivity of the less abundant (15)N by using polarisation transfer. In this work, we report (14)N and (15)N NMR spectroscopic studies of cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))] (1) and trans,trans,trans-[Pt(N(3))(2)(OH)(2)(X)(Y)], where X=Y=NH(3) (2); X=NH(3), Y=py (3) (py=pyridine); X=Y=py (4); and selected Pt(II) precursors. These studies provide the first (15)N NMR data for azido groups in coordination complexes. We discuss one- and three-bond J((15)N,(195)Pt) couplings for azido and am(m)ine ligands. The (14)N(α) (coordinated azido nitrogen) signal in the Pt(IV) azido complexes is extremely broad (W(1/2)≈2124 Hz for 4) in comparison to other metal azido complexes, attributable to a highly asymmetrical electric field gradient at the (14)N(α) atom. Through the use of anti-ringing pulse sequences, the (14)N NMR spectra, which show resolution of the broad (14)N(α) peak, were obtained rapidly (e.g., 1.5 h for 10 mM 4). The linewidths of the (14)N(α) signals correlate with the viscosity of the solvent. For (15) N-enriched samples, it is possible to detect azido (15)N resonances directly, which will allow photoreactions to be followed by 1D (15)N NMR spectroscopy. The T(1) relaxation times for 3 and 4 were in the range 5.7-120 s for (15)N, and 0.9-11.3 ms for (14)N. Analysis of the (1)J((15)N,(195)Pt) coupling constants suggests that an azido ligand has a moderately strong trans influence in octahedral Pt(IV) complexes, within the series 2-pic相似文献   

Influence of NO2 attachment on the nuclear magnetic shielding tensors of N and B nuclei in C30B15N15 heterofullerene: a DFT study

Adsorption of nitrogen dioxide in three different configurations on the exterior surface of C₃₀B₁₅N₁₅ is studied using density functional theory calculations. To this end, we optimized the structures of raw C₃₀B₁₅N₁₅ and nine NO₂–C₃₀B₁₅N₁₅ complexes at the B3LYP/6-31G^* level of theory and then calculated chemical shielding (CS) tensors at the GIAO-B3LYP/6-311G^** level for the optimized structures. The calculated chemical shielding isotropy (CSI), chemical shielding anisotropy (CSA), and orientation of CS tensors (Euler angles) reveal that the adsorption configurations (nitro, trans-nitrite, and cis-nitrite) have different effects on the electronic structure of C₃₀B₁₅N₁₅. Natural atomic charges based on natural population analysis (NPA) were used to justify the changes in CSI values after gas sorption.     

Electronic structure and spectra of nitrosyl complexes with cobalt and manganese porphyrins

The DFT calculations for nitrosyl manganese and cobalt porphyrins were carried out with the use of several density functionals. The binding energy of nitrosyl ligand and spin state of nitrosyl-free manganese porphyrin were determined. The best values of binding energy are obtained from the OLYP functional. The NBO analysis of metal?Cnitrosyl bonding was performed. Electronic spectra of nitrosyl cobalt and manganese porphyrin were calculated with the TDDFT method. The calculated electronic transitions agree well with the experimental data except for the Soret band of (Por)Mn(NO), where they are 0.3?C0.5?eV higher in energy than the experimental ones.     

Dynamic 15N NMR studies of iron phosphine complexes containing coordinated dinitrogen

The ¹⁵N‐labelled iron dinitrogen complexes trans‐[FeH(N₂)(PP)₂]⁺[BPh₄]^? (PP = dppe, depe, dmpe) and cis‐[FeH(N₂)(PP₃)]⁺[BPh₄]^? were prepared in situ by exchange of unlabelled coordinated dinitrogen with ¹⁵N₂. ¹⁵N NMR chemical shifts and coupling constants are reported. The ¹⁵N spectra exhibit separate signals for the metal‐bound and terminal nitrogen atoms of the coordinated N₂. The ¹⁵N resonances display ¹⁵N, ¹⁵N coupling as well as ³¹P, ¹⁵N coupling and long‐range ¹⁵N, ¹H coupling when there is a metal‐bound hydrido ligand. Exchange between free and coordinated dinitrogen was monitored by magnetization transfer between ¹⁵N‐labelled sites using an inversion–transfer–recovery experiment. Exchange between the metal‐bound and terminal nitrogen atoms of coordinated N₂ was also monitored by magnetization transfer and this could proceed by N₂ dissociation or by an intramolecular process. Copyright © 2003 John Wiley & Sons, Ltd.     

Variability of the 15N chemical shielding tensors in the B3 domain of protein G from 15N relaxation measurements at several fields. Implications for backbone order parameters

We applied a combination of 15N relaxation and CSA/dipolar cross-correlation measurements at five magnetic fields (9.4, 11.7, 14.1, 16.4, and 18.8 T) to determine the 15N chemical shielding tensors for backbone amides in protein G in solution. The data were analyzed using various model-independent approaches and those based on Lipari-Szabo approximation, all of them yielding similar results. The results indicate a range of site-specific values of the anisotropy (CSA) and orientation of the 15N chemical shielding tensor, similar to those in ubiquitin (Fushman, et al. J. Am. Chem. Soc. 1998, 120, 10947; J. Am. Chem. Soc. 1999, 121, 8577). Assuming a Gaussian distribution of the 15N CSA values, the mean anisotropy is -173.9 to -177.2 ppm (for 1.02 A NH bond length) and the site-to-site CSA variability is +/-17.6 to +/-21.4 ppm, depending on the method used. This CSA variability is significantly larger than derived previously for ribonuclease H (Kroenke, et al. J. Am. Chem. Soc. 1999, 121, 10119) or recently, using "meta-analysis" for ubiquitin (Damberg, et al. J. Am. Chem. Soc. 2005, 127, 1995). Standard interpretation of 15N relaxation studies of backbone dynamics in proteins involves an a priori assumption of a uniform 15N CSA. We show that this assumption leads to a significant discrepancy between the order parameters obtained at different fields. Using the site-specific CSAs obtained from our study removes this discrepancy and allows simultaneous fit of relaxation data at all five fields to Lipari-Szabo spectral densities. These findings emphasize the necessity of taking into account the variability of 15N CSA for accurate analysis of protein dynamics from 15N relaxation measurements.     

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.     

Quantitative 15N NMR spectroscopy

Line intensities in ¹⁵N NMR spectra are strongly influenced by spin-lattice and spin–spin relaxation times, relaxation mechanisms and experimental conditions. Special care has to be taken in using ¹⁵N spectra for quantitative purposes. Quantitative aspects are discussed for the ¹⁵N NMR of molecules with different nitrogen functional groups and also mixtures of nitrogen-containing compounds. It is shown that, in general, quantitative data are obtainable from integration of ¹⁵N lines in proton decoupled ¹⁵N NMR spectra using NOE suppression. Addition of paramagnetic relaxation reagents (PARR) under controlled conditions is frequently needed to accomplish the experiment within reasonable time limits.     

Proton shielding tensors in potassium hydrogen maleate

The proton NMR in single crystals of potassium hydrogen maleate has been sttudied by means of multiple-pulse line-narrowing techniques. The magnetic shielding tensors of all magnetically inequivalent protons in the unit cell could be determined independently. Two of these protons are carboxylic, forming hydrogen bonds. The orientations of the shift tensors are consistent with the position of the hydrogens at the midpoints of the 0–0 intervals. The range of anisotropy of 32 ppm, found for the shift tensor of the caboxylic hydrogen, is larger than that found for hydrogen bonds in acids and seems to be characteristics of acidic salts.The other protons in the unit cell are olefinic. Two features distinguish this type of protons from those studied so far: (1) The magnetic shielding tensor is not even approximately axially symmetric, the principal values being ?2.4, ?5.1, ?7.3 ± 05 ppm (from adamantane); and (2) the principal directions reflect all characteristic directions of the carboncarbon double bond (while the CH direction is of no importance). The principal value in the direction perpendicular to the sp² system is the least shielded one.     

Analyzing NMR shielding tensors calculated with two-component relativistic methods using spin-free localized molecular orbitals

A recently developed analysis method [J. Chem. Phys. 127, 124106 (2007)] for NMR spin-spin coupling constants employing two-component (spin-orbit) relativistic density functional theory along with scalar relativistic natural localized molecular orbitals (NLMOs) and natural bond orbitals (NBOs) has been extended for analyzing NMR shielding tensors. Contributions from a field-dependent basis set (gauge-including atomic orbitals) have been included in the formalism. The spin-orbit NLMO/NBO nuclear magnetic shielding analysis has been applied to methane, plumbane, hydrogen iodide, tetracholoplatinate(II), and hexachloroplatinate(IV).     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline amino acids

We have presented a systematic experimental investigation of carboxyl oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in crystalline amino acids. Three 17O-enriched amino acids were prepared: L-aspartic acid, L-threonine, and L-tyrosine. Analysis of two-dimensional 17O multiple-quantum magic-angle spinning (MQMAS), MAS, and stationary NMR spectra yields the 17O CS, EFG tensors and the relative orientations between the two tensors for the amino acids. The values of quadrupolar coupling constants (CQ) are found to be in the range of 6.70-7.60 MHz. The values of deltaiso lie in the range of 268-292 ppm, while those of the delta11 and delta22 components vary from 428 to 502 ppm, and from 303 to 338 ppm, respectively. There is a significant correlation between the magnitudes of delta22 components and C--O bond lengths. Since C--O bond length may be related to hydrogen-bonding environments, solid-state 17O NMR has significant potential to provide insights into important aspects of hydrogen bonds in biological systems.     

The calculation of NMR shielding tensors based on density functional theory and the frozen-core approximation

The application of the frozen-core approximation to the calculation of the shielding tensor of nuclear magnetic resonance (NMR) spectroscopy is discussed and an implementation is presented. A complete formulation of the shielding calculation within the frozen-core approximation is given, both in general terms and for the special case of density functional theory (DFT) and “gauge including atomic orbitals” (GIAOs). The practical implementation is validated by a detailed discussion of the consequences of the approximation. The general conclusion is drawn that the frozen-core approximation is a useful tool for shielding calculations—if the valence space is increased to contain at least the ns, np, (n − 1)p, (n − 1)d (fourth period and higher) shells, where n is the number of the given period in the periodic table of elements. The new method is applied to ⁷⁷Se shieldings and chemical shifts for a small number of compounds. The agreement between theory and experiment is good for relative shifts, whereas calculated absolute shieldings are generally too small by about 300–400 ppm. This difference is attributed to the relativistic contraction of the core density at the selenium atom that had been explicitly incorporated into the experimental absolute shielding scale. © 1996 John Wiley & Sons, Inc.     

Nitrosoruthenium nitrite-nitrate complexes in aqueous and nitric acid solutions as probed by 15N NMR

Aqueous and nitric acid solutions of Na₂[Ru¹⁵NO(¹⁵NO₂)₄OH] in the concentration range $c_{H^{15} NO_3 }$ = 0–3.3 mol/L have been studied by ¹⁵N NMR, dominating complex species have been identified, and the equilibrium constants for the nitrate ion incorporation into the inner coordination sphere of nitrosoruthenium have been estimated. The equilibration time for such equilibria is no more than 2 h at room temperature. In addition to the nitro complexes, isomeric nitritonitronitrosoruthenium compounds have been identified in solutions. In weak acidic solutions at $c_{HNO_3 }$ < 0.25 mol/L, nitro and nitritonitro complexes containing four and three coordinated nitrite ions predominate. At the HNO₃ concentration 0.4–1.7 mol/L, the vast majority of ruthenium presents in solution as fac-dinitronitrosoruthenium complexes containing coordinated water molecules and nitrate ions. In solutions with $c_{HNO_3 }$ > 1.5 mol/L, the fractions of dinitro- and mononitronitrosoruthenium complexes are comparable. In strong nitric acid solutions ( $c_{H^{15} NO_3 }$ = 10 mol/L) kept for three years in contact with air, nitro complexes are absent, and mononitrato- and dinitratoaquanitrosoruthenium complexes are dominating.     

Solid state 15N NMR study of 15N enriched melamine-formaldehyde resins

Molecular dynamics and structure of uncured and cured melamine-formaldehyde resins isotopically ¹⁵N enriched at amine sites were studied by solid-state ¹⁵N nuclear magnetic resonance (NMR). Spectra recorded with direct (DP) and cross-polarization (CP) pulse sequences reflect two motionally different regions arising from similar chemical structures. DP spectra of uncured resins at higher temperatures have narrow lines and the detection of slightly different structural units is possible. With increasing crosslinking resonances broaden and overlap and the direct detection of individual signals in cured resins is not possible. On the basis of variable contact time, variable spinning speed, and interrupted decoupling experiments three protonated and one nonprotonated group of signals are identified in the CP spectra for all samples. Short polarization-transfer rates, T_NH, for nonprotonated nitrogen in uncured and lightly cured samples reveal more effective hydrogen bonding in viscous and rubber-like resins compared to the highly cured rigid resins. The rigid portions of the resins exhibit longer T₁ and short T_1ρ relaxation times, while the shorter T₁ times and longer T_1ρ times are associated with the more mobile portion of samples. ©1995 John Wiley & Sons, Inc.     

Carbon-13 chemical shielding tensors in acetic acid

High resolution double resonance methods have been employed to study the anisotropic ¹³C chemical shifts in a single crystal of acetic acid. The pr     

Triphenylphosphineoxide nitrosyl complexes of molybdenum

Summary The tetramethylthiourea (TMTU) complexes of cobalt(II) and nickel(II) halides have been studied in the solid state by electronic, i.r. and far i.r. spectroscopy and magnetochemically. The tetrahedral Co(TMTU)₂X₂ (X = Cl, Br, 1) and Ni(TMTU)₂X₂ (X = Cl, Br) complexes have normal magnetic moments, electronic spectra and crystal field parameters; Ni₂ (TMTU)₃I₄ is diamagnetic. The cobalt complexes have normal (CoX) and (CoX) vibrational frequencies. Ni(TMTU)₂Cl₂ and Ni₂(TMTU)₃I₄ have (NiX) frequencies corresponding to long or bridging Ni-X bonds, while Ni(TMTU)₂Br₂ has normal (NiBr) frequencies for terminal Ni-Br bonds. The (MS) frequencies are similar to those of cobalt(II) and nickel(II) complexes of other thioureas.