Solution NMR Data Research Articles

Modeling of arylamide helix mimetics in the p53 peptide binding site of hDM2 suggests parallel and anti-parallel conformations are both stable.

The design of novel α-helix mimetic inhibitors of protein-protein interactions is of interest to pharmaceuticals and chemical genetics researchers as these inhibitors provide a chemical scaffold presenting side chains in the same geometry as an α-helix. This conformational arrangement allows the design of high affinity inhibitors mimicking known peptide sequences binding specific protein substrates. We show that GAFF and AutoDock potentials do not properly capture the conformational preferences of α-helix mimetics based on arylamide oligomers and identify alternate parameters matching solution NMR data and suitable for molecular dynamics simulation of arylamide compounds. Results from both docking and molecular dynamics simulations are consistent with the arylamides binding in the p53 peptide binding pocket. Simulations of arylamides in the p53 binding pocket of hDM2 are consistent with binding, exhibiting similar structural dynamics in the pocket as simulations of known hDM2 binders Nutlin-2 and a benzodiazepinedione compound. Arylamide conformations converge towards the same region of the binding pocket on the 20 ns time scale, and most, though not all dihedrals in the binding pocket are well sampled on this timescale. We show that there are two putative classes of binding modes for arylamide compounds supported equally by the modeling evidence. In the first, the arylamide compound lies parallel to the observed p53 helix. In the second class, not previously identified or proposed, the arylamide compound lies anti-parallel to the p53 helix.

Optimization of the additive CHARMM all-atom protein force field targeting improved sampling of the backbone φ, ψ and side-chain χ(1) and χ(2) dihedral angles.

While the quality of the current CHARMM22/CMAP additive force field for proteins has been demonstrated in a large number of applications, limitations in the model with respect to the equilibrium between the sampling of helical and extended conformations in folding simulations have been noted. To overcome this, as well as make other improvements in the model, we present a combination of refinements that should result in enhanced accuracy in simulations of proteins. The common (non Gly, Pro) backbone CMAP potential has been refined against experimental solution NMR data for weakly structured peptides, resulting in a rebalancing of the energies of the α-helix and extended regions of the Ramachandran map, correcting the α-helical bias of CHARMM22/CMAP. The Gly and Pro CMAPs have been refitted to more accurate quantum-mechanical energy surfaces. Side-chain torsion parameters have been optimized by fitting to backbone-dependent quantum-mechanical energy surfaces, followed by additional empirical optimization targeting NMR scalar couplings for unfolded proteins. A comprehensive validation of the revised force field was then performed against data not used to guide parametrization: (i) comparison of simulations of eight proteins in their crystal environments with crystal structures; (ii) comparison with backbone scalar couplings for weakly structured peptides; (iii) comparison with NMR residual dipolar couplings and scalar couplings for both backbone and side-chains in folded proteins; (iv) equilibrium folding of mini-proteins. The results indicate that the revised CHARMM 36 parameters represent an improved model for the modeling and simulation studies of proteins, including studies of protein folding, assembly and functionally relevant conformational changes.

Probing the Structure and Dynamics of Proteins by Combining Molecular Dynamics Simulations and Experimental NMR Data.

NMR experiments provide detailed structural information about biological macromolecules in solution. However, the amount of information obtained is usually much less than the number of degrees of freedom of the macromolecule. Moreover, the relationships between experimental observables and structural information, such as interatomic distances or dihedral angle values, may be multiple-valued and may rely on empirical parameters and approximations. The extraction of structural information from experimental data is further complicated by the time- and ensemble-averaged nature of NMR observables. Combining NMR data with molecular dynamics simulations can elucidate and alleviate some of these problems, as well as allow inconsistencies in the NMR data to be identified. Here, we use a number of examples from our work to highlight the power of molecular dynamics simulations in providing a structural interpretation of solution NMR data.

Thiol, Disulfide, and Trisulfide Complexes of Ru Porphyrins: Potential Models for Iron–Sulfur Bonds in Heme Proteins

Thirty-two Ru(porp)L(2) complexes have been synthesized, where porp = the dianion of meso-tetramesitylporphyrin (TMP) or meso-tetrakis(4-methylphenyl)porphyrin (H(2)T-pMe-PP), and L = a thiol, a sulfide, a disulfide, or a trisulfide. Species studied were with RSH [R = Me, Et, (n)Pr, (i)Pr, (t)Bu, Bn (benzyl), and Ph], RSR (R = Me, Bn), RSSR (R = Me, Et, (n)Pr, Bn) and MeSS(t)Bu, and RSSSR (R = Me, Bn). All the species except two, which were the isolated Ru(T-pMe-PP)((t)BuSH)(2) and Ru(TMP)(MeSSMe)(2), were characterized in situ. The disulfide complex was characterized by X-ray analysis. (1)H NMR data for the coordinated thiols are the first reported within metalloporphyrin systems, and are especially informative because of the upfield shifts of the axial sulfur-containing ligands due to the porphyrin π-ring current effect, which is also present in the di- and trisulfide species. The disulfide in the solid state structure of Ru(TMP)(MeSSMe)(2) is η(1)(end-on) coordinated, the first example of such bonding in a nontethered, acyclic dialkyl disulfide; (1)H-(1)H EXSY NMR data in solution show that the species undergoes 1,2-S-metallotropic shifts. Stepwise formation of the bis(disulfide) complex from Ru(TMP)(MeCN)(2) in solution occurs with a cooperativity effect, resembling behavior of Fe(II)-porphyrin systems where crystal field effects dominate, but ligand trans-effects are more likely in the Ru system. The η(1)(end-on) coordination mode is also favored for the trisulfide ligand. Discussed also are the remarkable linear correlations that exist between the ring-current shielding shifts for the axial ligand C(1) protons of Ru(porp)(RS(x)R)(2) and x (the number of S atoms). The Introduction briefly reviews literature on Ru- and Fe porphyrins (including heme proteins) with sulfur-containing ligands or substrates, and relationships between our findings and this literature are discussed throughout the paper.

Stereoelectronic Interactions and the One-Bond C–F Coupling Constant in Sevoflurane

The conformational preference of the widely utilized anesthetic fluoromethyl-1,1,1,3,3,3-hexafluoro-2-propyl ether (sevoflurane) has been investigated computationally and by NMR spectroscopy. Three conformational minima were located at the B3LYP/aug-cc-pVDZ level, but one is significantly more stable (by ca. 4 kcal/mol) than the other two. This is the case both for gas phase calculations and for solution NMR data. Although the main conformer is stabilized by electron delocalization (n(O) → σ*(C-F)), this type of hyperconjugation was not found to be the main driver for the conformer stabilization in the gas phase and, consequently, for the apparent anomeric effect in sevoflurane. Instead, more classical steric and electrostatic interactions appear to be responsible for the conformational energies. Also the (1)J(CF) coupling constants do not appear to be dominated by hyperconjugation; again, dipolar interactions are invoked instead.

Structure and Dynamics of Mycobacterium tuberculosis Truncated Hemoglobin N: Insights from NMR Spectroscopy and Molecular Dynamics Simulations

The potent nitric oxide dioxygenase (NOD) activity (trHbN-Fe²⁺-O₂ + (•)NO → trHbN-Fe³⁺-OH₂ + NO₃⁻) of Mycobacterium tuberculosis truncated hemoglobin N (trHbN) protects aerobic respiration from inhibition by (•)NO. The high activity of trHbN has been attributed in part to the presence of numerous short-lived hydrophobic cavities that allow partition and diffusion of the gaseous substrates (•)NO and O₂ to the active site. We investigated the relation between these cavities and the dynamics of the protein using solution NMR spectroscopy and molecular dynamics (MD). Results from both approaches indicate that the protein is mainly rigid with very limited motions of the backbone N-H bond vectors on the picoseconds-nanoseconds time scale, indicating that substrate diffusion and partition within trHbN may be controlled by side-chains movements. Model-free analysis also revealed the presence of slow motions (microseconds-milliseconds), not observed in MD simulations, for many residues located in helices B and G including the distal heme pocket Tyr33(B10). All currently known crystal structures and molecular dynamics data of truncated hemoglobins with the so-called pre-A N-terminal extension suggest a stable α-helical conformation that extends in solution. Moreover, a recent study attributed a crucial role to the pre-A helix for NOD activity. However, solution NMR data clearly show that in near-physiological conditions these residues do not adopt an α-helical conformation and are significantly disordered and that the helical conformation seen in crystal structures is likely induced by crystal contacts. Although this lack of order for the pre-A does not disagree with an important functional role for these residues, our data show that one should not assume an helical conformation for these residues in any functional interpretation. Moreover, future molecular dynamics simulations should not use an initial α-helical conformation for these residues in order to avoid a bias based on an erroneous initial structure for the N-termini residues. This work constitutes the first study of a truncated hemoglobin dynamics performed by solution heteronuclear relaxation NMR spectroscopy.

Flexible Diphosphine Ligands with Overall Charges of 0, +1, and +2: Critical Role of the Electrostatics in Favoring Trans over Cis Coordination

The influence of the formal electrostatic interaction on the cis/trans coordination mode at a PdCl(2) center is investigated in a family of isostructural flexible diphosphine ligands Ph(2)P-X-C(6)H(4)-Y-PPh(2), where X and Y stand for neutral or cationic N,C-imidazolylene linkers. While the neutral and monocationic diphosphine spontaneously behave as classical cis-chelating ligands, only the dicationic diphosphine, where the electrostatic repulsion between the formal positive charges specifically takes place, is observed to behave as a trans-chelating ligand. The crucial role of electrostatics is analyzed on the basis of (31)P NMR data in solution and X-ray diffraction data in the crystal state. Comparative theoretical studies of the cis- and trans-chelated complexes, including EDA, static (31)P NMR, MESP, and AIM analyses, have been undertaken on the basis of DFT calculations in the gas phase or in the acetonitrile continuum. Whereas the cis-coordination mode is shown to be thermodynamically favored for the neutral ligand, the trans-coordination mode is found to be preferred for the dicationic homologue. The stereochemical preference is thus shown to be parallel to the expected effect of the formal electrostatic interaction. The results open perspectives for control of the cis- and trans-chelating behavior of flexible bidentate ligands by more or less reversible charge transfer at the periphery of the coordination sphere of a metallic center.

새로운 유기주석 착물의 합성과 특성: 경쟁적인 N, O 및 S 주개 리간드에 기초한 Di(n-butyl)chloro[5-( p-dimethylaminobenzylidene)rhodanine]tin(IV)

A novel organotin(IV) complex has been prepared from and the N, O and S donor ligand, 5-(p-dimethylaminobenzylidene) rodanine (HL). The ligand is deprotonated in the presence of a base and the complex with the general formula is formed. This complex was fully characterized by IR, NMR and NMR and elemental analysis. Spectroscopic data indicate the ligand is coordinated through the oxygen atom to the tin and the coordination number of four is supported by NMR data in solution.

Reactions of the Bis(dialkylphosphino)methane Complexes Pd2X2(μ-R2PCH2PR2)2(X = halogen, R = Me or Et) with H2S, S8, COS, and CS2; Detection of Reaction Intermediates

The Pd(2)X(2)(dmpm)(2) complexes [X = Cl (1a), Br (1b), I (1c); dmpm = bis(dimethylphosphino)methane. In all the dipalladium complexes mentioned in this paper, the dmpm, depm, and dppm ligands (unless stated otherwise) are bridging, but for convenience the μ-symbol is omitted.] react with H(2)S to yield H(2) and the bridged-sulfido complexes Pd(2)X(2)(μ-S)(dmpm)(2) (2a-c), of which 2a and 2b are structurally characterized. With 1a, two rapid reversible equilibria are observed by NMR spectroscopy below -30 °C, and two reaction intermediates are detected; both are likely hydrido(mercapto) species. Reaction of 1a with 1 equiv of elemental sulfur also yields 2a. The reaction of 1a with COS results in the initial formation of Pd(2)Cl(2)(μ-COS)(dmpm)(2) (3) that undergoes decarbonylation to yield 2a and Pd(2)Cl(2)(μ-CO)(dmpm)(2) (4), which is also formed via reversible insertion of the CO into the Pd-Pd bond of 1a. The solid-state molecular structure of the previously reported complex Pd(2)Cl(2)(μ-CS(2))(dmpm)(2) (5), together with solution NMR data for 3 and 5, reveal that the bridging heterocumulene ligands coordinate in an η(2)-C,S fashion. Analogous findings were made for the corresponding Pd(2)X(2)(depm)(2) complexes [X = Cl (1a'), Br (1b'), I (1c'); depm = bis(diethylphosphino)methane], although no μ-COS species was detected. The Pd(2)X(2)(μ-S)(depm)(2) complex was structurally characterized. Differences in the chemistry of the previously studied, corresponding dppm systems (dppm = bis(diphenylphosphino)methane) are discussed.

Recognition of Multivalent Histone States Associated with Heterochromatin by UHRF1 Protein

Histone modifications and DNA methylation represent two layers of heritable epigenetic information that regulate eukaryotic chromatin structure and gene activity. UHRF1 is a unique factor that bridges these two layers; it is required for maintenance DNA methylation at hemimethylated CpG sites, which are specifically recognized through its SRA domain and also interacts with histone H3 trimethylated on lysine 9 (H3K9me3) in an unspecified manner. Here we show that UHRF1 contains a tandem Tudor domain (TTD) that recognizes H3 tail peptides with the heterochromatin-associated modification state of trimethylated lysine 9 and unmodified lysine 4 (H3K4me0/K9me3). Solution NMR and crystallographic data reveal the TTD simultaneously recognizes H3K9me3 through a conserved aromatic cage in the first Tudor subdomain and unmodified H3K4 within a groove between the tandem subdomains. The subdomains undergo a conformational adjustment upon peptide binding, distinct from previously reported mechanisms for dual histone mark recognition. Mutant UHRF1 protein deficient for H3K4me0/K9me3 binding shows altered localization to heterochromatic chromocenters and fails to reduce expression of a target gene, p16(INK4A), when overexpressed. Our results demonstrate a novel recognition mechanism for the combinatorial readout of histone modification states associated with gene silencing and add to the growing evidence for coordination of, and cross-talk between, the modification states of H3K4 and H3K9 in regulation of gene expression.

Structural and Thermodynamic Basis for Weak Interactions between Dihydrolipoamide Dehydrogenase and Subunit-binding Domain of the Branched-chain α-Ketoacid Dehydrogenase Complex

The purified mammalian branched-chain α-ketoacid dehydrogenase complex (BCKDC), which catalyzes the oxidative decarboxylation of branched-chain α-keto acids, is essentially devoid of the constituent dihydrolipoamide dehydrogenase component (E3). The absence of E3 is associated with the low affinity of the subunit-binding domain of human BCKDC (hSBDb) for hE3. In this work, sequence alignments of hSBDb with the E3-binding domain (E3BD) of the mammalian pyruvate dehydrogenase complex show that hSBDb has an arginine at position 118, where E3BD features an asparagine. Substitution of Arg-118 with an asparagine increases the binding affinity of the R118N hSBDb variant (designated hSBDb*) for hE3 by nearly 2 orders of magnitude. The enthalpy of the binding reaction changes from endothermic with the wild-type hSBDb to exothermic with the hSBDb* variant. This higher affinity interaction allowed the determination of the crystal structure of the hE3/hSBDb* complex to 2.4-Å resolution. The structure showed that the presence of Arg-118 poses a unique, possibly steric and/or electrostatic incompatibility that could impede E3 interactions with the wild-type hSBDb. Compared with the E3/E3BD structure, the hE3/hSBDb* structure has a smaller interfacial area. Solution NMR data corroborated the interactions of hE3 with Arg-118 and Asn-118 in wild-type hSBDb and mutant hSBDb*, respectively. The NMR results also showed that the interface between hSBDb and hE3 does not change significantly from hSBDb to hSBDb*. Taken together, our results represent a starting point for explaining the long standing enigma that the E2b core of the BCKDC binds E3 far more weakly relative to other α-ketoacid dehydrogenase complexes.

A Eukaryotic-Like Interaction of Soluble Cyanobacterial Sensory Rhodopsin Transducer with DNA

Anabaena sensory rhodopsin is a recently discovered membrane photosensor with a unique signal transduction cascade. It interacts with a soluble tetrameric transducer [ Anabaena sensory rhodopsin transducer (ASRT)] that can bind to promoter regions of several genes related to the utilization of light energy. Even though the X-ray crystal structure of ASRT is available, the mechanism of its interaction with DNA is still unknown. We used solution NMR to understand the mechanism of the DNA binding. Both X-ray crystal structures and solution NMR data reveal seven β-strands forming a rigid scaffold (β-face) and a flexible, partially disordered α-face, comprised by the C-termini and loops. We found that the conformation of the α-face in solution is very different from that in the crystals. While the C-termini of crystalline ASRT are solvent exposed and either α-helical or disordered, about half of ASRT monomers in solution feature buried C-terminal β-strand, with another half of C-tails being random coils. Titration of ASRT with a 20-bp fragment of the pec operon promoter showed that only monomers with β-structured C-tails bind the DNA. NMR signals suggest that specific Arg and Asn/Gln residues are involved in the interaction with DNA. The DNA binding occurs with micromolar affinity and a 1:1 stoichiometry (DNA:ASRT tetramer) and results in a significant ordering of the α-face involving the extension of the C-terminal β-strand and reorganization of the first loop. Such induced-fit type of interaction, which mainly utilizes loops between β-strands and results in the increase in their order, is typical for eukaryotic transcription factors of the immunoglobulin-like fold.

Selective hydrogenation of chloronitrobenzenes with an MCM-41 supported platinum allyl complex derived catalyst

A platinum precatalyst ( 1) has been prepared by reacting [( η 3-C 3H 5) 4Pt 4Cl 4] with surface functionalized MCM-41 with pendant –(CH 2) 3NH(CH 2) 2NH 2 groups. For the hydrogenation of o-, m- and p-chloronitrobenzenes to the corresponding chloroanilines, 1 is found to be a highly active catalyst with good selectivities for the m- and p-isomers. Its performance is superior to that of its palladium analogue and far superior to that of commercial (5%) Pt/C or (5%) Pt/Al 2O 3. Comparison of solid state and solution NMR data and other evidences indicate that on treatment with the functionalized MCM-41 support; [( η 3-C 3H 5) 4Pt 4Cl 4] loses the allyl ligand. XPS data show that in the fresh catalyst Pt is present in the 2+ oxidation state. Based on these and analytical data, co-ordination by surface diamine and hydroxo groups to Pt 2+ in 1 is suggested. In the used catalyst both Pt 2+ and Pt 0 are present but the amount of metallic platinum is ∼16% of the total.

Dictyostatin Flexibility Bridges Conformations in Solution and in the β-Tubulin Taxane Binding Site

Dictyostatin (DCT, 1) is a complex, flexible polyketide macrolide that demonstrates potent microtubule-polymerization activity. Both a solution structure (2a) and a possible binding mode for DCT (Conf-1) have been proposed by earlier NMR experiments. In the present study, the conformational landscape of DCT in DMSO-d(6) and methanol-d(4) was explored using extensive force-field-based conformational searches combined with geometric parameters derived from solution NMR data. The results portray a diversity of conformations for dictyostatin that illustrates the molecule's flexibility and excludes the previously suggested dominant solution conformation 2a. One conformation present in DMSO-d(6) with a 7% population (Conf-2, 0.6 kcal/mol above the global minimum at 298°) also satisfies the TR-NOESY NMR parameters of Canales et al. that characterize the taxane binding-site interaction between DCT and assembled microtubules in water. Application of several docking methods (Glide, Autodock, and RosettaLigand) has identified a low-energy binding model of the DCT/β-tubulin complex (Pose-2/Conf-2) that is gratifyingly compatible with the emerging DCT structure-activity data.

Complexations of 2-thiouracil and 2,4-dithiouracil with Cd(SeCN)2and Hg(SeCN)2: NMR and anti-bacterial activity studies

Cadmium and mercury selenocyanate complexes of 2-thiouracil (TU) and 2,4-dithiouracil (DTU) ligands have been synthesized to form complexes of the type [M(SeCN)2(TU)] and [M(SeCN)2(DTU)] (where M is Cd2+or Hg2+) and studied by various spectroscopic techniques such as IR,1H and13C NMR in solution and in the solid state for13C,15N, and113Cd nuclei. Based on IR, and solution and solid-state13C NMR data, stronger cadmium bonding to the thiouracil was observed compared to that of mercury. Anti-bacterial activities of these complexes have been investigated with standard type culture ofEscherichia coli(MTCC 443),Klebsiella pneumoniae(MTCC 109),Pseudomonas aeruginosa(MTCC 1688),Salmonella typhi(MTCC 733) andStaphylococcus aureus(MTCC 737) and show that ligands exhibit more anti-bacterial activities than that of the corresponding Cd(II) and Hg(II) complexes.

The Third Conformation of p38α MAP Kinase Observed in Phosphorylated p38α and in Solution

MAPKs engage substrates, MAP2Ks, and phosphatases via a docking groove in the C-terminal domain of the kinase. Prior crystallographic studies on the unphosphorylated MAPKs p38α and ERK2 defined the docking groove and revealed long-range conformational changes affecting the activation loop and active site of the kinase induced by peptide. Solution NMR data presented here for unphosphorylated p38α with a MEK3b-derived peptide (p38α/pepMEK3b) validate these findings. Crystallograhic data from doubly phosphorylated active p38α (p38α/T∗GY∗/pepMEK3b) reveal a structure similar to unphosphorylated p38α/MEK3b, and distinct from phosphorylated p38γ (p38γ/T∗GY∗) and ERK2 (ERK2/T∗EY∗). The structure supports the idea that MAP kinases adopt three distinct conformations: unphosphorylated, phosphorylated, and a docking peptide-induced form.

A novel organotellurium halide with tellurium presenting mixed oxidation states: Synthesis and structural characterization

In this work we have investigated the reaction for the obtention of an uncommon complex salt of the organotellurium bromide class. The reaction was carried out without the isolation of the synthetic intermediates in one-pot procedure. The complex salt obtained presents the tellurium atoms with mixed oxidation states (Te II and Te IV). The cationic unit [5-Br-2-CH 3O–C 6H 3Te(ETU)] + presents the Te II atom with a “T” shape coordination geometry, and the anionic unit [4-CH 3O–C 6H 4TeBr 4] − presents the Te IV atom with an octahedral coordination geometry considering the Te···Br interaction of 3.600(6) Å from the dimeric arrangement presented in the solid state. Intermolecular (Te···Br) secondary bonds, built up the crystalline/molecular structure. Solution NMR data are presented and discussed in the light of the X-ray structure.

The electrochemical behaviour of [Co(sep)]3+ bound with p-sulfonatothiacalix[4]arene and tetracarboxy-p-sulfonatocalix[4]arene in correlation with inclusive and non-inclusive binding modes

The comparative study of the ion-pairing of Co(III) sepulchrate ([Co(sep)]3+) with p-sulfonatothiacalix[4]arene and p-sulfonatocalix[4]arene tetracarbonic acid by 1H NMR and pH-metric data in solution and X-ray data in solid state elucidates pH dependent effect of carboxylate groups on the stoichiometry and the mode of the ion pairing. The electrochemical data of [Co(sep)]3+ bound with both calix[4]arenes have been analyzed in the correlation with stoichiometry and the mode of their binding. The pH-dependent effect of tetracarboxy-p-sulfonatocalix[4]arene on the electrochemical behavior of [Co(sep)]3+ has been found to correlate with the participation of carboxylate groups in the host–guest binding.

ChemInform Abstract: Discrete Fulleride Anions and Fullerenium Cations

Chem. Rev. 2000, 100, 1075−1120 Discrete Fulleride Anions and Fullerenium Cations Christopher A. Reed* and Robert D. Bolskar Department of Chemistry, University of CaliforniasRiverside, Riverside, California 92521-0403 Received June 22, 1999 Contents I. Introduction, Scope, and Nomenclature II. Electrochemistry A. Reductive Voltammetry B. Oxidative Voltammetry III. Synthesis A. Chemical Reduction of Fullerenes to Fullerides i. Metals as Reducing Agents ii. Coordination and Organometallic Compounds as Reducing Agents iii. Organic/Other Reducing Agents B. Electrosynthesis of Fullerides C. Chemical Oxidation of Fullerenes to Fullerenium Cations IV. Electronic (NIR) Spectroscopy A. Introduction B. C 60 n- Fullerides C. C 70 and Higher Fullerenes D. Fullerenium Cations E. Diffuse Interstellar Bands V. Vibrational Spectroscopy A. Infrared Spectroscopy B. Raman Spectroscopy VI. X-ray Crystallography A. Introduction B. [PPN] 2 [C 60 ] and Related C 602- Structures C. C 60 - Structures D. C 603- Structures E. Comparison of Discrete and Extended Structures VII. Magnetic Susceptibility and Spin States VIII. NMR Spectroscopy A. Introduction B. 13 C NMR Data in Solution C. Interpretation of Solution NMR Data D. 13 C NMR Data in the Solid State E. Knight Shift in A 3 C 60 F. 3 He NMR of Endohedral Fullerides G. 13 C NMR of Derivatized Fullerenes H. 13 C NMR of Fullerenium Cations IX. EPR Spectroscopy A. Introduction B. Features of the C 60 - Spectrum i. The Low g Value ii. Temperature Dependence of the Line Width (∆H pp ) X. XI. XII. XIII. iii. Anisotropy iv. Problem of the Sharp Signals v. Origins of Sharp Signals vi. The C 120 O Impurity Postulate vii. The Dimer Postulate C. Features of the C 602- EPR Spectrum D. Features of the C 603- EPR Spectrum E. Features of C 604- and C 605- EPR Spectra F. EPR Spectra of Higher Fullerides G. EPR Spectra of Fullerenium Cations Chemical Reactivity A. Introduction B. Fulleride Basicity C. Fulleride Nucleophilicity/Electron Transfer D. Fullerides as Intermediates E. Fullerides as Catalysts F. Fullerides as Ligands G. Fullerenium Cations Conclusions and Future Directions Acknowledgments References I. Introduction, Scope, and Nomenclature The electron-accepting ability of C 60 , the archetypal fullerene, is its most characteristic chemical property. It is a natural consequence of electronic structure and was anticipated in early molecular orbital calcula- tions 1 which place a low-lying unoccupied t 1u level about 2 eV above the h u HOMO: 2-4 Early in the gas-phase investigations of fullerenes, the electron affinity of C 60 was measured and found to be high (2.69 eV). 5-7 When the macroscopic era of C 60 chemistry began in 1990, this property was soon found to translate into the solution phase. 8 In a rather remarkable cyclic voltammogram (see Figure 1), the reversible stepwise addition of up to six electrons was soon demonstrated electrochemically. 9,10 10.1021/cr980017o CCC: $35.00 © 2000 American Chemical Society Published on Web 02/16/2000

Platinum(IV) DMSO Complexes: Synthesis, Isomerization, and Agostic Intermediates

Oxidation of cyclometalated Pt(II) complexes with S-bound DMSO ligands initially results in Pt(IV) complexes that retain the S-bound DMSO ligands in the same relative position. Isomerization reactions result in a rearrangement of the ligands to give O-bound DMSO complexes, with the DMSO trans to a cyclometalated carbon. X-ray structures representing the only two known examples of Pt(IV) complexes with O-bound DMSO ligands have been solved. The rate of isomerization of complexes without a pendant alkyl chain is strongly solvent dependent, consistent with the need to stabilize a coordinatively unsaturated intermediate. Pt(IV) complexes with a pendant alkyl chain show little dependence on isomerization rate with solvent, with solution NMR data strongly suggesting the presence of agostic complexes. DFT calculations provide support for the presence of agostic complexes, with the same interactions being used to account for the loss of DMSO from the O-bound DMSO complexes.