HMBC-RELAY: a combined technique for the differentiation of simultaneously detected 2J(C,H) and (n)J(C,H) connectivities

We present a new pulse sequence that detects simultaneously (n)J(C,H) and 2J(C,H) connectivities. The corresponding coherences are created along independent pathways and therefore can be separated into two different subspectra. One spectrum is to show all (n)J(C,H) connectivities and the other is to show exclusively 2J(C,H) connectivities. In contrast to the previously published 2J/(n)J experiment, this sequence detects the 2J(C,H) connectivities via a C,H,H-RELAY pathway leading to an intensification of the 2J(C,H) signals. Strictly, the 2J(C,H) spectrum does not show 2J(C,H) but 3J(H,H) coupling interactions within 13CH(k)-12CH(l) fragments. Therefore, 2J(C,H) signals can appear even if the corresponding 2J(C,H) coupling constant is zero.     

Structural and electronic effects on one-bond spin-spin coupling constants 1J(B-N), 1J(B-H), and 1J(B-F) for complexes of nitrogen bases with BH3 and its fluoro-substituted derivatives

Ab initio equation-of-motion coupled cluster (EOM-CCSD) one-bond spin-spin coupling constants (1)J(B-N), (1)J(B-H), and (1)J(B-F) have been evaluated for complexes X:BH(n)F(3-n) with X = N(2), NCH, NCLi, H(2)CNH, NF(3), and NH(3), for n = 0-3. These complexes can be classified as either covalent or van der Waals complexes, on the basis of their binding energies and B-N distances. (1)J(B-N) for covalent complexes varies significantly from -19 to +9 Hz, whereas (1)J(B-N) is less than 2 Hz for van der Waals complexes. An absolute value of (1)J(B-N) of 3 Hz or greater indicates that the complex is covalently bonded, but a small value of this coupling constant does not necessarily mean that it is a van der Waals complex, in view of the variation among these complexes found for (1)J(B-N) as a function of the B-N distance. Deformation of the boron acid upon complex formation and electron donation by the nitrogen base has opposing effects on both (1)J(B-H) and (1)J(B-F). These effects are relatively small in van der Waals complexes. In covalent complexes, electron donation has the dominant effect on (1)J(B-H), and on (1)J(B-F) in complexes with BH(2)F and BHF(2), but acid deformation has the dominant effect on (1)J(B-F) in complexes with BF(3). Values of both (1)J(B-H) and (1)J(B-F) reflect the van der Waals or covalent nature of the B-N bond.     

(13)C-(1)H and (13)C-(13)C NMR J-couplings in (13)C-labeled N-acetyl-neuraminic acid: correlations with molecular structure

N-acetyl-neuraminic acid (Neu5Ac, 2) was prepared enzymatically containing single sites of (13)C-enrichment at C1, C2, and C3. Aqueous solutions of the three (13)C isotopomers were studied by (1)H and (13)C NMR spectroscopy at p(2)H 2 and pH 8 to obtain J(CH) and J(CC) values involving the labeled carbons. Experimental studies were complemented by DFT calculations of the same set of J-couplings in protonated and ionized structural mimics of 2 to determine how well theoretical predictions match the experimental findings in saccharides bearing ionizable functionality. Results show that: (a) (2)J(C2,H3ax/eq) values in 2 depend on anomeric configuration, thus complementing (3)J(C1,H3ax/eq) behavior, (b) J(CH) and J(CC) values involving C2 depend on anomeric configuration, the C1-C2 bond torsion, and solution pH, and (c) long-range (4)J(C2,H7) is sensitive to glycerol side-chain conformation. Intraring J(HH) and most (2)J(CH), (3)J(CH), (2)J(CC), and (3)J(CC) involving C1-C3 of 2 appear largely unaffected by the ionization state of the carboxyl group. In vacuo and solvated DFT calculations of geminal and vicinal J(CH) and J(CC) values are similar and reproduce the experimental data well, but better agreement with experiment was observed for (1)J(C1,C2) in the solvated calculations. The present work provides new information for future treatments of trans-glycoside couplings involving Neu5Ac residues by (a) providing new standard values of intraring J(CC) for coupling pathways that mimic those for trans-glycoside J(CC), (b) identifying potential effects of solution pH on trans-glycoside couplings inferred through the behavior of related intraring couplings, and (c) providing specific guidelines for more reliable DFT predictions of J(CH) and J(CC) values in ionizable saccharides.     

Variation in protein C(alpha)-related one-bond J couplings

Four types of polypeptide (1)J(C alpha X) couplings are examined, involving the main-chain carbon C(alpha) and either of four possible substituents. A total 3105 values of (1)J(C alpha H alpha), (1)J(C alpha C beta), (1)J(C alpha C'), and (1)J(C alpha N') were collected from six proteins, averaging 143.4 +/- 3.3, 34.9 +/- 2.5, 52.6 +/- 0.9, and 10.7 +/- 1.2 Hz, respectively. Analysis of variances (ANOVA) reveals a variety of factors impacting on (1)J and ranks their relative statistical significance and importance to biomolecular NMR structure refinement. Accordingly, the spread in the (1)J values is attributed, in equal proportions, to amino-acid specific substituent patterns and to polypeptide-chain geometry, specifically torsions phi, psi, and chi(1) circumjacent to C(alpha). The (1)J coupling constants correlate with protein secondary structure. For alpha-helical phi, psi combinations, (1)J(C alpha H alpha) is elevated by more than one standard deviation (147.8 Hz), while both (1)J(C alpha N') and (1)J(C alpha C beta) fall short of their grand means (9.5 and 33.7 Hz). Rare positive phi torsion angles in proteins exhibit concomitant small (1)J(C alpha H alpha) and (1)J(C alpha N') (138.4 and 9.6 Hz) and large (1)J(C alpha C beta) (39.9 Hz) values. The (1)J(C alpha N') coupling varies monotonously over the phi torsion range typical of beta-sheet secondary structure and is largest (13.3 Hz) for phi around -160 degrees. All four coupling types depend on psi and thus help determine a torsion that is notoriously difficult to assess by traditional approaches using (3)J. Influences on (1)J stemming from protein secondary structure and other factors, such as amino-acid composition, are largely independent.     

Dynamic stereochemistry of erigeroside by measurement of 1H-1H and 13C-1H coupling constants

Erigeroside was extracted from Satureja khuzistanica Jamzad (Marzeh Khuzistani in Persian, family of lamiaceae), and (1)H, (13)C, (13)C{(1)H}, (1)H-(1)H COSY, HMQC and J-HMBC were obtained to identify this compound and determine a complete set of J-coupling constants ((1)J(C-H), (2)J(C-H), (3)J(C-H) and (3)J(H-H)) values within the exocyclic hydroxymethyl group (CH(2)OH) and anomeric center. In parallel, density functional theory (DFT) using B3LYP functional and split-valance 6-311++G** basis set has been used to optimized the structures and conformers of erigeroside. In all calculations solvent effects were considered using a polarized continuum (overlapping spheres) model (PCM). The dependencies of (1)J, (2)J and (3)J involving (1)H and (13)C on the C(5')-C(6') (omega), C(6')-O(6') (theta) and C(1')-O(1') (phi) torsion angles in erigeroside were computed using DFT method. Complete hyper surfaces for (1)J(C1',H1'), (2)J(C5',H6'R), (2)J(C5',H6'S), (2)J(C6',H5'), (3)J(C4',H6'R), (3)J(C4',H6'S) and (2)J(H6'R-H5'S) as well as (3)J(H5',H6'R) were obtained and used to derive Karplus equations to correlate these couplings to omega, theta and phi. These calculated J-couplings are in agreement with experimental values. These results confirm the reliability of DFT calculated coupling constants in aqueous solution.     

Structural dependencies of h3JNC' scalar coupling in protein H-bond chains

The H-bond ((h3)J(NC')) and peptide bond ((1)J(NC')) scalar couplings establish connectivity of the electronic structure in the H-bond chains of proteins. The correlated changes of (h3)J(NC') and (1)J(NC') couplings extend over several peptide groups in the chains. Consequently, the electronic structure of the H-bond chains can affect (h3)J(NC') in a manner that is independent of the local H-bond geometry. By taking this into account, and by using a more complete set of H-bond geometry parameters, we have predicted (h3)J(NC') couplings in the H-bond chains with deviations commensurate to the standard deviations of the experimentally determined values. We have created a comprehensive database of (h3)J(NC') and (1)J(NC') couplings by measuring the coupling constants in ubiquitin (alphabeta-fold) intestinal fatty acid binding protein (beta-barrel) and carp parvalbumin (alpha-helical).     

An unprecedented asymmetric end-on azido-bridged copper(II) imino nitroxide complex: structure, magnetic properties, and density functional theory analysis

The dinuclear copper(II) complex [Cu2(mu(1,1)-N3)2(im-2py)2(N3)2] [im-2py = 2-(2-pyridyl)-4,4,5,5-tetramethylimidazolinyl-1-oxy] has been prepared and structurally characterized. The crystal structure consists of a dinuclear unit in which the Cu(II) ions are bridged by two azido ions in a end-on asymmetric fashion and the imino nitroxide radicals are chelating by the two imino N atoms. Accordingly, the magnetic susceptibility data were analyzed considering a linear spin-coupling scheme rad(1)-Cu(2)-Cu(3)-rad(4) (with Si = 1/2, i = 1-4), where the Heisenberg spin Hamiltonian assumes the general form -2Sigma(i)<(j)S(i)S(j). Considering only first-neighbor spin-coupling constants (J13 = J24 = J14 = 0), magnetic susceptibility measurements show that the copper(II) imino nitroxide rad-Cu-(Cu-rad)(rad-Cu)-Cu-rad exchange coupling is ferromagnetic and large (J12 = J34 = J1 > +190 cm(-1)), as is expected for copper imino nitroxide species, and the copper-copper (rad)-Cu-Cu-(rad) coupling through the asymmetric double end-on azide bridges appeared antiferromagnetic and rather large [J23 = J2 = -43(2) cm(-1)]. By contrast, a density functional theory analysis of the system through the computation of broken-symmetry-state energies resulted in J2 approximately 0 cm(-1). This apparent paradox is resolved by introducing a second-neighbor rad-(Cu)-Cu-(rad)(rad)-Cu-(Cu)-rad spin-coupling constant J13 = J24 = J3, which turns out to be antiferromagnetic both experimentally (when J2 is set equal to zero) and computationally.     

Electronic spectra and crystal field analysis of energy levels of Ho3+ in HoF6(3-)

The electronic absorption spectra of single crystals of Cs(2)NaHoF(6) have been recorded in the spectral region between 4700 and 42000 cm(-1) at temperatures down to 10 K. The structure in the (5)I(8) → (5)I(J) (J = 7-4), (5)F(J) (J = 5-1), (5)S(2), (5)G(J) (J = 4-6), (3)K(J) (J = 7, 8) transitions has been analyzed and assigned. The emission spectra (5)S(2) → (5)I(J) (J = 6-8) and (5)G(4) → (5)I(J) (J = 5-7), (5)F(5) have also been recorded at 10 K for crystals of Cs(2)NaHoF(6) and partly also for samples of Cs(2)NaHoF(6):Yb(3+). The spectra comprise magnetic dipole zero phonon lines and electric dipole allowed one-phonon vibronic sidebands. From the detailed interpretation of the emission and absorption spectra, aided by a clear understanding of the vibrational behavior of the HoF(6)(3-) moiety and by magnetic dipole intensity calculations, a data set of 59 energy levels spanning 17 multiplet terms was derived. Crystal field calculations were then performed using a 4f(10) basis, as well as including the configuration interaction with a p-electron configuration. The latter calculation, which employed 14 parameters, gave better agreement with experiment and the mean deviation was 13.5 cm(-1). A comparison with the energy level fittings for Cs(2)NaHoCl(6) has been included. The crystal field parameters for the fluoro- and chloro-systems followed empirically predicted ratios.     

Correlated C-C and C-O bond conformations in saccharide hydroxymethyl groups: parametrization and application of redundant 1H-1H, 13C-1H, and 13C-13C NMR J-couplings

Methyl alpha- and beta-pyranosides of d-glucose and d-galactose 1-4 were prepared containing single sites of (13)C-enrichment at C4, C5, and C6 (12 compounds), and (1)H and (13)C[(1)H] NMR spectra were obtained to determine a complete set of J-couplings ((1)J, (2)J, and (3)J) involving the labeled carbon and nearby protons and carbons within the exocyclic hydroxymethyl group (CH(2)OH) of each compound. In parallel theoretical studies, the dependencies of (1)J, (2)J, and (3)J involving (1)H and (13)C on the C5-C6 (omega) and C6-O6 (theta;) torsion angles in aldohexopyranoside model compounds were computed using density functional theory (DFT) and a special basis set designed to reliably recover the Fermi contact contribution to the coupling. Complete hypersurfaces for (1)J(C5,C6), (2)J(C5,H6)(R), (2)J(C5,H6)(S), (2)J(C6,H5), (2)J(C4,C6), (3)J(C4,H6)(R), (3)J(C4,H6)(S), and (3)J(C6,H4), as well as (2)J(H6)(R)(,H6)(S), (3)J(H5,H6)(R), and (3)J(H5,H6)(S), were obtained and used to parametrize new equations correlating these couplings to omega and/or theta;. DFT-computed couplings were also tested for accuracy by measuring J-couplings in (13)C-labeled 4,6-O-ethylidene derivatives of d-glucose and d-galactose in which values of omega and theta; were constrained. Using a new computer program, Chymesa, designed to utilize multiple J-couplings sensitive to exocyclic CH(2)OH conformation, the ensemble of experimental couplings observed in 1-4 were analyzed to yield preferred rotamer populations about omega and theta;. Importantly, due to the sensitivity of some couplings, most notably (2)J(H6)(R)(,H6)(S), (2)J(C5,H6)(R), and (2)J(C5,H6)(S), to both omega and theta;, unique information on correlated conformation about both torsion angles was obtained. The latter treatment represents a means of evaluating correlated conformation in 1,6-linked oligosaccharides, since psi and theta; are redundant in these linkages. In the latter regard, multiple, redundant scalar couplings originating from both sides of the glycosidic linkage can be used collectively to evaluate conformational correlations between psi/theta; and C5-C6 bond rotamers.     

Magnetic polyoxometalates: anisotropic antiferro- and ferromagnetic exchange interactions in the pentameric cobalt(II) cluster

The ground-state properties of the pentameric Co(II) cluster [Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)](12-) were investigated by combining magnetic susceptibility and low-temperature magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na(12)[Co(3)W(D(2)O)(2)(CoW(9)O(34))(2)].46D(2)O. The encapsulated magnetic Co(5) unit consists of three octahedral and two tetrahedral oxo-coordinated Co(II) ions. Thus, two different types of exchange interactions are present within this cluster: a ferromagnetic interaction between the octahedral Co(II) ions and an antiferromagnetic interaction between the octahedral and the tetrahedral Co(II) ions. As a result of the single-ion anisotropy of the octahedral Co(II) ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co(5) spin cluster is anisotropic and is expressed as H = -2 summation operator(i= x,y,z)J(1)(i)[S(1)(i)S(2)(i) + S(2)(i)S(3)(i)] + J(2)(i)[S(1)(i)S(5)(i) + S(2)(i)S(5)(i) + S(2)(i)S(6)(i) + S(3)(i)S(6)(i)], where J(1)(i) are the components of the exchange interaction between the octahedral Co(II) ions and J(2)(i) are the components of the exchange interaction between the octahedral and tetrahedral Co(II) ions (see Figure 1d). The study of the exchange interactions in the two structurally related polyoxoanions [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10)(-) and [Co(3)W(H(2)O)(2)(ZnW(9)O(34))(2)](12)(-) allowed an independent determination of the ferromagnetic exchange parameters J(1)(x) = 0.70 meV, J(1)(y) = 0.43 meV, and J(1)(z) = 1.51 meV (set a) and J(1)(x) = 1.16 meV, J(1)(y) = 1.16 meV and J(1)(z) = 1.73 meV (set b), respectively. Our analysis proved to be much more sensitive to the size and anisotropy of the antiferromagnetic exchange interaction J(2). We demonstrate that this exchange interaction exhibits a rhombic anisotropy with exchange parameters J(2)(x) = -1.24 meV, J(2)(y) = -0.53 meV, and J(2)(z) = -1.44 meV (set a) or J(1)(x) = -1.19 meV, J(1)(y) = -0.53 meV, and J(1)(z) = -1.44 meV (set b). The two parameter sets reproduce in a satisfactory manner the susceptibility, magnetization, and INS properties of the title compound.     

Time evolution of a single spin inhomogeneously coupled to an interacting spin environment

We study the time evolution of a single spin coupled by exchange interaction to an environment of interacting spin bath modeled by the XY Hamiltonian. By evaluating the spin correlator of the single spin, we observed that the decay rate of the spin oscillations strongly depends on the relative magnitude of the exchange coupling between the single spin and its nearest neighbor J(') and coupling among the spins in the environment J. The decoherence time varies significantly based on the relative coupling magnitudes of J and J('). The decay rate law has a Gaussian profile when the two exchange couplings are of the same order J(') approximately J but converts to exponential and then a power law as we move to the regimes of J(')>J and J(')相似文献   

DFT analysis of NMR scalar interactions across the glycosidic bond in DNA

The relationship between the glycosidic torsion angle chi, the three-bond couplings (3)J(C2/4-H1') and (3)J(C6/8-H1'), and the one-bond coupling (1)J(C1'-H1') in deoxyribonucleosides and a number of uracil cyclo-nucleosides has been analyzed using density functional theory. The influence of the sugar pucker and the hydroxymethyl conformation has also been considered. The parameters of the Karplus relationships between the three-bond couplings and chi depend strongly on the aromatic base. (3)J(C2/4-H1') reveals different behavior for deoxyadenosine, deoxyguanosine, and deoxycytidine as compared to deoxythymidine and deoxyuridine. In the case of (3)J(C6/8-H1'), an opposite trans to cis ratio of couplings is obtained for pyrimidine nucleosides in contrast to purine nucleosides. The extremes of the Karplus curves are shifted by ca. 10 degrees with respect to syn and anti-periplanar orientations of the coupled nuclei. The change in the sugar pucker from S to N decreases (3)J(C2/4-H1') and (3)J(C6/8-H1'), while increasing (1)J(C1'-H1') for the syn rotamers, whereas all of the trends are reversed for the anti rotamers. The influence of the sugar pucker on (1)J(C1'-H1') is interpreted in terms of interactions between the n(O4'), sigma*(C1'-H1') orbitals. The (1)J(C1'-H1') are related to chi through a generalized Karplus relationship, which combines cos(chi) and cos(2)(chi) functions with mutually different phase shifts that implicitly accounts for a significant portion of the related sugar pucker effects. Most of theoretical (3)J(C2/4-H1') and (3)J(C6/8-H1') for uracil cyclo-nucleosides compare well with available experimental data. (3)J(C6/8-H1') couplings for all C2-bridged nucleosides are up to 3 Hz smaller than in the genuine nucleosides with the corresponding chi, revealing a nonlocal aspect of the spin-spin interactions across the glycosidic bond. Theoretical (1)J(C1'-H1') are underestimated with respect to the experiment by ca. 10% but reproduce the trends in (1)J(C1'-H1') vs chi.     

Structures, energies, and spin-spin coupling constants of fluoro-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes: four-member B-P-B-P rings B2P2F(n)H(8-n) with n = 0, 1, 2, 4

An ab initio study has been carried out to determine the structures, relative stabilities, and spin-spin coupling constants of a set of 15 fluoro-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes B(2)P(2)F(n)H(8-n), for n = 0, 1, 2, 4, with four-member B-P-B-P rings. Except for B(2)P(2)F(4)H(4) with four fluorines bonded to two borons, these rings are puckered in a butterfly conformation. For a fixed number of fluorines, the isomers with B-F bonds are significantly more stable than those with P-F bonds. As the number of fluorines increases, the energy difference between the most stable isomer and the other isomers increases. Transition structures which interconvert axial and equatorial positions present relatively small inversion barriers. Coupling constants involving (31)P, namely, (1)J(B-P), (1)J(P-F), (2)J(P-P), (2)J(P-F), and (3)J(P-F) are large and are capable of providing structural information. They are sensitive to the number of fluorines present and can discriminate between axial, equatorial, and geminal B-F and P-F bonds, although not all do this to the same extent. (1)J(B-P) and (2)J(P-P) are similar in equilibrium and transition structures. Although transition structures no longer discriminate between axial and equatorial bonds, (1)J(P-F) and (3)J(P-F) remain sensitive to the number of fluorine atoms present.     

Magnetic polyoxometalates: anisotropic exchange interactions in the moiety of [(NaOH2)Co3(H2O)(P2W15O56)2]17-

The magnetic exchange interactions in a C0(3)(11) moiety encapsulated in Na(17) [(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)] (NaCo(3)) were studied by a combination of magnetic measurements (magnetic susceptibility and low-temperature magnetization), with a detailed Inelastic Neutron Scattering (INS) investigation. The novel structure of the salt was determined by X-ray crystallography. The ferromagnetic Co(3)O(14) triangular cluster core consists of three octahedrally oxo-coordinated Co(II) ions sharing edges. According to the single-ion anisotropy and spin-orbit coupling usually assumed for octahedral Co(II) ions, the appropiate exchange Hamiltonian to describe the ground-state properties of the isosceles triangular Co(3) spin cluster is anisotropic and is expressed as H = - 2sigma(alpha)(=)(x,y,z)(J(alpha)(12)S(1alpha)S(2alpha) + J(alpha)(23)S(2alpha)S(3alpha) + J(alpha)(13)S(1alpha)S(3alpha)), where J(alpha) are the components of the exchange interactions between the Co(II) ions. To reproduce the INS data, nonparallel anisotropic exchange tensors needed to be introduced, which were directly connected to the molecular symmetry of the complex. The following range of parameters (value +/- 0.5 cm(-1)) was found to reproduce all experimental information while taking magnetostructural relations into account: J(x)(12) = J(y)(13) = 8.6 cm(-1); J(y)(12) = J(x)(13) = 1.4 cm(-1); J(z)(12) = J(z)(13) = 10.0 cm(-1); J(x)(23) = J(y)(23) = 6.5 cm(-1) and = 3.4 cm(-1).     

Dose and timing of the first light fraction in two-fold illumination schemes for topical ALA-mediated photodynamic therapy of hairless mouse skin

A fractionated illumination scheme in which a cumulative fluence of 100 J cm(-2) is delivered in two equal light fractions separated by a dark interval of 2 h has been shown to considerably increase the efficacy of 5-aminolevulinic acid (ALA)-photodynamic therapy (PDT). The efficacy of such a scheme is further increased if the fluence of the first light fraction is reduced to 5 J cm(-2). We have investigated the relationship between the PDT response and the kinetics of protoporphyrin IX (PpIX) fluorescence in the SKH1 HR hairless mouse for first fraction fluences below 5 J cm(-2) delivered 4 h after the application of ALA and 10 J cm(-2) delivered 2 h after the application of ALA. Illumination is performed using 514 nm at a fluence rate of 50 mW cm(-2). Reducing the fluence of the first fraction to 2.5 J cm(-2) does not result in significantly different visual skin damage. The PDT response, however, is significantly reduced if the fluence is lowered to 1 J cm(-2), but this illumination scheme (1 + 99 J cm(-2)) remains significantly more effective than a single illumination of 100 J cm(-2). A first light fraction of 10 J cm(-2) can be delivered 2 h earlier, 2 h after the application of ALA, without significant reduction in the PDT response compared with 5 + 95 J cm(-2) delivered 4 and 6 h after the application of ALA. The kinetics of PpIX fluorescence are consistent with those reported previously by us and do not explain the significant increase in PDT response with a two-fold illumination scheme. Histological sections of the illuminated volume showed a trend toward increasing extent and depth of necrosis for the two-fold illumination scheme in which the first light fraction is 5 J cm(-2), compared with a single illumination scheme.     

Structural classification of acyl-substituted Quillaja saponins by electrospray ionisation ion trap multiple-stage mass spectrometry in combination with multivariate analysis

Thirty-eight saponins in two chromatographic fractions (QH-B and QH-C) from Quillaja saponaria Molina have been separated by a two-step high-performance liquid chromatography (HPLC) procedure and investigated by electrospray ionisation ion trap multiple-stage mass spectrometry (ESI-ITMS(n)) in positive ion mode. MS(2) and MS(3) spectra of the compounds were investigated by principal component analysis (PCA) and could be classified by partial least squares - discriminant analysis (PLS-DA) according to the structures of the oligosaccharides at C-3 and C-28 of the saponins. Four minor components with novel structures were found in a previously non-investigated fraction of QH-C. The structures of two of these components, J1 and J1a, were predicted by PLS-DA whereas the structures of the two others, J2 and J3, were only partly predicted. The structures of J1 and J1a were composed of structural elements found in the 34 known saponins whereas a new acyl substituent, not included in the training set used for calibration of the PLS-DA models, was found in J2 and J3, making these two components outliers. The complete structures of the four components were confirmed by monosaccharide analysis, MS(n) data and (1)H NMR spectroscopy.     

Spin-spin coupling across intramolecular N-H(+)-N hydrogen bonds in models for proton sponges: an ab initio investigation

Ab initio calculations have been performed to obtain structures and coupling constants (1)J(N-H), (1h)J(H-N), and (2h)J(N-N) for models of proton sponges with symmetric and asymmetric N-H(+)-N intramolecular hydrogen bonds (IMHBs). For a given model, the asymmetric structure has a lower energy, a longer N-N distance, and a hydrogen bond which has a greater deviation from linearity. The computed values of (2h)J(N-N) for the models are significantly less than predicted values based on the distance dependence of (2h)J(N-N) for complexes with intermolecular N-H(+)-N hydrogen bonds. However, the reduced values of (2h)J(N-N) cannot be attributed solely to the distortion of the hydrogen bond in the models, but also reflect differences in s electron populations at the nitrogens in both the ground state and the excited states which couple to it through the Fermi-contact (FC) operator. Values of (2h)J(N-N) for IMHBs can be related quadratically to the N-N distances in the models, and demonstrate that there is no discrepancy between computed values of (2h)J(N-N) at the short N-N distances found in these systems and experimental data for proton sponges.     

J-coupling constants for a trialanine peptide as a function of dihedral angles calculated by density functional theory over the full Ramachandran space

We present 13 (3)J, seven (2)J and four (1)J coupling constants (24 in all) calculated using B3LYP/D95** as a function of the φ and ψ Ramachandran dihedral angles of the acetyl(Ala)(3)NH(2) capped trialanine peptide over the entire Ramachandran space. With the exception of three of these J couplings, all show significant dependence upon both dihedral angles. For each J coupling considered, a two dimensional grid with respect to φ and ψ angles can be used to interpolate the values for any pair of φ and ψ values. Such simple interpolation is shown to be very accurate. Most of these calculated J couplings should prove useful for improving the accuracy of the determination of peptide and protein structures from NMR measurements in solution over that provided by the common procedure of treating the J couplings as functions of a single dihedral angle by means of Karplus-type fittings.     

Magnetic exchange couplings in the single-molecule magnet of Mn(12)-Ac

Four types of isotropic exchange interactions of Mn(12)-Ac are obtained by using the classical Monte Carlo simulations. The equilibrium susceptibilities are well reproduced in the temperature range between 10 and 100 K. The calculated effective spin at 0.1 K coincides with the ground-state spin. Our results show that J(1) and J(2) are strong antiferromagnetic, but the magnitude of J(2) is much smaller than that of J(1). Both J(3) and J(4) favor weakly antiferromagnetic couplings. The effects of the exchange couplings on the magnetic properties and ground-state spin are investigated too. The magnetic susceptibilities below 100 K depend more on J(2) rather than on the stronger J(1). The weak exchange couplings J(3) and J(4) have significant frustration effects on the ground-state configuration.     

Three-bond sugar-base couplings in purine versus pyrimidine nucleosides: a DFT study of Karplus relationships for (3)J(C2/4-H1') and (3)J(C6/8-H1') in DNA

(3)J(C2/4-H1') and (3)J(C6/8-H1') scalar spin-spin coupling constants have been calculated for deoxyadenosine, deoxyguanosine, deoxycytidine, and deoxythymidine as functions of the glycosidic torsion angle chi by means of density functional theory. Except for deoxythymidine, (3)J(C2/4-H1') depends little on the base type. On the contrary, (3)J(C6/8-H1') follows the usual trans to cis ratio ((3)J(C-H(cis)) < (3)J(C-H(trans))) for purine nucleosides, but reveals the opposite relation ((3)J(C-H(cis)) > (3)J(C-H(trans))) for pyrimidine nucleosides. Our results compare well with the experiment for deoxyguanosine and predict a novel trend in the case of pyrimidine bases for which no NMR results are available in the syn region. A breakdown of the key Fermi contact part of (3)J(C6/8-H1') into MO contributions rationalizes this trend in terms of an unusual coupling mechanism in the syn orientation that is very effective for pyrimidine nucleosides and considerably weaker for purine nucleosides.