Proton Resonance Spectra Research Articles

Pulsed EPR studies of nonexchangeable protons near the Mo(V) center of sulfite oxidase: direct detection of the alpha-proton of the coordinated cysteinyl residue and structural implications for the active site.

Pulsed electron nuclear double resonance (ENDOR) spectra of nonexchangeable protons in the vicinity of the Mo(V) center of the high pH (hpH) and low pH (lpH) forms of native chicken liver sulfite oxidase (SO) and recombinant human SO have been obtained and analyzed for the first time. The close similarity of the spectra for the chicken and human enzymes indicates that the structures of their molybdenum centers are essentially identical. For lpH SO, the closest nonexchangeable proton is found to be approximately 2.8 A from the Mo atom. To more accurately determine the distance to this proton and facilitate its assignment, the C-band electron spin-echo envelope modulation (ESEEM) spectra of lpH SO were also analyzed. From the obtained distance and comparison with the X-ray structure, this closest nonexchangeable proton is assigned to the alpha-proton of the coordinated conserved cysteinyl residue (Cys185 in chicken, Cys207 in human). The closest Mo...H distance for the nonexchangeable protons of hpH SO is found to be approximately 3.3 A. For the cysteinyl alpha-proton, such an increase in the Mo...H distance only requires a very small change in torsional angles. This study demonstrates that details of the enzyme structural rearrangements with pH can be monitored by ENDOR spectroscopy and suggests that a similar approach may be routinely used to probe the orientation of the coordinated cysteinyl residue in mutant forms of SO that are catalytically compromised.

Structure of 9,10-di(9,9-dioctylfluorenyl)anthracene in the solid and in solution.

A new example of magnetic nonequivalence of chemically equivalent atoms is identified from the proton and carbon resonance spectra of 9,10-di(9,9-dioctylfluorenyl)anthracene with the aid of its conformation in the crystalline state. Molecular modeling suggests that it has a similar conformation in solution.

Differentiation between 1,2,4,5‐tetraoxanes and 1,2,4,5,7,8‐hexaoxonanes using 1H and 13C NMR analyses

AbstractThe proton and carbon resonance spectra of ten pairs of 1,2,4,5‐tetraoxanes and 1,2,4,5,7,8‐hexa‐oxonanes derived from acyclic and cyclic ketones have been recorded. A reliable method for distinguishing between 1,2,4,5‐tetraoxanes and 1,2,4,5,7,8‐hexaoxonanes by inspection of their chemical shifts, signal number, signal shape, and dynamic features has been developed. This method provides a rapid structural assignment of 1,2,4,5‐tetraoxanes and 1,2,4,5,7,8‐hexaoxonanes without using vapor pressure osmometry, isotope labeling or low temperature NMR techniques.

Proton chemical shifts in NMR. Part 16.1 Proton chemical shifts in acetylenes and the anisotropic and steric effects of the acetylene group

The proton resonance spectra of a number of acetylenes of fixed geometry were recorded in dilute CDCl3 solution and assigned. These were acetylene, equatorial- and axial-cyclohexylacetylene at −60 °C, 1,4-di-1-adamantylbutadiyne, 1-ethynyl-t- and -c-4-tert-butylcyclohexan-r-1-ol, 2-exo-ethynylnorbornan-2-oland 2,2′-ethyne-1,2-diyldibornan-2-ol. The aromatic acetylenes measured were phenylacetylene, o-ethynyltoluene, 2-ethynylnaphthalene and 9-ethynylanthracene. This data together with previous literature data for but-1-yne, but-2-yne, pent-1-yne, tert-butylacetylene, p-ethynyltoluene, 1-ethynylnaphthalene and 2-ethynylpropene allowed the determination of the acetylene substituent chemical shifts (SCS) in a variety of molecules. These SCS were analysed in terms of the magnetic anisotropy and steric effects of the acetylene group together with a model (CHARGE7) for the calculation of the two-bond and three-bond electronic effects. For the aromatic acetylenes ring current and π electron effects were included.

Proton chemical shifts in NMR. Part 15?proton chemical shifts in nitriles and the electric field and ?-electron effects of the cyano group

The proton resonance spectra of a number of nitriles of fixed geometry were recorded in dilute CDCl3 solution and assigned. These were trans- and cis-4-tert-butylcylohexanecarbonitrile, axial and equatorial cyclohexanecarbonitrile and ax–ax- and eq–eq-trans-1,4-dicyanocyclohexane, the latter compounds at −60 °C. The aromatic nitriles measured were benzonitrile, o-, m- and p-dicyanobenzene, 1- and 2-cyanonaphthalene and 9-cyanoanthracene. This plus previous literature data allowed the determination of the cyano substituent chemical shifts (SCS) in a variety of molecules. These SCS were analysed in terms of the CN electric field, magnetic anisotropy and steric effects for protons more than three bonds removed together with a model (CHARGE7) for the calculation of the two- and three-bond SCS. For the aromatic nitriles ring current and π-electron effects were included. The anisotropic and steric effects of the cyano group were negligible in all the compounds investigated and in the aliphatic nitriles the SCS were due only to the CN electric field plus for near protons electronic effects. For the aromatic nitriles the π-electron effects were calculated from Hückel theory with the values of the exchange and resonance integrals adjusted to give π-electron densities in agreement with those obtained by ab initio calculations. The ring current shifts of the cyano derivatives were assumed to be the same as those of the parent hydrocarbons. The model gives the first comprehensive calculation of the SCS of the cyano group. For the data set of 93 proton chemical shifts from 1 to 9δ, the r.m.s. error (observed vs calculated shifts) was 0.088 ppm. The breakdown of the CN SCS in the aromatic nitriles showed good agreement with the Swain and Lupton field and resonance (F and R) components of substituent effects. Copyright © 2000 John Wiley & Sons, Ltd.

Proton chemical shifts in NMR. Part 14. Proton chemical shifts, ring currents and π electron effects in condensed aromatic hydrocarbons and substituted benzenes

The proton resonance spectra of a variety of condensed aromatic compounds including benzene, naphthalene, anthracene, phenanthrene, pyrene, acenaphthylene and triphenylene were obtained in dilute CDCl3 solution. Comparison of the proton chemical shifts obtained with previous literature data for CCl4 solution shows small but significant differences. A previous model (CHARGE6) for calculating the proton chemical shifts of aliphatic compounds was extended to aromatic compounds. This was achieved by including an automatic identification of both five- and six-membered aromatic rings based on atomic connectivities plus a dipole calculation of the aromatic ring current. The ring current intensity in the molecules was calculated by two alternative methods. a) The ring current intensity in the individual benzenoid rings was a function of the number of adjoining rings and b) the molecular ring current was proportional to the molecular area divided by the molecular perimeter. This, plus the inclusion of deshielding steric effects for the crowded protons in these molecules, gave a good account of the observed chemical shifts. The model was also applied successfully to the non-alternant hydrocarbons of fulvene and acenaphthylene and to the aliphatic protons near to and above the benzene ring in tricyclophane and [10]cyclophane.The Huckel calculation of the π electron densities in CHARGE6 was used to calculate the π electron densities in substituted benzenes. The π-inductive effect was used to simulate the effect of CX3 groups (X = H, Me, F) on the benzene ring. These together with the long range effects of the substituent groups identified previously allowed a precise calculation of the SCS of a variety of substituents on all the benzene ring protons.The model gives the first accurate calculation of the proton chemical shifts of condensed aromatic compounds and of the proton SCS in the benzene ring. For the data set of 55 proton chemical shifts spanning 3 ppm the rms error of the observed vs. calculated shifts was ca. 0.1 ppm. The model also allows the interpretation of the shifts in terms of the separate interactions calculated in the programme, i.e. π electron densities and steric, anisotropic and electric field effects. Previous correlations of the proton SCS with π electron densities and substituent parameters are shown to be over simplified. The relative proportions of these different interactions are very different for each substituent and for each ring proton. p

Proton chemical shifts in NMR. Part 13.1 Proton chemical shifts in ketones and the magnetic anisotropy and electric field effect of the carbonyl group

The proton resonance spectra of a variety of cyclic ketones including 2-tert-butylcyclohexanone, 4-tert-butylcyclohexanone, fenchone, trans-1-decalone, androstan-3-one, androstan-17-one, androstane-3,17-dione and androstane-3,11,17-trione were obtained and completely assigned. These data together with previous literature data allowed the determination of the carbonyl substituent chemical shifts (SCS) in a variety of cyclic molecules. These SCS were analysed in terms of the carbonyl electric field, magnetic anisotropy and steric effect for long-range protons together with a model (CHARGE6) for the calculation of the two-bond and three-bond effects.

Proton chemical shifts in NMR. Part 12.1 Steric, electric field and conformational effects in acyclic and cyclic ethers

The proton resonance spectra of tetrahydropyran, at room temperature and –85 °C where the ring inversion is slow on the NMR timescale, 2-methoxy-, 3-methyl- and several 4-substituted tetrahydropyrans, 2-methyl-1,3-dioxolane and the rigid cyclic ethers 7-oxabicyclo[2.2.1]heptane and 1,8-cineole have been recorded and completely analysed.These results together with literature data on acyclic and cyclic ethers (1,3- and 1,4-dioxane, dioxolane, 4-oxa-5α-androstane etc.) have allowed the determination of the oxygen substituent chemical shifts (SCS) in these systems. This data set consisting of 78 proton chemical shifts in 17 compounds has been used to test the application of a previous theoretical model of proton chemical shifts to these compounds.It is shown that the model gives a very good account of the proton chemical shifts in these systems. The ether oxygen SCS are due to both steric and electrostatic terms, the steric term predominating at short distances (e.g. in the 1,3-diaxial interactions in methoxycyclohexanes).Conformational isomerism in these compounds has also been investigated. Low temperature NMR gave ΔG (eq–ax) +1.0 kcal mol–1 for 4-hydroxy-THP. Analysis of the couplings in the CHCH2OH side chain of 2-(hydroxymethyl)-THP has shown that the preferred conformer is gt in both chloroform and acetone solvents.

Calcium-Dependent Solvation of the Myristoyl Group of Recoverin

Recoverin is an N-myristoylated calcium-binding protein present in the photoreceptor cells of the mammalian retina. It is believed to function as a calcium sensor in visual signal transduction by coupling the kinetics of the recovery phase of the photoresponse to changes in the levels of intracellular Ca2+. Upon binding Ca2+, recoverin undergoes a conformational change that allows it to associate with membranes in a manner that requires N-myristoyl modification. It has been proposed that, in the Ca(2+)-free conformation, the myristoyl group is sequestered in a hydrophobic part of the protein, and in the Ca(2+)-bound conformation, the myristoyl group is exposed to solution. The crystal structure of Ca(2+)-bound recoverin reveals an exposed cluster of hydrophobic residues, raising the possibility that residues in this region may function as part of an intramolecular myristoyl binding site. Fluorescence spectroscopy analysis of interactions between recoverin and 1-anilinonaphthalene-8-sulfonate (ANS) shows that an increase in solvent-accessible hydrophobic surface accompanies Ca2+ binding. 1H nuclear magnetic resonance (NMR) spectra of myristoyl protons show dispersed chemical shifts in the Ca(2+)-free conformation that become relatively uniform upon the addition of Ca2+. Two-dimensional nuclear Overhauser effect (NOE) spectra of Ca(2+)-free recoverin show NOE contacts between myristoyl protons and aromatic ring protons. Tryptophan fluorescence quenching by acrylamide indicates that the myristoyl group is in proximity to a tryptophan residue only in the Ca(2+)-free conformation. These results indicate that the myristoyl group is in contact with residues in the hydrophobic cluster in Ca(2+)-free recoverin and that it is exposed to solution in the Ca(2+)-bound conformation.

Proton resonance spectra and substituent‐induced chemical shifts of 3‐halocamphors

AbstractComplete signal assignments of high‐field 1H NMR spectra for 3‐halosubstituted camphors (endo and exo) are presented, allowing a refinement of a previous analysis for the chloro (endo and exo) and bromo (endo) derivatives. In addition, still unpublished data for the dichloro and exo‐fluoro derivatives and for three new compounds (endo‐fluoro, endo‐ and exo‐iodo), in addition to a revised assignment of the camphor parent molecule, are reported. The precise substituent‐induced chemical shifts (SCS) obtained for these difunctional systems are examined on the basis of well known transmission mechanisms aided by correlations with steric and electronic parameters, and taking into account SCS data from monohaloalkanes and the calculated molecular geometry data (using the AM1 method).

Synthesis, characterization and high-resolution proton resonance spectra of [AgL 2]ClO 4 (L = 2-arylpyridinecar☐yaldimine) and their reactivities

The synthesis and characterization of bischelated silver(I) complexes of L(1) (L = 2-arylpyridinecar☐aldimine), [AgL 2]ClO 4 is reported. Solutions of the complexes are stable in methanol and chloroform, but unstable in acetonitrile. The chemical reactions of the silver complexes with chlorides of iron(III), cobalt(II) and nickel(II) have been examined. In all cases metal-exchange reactions take place to yield ML 3 2+ [M = iron(II), cobalt(II) and nickel(II)]. These compounds have been characterized by comparison of their spectral data with those of authentic samples. The redox properties of the compounds are also briefly noted.

Restricted chain rotation in polymers containing pendant groups: structural characterization of hydroxyl-functionalized polysulfones

Proton and carbon resonance spectra have been obtained for 9- and 9,9'-substituted Udel polysulfones with progressively bulkier substituents. The degree of substitution and the changes in stereochemistry caused by the substituents have been determined by analysis of the spectra

Isomeric complexes of ruthenium(II) with neutral heterocyclic schiff base ligands. High resolution proton resonance spectra of trans- cis isomeric pairs of RuX 2L 2 (L  2-arylpyridinecarboxaldimine, X  Cl, Br) and comparison of their physical properties

The reaction of 2-arylpyridinecarboxaldimine [RH 4C 6NC(H)Py, L ( 1)] with hydrated RuX 3 (X  Cl, Br) in boiling C 2H 5OH affords dark crystals of RuX 2L 2. Two geometrical isomers of the compound have been isolated and characterized by analytical and spectroscopic data. The trans isomer of RuCl 2L 2 shows a single sharp band for ν(RuCl), whereas two bands are observed for the corresponding cis isomer. The highresolution 1H NMR spectra of the isolated complexes are reported and completely assigned. All the complexes have multiple t 2→π *(L) transitions in the visible region. Each of the complexes display a quasi-reversible oxidative response due to an Ru III/Ru II couple in the range 0.25–0.40 V vs S.C.E. at a platinum working electrode. The formal potentials of this couple obey the Hammett relationship. The ligand-based irreversible oxidations are also briefly noted.

Increased spectral dispersion via triple-quantum nuclear magnetic resonance

Triple-quantum proton resonance spectra were collected and assigned for several spin systems of common occurrence in organic compounds. The increased dispersion in the ω1 dimension allows one easily to resolve the CH3 signals in a mixture of CH3CH2CH2OH and CH3CH2CH2CHO.

Proton NMR comparison of the Saccharomyces cerevisiae ferricytochrome c isozyme-1 monomer and covalent disulfide dimer

Proton NMR studies of Saccharomyces cerevisiae (bakers yeast) isozyme-1 monomer and dimer ferricytochrome c have been carried out. The dimer is formed via a disulfide bridge between the Cys-102 residues of monomer proteins. Nuclear Overhauser effect (NOE) experiments have led to resonance assignments for many of the heme and axial ligand (Met-80; His-18) protons in both protein forms. Resonances of the following amino acids have also been assigned in both forms: Phe-10; Pro-30; Phe-82; Trp-59; Leu-68. The proton NOE connectivity patterns of the monomer of yeast isozyme-1 ferricytochrome c are similar to those of horse, tuna, and yeast isozyme-2 ferricytochromes c, even though the observed hyperfine resonance spectra are significantly different for the various cytochromes. The pattern of dimer proton hyperfine resonances is distinct from the isozyme-1 monomer pattern, which indicates that the formation of a disulfide bridge via Cys-102 is detected at the heme site, approximately 10 A distant. It appears that a specific structural change is induced upon dimerization, which, in turn, causes specific perturbations in the vicinity of the heme. However, the general features of the NOE connectivity pattern in the dimer are the same as for the monomer indicating that dimerization does not result in drastic structural disruption. Furthermore, the 1H NMR spectrum of the dimer can be mimicked by the monomer form that results when the -SH group of Cys-102 is chemically modified with certain types of bulky, or hydrophilic reagents (i.e. 5,5'-dithiobis[2-nitrobenzoate], indicating that perturbations of the yeast isozyme-1 ferricytochrome c proton resonance spectrum observed upon dimerization are essentially due to changes in intramolecular, rather than intermolecular, interactions. These results suggest that a possible regulatory site for yeast isozyme-1 cytochrome c exists at position 102, which could conceivably have a physiological role in altering the conformation of the molecule.

SYNTHESES OF FERROCENYLFLUOROPHOSPHINIC ACID AND RELATED COMPOUNDS1

Abstract Ferrocenylfluorophosphinic acid (η5-C5H5)Fe(η5-C5H4POH(O)F), ferrocenyldifluorophosphine oxide (η5-C5H5)Fe(η5-C5H4POF2) and ferrocenyltetrafluorophosphorane (η5-C5H5)Fe(η5-C5H4PF4) are prepared from the reaction of ferrocene with PF5 in CH2Cl2 and are characterized by proton, carbon, fluorine and phosphorus resonance spectra.

Fe6Se9(SR)2]4- Clusters (R = Me, CH2Ph): Synthesis and Characterization, and the X-Ray Structure of (PhCH2NEt3)4[Fe6Se9(SMe)2

(1) The compounds (PhCH2NEt3)4[Fe6Se9(SMe)2] and (Et4N)4[Fe6Se9(SCH2Ph)2] (2) have been isolated in good yields from Fe(SR)3 (R = Me, CH2Ph)/“Na2Se2” reaction mixtures in methanol/N,N-dimethylformamide after addition of the appropriate tetraalkylammonium chlorides. 1 is monoclinic, space group P21/c, Z = 4, with a = 18.697(14), b - 22.606(16), c = 15.989(11) Å, and β = 94.15(6)° at 140 K. Its structure has been refined to R (Rw) = 0.092 (0.075). The [Fe6Se9(SMe)2]4- anion contains six coplanar iron atoms which are present in distorted tetrahedral Se4 and Se3(SMe) ligand surroundings. The Se atoms are μ, μ3 and μ4 bridging. The four central Fe atoms and the Se atoms constitute a Fe4Se9 group that is structurally related to 1 ∞[Fe2Se3]2- double tetrahedral chains and 2 ∞[FeSe] layers. The Fe atoms in the [Fe6Se9(SMe)2]4- cluster are antiferromagnetically coupled. 1 has a room-temperature magnetic moment of 1.22 μB/Fe. Cyclic voltammograms, and electronic, infrared, and proton resonance spectra of 1 and 2 have been measured.

Substituent chemical shifts in NMR. 1—Proton resonance spectra and geometries of norbornene, norbornane and adamantane

AbstractThe molecular geometries and proton NMR spectra of norbornene, norbornane and adamantane are reported. The molecular geometries were refined from previous experimental determinations using MNDO and ab initio techniques to determine the positions of the hydrogen atoms. No evidence of any endo—endo interactions in norbornane was obtained from these calculations.High‐field proton NMR spectra allow the refinement of previous spectral analyses of adamantane and norbornane, and the complete analysis of the olefin decoupled eight spin system of norbornene. The chemical shifts of the parent compounds and cyclohexane under comparable conditions show minor but significant differences from previous values. The methine—methylene coupling in adamantane is 3.30 Hz. The long‐range couplings in norbornene were compared with those calculated by FPT/INDO and EHMO/VB methods. Only moderate agreement was obtained and possible reasons for this are examined.

NMR technique for assessing contributions of heavy and light chains to an antibody combining site.

Nuclear magnetic resonance (NMR) has been used extensively to study the structure of antibody combining sites. In recent studies we have observed the proton resonance spectra of the Fab fragment of a monoclonal anti-spin-label antibody derived from a hybridoma grown on various specifically deuterated amino acids. The broadening of the proton resonance signals by the paramagnetic hapten, together with selective deuteration, has allowed the identification of most of the amino acids in the combining-site region of this antibody and has also provided estimates of distances between amino-acid protons and the unpaired electron. Here we show how recombination of specifically deuterated heavy and light chains permits the assignment of single amino-acid proton resonance signals to either the heavy or light chain. In addition, the spectra of such recombinants demonstrate that their combining-site structures must be almost identical to the native structure.

PMR studies based on lanthanoid shift reagents: Aurone and thioaurone steric structures

The proton resonance spectra have been used along with the interactions with the lanthanoid shift reagent tris-1,1,1,2,2,3,3-heptofluoro-7,7-dimethyloctane-2,6-dione-Eu(III) to establish the isomeric forms and preferred conformations of substituted aurones and thioaurones. Calculations have been performed on the geometrical structures of the LSR-substrate adducts for certain compounds.