Distinct Chemical Shifts Research Articles

Investigation of water environments in a C18 bonded silica phase using 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy.

High resolution 1H magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra have been obtained on typical C18 bonded silicas used in chromatographic solid-phase extraction separations. It has been shown for the first time that water molecules distributed in distinct physico-chemical environments within the chromatographic system can be detected directly using a simple 1H MAS NMR measurement. The resonances assigned to water protons in differing physico-chemical environments have distinct chemical shifts, line widths, relaxation times (T1 and T2) and also exhibit temperature dependent coalescence behaviour. This novel MAS approach may lead to a better understanding of the environments of other analytes in mixtures during such separations.

99Ru NMR Spectroscopy of Organometallic and Coordination Complexes of Ruthenium(II)

The ruthenium(II) organometallic and coordination complexes of type Ru(X)(Y)(CO)2(i-Pr-DAB) (i-Pr-DAB = 1,4-diisopropyl-1,4-diaza-1,3-butadiene; X, Y = Cl, Br, I, Me, Et, i-Pr, neo-Pe, Ph3Sn, Me3Sn, Ph3Pb, Ph3Ge, Mn(CO)5, RuCp(CO)2) have been analyzed by means of 99Ru NMR spectroscopy. The 99Ru NMR resonances for these compounds are generally sharp, allowing scalar couplings to group 14 nuclei to be observed. Correlation times τc have been determined from NOE factors and 13C T1 values for selected compounds and were found to parallel the molecular weight of the compounds studied. The M−Ru−M‘ and M−Ru−X complexes have 99Ru NMR resonances in distinct chemical shift regions. In the series M−Ru−M‘ a correlation between the inverse of the crystal field splitting energy ΔE and δ(99Ru) exists. These complexes have a HOMO which is delocalized over the M−Ru−M‘ σ system and a LUMO which has predominantly π*-DAB character; therefore, (ΔE)-1 is dominant in the paramagnetic shielding of the ruthenium nucleus. The diff...

15N NMR and electronic properties of S-nitrosothiols

Investigation of the 15N NMR of S-nitrosothiols showed that primary and tertiary RSNOs have distinct 15N chemical shifts around 730 and 790 ppm, respectively. Using 15N NMR technique, the equilibrium constant of NO transfer between SNAP and GSH was found to be 0.74. For primary RSNOs, linear relationships exist among 15N NMR chemical shifts, reduction potentials, and the p K as of their parent thiols.

Oxidation states of submonolayer copper islands on a Pd(111) surface exposed to oxygen

The adsorption of O 2 on a monoatomic copper layer on Pd(111) leads to the formation of two distinctive copper oxide features which, in scanning tunneling microscopy (STM) images, appear in dark hexagonal form (A) and bright triangular form (B). By providing either the dark hexagonal or the bright triangular patches at a time, the chemical (oxidation) states of copper were studied by X-ray photoelectron specroscopy (XPS). The Cu LMM Auger peaks from these copper oxide islands exhibited distinct chemical shifts relative to the metallic copper islands on Pd(111): the kinetic energy was 0.6 eV lower for the copper oxide islands of type A and 0.3 eV lower for B. No shake-up satellite accompanying the Cu 2p photoelectron peaks was observed, indicating that the chemical state of copper atoms was not the Cu 2+ but the Cu + oxidation state (Cu 2O). By combining photoelectron spectra of the surfaces with STM results, it was deduced that the dark hexagonal patches (A) are a Cu 2O monolayer lying on a Pd(111) surface and the bright triangular patches (B) are a Cu 2O monolayer grown on a copper layer formed on Pd(111).

A silicon-29 MAS–NMR study of α-silicon nitride and amorphous silicon oxynitride fibres

29Si–MAS–NMR with the support of elemental and XRD analyses has been used to assess the structure, the chemical and the phase composition of two types (C and A) of Si–N–(O) fibres prepared according to a gas phase process. The type C fibre consists of α-Si 3N 4 and traces of β-Si 3N 4. It however contains 1–2 at% of oxygen which has been suggested to be distributed within the α-Si 3N 4 structure as a silicon oxynitride. The 29Si–NMR peak at −56·4 ppm may originate from the original Si–N 3O tetrahedral site involved in this structure. The type A fibre consists of a single amorphous silicon oxynitride phase of composition Si–N 4 (1− x)/3 O 2 x . with x=0·14–0·16 (12–13 at% of oxygen). Silicon atoms in this material are in a tetrahedral environment of statistically distributed O and N atoms (Si–N 4– n O n with n=0, 1, …, 4). These different environments give rise to characteristic 29Si–NMR bands, with distinct chemical shifts. Because of the amorphous structure, these bands overlap in the case of the Type A fibre, yielding a single broad NMR band. Their respective intensities and therefore, the shape of the band, are related to the oxygen concentration of the fibre.

Identification of reaction products and intermediates of aromatic-amine dehydrogenase by 15N and 13C NMR.

13C- and 15N-NMR studies of the reaction of aromatic amine dehydrogenase (AADH) with methylamine demonstrated that the products of the reductive half-reaction are an equivalent of formaldehyde hydrate and a reduced aminoquinol form of the tryptophan tryptophylquinone (TTQ) cofactor which contains covalently bound substrate-derived N. These data are consistent with the Ping Pong kinetic mechanism and aminotransferase-type chemical reaction mechanism which have been previously proposed for AADH. Comparison of the 15N-NMR spectra of the aminoquinol TTQ intermediates of AADH and methylamine dehydrogenase (MADH) revealed that the substrate-derived aminoquinol N of AADH and MADH exhibited distinct 15N chemical shifts which are separated by approx. 7 p.p.m. In each case, the signal for the substrate-derived aminoquinol N appears optimally with short pulse delay and exhibits a relaxation time and chemical shift which are consistent with 15N covalently bound to an aromatic ring (i.e. aminoquinol) which is attached to a rigid protein matrix. The aminoquinol of AADH is less stable against reoxidation than that of MADH. These data suggest that differences in the active-site mediated electrostatic environments of the aminoquinol N in the respective enzymes may influence both the observed 15N chemical shift and the relative reactivities of the TTQ aminoquinols towards oxygen. These data also demonstrate the utility of 13C- and 15N-NMR spectroscopy as a tool for monitoring the intermediates and products of enzyme-catalysed transformations.

Separation of a Pair of Interleaved Sideband Families in Magic Angle Spinning NMR by a Pair of One-Dimensional Experiments

Interleaved sideband families that arise from distinct isotropic chemical shifts and the associated chemical shift anisotropy (CSA) tensors in magic angle spinning NMR are conventionally disentangled using two-dimensional schemes. However, for the case of just a pair of distinct chemical sites, a scheme is suggested that involves a pair of one-dimensional experiments whose sum and difference spectra separate the interleaved sideband families. This method was applied to phospholipid bilayers. The 31P NMR sidebands of phosphatidylcholine and dioleoylphosphatidylethanolamine are separated and CSA values for each phospholipid were calculated. © 1997 by John Wiley & Sons, Ltd.

Proton NMR spectroscopy assignment of d-glucose residues in highly acetylated starch

1H NMR spectroscopy assignments have been obtained for starch acetates using COSY and HOHAHA experiments by comparison with the spectra of amylose triacetate and of peracetylated malto-oligosaccharides (maltotriose, maltotetraose, maltoheptaose). These assignments are valuable for the location and evaluation of the substitution pattern in modified starches. The bulk of the 1H NMR spectra of highly acetylated starch strongly resembles the spectrum of amylose triacetate in which all protons are identified and display distinct chemical shifts. The resolving power of the HOHAHA experiment allowed the distinction of minor spin systems. Beside these strong signals pertaining to an average 2,3,6-tri- O-acetyl- α-(1 → 4) linked d-glucopyranose unit in an infinite chain, the combination of COSY and HOHAHA experiments allowed the identification of these systems to the terminal, n-1, n-2, and to partially acetylated glucopyranosyl units. As an example, two different preparations of starch acetates with degrees of substitution 2.74 and 2.63 were examined. In one case, NMR demonstrates that the defects of acetylation are random on the polymeric chain (with corresponding signals for unacylated secondary hydroxyl positions at δ 3.61 and 3.40) while in another case, these signals are not detectable, probably due to the presence of clusters of non-acetylated residue forming solid-like zones.

Kinetic Analysis of the Stepwise Platination of Single‐ and Double‐Stranded GG Oligonucleotides with Cisplatin and cis‐[PtCl(H2O)(NH3)2]+

AbstractWe report the first direct comparison of the kinetics of platination of defined single‐ and double‐stranded DNA with the anticancer drug cisplatin. The courses of the reactions of the 14‐mer duplex d(A‐T‐A‐C‐A‐T‐G‐G‐T‐A‐C‐A‐T‐A)·d(T‐A‐T‐G‐T‐A‐C‐C‐A‐T‐G‐T‐A‐T) with [15N]cisplatin and cis‐[PtCl(H2O)‐(15NH3)2]+ and of each of the single strands with [15N]cisplatin have been studied at 298 K, pH 6, by [1H, 15N] NMR spectroscopy. As expected the reactions of cisplatin proceed via cis‐[PtCl(H2O)(NH3)2]+, and lead to two monofunctional adducts on the duplex and two on the GG single strand. In both the GG single strand and the duplex, one of the two G's is platinated faster than the other (by a factor of ca. 4). Remarkably, ring closure on the duplex to form the GG chelate occurs about an order of magnitude faster for one monofunctional adduct than for the other. The latter monofunctional adduct has distinctive 1H and 15N NMR chemical shifts for Pt‐NH3, and is very long‐lived (persists for >5 d). The Pt‐Cl bond in this monofunctional adduct is protected from hydrolysis by the duplex. In contrast, the two monofunctional adducts on the GG single strand undergo ring closure at about the same rate. Equilibria between kinked and distorted forms of the GG platinated duplex, the platination of G's on the complementary strand, and the potential biological significance of long‐lived monofunctional adducts of platinum drugs with DNA are discussed.

NMR evidence for helix geometry modifications by a G-U wobble base pair in the acceptor arm of E. coli tRNA Ala

A ribooligonucleotide duplex representing the acceptor stem of E. coli RNA Ala with a G3-U70 wobble base pair, which is the main identity element for the recognition by the alanine-tRNA synthetase, has been characterized by 2D-NMR, as having two sequence variants with a regular Watson-Crick G3-C70 and an I3-U70 wobble pair, respectively. As compared to a regular A-RNA, the G-U base pair gives rise to variations of the local helix geometry which are reflected in distinct local chemical shift changes. Structural differences between the duplex possessing an 13-U70 base pair and the wild-type G3-U70 sequence have also been found. The nucleotides in the ubiquitous single-stranded NCCA terminus display a surprisingly high degree of stacking order, especially between A73, C74, and C75.

19F and 27Al n.m.r. spectroscopic study of the fluoro complexes of aluminum in aqueous solution and in zeolites: Dealumination of zeolites by fluoride ions

The addition of fluoride ions to aluminum〈III〉 salts or to zeolites in aqueous solutions produces fluoro complexes of the general form [AlFitn](3itn), which have been characterized by 27Al and 19F n.m.r. Each fluoroaluminate complex has distinct 19F chemical shifts, although the 27Al chemical shifts of all of the complexes are approximately the same. The 27Al n.m.r. line width, however, changes as different complexes are formed. Zeolite reacts with aqueous fluoride ion solution at approximately neutral pH. n.m.r., FTi.r. and X-ray powder diffraction measurements suggest that the formation of fluoroaluminate species such as K2NaAlF6 at high fluoride ion concentration inside zeolite is an important dealumination mechanism.

Molecular Weight of Heparin Using 13C Nuclear Magnetic Resonance Spectroscopy

Heparin is a polydisperse, heterogeneous polysaccharide that has been used as an anticoagulant for the past 50 years. The molecular weight determination of this important drug has traditionally relied on gel permeation chromatography, which requires the use of well‐defined molecular weight standards that are not easily obtained. We have investigated the use of13C‐NMR spectroscopy for measuring the number average molecular weight of heparin. The signal intensities of the reducing end and internal anomeric carbons, having distinctive chemical shifts in the13C‐NMR spectrum, were used to determine the molecular weight. Distortionless enhancement polarization transfer was found to provide a better quantitation of signal intensities of anomeric carbons than broad band decoupling or selective decoupling of anomeric protons. Signal averaging over 300 000 transients, requiring approximately 48 h on a 360 MHz NMR spectrometer, resulted in the measurement of the number average molecular weight (˜10 000 Da) of heparin.13C‐NMR spectroscopy does not require the use of difficult to obtain molecular weight standards and thus is particularly well‐suited for workers in the pharmaceutical industry.

Molecular weight of low molecular weight heparins by 13C nuclear magnetic resonance spectroscopy

Heparin and low molecular weight heparins are polydisperse polysaccharides with a degree of polymerization ranging from 4 to ∼ 40. The determination of their average molecular weights has traditionally relied on size exclusion chromatography involving the use of oligosaccharides of known size and molecular weight as standards. 13C NMR spectroscopy is applied for the first time to obtain the molecular weights of low molecular weight heparins. The signal intensities of the reducing end and internal anomeric carbons, having distinctive chemical shifts in the 13C NMR spectrum, are measured to determine the molecular weight. Compared to techniques utilizing broad band decoupling or selective decoupling of anomeric protons, distortionless enhancement polarization transfer pulse sequence gave better quantitation of signal intensities of anomeric carbons. Molecular weight was calculated from the calibrated ratio of signal intensities of the anomeric carbons of reducing end groups and internal residues, and the disaccharide compositional analysis. The calibrated signal intensity ratio is determined using the T 1 relaxation rates of anomeric carbons of model oligosaccharides. The disaccharide composition of low molecular weight-heparins is obtained using capillary electrophoresis. Signal averaging over 40 000–90 000 transients, requiring a total of 12–18 h on a 360-MHz NMR spectrometer was adequate to measure molecular weights in the range of 3000–7000. The measured molecular weights of twelve low molecular weight heparins, analyzed by this 13C NMR spectroscopic technique, correlated well with the number average molecular weights obtained using high performance-gel permeation chromatography and gradient polyacrylamide gel electrophoresis. In addition to establishing the number average molecular weight, the 13C NMR spectra helped distinguish the structural properties of different commercially prepared low molecular weight heparins.

Discrimination of fluorinated uridine metabolites in N‐417 small cell lung cancer cell extracts via 19F‐ and 31P‐NMR

31P-NMR extract spectra of N-417 Small Cell Lung Cancer (SCLC) cells cultured with fluorouridine (FUrd) reveal new peaks with chemical shifts in the diphosphodiester and nucleoside triphosphate regions. These peaks were identified as FUTP, FUDP, FUDP-glucose, FUDP-glucuronate, FUDP-GlcNAc, and FUDP-GalNAc via enzymatic conversion and 19F- and 31P-NMR analysis. Distinct 19F chemical shifts were assigned for FUTP, FUDP, and the FUDP-sugars.

Mapping the nucleotide-dependent conformational change of human N-ras p21 in solution by heteronuclear-edited proton-observed NMR methods.

Heteronuclear-edited proton-detected NMR methods are used to study the nucleotide-dependent conformational change between GDP- and GTP gamma S-bound forms of human N-ras p21. Amide groups of Asp are used as sensitive probes. When GTP gamma S is substituted for GDP in cellular N-ras p21, the chemical shifts of resonances Asp-47, -126, -154, and Asn-172, as well as Gly-77 and -151, are not sensitive to nucleotide exchange, whereas Asp-30, -33, -38, -54, -57, -69, -92, -105, and -119 are affected. Distinct chemical shift changes of Asp-33, -38, and -69 indicate that substantial structure changes occur in the effector-binding region and the switch II region. Crystallographic studies of H-ras p21 have indicated that the conformational differences are confined to residues 32-38 and 60-76. Our observations indicate that the nucleotide-dependent structural transitions of the protein in solution may not be identical to those in the crystal. They suggest that the peptide beyond Glu-76 participates in a conformational switch, and possibly is involved in effector function. We propose that the region roughly from Asp-92 to -105, and the region of guanine base-binding motif(s), e.g., 116NKXD, are candidate sites recognized by either a GDP/GTP release factor or a GTPase-affected protein.

Phosphorus-31 and aluminum-27 NMR investigations of highly acidic, aqueous solutions containing aluminum and phosphorus

[sup 31]P and [sup 27]Al NMR spectroscopies have been used to characterize acidic, aqueous solutions of orthophosphoric acid, aluminum chloride, and tetramethylammonium (TMA) hydroxide. The final compositions of the solutions range from 0.1 to 1 mol % P, 0.0 to 20 mol % HCl, P/Al = 0.1 to 20, and P/(TMA)[sub 2]O = 2 to 20. Soluble aluminophosphate cations form reactions of hexaaqua Al monomeric cations, [Al(H[sub 2]O)[sub 6]][sup 3+], with phosphate ligands (i.e., H[sub 3]PO[sub 4], H[sub 2]PO[sub 4][sup [minus]], and acid dimers H[sub 6]P[sub 2]O[sub 8] and H[sub 5]P[sub 2]O[sub 7][sup [minus]]). [sup 31]P NMR spectra of aluminophosphate solutions reveal five distinct chemical shift regions. A correlation between the partial charge on P nuclei (calculated using semiempirical molecular orbital theory, MNDO [modified neglect of differential overlap]) and the [sup 31]P NMR chemical shift confirms the assignments of the [sup 31]P NMR resonances. Regardless of the total charge of the complex or the connectivity of the Al to -O-P units, the MNDO calculations show that P atoms in the same type of phosphate ligand bonded to Al monomers have similar partial charges, and, therefore, exhibit nearly identical [sup 31]P chemical shifts. [sup 27]Al NMR spectra of aluminophosphate solutions displaymore » four resonances. The assignment of the [sup 27]Al NMR resonances is based on the manner in which the [sup 27]Al and [sup 31]P NMR peaks change in intensity with pH, P, and Al concentration. Estimates of the number of Al-O-P bonds are calculated independently from [sup 31]P and [sup 27]Al NMR spectra and agree quite closely for a range of pH's and both Al and P concentrations. 44 refs., 17 figs., 3 tabs.« less

The conformation of apolipoprotein A-I in discoidal and spherical recombinant high density lipoprotein particles. 13C NMR studies of lysine ionization behavior.

To elucidate the molecular details of how high density lipoprotein (HDL) microstructure affects the conformation of apolipoprotein (apo) A-I in various classes of HDL particles, apoA-I structure in homogeneous recombinant HDL (rHDL) complexes containing palmitoyl-oleoyl phosphatidylcholine (POPC) and cholesteryl oleate has been investigated by NMR spectroscopy of [13C]lysine-labeled apoA-I. All Lys residues in rHDL apoA-I were labeled with 13C by reductive methylation, and then their ionization behavior was characterized by 13C NMR spectroscopy. Four discoidal particles were prepared to contain from 64 to 256 molecules of POPC and 2 molecules of apoA-I; their major diameters ranged from 9.3 to 12.1 nm. (13CH3)2-Lys resonances from apoA-I in discoidal complexes exhibit six distinct chemical shifts at pH 10. The various Lys have pKa values ranging from 8.3 to 10.5, indicating that they exist in different microenvironments. More than 80% of the Lys residues in small (9.3 nm) discoidal particles titrate at a significantly lower pH than in the large (12.1 nm) discoidal particles. This indicates that apoA-I has a different conformation on the differently size discs. Two spherical particles were prepared with POPC:cholesteryl oleate:apoA-I molar stoichiometries of 56:16:2 and 232:84:4 and diameters of 7.4 and 12.6 nm, respectively. On spherical rHDL, apoA-I (13CH3)2-Lys resonances exhibit five distinct chemical shifts at pH 10. The titration behavior of apoA-I Lys residues is the same in small and large spherical particles, indicating that apoA-I conformation is similar on the two particles. The Lys microenvironments indicate that the conformation of apoA-I in discoidal complexes is dependent on particle size and that these conformations are substantially different from that of apoA-I on spherical complexes. Lys microenvironments in discoidal complexes differ from that of spherical complexes by 4 to 5 ysines which titrate with relatively low pKa values on discs. This reflects apparent differences in conformation in the NH2-terminal one-third of apoA-I on discs and spheres.

Lithographic processing of YBa2Cu3O7 thin films: Impact on electrical contacts

Silver contact pads to c-axis-oriented thin films of YBa2Cu3O7 (YBCO) were defined using three different techniques: (1) deposition through stencil masks; (2) liftoff; and (3) Ar plasma etching of in situ deposited Ag. A specific contact resistance of ρc(77 K) = 1 × 10−7 Ω cm2 was found for the in situ deposited contacts. Contacts defined with stencil masks and liftoff showed a ρc(77 K) ∼ 10−5 and ∼ 10−2 Ω cm2, respectively. Core-level photoelectron spectroscopy measurements indicate that the resist and acetone applied to the YBCO thin film during patterning do not introduce surface impurity phases in addition to those formed upon air exposure. Distinct chemical shifts originating from process-induced surface impurity phases were observed in photoelectron spectra taken on films prepared for contact definition by liftoff, i.e., exposed to the lithographic developer and deionized water. These findings correlate with the measured increase in contact resistance. Cross-sectional transmission electron microscopy images of an in situ deposited Ag contact unveil a 20–30-Å-thick interface layer, in which the 12 Å periodicity along the YBCO c axis vanishes. The presence of this layer could explain the absence of Josephson effects in YBCO/noble-metal/low-Tc superconductor junctions where the currents are forced to flow perpendicular to the a-b planes.

Nuclear magnetic resonance relaxation studies of plant polyester dynamics. 2. Suberized potato cell wall

Measurements of the proton rotating-frame relaxation time T 1ρ (H) indicate that wound-healing involves the formation of spatially separated suberin and cell-wall domains. Suberin appears to be attached to the cell-wall domain at discrete sites, with both its methylene and phenylpropanoid groups divided into two populations having distinct chemical shifts and spin relaxation characteristics. Measurements of T 1ρ (C) and T 1 (C) in this biopolymer system sho significant site-specific and tissue-specific variations .

Synthesis, nuclear magnetic resonance and crystallographic studies of six-co-ordinate bis(bidentate ligand)-dihalogenotin(IV) complexes

Six co-ordinate tin(IV) complexes of the type SnL2X2 have been prepared (where HL is a monobasic chelating ligand and X = Cl, Br or I). They have been characterised by elemental analysis, 1H and 119Sn NMR, positive-ion fast atom bombardment and electron-impact mass and vibrational spectroscopy. Three distinct 119Sn NMR chemical shift ranges are observed depending on the halogen, demonstrating the relationship between the shielding of the tin nucleus and the electronegativity of the halogens. Geometrical isomerism was normally slow on the NMR time-scale and in some cases slow- and fast-exchange spectra were obtained by variable-temperature 119Sn NMR spectroscopy. The crystal structures of two examples are reported. Both have non-crystallographic C2 symmetry with the halides cis. The structures represent different configurational isomers further illustrating the facile interconversion between isomers in solution.