Weak alignment of paramagnetic proteins warrants correction for residual CSA effects in measurements of pseudocontact shifts

Paramagnetic metal ions can induce molecular alignment with respect to the magnetic field. This alignment generates residual anisotropic chemical shifts (RACS) due to nonisotropic averaging over the molecular orientations. Using a 30 kDa protein-protein complex, the RACS effects are shown to be significant for heteronuclear spins with large chemical shift anisotropies, lanthanide ions with large anisotropic magnetic susceptibility tensors, and measurements at high magnetic field. Therefore, RACS must be taken into account when pseudocontact shifts are measured by comparison of chemical shifts observed between complexes with paramagnetic and diamagnetic lanthanide ions. The results are of particular importance when different pseudocontact shifts measured for the 1HN, 15N, and 13C' spins of a peptide group are used to restrain its orientation with respect to the electronic magnetic susceptibility tensor in structure calculations.     

乙基醋酸纤维素液晶态条带织构的形成机理

本文应用平行板移动式和转动式二种剪切装置研究了乙基醋酸纤维素的二氯乙酸体系液晶态受剪过程形成条带织构的临界剪切速率,条带织构的形成机理以及所形成的条带织构在升降温过程中的变化。结果表明,高分子液晶态受剪过程条带织构不是在受剪时产生,而是在受剪停止后的弛豫过程中形成的。可以观察到条带织构出现的诱导时间(t_b),其值的大小与溶液的浓度、剪切速率等因素有关。最后提出一模型来解释高分子液晶态受剪切过程条带织构的形成机理。     

Structural evolution of liquid-crystalline solutions of hydroxypropyl cellulose and hydroxypropyl cellulose-based nanocomposites during flow

The phase state and orientation and dissipative characteristics of biphasic LC HPC-water solutions and filled systems formed on their basis during shear flow are studied by various methods. The concentration of solutions is selected on the basis of the corrected phase diagram constructed with the use of optical interferometry. Flow curves and concentration dependences of viscosity provide additional information about the phase state and structure of the samples and the role of fillers in the rheological properties of solutions. X-ray diffraction data are obtained with the use of a Couette cell consisting of two coaxial capillaries. In the case of a clay suspension in water, practically no orientation is attained. However, in the isotropic 30% solution, clay particles easily orient, a result that indicates an important role of the viscoelasticity of a medium in the orientation process. The development of orientation of HPC macromolecules and clay particles in relation to the shear rate is analyzed separately for systems with the biphasic matrix (LC + isotropic phase). In addition, the time decay of the orientation parameter during relaxation is investigated. It is shown that higher shear rates cause a more rapid relaxation of orientation, for which recovery of the cholesteric helix typical for LC solutions of cellulose derivatives in the equilibrium state plays an important role. Order parameters (separately for the two components) are calculated, and their evolution with the shear rate and total deformation is investigated for systems containing clay nanoparticles (also the structure-active component) in LC solutions. On the basis of these data, it is hypothesized that clay particles form the columnar mesophase, which, under certain conditions, may transform into the discotic mesophase. This transition is responsible for a certain decrease in the order parameter of HPC apparently due to the instability effect of the director. It is found that shearing substantially affects the structure of the system composed of two mesophase species; specifically, it either facilitates the reinforcement of one of them or provokes structural transitions.     

Poly(epichlorohydrin) modified with 3,4,5‐tris(dodecyloxy)benzoate: The structure and dynamics of the aliphatic side chains in the columnar mesophase

We investigated the dynamics and structure of the aliphatic side chains of a randomly grafted copolymer, obtained through the chemical modification of poly(epichlorohydrin) with potassium 3,4,5‐tris(dodecyloxy)benzoate, with solid‐state ¹³C NMR. Below 283 K, the aliphatic chains partially crystallized in an all‐anti conformation. The calorimetric data were compatible with an orthorhombic packing. Below 323 K, the polymer exhibited a columnar mesophase. Spin–lattice relaxation times were determined in this temperature range and at 333 K, that is, in the isotropic phase. In the liquid‐crystalline state, some carbons exhibited a double decay in the spin–lattice relaxation, and this was attributed to presence of the liquid‐crystalline phase. This hypothesis was supported by a conformational analysis performed by molecular modeling. The activation energies of the relaxation processes in the mesophase were also estimated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2099–2111, 2005     

Magnetic susceptibility tensor and heme contact shifts determinations in the Rhodobacter capsulatus ferricytochrome c': NMR and magnetic susceptibility studies.

The 1H and 15N resonances of the carbon monoxide complex of ferrocytochrome c' of Rhodobacter capsulatus, a ferrous diamagnetic heme protein, have been extensively assigned by TOCSY-HSQC, NOESY-HSQC, and HSQC-NOESY-HSQC 3D heteronuclear experiments performed on a 7 mM sample labeled with 15N. Based on short-range and medium-range NOEs and H(N) exchange rates, the secondary structure consists of four helices: helix 1 (3-29), helix 2 (33-48), helix 3 (78-101), and helix 4 (103-125). The 15N, 1HN, and 1H(alpha) chemical shifts of the CO complex form are compared to those of the previously assigned oxidized (or ferric) state. From the chemical shift differences between these redox states, the orientation and the anisotropy of the paramagnetic susceptibility tensor have been determined using the crystallographic coordinates of the ferric state. The chi-tensor is axial, and the orientation of the z-axis is approximately perpendicular to the heme plane. The paramagnetic chemical shifts of the protons of the heme ligand have been determined and decomposed into the Fermi shift and dipolar shift contributions. Magnetic susceptibility studies in frozen solutions have been performed. Fits of the susceptibility data using the model of Maltempo (Maltempo, M. M. J. Chem. Phys. 1974, 61, 2540-2547) are consistent with a rather low contribution of the S = 3/2 spin state over the range of temperatures and confirm the value of the axial anisotropy. Values in the range 10.4-12.5 cm(-1) have been inferred for the axial zero-field splitting parameter (D). Analysis of the contact shift and the susceptibility data suggests that cytochrome c' of Rb. capsulatus exhibits a predominant high-spin character of the iron in the oxidized state at room temperature.     

Thermotropic homopolyesters. IV. Study of fiber formation

We report a melt spinning and viscosity study of two semiflexible homopolyesters containing both rigid and flexible segments in the repeating unit. Single filaments of the polyesters formed from 4,4′-diacetoxybiphenyl and azelaic acid (PB7) and sebacic acid (PB8), and from 4′-hydroxyphenyl-4-hydroxycinnamate and azelaic acid (C7), were spun at temperatures between 205 and 255°C. The temperature dependence of the Newtonian melt viscosity of PB7 and C7 was investigated, and a range of molecular weights was studied for PB7. The spinning parameters, fiber characteristics, and viscosity-temperature behavior are related to the type of mesophase formed. The mechanical properties of fibers spun from both the nematic and smectic phases of these semiflexible chain polymers were poor. Increasing the polymer molecular weight or extrusion rate only afforded a modest improvement in fiber properties. Most polymers could not be spun at temperatures corresponding to the existence of the single mesophase. Hence the low viscosity typical of the nematic mesophase is not necessarily an advantage in fiber formation from the melt. It appears from these results that this type of polyester does not possess adequate chain extension to develop ultrahigh-modulus properties. The director, which describes the local orientation of molecules within the mesophase, may undergo more frequent variations than is the case for rigid chain polyesters. Mechanisms relevant to flexible polymers may contribute to the development of orientation for this class of nematogenic melts.     

Spectroscopic analysis of post drawing relaxation in poly(lactic acid) with oriented mesophase

Uniaxial deformation of amorphous poly(lactic acid) (PLA) film was performed at 60 °C (around the glass transition temperature). The deformed samples revealed a strain-induced mesophase, and its fraction and thermal stability increased with draw strain. Further annealing was performed in situ at constant length, at the drawing temperature for the films drawn to strains of 100% and 230%. Interestingly, the orientation of amorphous phase relaxed more rapidly for the 100% sample compared with the 230% one. This could be ascribed to the constraint effect of mesophase on the amorphous chains. In addition, the chains of mesophase relaxed slightly for the 100% sample while it retained high orientation for the 230% sample. Meanwhile, the mesophase fraction decreased, and the trend was more significant for the sample drawn to 100%. These effects can be ascribed to the melting of mesophase and the different thermal stabilities of the mesophases.     

A low-dimensional molecular spin system with two steps of magnetic transitions and liquid crystal property

A low-dimensional compound [C(6)-Apy][Ni(mnt)(2)] (1, where mnt(2-) = maleonitriledithiolate, C(6)-Apy(+) = 4-amino-1-hexylpyridinium) has been designed and synthesized, which has layer arrangement of anions and cations and shows two steps of magnetic transitions. The low temperature magnetic transition has an uncommon hysteresis loop, while the crystal structure investigations disclosed no structural transition with the magnetic transition. The high temperature magnetic transition exhibits two remarkable features: (1) it synchronously occurs with a crystalline-to-mesophase transition in the first heating process and (2) the structural changes that accompany the solid-mesophase transition are irreversible. A diamagnetic and isostructural compound, [C(6)-Apy][Cu(mnt)(2)], is further characterized by structure, DSC and POM techniques, which revealed also the existence of an irreversible crystalline-to-mesophase transition in the same temperature interval of [C(6)-Apy][Ni(mnt)(2)]. Therefore, the high-temperature magnetic transition in 1 is driven by release of the structural strains, but not magnetoelastic interactions. The mesophase exhibits the characteristic of smectic A phase, and the alkyl chain melting in the cation layers probably lead to the formation of mesophase. It is noticeable that the finding of a mesophase occurring in a hexyl hydrocarbon chain molecular system is in contrast to a suggested rule that at least a dodecyl chain is required. Our results will shed a light on the design and preparation of a new low-dimensional molecular system combining magnetic transition and liquid crystal properties.     

Dynamics and structure of a flexible columnar liquid crystal based on tetrabenzocyclododecatetraene

The dynamical and conformational behaviour of a flexible tetrabenzocyclododecatetraene derivative exhibiting a columnar mesophase has been studied by a combination of deuteron solid state NMR spectroscopy and molecular dynamics (MD) simulations. As shown by two-dimensional (2D) exchange NMR, the mesophase is characterized by slow axial reorientations (∼10^-3s) of single molecular units where the phenylene rings exhibit a well-defined quasi-fourfold potential, while the 2D spectra of the core methylene sites are sensitive to the molecular conformation and reorientation mechanism. Motional narrowing of one-dimensional (1D) spectra reveals additional fast librations due to the internal flexibility of the mesogenic moiety. The various reorientation pathways comprising interconversions and pseudo-rotations between different energetically stable conformations are elucidated on a microscopic level by molecular dynamics simulations. The mesophase dynamics is ascribed to a complex axial motion involving rotational jumps combined with a pseudo-rotation between two symmetry related sofa forms. This is confirmed quantitatively by comparing the experimental 2D NMR spectra of the core methylene sites and the simulations which are based on the molecular geometries obtained by MD simulations. The lineshapes of one- and two-dimensional spectra of magnetically aligned samples specific to the orientation behaviour of the sofa conformer are discussed.     

Polymesomorphism and orientation in liquid crystalline poly(triethylene glycol p,p′-bibenzoate)

The phase transitions and the orientational behavior of liquid crystalline poly(triethylene glycol p,p′-bibenzoate) have been studied. The real-time synchrotron diffraction results indicate that, on cooling from the isotropic melt, an orthogonal SmA mesophase is formed first, and later it is transformed into a tilted SmC mesophase. However, the SmA mesophase is stable in a rather wide temperature interval, and the transformation into the SmC phase occurs at temperatures close to the glass transition, so that not very high tilting angles are attained. The uniaxial deformation of the SmC mesophase indicates that usual parallel orientation of the molecular axes in relation to the stretching direction is obtained at high strain rates, while anomalous perpendicular orientation occurs at low deformation rates, with the smectic layers aligned with the stretching direction and the molecular axes almost perpendicular. A mixture of the two types of orientation is observed at intermediate rates, with rather interesting features.     

Crystallization behavior of polyethers containing odd numbers of methylene spacers from the isotropic and liquid crystalline states

A series of thermotropic polyethers synthesized from 1-(4-hydroxyphenyl) - 2 - (2 - methyl - 4 - hydroxyphenyl) - ethane and α,ω-dibromo-n-alkanes with odd numbers of methylene units (MBPE-n = odd) shows monotropic mesophase behavior. In isothermal differential scanning calorimetry (DSC) experiments, two—sometimes even three—exothermic transition processes can be observed when the crystallization temperature is below the mesophase transition temperature, while only one exothermic process is present above the mesophase transition temperature. The melting behavior of the crystals grown from the mesophase and from the isotropic melt states is different. The crystals grown from the mesophase state exhibit a larger overall heat of transition and a higher transition temperature compared with those grown from the isotropic melt. This may be attributed to the molecular interfacial connections between the crystal and amorphous regions when MBPEs crystallize from the mesophase state. The difference in morphology between the crystals grown from the different states has also been studied with polarized light microscopy (PLM) and transmission electronic microscopy (TEM). The structures of the crystals grown from the different states are, however, the same, as evidenced through wide-angle X-ray diffraction (WAXD) measurements. From the banded morphology of MBPE samples observed from PLM, the defect textures observed through TEM and the results of WAXD experiments, this mesophase can be identified as a nematic liquid crystal state.     

Spin Crossover Studies on Bis{hydro-tris(1,2,4-triazolyl)borato}iron(II)

The spin-crossover behavior of bis{hydro-tris(1,2,4-triazolyl)borato}iron(II) is investigated in bulk and as a magnetically diluted sample in the solid state and in solution as a function of temperature by magnetic susceptibility measurements and differential scanning calorimetry. In aqueous solution the low-spin to high-spin transition is shown in turn to decrease the longitudinal relaxation time T₁ of water protons with increasing temperature. The solid-state magnetically diluted samples were prepared by cocrystallization with the isostructural zinc complex.     

Magnetic orientation of 1,3,5-triphenyl-6-oxoverdazyl radical crystals

The magnetic orientation has been studied for paramagnetic organic radical crystals 1,3,5-triphenyl-6-oxoverdazyl and 1,5-di-p-tolyl-3-phenyl-6-oxoverdazyl in magnetic fields of 2-80 kOe at temperatures of 77-343 K. The X-ray diffraction measurement has revealed that the crystals are oriented with the crystallographic c axis perpendicular to the field. The anisotropic diamagnetic susceptibility arising from the benzene rings has been estimated for the crystals along the principal magnetic chi 1, chi 2, and chi 3 axes. (The chi 1 axis is at a small angle to the a axis in the monoclinic ac plane, and the chi 3 axis is along the b axis.) Since the paramagnetic susceptibility originating from the verdazyl ring is isotropic (though a large absolute value), it is shown that the magnetic orientation occurs by the anisotropy of the diamagnetic susceptibility in the crystals. The diamagnetic susceptibility is found to have a relation of chi 2 < chi 1 < chi 3 < 0.     

Continuous-saturation study of spin-lattice relaxation on the “main” and “satellite” lines of an oriented free radical

Electron-spin relaxation is investigated, through line-shape and continuous-saturation measurements, in a γ-irradiated single crystal of glycolic acid at room temperature. For the only orientation which gives fair line resolution, the experimental data allow determination of the ratio of the effective spin-lattice relaxation times for the “main” and “satellite” lines. Comparison of this ratio with those to be expected from various possible mechanisms suggests that spin-lattice relaxation in this system originates mainly from random modulation, by molecular torsional oscillations, of the intramolecular dipolar hyperfine interaction responsible for the spectral pattern.     

价态互变Co配合物的磁性能和光诱导弛豫动力学

合成一种新型具有热致和光致自旋交叉及价态互变性能的配合物Co[HN(C₅H4N)₂](3,5-DBSQ)₂.对该配合物的热致和光致磁性变化以及光照后光诱导弛豫动力学进行了研究.自旋交叉和价态互变的相变起始温度约300K.低温下光照后,低自旋Co(Ⅲ)从配体3,5-DBCAT得到1个电子变成高自旋的Co(Ⅱ),3,5-DBCAT转化为3,5-DBSQ,分子的磁矩升高.在5～30K范围内,高自旋态的弛豫速度常数k_VT与温度无关,弛豫行为表现为隧道效应;而在30～70K之间,光照射后高自旋态弛豫的活化能为252cm^-1.     

Magnetic orientation of diamagnetic amorphous polymers

Molecular order in an amorphous polymer with anisotropic magnetic susceptibility is altered by applying external magnetic fields. Disks of atactic polystyrene (a‐PS) are prepared by solvent casting outside or inside a magnet. The effect of the magnetic field on the polymer samples is investigated by magnetic levitation and solid state NMR spectroscopy. Magnetic levitation of the a‐PS disks shows that the orientation of disk symmetry axis with respect to the magnetic field gradient depends on the magnitude and direction of the applied field during casting. Similarly, carbon‐13 two‐dimensional cross‐polarization/magic angle spinning rotor‐synchronized NMR measurements in these samples show modulation patterns of the spinning side bands only on disks prepared in the presence of a magnetic field. These findings suggest that macromolecular order could be induced in a fluid or fluid–solid phase transition with cooperative segmental motions reorienting the diamagnetic susceptibility tensor to minimize the magnetic contribution to free energy of the sample. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1009–1015, 2010     

Magnetically driven growth of anthracene thin films by organic molecular beam deposition

The possible use of a static magnetic field during organic molecular beam deposition of thin molecular films for inducing some preferential growth is discussed and the magnetic properties of diamagnetic molecules and molecular crystals are recalled. Considering prototypical materials, namely anthracene molecules and potassium phthalate substrates, which interact and may give rise to polycrystalline films with specific orientations, we show that in the presence of a magnetic field the films display a macroscopic preferential orientation as a result of minimization of the magnetic energy contribution. A very good agreement between the results of optical spectroscopy, atomic force microscopy, and predictions made on the basis of the anisotropic magnetic susceptibility of anthracene is found.     

Probing the electric field alignment of a thermotropic liquid crystalline polymer by synchrotron radiation

The orientation of a cyclic side-chain thermotropic liquid crystalline material in an AC field was monitored in real-time using synchrotron radiation. Monitoring the realignment processes in the millisecond-to-minute time-scale was made possible by the high X-ray flux. Orientation parameters and response times were calculated as a function of temperature and frequency. Response times decreased exponentially with temperature due to a decrease in the viscosity. Very little dependence of the response time on frequency was observed, except at low temperatures, where a switch from homeotropic to planar alignment of the molecules was detected. This reorientation of the director was studied in real-time and the resulting complex diffraction patterns were due to equal but opposite director rotations from an alignment parallel to the applied electric field to an alignment perpendicular to the applied electric field. The orientation parameters were highest in the central portion of the mesophase temperature range. At temperatures near clearing, the net degree of orientation diminished. Cooling through the mesophase with an applied electric field resulted in much larger orientation parameters than could be obtained by aligning at a fixed temperature in the mesophase.