Molecular insight into the electrostatic membrane surface potential by 14n/31p MAS NMR spectroscopy: nociceptin-lipid association

Exploiting naturally abundant (14)N and (31)P nuclei by high-resolution MAS NMR (magic angle spinning nuclear magnetic resonance) provides a molecular view of the electrostatic potential present at the surface of biological model membranes, the electrostatic charge distribution across the membrane interface, and changes that occur upon peptide association. The spectral resolution in (31)P and (14)N MAS NMR spectra is sufficient to probe directly the negatively charged phosphate and positively charged choline segment of the electrostatic P(-)-O-CH(2)-CH(2)-N(+)(CH(3))(3) headgroup dipole of zwitterionic DMPC (dimyristoylphosphatidylcholine) in mixed-lipid systems. The isotropic shifts report on the size of the potential existing at the phosphate and ammonium group within the lipid headgroup while the chemical shielding anisotropy ((31)P) and anisotropic quadrupolar interaction ((14)N) characterize changes in headgroup orientation in response to surface potential. The (31)P/(14)N isotropic chemical shifts for DMPC show opposing systematic changes in response to changing membrane potential, reflecting the size of the electrostatic potential at opposing ends of the P(-)-N(+) dipole. The orientational response of the DMPC lipid headgroup to electrostatic surface variations is visible in the anisotropic features of (14)N and (31)P NMR spectra. These features are analyzed in terms of a modified "molecular voltmeter" model, with changes in dynamic averaging reflecting the tilt of the C(beta)-N(+)(CH)(3) choline and PO(4)(-) segment. These properties have been exploited to characterize the changes in surface potential upon the binding of nociceptin to negatively charged membranes, a process assumed to proceed its agonistic binding to its opoid G-protein coupled receptor.     

Molecular structure of 1,5-diazabicyclo[3.1.0]hexane as determined by gas electron diffraction and quantum-chemical calculations

The equilibrium molecular structure and conformation of 1,5-diazabicyclo[3.1.0]hexane (DABH) has been studied by the gas-phase electron-diffraction method at 20 degrees C and quantum-chemical calculations. Three possible conformations of DABH were considered: boat, chair, and twist. According to the experimental and theoretical results, DABH exists exclusively as a boat conformation of C s symmetry at the temperature of the experiment. The MP2 calculations predict the stable chair and twist conformations to be 3.8 and 49.5 kcal mol(-1) above the boat form, respectively. The most important semi-experimental geometrical parameters of DABH (r(e), A and angle)e), deg) are (N1-N5) = 1.506(13), (N1-C6) = 1.442(2), (N1-C2) = 1.469(4), (C2-C3) = 1.524(7), (C6-N1-C2) = 114.8(8), (N5-N1-C2) = 107.7(4), (N1-C2-C3) = 106.5(9), and (C2-C3-C4) = 104.0(10). The natural bond orbital (NBO) analysis has shown that the most important stabilization factor in the boat conformation is the n(N) --> sigma*(C-C) anomeric effect. The geometry calculations and NBO analysis have been performed also for the bicyclohexane molecule.     

Study of the thymine molecule: equilibrium structure from joint analysis of gas-phase electron diffraction and microwave data and assignment of vibrational spectra using results of ab initio calculations

Thymine is one of the nucleobases which forms the nucleic acid (NA) base pair with adenine in DNA. The study of molecular structure and dynamics of nucleobases can help to understand and explain some processes in biological systems and therefore it is of interest. Because the scattered intensities on the C, N, and O atoms as well as some bond lengths in thymine are close to each other the structural problem cannot been solved by the gas phase electron diffraction (GED) method alone. Therefore the rotational constants from microvawe (MW) studies and differences in the groups of N-C, C=O, N-H, and C-H bond lengths from MP2 (full)/cc-pVQZ calculations were used as supplementary data. The analysis of GED data was based on the C(s) molecular symmetry according to results of the structure optimizations at the MP2 (full) level using 6-311G (d,p), cc-pVTZ, and cc-pVQZ basis sets confirmed by vibrational frequency calculations with 6-311G (d,p) and cc-pVTZ basis sets. Mean-square amplitudes as well as harmonic and anharmonic vibrational corrections to the internuclear distances (r(e)-r(a)) and to the rotational constants (B(e)(k)-B(0)(k), where k = A, B, C) were calculated from the quadratic (MP2 (full)/cc-pVTZ) and cubic (MP2 (full)/6-311G (d,p)) force constants (the latter were used only for anharmonic corrections). The harmonic force field was scaled using published IR and Raman spectra of the parent and N1,N3-dideuterated species, which were for the first time completely assigned in the present work. The main equilibrium structural parameters of the thymine molecule determined from GED data supplemented by MW rotational constants and results of MP2 calculations are the following (bond lengths in Angstroms and bond angles in degrees with 3sigma in parentheses): r(e) (C5=C6) = 1.344 (16), r(e) (C5-C9) = 1.487 (8), r(e) (N1-C6) = 1.372 (3), r(e) (N1-C2) = 1.377 (3), r(e) (C2-N3) = 1.378 (3), r(e) (N3-C4) = 1.395 (3), r(e) (C2=O7) = 1.210 (1), r(e) (C4=O8) = 1.215 (1), angle e (N1-C6=C5) = 123.1 (5), angle e (C2-N1-C6) = 123.7 (5), angle e (N1-C2-N3) = 112.8 (5), angle e (C2-N3-C4) = 128.0 (5), angle e (N3-C4-C5) = 114.8 (5), angle e (C6=C5-C9) = 124.4 (9). The experimental structural parameters are in good agreement with those from MP2 (full) calculations with use of cc-pVTZ and cc-pVQZ basis sets.     

Conformational response of the phosphatidylcholine headgroup to bilayer surface charge: torsion angle constraints from dipolar and quadrupolar couplings in bicelles

The effects of bilayer surface charge on the conformation of the phosphocholine group of phosphatidylcholine were investigated using a torsion angle analysis of quadrupolar and dipolar splittings in, respectively, (2)H and (13)C NMR spectra of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) labelled in the phosphocholine group with either deuterons (POPC-alpha-d(2), POPC-beta-d(2) and POPC-gamma-d(9)) or carbon-13 (POPC-alpha-(13)C and POPC-alphabeta-(13)C(2)) and incorporated into magnetically aligned bicelles containing various amounts of either the cationic amphiphile 1,2-dimyristoyl-3-trimethylammoniumpropane (DMTAP) or the anionic amphiphile 1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG). Three sets of quadrupolar splittings, one from each of the three deuteron labelling positions, and three sets of dipolar splittings ((13)C(alpha)-(31)P, (13)C(alpha)-(13)C(beta), (13)C(beta)-(14)N), were measured at each surface charge, along with the (31)P residual chemical shift anisotropy. The torsion angle analysis assumed fast anisotropic rotation of POPC about its long molecular axis, thus projecting all NMR interactions onto that director axis of motion. Dipolar, quadrupolar and chemical shift anisotropies were calculated as a function of the phosphocholine internal torsion angles by first transforming into a common reference frame affixed to the phosphocholine group prior to motional averaging about the director axis. A comparison of experiment and calculation provided the two order parameters specifying the director orientation relative to the molecule, plus the torsion angles alpha(3), alpha(4) and alpha(5). Surface charge was found to have little effect on the torsion angle alpha(5) (rotations about C(alpha)-C(beta)), but to have large and inverse effects on torsion angles alpha(3) [rotations about P-O(11)] and alpha(4) [rotations about O(11)-C(alpha)], yielding a net upwards tilt of the P-N vector in the presence of cationic surface charge, and a downwards tilt in the presence of anionic surface charge, relative to neutrality.     

Simulated and NMR-derived backbone dynamics of a protein with significant flexibility: a comparison of spectral densities for the betaARK1 PH domain.

A 7.6 ns molecular dynamics trajectory of the betaARK1 PH domain in explicit water with appropriate ions was calculated at 300 K. Spectral densities at omega = 0, omega(N), and 0.87omega(H) and the model-free parameters were evaluated from the experimental as well as the simulated data, taking the anisotropic overall motion of the protein into account. Experimental and simulated spectral densities are in reasonable general agreement for NH bond vectors, where the corresponding motions have converged within the simulation time. A sufficient sampling of the motions for NH bonds within flexible parts of the protein requires a longer simulation time. The simulated spectral densities J(0) and J(omega(N)) are, on average, 4.5% and 16% lower than the experimental data; the corresponding numbers for the core residues are about 6%; the high-frequency spectral densities J(0.87omega(H)) are lower by, on average, 16% (21% for the core). The simulated order parameters, S(2), are also lower, although the overall disagreement between the simulation and experiment is less pronounced: 1% for all residues and 6% for the core. The observed systematic decrease of simulated spectral density and the order parameters compared to the experimental data can be partially attributed to the ultrafast librational motion of the NH bonds with respect to their peptide plane, which was analyzed in detail. This systematic difference is most pronounced for J(0.87omega(H)), which appears to be most sensitive to the slow, subnanosecond time scale of internal motion, whereas J(0) and J(omega(N)) are dominated by the overall rotational tumbling of the protein. Similar discrepancies are observed between the experimentally measured (15)N relaxation parameters (R(1), R(2), NOE) and their values calculated from the simulated spectral densities. The analysis of spectral densities provides additional information regarding the comparison of the simulated and experimental data, not available from the model-free analysis.     

Interaction tensors and local dynamics in common structural motifs of nitrogen: a solid-state (14)n NMR and DFT study

(14)N solid-state NMR powder patterns have been obtained at high field (21.1 T) using broadband, frequency-swept pulses and a piecewise acquisition method. This approach allowed the electric field gradient (EFG) tensor parameters to be obtained from model organic and inorganic systems featuring spherically asymmetric nitrogen environments (C(Q) values of up to ca. 4 MHz). The advantages and limitations of this experimental approach are discussed, and the observation of (14)N T(2) relaxation anisotropy in certain systems is also reported, which can shed light on dynamic processes, allowing motional geometries and jump rates to be probed. In particular, we show that observable effects of dynamics on (14)N spectra can be mediated by modulation of either the EFG tensor or heteronuclear dipolar couplings. It is demonstrated that the QCPMG protocol can be used to selectively enhance certain types of nitrogen environments on the basis of differences in T(2). We also present the results of extensive density functional theory calculations on these systems, which show remarkably good correlation with the experimental results and allow the prediction of tensor orientations, assignment of parameters to crystallographic sites, and a rationalization of the origin of the EFG tensors in terms of contributions from individual molecular orbitals. This work demonstrates that ultra-wideline (14)N solid-state NMR can, under favorable circumstances, be a straightforward, useful, and informative probe of molecular structure and dynamics.     

N-甲基吡咯-2-甲醛激发态结构动力学及其溶剂效应的共振拉曼光谱和密度泛函理论研究

获取了覆盖N-甲基吡咯-2-甲醛(NMPCA)A-带和B-带电子吸收共7个激发波长的共振拉曼光谱,并结合含时密度泛函理论(TD-DFT)方法研究了的A-带和B-带电子激发和Franck-Condon区域结构动力学.TD-B3LYP/6-311++G(d,p)计算表明:A-带和B-带电子吸收的跃迁主体为π→π*.共振拉曼光谱可以指认为,11-13振动模式(A-带激发)或者7-11振动模式(B-带激发)的基频、倍频和组合频,其中C=O伸缩振动(ν7)、环的变形振动+N1-C6伸缩振动(ν17)、环的变形振动(ν21)和C6-N1-C2/C2-C3-C4不对称伸缩振动(ν14)占据了绝大部分.这表明NMPCA的Sπ激发态结构动力学主要沿C=O伸缩振动、环的变形振动和环上N1-C6伸缩振动等反应坐标展开.在同一溶剂的共振拉曼光谱中随激发波长由长变短,ν7与ν14的强度比呈现出由强变弱再变强的现象,这种变化规律被认为与Franck-Condon区域Sn/Sπ态混合或势能面交叉有关.溶剂对Sn/Sπ态混合或势能面交叉具有调控作用.     

Study of internal ring rotation and molecular reorientation in the liquid crystal 5O.7 by deuterium N.M.R. spectroscopy

The spectral densities of motion were determined by deuterium N.M.R. relaxation measurements in the nematic, smectic A and smectic C phases of 4-n-pentyloxybenzylidene-d₁-4'-heptylaniline and 4-n-pentyloxybenzylidene-4'-heptylaniline-2,3,5,6-d₄. By examining two atomic sites on a 5O.7 molecule, we were able to gain information on the reorientation motion and internal rotation of the aniline ring. It was also found that director fluctuations make some contribution to the spectral density J₁ (ω). We use the superimposed rotations model to account for the internal ring motion and the small step rotational diffusion model for the molecular reorientation. The derived rotational diffusion constants for the spinning and tumbling motions appear to give physically plausible activation energies in the mesophases of 5O.7.     

Nuclear magnetic resonance investigation of the micellar properties of two-headed surfactant systems: the disodium 4-alkyl-3-sulfonatosuccinates. 2. The dynamics of the chains comprising the interior of two-headed surfactant micelles

The dynamics of the carbons comprising the micelles of two members of the family of two-headed surfactants, the disodium 4-alkyl-3-sulfonatosuccinates, has been determined via the application of the two-step model to the 13C relaxation rates and the nuclear Overhauser enhancements (nOe's) at 200 MHz. The NMR relaxation times, determined from the inversion recovery method, increase steadily as we descend the chain from the headgroup region. The relaxation rate profiles and the order parameters have been calculated from the two-step model for the micellar sulfosuccinate aggregates. We note that the order parameter profile and the fast motion correlation time profile for these two-headed surfactants are distinctly different from those of a typical single-headed, single-tailed surfactant such as dodecyltrimethylammonium bromide, particularly in the headgroup region of the micelle. All these results are interpreted in terms of the effect of adding a second headgroup to a single-headed, single-tailed surfactant.     

Sensitivity of 2H NMR spectroscopy to motional models: proteins and highly viscous liquids as examples

In order to study to what extent mechanisms of molecular motion can be unambiguously revealed by (2)H NMR spectroscopy, (2)H spectra for proteins (chicken villin protein headpiece HP36, selectively methyl-deuterated at leucine-69, C(δ) D(3)) and binary systems of high viscosity (benzene-d(6) in tricresyl phosphate) have been carefully analyzed as illustrative examples (the spectra are taken from the literature). In the first case, a model of restricted diffusion mediated by jumps between rotameric orientations has been tested against jump- and free diffusion models which describe rotational motion combined with jump dynamics. It has been found that the set of (2)H spectra of methyl-deuterated at leucine-69 chicken villin protein headpiece HP36 can be consistently explained by different motional models as well as by a gaussian distribution of correlation times assuming isotropic rotation (simple brownian diffusion model). The last finding shows that when the possible distribution of correlation times is not very broad one might not be able to distinguish between heterogeneous and homogenous (but more complex) dynamics by analyzing (2)H lineshapes. For benzene-d(6) in tricresyl phosphate, the dynamics is heterogeneous and it has been demonstrated that a gaussian distribution of correlation times reproduces well the experimental lineshapes, while for a Cole-Davidson distribution the agreement is somewhat worse. For inquires into the sensitivity of quadrupolar NMR spectral analysis (by "quadrupolar NMR spectroscopy we understand NMR spectroscopy of nuclei possessing quadrupole moment), the recently presented theoretical approach [Kruk et al., J. Chem. Phys. 135, 224511 (2011)] has been used as it allows simulating quadrupolar spectra for arbitrary motional conditions by employing the stochastic Liouville equation.     

Insights into the mobility of methyl-bearing side chains in proteins from (3)J(CC) and (3)J(CN) couplings

Side-chain dynamics in proteins can be characterized by the NMR measurement of (13)C and (2)H relaxation rates. Evaluation of the corresponding spectral densities limits the slowest motions that can be studied quantitatively to the time scale on which the overall molecular tumbling takes place. A different measure for the degree of side-chain order about the C(alpha)-C(beta) bond (chi(1) angle) can be derived from (3)J(C)(')(-)(C)(gamma) and (3)J(N)(-)(C)(gamma) couplings. These couplings can be measured at high accuracy, in particular for Thr, Ile, and Val residues. In conjunction with the known backbone structures of ubiquitin and the third IgG-binding domain of protein G, and an extensive set of (13)C-(1)H side-chain dipolar coupling measurements in oriented media, these (3)J couplings were used to parametrize empirical Karplus relationships for (3)J(C)(')(-)(C)(gamma) and (3)J(N)(-)(C)(gamma). These Karplus curves agree well with results from DFT calculations, including an unusual phase shift, which causes the maximum (3)J(CC) and (3)J(CN) couplings to occur for dihedral angles slightly smaller than 180 degrees, particularly noticeable in Thr residues. The new Karplus curves permit determination of rotamer populations for the chi(1) torsion angles. Similar rotamer populations can be derived from side-chain dipolar couplings. Conversion of these rotamer populations into generalized order parameters, S(J)(2) and S(D)(2), provides a view of side-chain dynamics that is complementary to that obtained from (13)C and (2)H relaxation. On average, results agree well with literature values for (2)H-relaxation-derived S(rel)(2) values in ubiquitin and HIV protease, but also identify a fraction of residues for which S(J,D)(2) < S(rel)(2). This indicates that some of the rotameric averaging occurs on a time scale too slow to be observable in traditional relaxation measurements.     

Conjecture: imines as unidirectional photodriven molecular motors-motional and constitutional dynamic devices

Compounds containing the C==N group, such as imines and their derivatives, may undergo syn-anti isomerization by two different routes: 1) photochemically, by out-of-plane rotation around the carbon-nitrogen double bond through a "perpendicular" form, and 2) thermally, by in-plane nitrogen inversion through a "linear" transition state. When the two interconversions occur in sequence, a full, closed process is accomplished, restoring the initial state of the system along two different steps. In a chiral imine-type compound, for example, with an asymmetric center next to the C==N function, photoinduced rotation may be expected to occur in one sense in preference to the opposite one. Thus, photoisomerization followed by thermal isomerization in a chiral imine compound generates unidirectional molecular motion. Generally, imine-type compounds represent unidirectional molecular photomotors converting light energy into mechanical motion. As they are also able to undergo exchange of the carbonyl and amine partners, they present constitutional dynamics. Thus, imine-type compounds are double dynamic, motional, and constitutional devices.     

Synthesis and Structure of 6-(4- Fluorobenzylamino)-3-methylthio-1,5- diphenyl-1H-pyrazolo[3,4-d]pyrimidin-4(5H)-one

1 INTRODUCTION It was reported that the pyrazolopyrimidinone derivatives play a very important role in the bio- chemistry of living cell. Many potential drugs[1～3] and agrochemicals[4, 5] have been modeled on the compound, and the study on derivatives …     

Effect of fluoro substitution on the fragmentation of the K-shell excited/ionized pyridine studied by electron impact

Fragmentation of the pyridine ring followed by K-shell excitation/ionization has been studied with 2-fluoropyridine (2FPy) by electron impact. Ab initio molecular orbital (MO) calculations were also carried out to investigate the electronic states correlating with specific fragment ions. The fragment ions are produced characteristically at the N 1s edge, while the spectra observed at the F 1s and C 1s edges exhibit a small difference from that at the valence ionization. The production of the C(4)H(2)(+), C(4)H(3)(+) and C(4)H(2)F(+) ions indicates that the cleavage of the N-C6 and C2-C3 bonds or the N-C2 and C5-C6 bonds is likely to occur after the N 1s excitation/ionization. Ab initio MO calculations indicate that the former fission is likely to proceed through the n(N)(1)π(2)(1)π(3)(2) and n(N)(0)π(2)(2)π(3)(2) excited states of the parent molecular dication. On the other hand, the breakage of the N-C2 and C4-C5 bonds, which specifically proceeds at the N 1s edge for 2-methylpyridine, does not occur for 2FPy. The present calculation reveals that the products of this channel are unstable by the electronegativity of fluorine and that the relative energy of the Auger-final states of 2FPy is lowered by the reorganization and electron correlation effects.     

Application of Schrödinger equation to study the tunnelling dynamics of proton transfer in the hydrogen bond of 2,5-dinitrobenzoic acid: proton T1 T1rho, and deuteron T1 relaxation methods

Temperature measurements of proton T1 (24.7 MHz), deuteron (deuterated hydroxyl group) T1 (55.2 MHz), and proton T1(rho) (B1 = 9 G) spin-lattice relaxation times of 2,5-dinitrobenzoic acid have been performed. An analysis of present experimental data together with previously published proton T1 (55.2 MHz) data has revealed the following molecular motions: proton/deuteron transfer in the hydrogen bond and two-site hopping of the whole dimer. It is shown that the proton-transfer dynamics are characterized by two correlation times tau(ov) and tau(tu), describing two fundamentally different motional processes, namely, thermally activated jumps over the barrier and tunneling through the barrier. The temperature dependence of 1/tau(tu) is the solution of Schr?dinger's equation, which also yields the temperature T(tun), where begins the tunnel pathway for proton transfer. A new equation for the spectral density function of complex motion consisting of the three motions is derived. The third motion (two-site hopping of the whole dimer characterized by tau(lib) correlation time) is responsible for a proton T1(rho) minimum in high temperatures, just below the melting point. Such a minimum is not reached by T1 temperature dependencies. The minimum of T1(rho) assigned to the classical hopping of a hydrogen-bonded proton occurs in the same low-temperature regime in which the flattening of the temperature dependencies of T1 points to the dominance of incoherent tunneling. This experimental fact denies the known theories predicting the intermediate temperature regime where a smooth transition between classical and quantum tunneling dynamics is expected. The fit of the derived theoretical equations to the experimental data T1(rho) and T1 is satisfactory. The correlation times obtained for deuterons indicate deuteron-transfer dynamics much slower than proton-transfer dynamics. It is concluded that the classical proton transfer takes place over the whole temperature regime, while the incoherent tunneling occurs below 46.5 (hydrogen) or 87.2 K (deuterium) only.     

Isotope effects, dynamics, and the mechanism of solvolysis of aryldiazonium cations in water

The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant (13)C kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C(2)-C(3) and C(5)-C(6) bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple S(N)1 heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N(2) by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the (13)C isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between S(N)2Ar and S(N)1 mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.     

Mass spectra of some 4,5-dihydroimidazole derivatives

The mass spectra of a series of 4,5-dihydroimidazole derivatives were determined. Deuterium labelling and high resolution mass spectrometry were utilized in order to elucidate the mechanism of a number of fragmentations. The most significant ions arise from the following processes: (1) elimination of the C-4 substituent; (2) cleavage of the 1,2-N? C and 4,5-C? C bonds with or without hydrogen migration; (3) cleavage of the 1,5-N? C and 2,3-C? N bonds with charge retention on the N-1? C-2 moiety.

1 In this paper the standard numbering of the imidazole ring (see structures) is retained for the fragment ions.

Anomalous behaviour is shown by the t-butoxy derivative; the reasons for this behaviour are discussed.     

Composite spectral density functions for the interpretation of the 13C NMR relaxation behavior of polymers containing hydrocarbon side chains. Free and restricted side chain motion

¹³C NMR NT₁ and NOE have been calculated by using composite spectral density functions describing polymer chain segmental motion and internal rotation of a hydrocarbon side chain attached to the polymer backbone. Numerical results at two magnetic fields are presented as a function of the various motional parameters characterizing the various models. NT₁ and NOE relaxation parameters are well behaved and appear to have practical value for describing the dynamics of these systems. The models have been applied to the relaxation data of poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) in toluene solutions. The dynamics of the two polymers are characterized by a very localized backbone motion and restricted internal rotation about successive C? C bonds of the side chains. © 1995 John Wiley & Sons, Inc.     

两种单齿硫醇为配位体的钼络合物的合成和结构表征

本文报导利用MoC1_5、NaOMe、C_6H_5CH_2SH和NaHS在无氧、无水,室温条件下在甲醇介质中合成(Et_4N)_2[Mo_2O_2S_2(SCH_2C_6H_5)_4](化合物Ⅰ)和(Et_4N)-[Mo_2O_2(μ-SCH_2C_6H_5)_3(SCH_2C_6H_5)_4](化合物Ⅱ)的反应条件,分离步骤。本文同时报导这两种化合物的核磁共振谱、红外光谱、电子光谱等结构表征,同时也对化合物形成的可能机理作了探索性的讨论。     

A new spin probe of protein dynamics: nitrogen relaxation in 15N-2H amide groups

(15)N spin relaxation data have provided a wealth of information on protein dynamics in solution. Standard R(1), R(1)(rho), and NOE experiments aimed at (15)N[(1)H] amide moieties are complemented in this work by HA(CACO)N-type experiments allowing the measurement of nitrogen R(1) and R(1)(rho) rates at deuterated (15)N[(2)D] sites. Difference rates obtained using this approach, R(1)((15)N[(1)H]) - R(1)((15)N[(2)D]) and R(2)((15)N[(1)H]) - R(2)((15)N[(2)D]), depend exclusively on dipolar interactions and are insensitive to (15)N CSA and R(ex) relaxation mechanisms. The methodology has been tested on a sample of peptostreptococcal protein L (63 residues) prepared in 50% H(2)O-50% D(2)O solvent. The results from the new and conventional experiments are found to be consistent, with respect to both local backbone dynamics and overall protein tumbling. Combining several data sets permits evaluation of the spectral density J(omega(D) + omega(N)) for each amide site. This spectral density samples a uniquely low frequency (26 MHz at a 500 MHz field) and, therefore, is expected to be highly useful for characterizing nanosecond time scale local motions. The spectral density mapping demonstrates that, in the case of protein L, J(omega(D) + omega(N)) values are compatible with the Lipari-Szabo interpretation of backbone dynamics based on the conventional (15)N relaxation data.