Solution-state NMR Spectroscopy Research Articles

Combined Analysis of 15N Relaxation Data from Solid- and Solution-State NMR Spectroscopy

It is well-known that structures of globular proteins in liquid and in crystalline solid are essentially identical. Many lines of evidence suggest that internal dynamics are also similar (assuming that the solid sample is well-hydrated and the measurements are conducted at the same temperature). On the basis of this premise, we undertake a combined analysis of solid- and liquid-state 15N relaxation data from a small globular protein, α-spectrin SH3 domain. The interpretation using the extended Lipari−Szabo model demonstrates that liquid R1, R2, NOE, and solid R1 data measured at multiple fields are mutually consistent. To validate these results, we prepared a series of samples where the protein is dissolved in a water−glycerol solvent. The presence of glycerol ensures that the overall protein tumbling is slowed, thus increasing the visibility of nanosecond time-scale internal motions. When additional data are included in the fitting procedure, a credible picture of protein dynamics is obtained. In particular, the analysis suggests that ns time-scale motions with very low amplitude, S2≈ 0.95, are present throughout the protein. It is envisaged that combined analyses of liquid- and solid-state data can provide an efficient method for detailed characterization of internal dynamics in proteins at multiple time scales.

NMR Solution Structure of Neurotensin in Membrane-Mimetic Environments: Molecular Basis for Neurotensin Receptor Recognition

Neurotensin (NT) is a 13-residue neuropeptide that exerts multiple biological functions in the central and peripheral nervous system. Little is known about the structure of this neuropeptide, and what is known only concerns its C-terminal part. We determined here for the first time the structure of the full-length NT in membrane-mimicking environments by means of classical proton-proton distance constraints derived from solution-state NMR spectroscopy. NT was found to have a structure at both its N and C termini, whereas the central region of NT remains highly flexible. In TFE and HFIP solutions, the NT C-terminus presents an extended slightly incurved structure, whereas in DPC it has a beta turn. The N-terminal region of NT possesses great adaptability and accessibility to the microenvironment in the three media studied. Altogether, our work demonstrates a structure of NT fully compatible with its NTR-bound state.

Speciation of hydroxyl-Al polymers formed through simultaneous hydrolysis of aluminum salts and urea

Urea hydrolysis has always been used to prepare alumina gels and little attention has been paid to the reactive polymeric Al species that is formed before alumina sol–gels occur. Based on the hydrolysis process of aluminum in urea solution at 90 °C, speciation and transformation of the reactive hydroxyl-Al polymers obtained by urea hydrolysis was investigated with Ferron assay, solution-state and solid-state 27Al NMR spectroscopy. Unlike the traditional viewpoint, Keggin-Al 13 can form in solution without localized high alkalinity. The reaction of oligomers with Al(OH) 4 − resulted from the dissolving of colloidal Al hydroxides is accountable for the formation of Keggin-Al 13. Al p1, the defected structure of Al 13, was considered as the transient species in the transformation from Al 13 to crystalline Al hydroxides. Besides Al 13, some other reactive polymers with hexameric ring structure also exist. The two categories of Al species have different transformation models (i.e., forced hydrolysis and spontaneous hydrolysis). The forced hydrolysis model should be the main transformation pattern for Al species under the condition of strong base addition into Al solution. Sulphate ions inhibit the formation of high polymeric Al species and affect the crystal structure of the final Al precipitate.

Oligomerization of the Human Prion Protein Proceeds via a Molten Globule Intermediate

The conformational transition of the human prion protein from an alpha-helical to a beta-sheet-rich structure is believed to be the critical event in prion pathogenesis. The molecular mechanism of misfolding and the role of intermediate states during this transition remain poorly understood. To overcome the obstacle of insolubility of amyloid fibrils, we have studied a beta-sheet-rich misfolded isoform of the prion protein, the beta-oligomer, which shares some structural properties with amyloid, including partial proteinase resistance. We demonstrate here that the beta-oligomer can be studied by solution-state NMR spectroscopy and obtain insights into the misfolding mechanism via its transient monomeric precursor. It is often assumed that misfolding into beta-sheet-rich isoforms proceeds via a compatible precursor with a beta-sheet subunit structure. We show here, on the contrary, evidence for an almost natively alpha-helix-rich monomeric precursor state with molten globule characteristics, converting in vitro into the beta-oligomer. We propose a possible mechanism for the formation of the beta-oligomer, triggered by intermolecular contacts between constantly rearranging structures. It is concluded that the beta-oligomer is not preceded by precursors with beta-sheet structure but by a partially unfolded clearly distinguishable alpha-helical state.

Structure of the First Transmembrane Domain of the Neuronal Acetylcholine Receptor β2 Subunit

The recent cryoelectron microscopy structure of the Torpedo nicotinic acetylcholine receptor (nAChR) at 4-Å resolution shows long helices for all transmembrane (TM) domains. This is in disagreement with several previous reports that the first TM domain of nAChR and other Cys-loop receptors are not entirely helical. In this study, we determined the structure and backbone dynamics of an extended segment encompassing the first TM domain (TM1e) of nAChR β 2 subunit in dodecylphosphocholine micelles, using solution-state NMR and circular dichroism (CD) spectroscopy. Both CD and NMR results show less helicity in TM1e than in Torpedo nAChR structure (Protein Data Bank: 2BG9). The helical ending residues at the C-terminus are the same in the TM1e NMR structure and the Torpedo nAChR structure, but the helical starting residue (I-217) in TM1e is seven residues closer to the C-terminus. Interestingly, the helical starting residue is two residues before the highly conserved P-219, in accordance with the hypothesis that proline causes helical distortions at three residues preceding it. The NMR relaxation measurements show a dynamics pattern consistent with TM1e structure. The substantial nonhelical content adds greater flexibilities to TM1e, thereby implicating a different molecular basis for nAChR function compared to a longer and more rigid helical TM1.

αB-crystallin competes with Alzheimer’s disease β-amyloid peptide for peptide–peptide interactions and induces oxidation of Abeta-Met35

Alzheimer’s disease (AD) is associated with plaque deposition in the brain of AD patients. The major component of the aggregate is a 39–42 long peptide termed β-amyloid (Aβ). Except for Aβ, plaques contain several other components which co-precipitate together with Aβ. One such component is the small heat shock protein (sHSP) αB-crystallin. Instead of preventing the cell from the neurotoxicity of Aβ, αB-crystallin induces an increased neurotoxicity. We find – using solution state NMR spectroscopy – that αB-crystallin competes efficiently for Aβ monomer–monomer interactions. Interactions between Aβ and αB-crystallin involve the hydrophobic core residues 17–21 as well as residues 31–32 of Aβ, and thus the same chemical groups which are important for Aβ aggregation. In the presence of αB-crystallin, Met35 in Aβ becomes efficiently oxidized. In order to quantify the redox properties of the different complexes consisting of Aβ/αB-crystallin/copper, we suggest an NMR assay which allows to estimate the electrochemical properties indirectly by monitoring the rate of glutathion (GSH) auto-oxidation. The oxidation of the side chain Met35 in Aβ might account for the increased neurotoxicity and the inability of Aβ to form fibrillar structures, which has been observed previously in the presence of αB-crystallin [Stege, G.J. et al. (1999) The molecular chaperone αB-crystallin enhances amyloid-beta neurotoxicity. Biochem. Biophys. Res. Commun. 262, 152–156.].

MTH187 from Methanobacterium thermoautotrophicum has three HEAT-like Repeats

With the completion of genome sequencing projects, there are a large number of proteins for which we have little or no functional information. Since protein function is closely related to three-dimensional conformation, structural proteomics is one avenue where the role of proteins with unknown function can be investigated. In the present structural project, the structure of MTH187 has been determined by solution-state NMR spectroscopy. This protein of 12.4 kDa is one of the 424 non-membrane proteins that were cloned and purified for the structural proteomic project of Methanobacterium thermoautotrophicum [Christendat, D., Yee, A., Dharamsi, A., Kluger, Y., Gerstein, M., Arrowsmith, C.H. and Edwards, A.M. (2000) Prog. Biophys. Mol. Biol., 73, 339-345]. Methanobacterium thermoautotrophicum is a thermophilic archaeon that grows optimally at 65 degrees C. A particular characteristic of this microorganism is its ability to generate methane from carbon dioxide and hydrogen [Smith, D.R., Doucette-Stamm, L.A., Deloughery, C., Lee, H., Dubois, J., Aldredge, T., Bashirzadeh, R., Blakely, D., Cook, R., Gilbert, K., Harrison, D., Hoang, L., Keagle, P., Lumm, W., Pothier, B., Qiu, D., Spadafora, R., Vicaire, R., Wang, Y., Wierzbowski, J., Gibson, R., Jiwani, N., Caruso, A., Bush, D., Reeve, J. N. et al. (1997) J. Bacteriol., 179, 7135-7155].

Secondary structure and dynamics of micelle bound β‐ and γ‐synuclein

We have used solution state NMR spectroscopy to characterize the secondary structure and backbone dynamics of the proteins beta- and gamma-synuclein in their detergent micelle-bound conformations. Comparison of the results with those previously obtained for the Parkinson's disease-linked protein alpha-synuclein shows that structural differences between the three homologous synuclein family members are directly related to variations in their primary amino acid sequences. An 11-residue deletion in the lipid-binding domain of beta-synuclein leads to the destabilization of an entire segment of the micelle-bound helical structure containing the deletion site. The acidic C-terminal tail region of gamma-synuclein, which displays extensive sequence divergence, is more highly disordered than the corresponding regions in the other two family members. The observed structural differences are likely to mediate functional variations between the three proteins, with differences between alpha- and beta-synuclein expected to revolve around their lipid interactions, while differences in gamma-synuclein function are expected to result from different protein-protein interactions mediated by its unique C-terminal tail.

Investigation of Group 12 Metal Complexes with a Tridentate SNS Ligand by X-ray Crystallography and 1H NMR Spectroscopy

Two series of zinc triad complexes containing the ligand 2,6-bis(methylthiomethyl)pyridine (L1) were synthesized and characterized by X-ray crystallography and solution-state 1H NMR spectroscopy. The distorted meridional octahedral M(L1)2(ClO4)2 series includes the first structurally characterized Zn(II) and Cd(II) complexes with N2(SR2)4 coordination spheres. Coordination of HgCl2 and ZnCl2 with 1 equiv of ligand afforded mononuclear, five-coordinate species Hg(L1)Cl2 and Zn(L1)Cl2, respectively, with distorted square-pyramidal and trigonal-bipyramidal geometries. With CdCl2, the dimeric [Cd(L1)Cl(mu-Cl)]2 complex was obtained. The distorted octahedral coordination geometry of each Cd(II) center in this complex is formed by one tridentate ligand, two bridging chloride ions, and one terminal chloride ion. NMR spectra indicate that the intermolecular ligand-exchange rate of [M(L1)2](2+) decreased in the order Cd(II) > Zn(II) > Hg(II). Slow intermolecular ligand-exchange conditions on the chemical-shift time scale were found for 1:2 metal-to-ligand complexes of L(1) with Hg(II) and Zn(II) but not Cd(II). Slow intermolecular ligand-exchange conditions in acetonitrile-d(3) solutions permitting detection of (3-5)J(199Hg1H) were found for 1:1 and 1:2 Hg(ClO4)2/L1 complexes, but not for the related Cd(ClO4)2) complexes. The magnitudes of J(199Hg1H) for equivalent protons were smaller in [Hg(L1)2](2+) than in [Hg(L1)(NCCH3)x](2+). The relative intermolecular ligand-exchange rates of the zinc triad complexes investigated here suggest that the toxicity of Hg(II) is accentuated by the relative difficulty of displacing it from the coordination sites encountered.

Induction of Morphological Changes in Model Lipid Membranes and the Mechanism of Membrane Disruption by a Large Scorpion-Derived Pore-Forming Peptide

The membrane disruption mechanism of pandinin 1 (pin1), an antimicrobial peptide isolated from the venom of the African scorpion, was studied using 31P, 13C, 1H solid-state and multidimensional solution-state NMR spectroscopy. A high-resolution NMR solution structure of pin1 showed that the two distinct α-helical regions move around the central hinge region, which contains Pro 19. 31P NMR spectra of lipid membrane in the presence of pin1, at various temperatures, showed that pin1 induces various lipid phase behaviors depending on the acyl chain length and charge of phospholipids. Notably, it was found that pin1 induced formation of the cubic phase in shorter lipid membranes above T m. Further, the 13C NMR spectra of pin1 labeled at Leu 28 under magic angle spinning (MAS) indicated that the motion of pin1 bound to the lipid bilayer was very slow, with a correlation time of the order of 10 −3 s. 31P NMR spectra of dispersions of four saturated phosphatidyl-cholines in the presence of three types of pin1 derivatives, [W4A, W6A, W15A]-pin1, pin1(1-18), and pin1(20-44), at various temperatures demonstrated that all three pin1 derivatives have a reduced ability to trigger the cubic phase. 13C chemical shift values for pin1(1-18) labeled at Val 3, Ala 10, or Ala 11 under static or slow MAS conditions indicate that pin1(1-18) rapidly rotates around the average helical axis, and the helical rods are inclined at ∼30° to the lipid long axis. 13C chemical shift values for pin1(20-44) labeled at Gly 25, Leu 28, or Ala 31 under static conditions indicate that pin1(20-44) may be isotropically tumbling. 1H MAS chemical shift measurements suggest that pin1 is located at the membrane-water interface approximately parallel to the bilayer surface. Solid-state NMR results correlated well with the observed biological activity of pin1 in red blood cells and bacteria.

The Synthesis, Structural, and Spectroscopic Characterization of Uranium(IV) Perrhenate Complexes

We report the synthesis, structural, and spectroscopic characterization of a series of uranium(IV)-perrhenato complexes. Three isostructural complexes with general formula [U(ReO4)4(L)4] (where L = tri-n-butylphosphine oxide/TBPO (2), triethyl phosphate/TEP (3), or tri-iso-butyl phosphate/TiBP (4)), have been synthesized, both through the photoreduction of ethanolic {UO2}2+ solutions and also via a novel U(IV) starting material, U(ReO4)4.5H2O (1). Compound 1 has also been used in the preparation of [U(ReO4)4(TPPO)3(CH3CN)].2CH3CN (5) and [U(ReO4)(DPPMO2)3(OH)][ReO4]2.2CH3CN (6), where TPPO represents triphenylphosphine oxide and DPPMO2 represents bis(diphenylphosphino)methane dioxide. All six complexes have been spectroscopically characterized using NMR, UV-vis-NIR, and IR techniques, with 2, 3, 5, and 6 also fully structurally characterized. The U atoms in compounds 2-6 all exhibit eight-coordinate geometry with up to four perrhenate groups in addition to three (DPPMO2 and TPPO) or four (TEP, TiBP, TBPO) coordinated organic ligands. In the case of compounds 5 and 6, the coordination of eight ligands to the U(IV) center is completed by the binding of a solvent molecule (CH3CN) and OH-, respectively. Solid-state physical analysis (elemental and thermogravimetric) and infrared spectroscopy are in agreement with the structural studies. The crystallographic data suggest that the strength of the U(IV)-O-donor ligand bonds decreases across the series R3PO > [ReO4]- > (RO)3PO. Solution-state IR and 31P NMR spectroscopy appear to be in agreement with these solid-state results.

Steric compression and twist in o-hydroxy acyl aromatics with intramolecular hydrogen bonding

A series of o-hydroxy acyl aromatics of the type 1,3-diacetyl-2,4,6-trihydroxybenzene ( 1), 6-methoxy-1,3-diacetyl-2,4-dihydroxy- ( 2), 2,4,6-trihydroxy-1,3,5-triacetylbenzene ( 3) and 1-acetyl-2-naphthol ( 4) have been investigated by means of single crystal X-ray diffraction, solution and solid state NMR spectroscopy and theoretical calculations. The structures of 1 and 2 exhibit interesting hydrogen bonds, planar structures and have, as a consequence, unexpected geometrical parameters (interatomic distances, bond lengths and valence angles) and depletion of electron density of the aromatic rings. Also significant packing effects are present. For 4, a twist of the carbonyl group is observed together with an out of plane bending of the C–C O bond leading to the formation of a C O⋯H–O–C hydrogen bond which is almost coplanar with the naphthalene rings. Solid state NMR spectra show lack of C 3 symmetry for 3. Solution NMR spectra show-in the case of 1 and 2-quite different behaviour. A complex averaging-observed for 1 in solution-is unravelled at low temperature. Compounds 1– 3 show large two-bond deuterium isotope effects, 2ΔC-2(OD), on 13C chemical shifts. This indicates strong hydrogen bonds. These can be understood in terms of an electronic effect caused by bond localisation of the benzene ring and a steric effect caused by either neighbouring CH 3CO, OH or OCH 3 groups leading to shorter OH⋯O and O⋯O distances and, consequently, stronger hydrogen bonds. A general scheme for distinguishing between steric twist (as seen in 4) and steric compression as seen in 1– 3 is suggested. An experimental method based on isotope effects at the chelate proton of compounds deuteriated at the CD 3CO groups is demonstrated. A Bader atom in molecules is done to investigate hydrogen bonding.

Heteronuclear Solution-State NMR Studies of the Chromophore in Cyanobacterial Phytochrome Cph1

Precise structural information regarding the chromophore binding pocket is essential for an understanding of photochromicity and photoconversion in phytochrome photoreceptors. To this end, we are studying the 59 kDa N-terminal module of the cyanobacterial phytochrome Cph1 from Synechocystis sp. PCC 6803 in both thermally stable forms (Pr and Pfr) using solution-state NMR spectroscopy. The protein is deuterated, while the chromophore, phycocyanobilin (PCB), is isotopically labeled with (15)N or (13)C and (15)N. We have established a simple approach for preparing labeled PCB based on BG11 medium supplemented with an appropriate buffer and NaH(13)CO(3) and Na(15)NO(3) as sole carbon and nitrogen sources, respectively. We show that structural details of the chromophore binding pocket in both Pr and Pfr forms can be obtained using multidimensional heteronuclear solution-state NMR spectroscopy. Using one-dimensional (15)N NMR spectra, we show unequivocally that the chromophore is protonated in both Pr and Pfr states.

The identification of some new antimony(III) compounds containing fluoroxyl ligands by 19F solution-state NMR spectroscopy: crystal and molecular structure of [formula omitted] (Ar′ = 2,6-(CF 3) 2C 6H 3; Ar″ = 2,4-(CF 3) 2C 6H 3)

Reaction between 2,6-(CF 3) 2C 6H 3Li (Ar′Li) and/or 2,4-(CF 3) 2C 6H 3Li (Ar″Li) and SbCl 3 has afforded a series of new antimony(III) derivatives, which have all been identified in solution by 19F NMR spectroscopy. The unusual compound Ar ′ Ar 2 ″ Sb has been isolated, and its crystal and molecular structures determined by single-crystal X-ray diffraction. Ar ′ Ar 2 ″ Sb is only the second example of a compound containing three fluoroxyl ligands to be structurally characterised.

Solution structure of Escherichia coli Par10: The prototypic member of the Parvulin family of peptidyl-prolyl cis/trans isomerases.

E. coli Par10 is a peptidyl-prolyl cis/trans isomerase (PPIase) from Escherichia coli catalyzing the isomerization of Xaa-Pro bonds in oligopeptides with a broad substrate specificity. The structure of E. coli Par10 has been determined by multidimensional solution-state NMR spectroscopy based on 1207 conformational constraints (1067 NOE-derived distances, 42 vicinal coupling-constant restraints, 30 hydrogen-bond restraints, and 68 phi/psi restraints derived from the Chemical Shift Index). Simulated-annealing calculations with the program ARIA and subsequent refinement with XPLOR yielded a set of 18 convergent structures with an average backbone RMSD from mean atomic coordinates of 0.50 A within the well-defined secondary structure elements. E. coli Par10 is the smallest known PPIase so far, with a high catalytic efficiency comparable to that of FKBPs and cyclophilins. The secondary structure of E. coli Par10 consists of four helical regions and a four-stranded antiparallel beta-sheet. The N terminus forms a beta-strand, followed by a large stretch comprising three alpha-helices. A loop region containing a short beta-strand separates these helices from a fourth alpha-helix. The C terminus consists of two more beta-strands completing the four-stranded anti-parallel beta-sheet with strand order 2143. Interestingly, the third beta-strand includes a Gly-Pro cis peptide bond. The curved beta-strand forms a hydrophobic binding pocket together with alpha-helix 4, which also contains a number of highly conserved residues. The three-dimensional structure of Par10 closely resembles that of the human proteins hPin1 and hPar14 and the plant protein Pin1At, belonging to the same family of highly homologous proteins.

High-performance selective excitation pulses for solid- and liquid-state NMR spectroscopy.

Computer-optimized selective pulses are routinely used in solution-state NMR spectroscopy. At the same time, their utility and importance for solid-state applications has yet to be fully realized. We suggest a new computational approach that makes the design of soft selective pulses with desired properties relatively straightforward. By applying this technique to the generic selective excitation problem, we have arrived at a family of high performance selective excitation pulses, dubbed E-Family, that allows more flexibility and better performance than analogous pulses previously reported in the literature. The new pulses have been successfully tested in both solid- and solution-state NMR experiments. A theoretical treatment of the effects of chemical shift anisotropy (CSA) on the selective excitation in magic-angle spinning (MAS) experiments in solids is presented. The set of heuristics that comprise our new strategy were incorporated into a general NMR simulation program SPINEVOLUTION.

Ring‐opening polymerisation of coordination rings and cages

AbstractDespite the great interest in crystalline coordination polymers (sometimes called metal organic frameworks) surprisingly little is known about how they form. However, there has recently been some attention given to characterising their solution‐based precursors. Of particular interest is the formal ring‐opening polymerisation (ROP) relationship between the structures of some precursors and polymers, and the actual observation of ring‐opened oligomers by solution state NMR spectroscopy. These points are highlighted and discussed along with related aspects of isomerism in discrete and polymeric coordination structures, and polymer‐polymer interconversion.

Towards novel C–N materials: crystal structures of two polymorphs of guanidinium dicyanamide and their thermal conversion into melamine

Two modifications of the novel guanidinium dicyanamide have been obtained by means of ion-exchange reaction in aqueous or methanolic solution. The hygroscopic compounds were characterized by solution state NMR, mass spectrometry and vibrational spectroscopy. The crystal structures of the polymorphs were elucidated by means of single-crystal X-ray diffraction at 200 K {β-[C(NH2)3][N(CN)2]: Pna21, Z=8, a=1373.1(3), b=495.5(1), c=1802.9(4) pm, U=1226.7(4)×106 pm3; α-[C(NH2)3][N(CN)2]: P21/c, Z=8, a=1924.9(4), b=496.0(1), c=1372.4(3) pm, β=110.46(3)°, U=1227.5(4)×106 pm3} and were found to be largely equivalent in terms of the overall assembly of the molecular ions. Thermodynamic and kinetic aspects of the temperature behaviour of the polymorphs, which is characterized by a succession of thermal events in the temperature region between 240 and 440 K, were assessed by means of temperature-dependent X-ray powder diffraction and thermal analysis. Due to the chemical composition of the novel dicyanamide (C3N6H6), which is formally identical with that of melamine C3N3(NH2)3, and its thermal reactivity, which is represented by the facile conversion into melamine around 400 K, guanidinium dicyanamide may be ideally suited as a molecular precursor for chemical approaches toward highly condensed graphitic carbon nitride materials.

Comparison of the Quaternary Aromatic Carbon Contents of a Coal, a Coal Extract, and Its Hydrocracking Products by NMR Methods

Samples of a coal extract and its hydrocracked products have been examined by solution-state and solid-state NMR spectroscopy to establish if these methods could follow the reaction process that is expected to convert bridgehead C atoms into hydrogen-substituted C atoms. The changes have also been followed by size-exclusion chromatography (SEC), elemental analysis, basic −OH titration, and UV-fluorescence spectroscopy. NMR spectra in CDCl3 indicated that the products contained more bridgehead C atoms than the extract. Spectra obtained using a better solvent, 1-methyl-2-pyrrolidinone (NMP), did indicate that the extract contained more bridgehead C atoms than the products. The basic −OH measurement indicated rapid removal of the group by hydrocracking; however, this was a relatively minor correction to the overall quaternary carbon concentration. The evidence from SEC and UV-fluorescence suggested that the large molecules that were indicated to be present in the coal extract and the products did not give good NMR spectra, because the solvents did not dissolve them. Therefore, results from solution-state NMR of such coal liquids must be considered as only partial results and not covering the entire sample. Solid-state 13C NMR methods showed that the process solvent could not dissolve the most aromatic components of the coal, with a significant loss of aromaticity in the digestion stage. Nonquaternary suppression (NQS) spectra of the microbomb product indicated the formation of methyl groups by the formation of methyl-substituted five-membered rings from tetralin. Solid-state 1H spectra of the coal extract and products showed a decrease of aromatic to aliphatic hydrogen on reaction, as expected.

Microstructure of Maleic Anhydride Grafted Polyethylene by High-Resolution Solution-State NMR and FTIR Spectroscopy

The microstructure of maleic anhydride (MA) grafted polyethylene (PE), MA-g-PE, has been successfully characterized by high-resolution NMR and FT-IR spectroscopy. The carbonyl asymmetric and symmetric stretchings of MA-g-PE exhibit lower shifts with an increase in the grafting degree of MA, suggesting stronger inductive interactions between different grafts in a solid state. The 1H and 13C NMR spectroscopy of [2,3-13C2]MA grafted PE with a low molecular weight (lmPE), [2,3-13C2]MA-g-lmPE, demonstrates that [2,3-13C2]MA-g-lmPE contains fewer succinic anhydride (SA) oligomeric grafts with a terminal unsaturated MA ring (oligo-MA) in addition to more SA oligomeric grafts terminated with a saturated SA ring (oligo-SA), the only structure that had been observed prior to this study. The formation of oligo-MA and oligo-SA can be explained by a mechanism based on two different termination processes: disproportionation between a grafting radical and a macroradical on a secondary carbon in lmPE or another grafting...