Natural Spin Research Articles

Rheological behaviour of native silk feedstocks

Whilst much is known about the properties of silks, the means by which native silk feedstocks are spun still represent a gap in our knowledge. Rheology of the native silk feedstocks is germane to an understanding of the natural spinning process. Yet, an overview of the literature reveals subtle limitations and inconsistencies between studies, which has been largely attributed to sample-to-sample variation when testing these exquisitely flow-sensitive materials. This ambiguity has prevented reliable, consistent inferences from standard polymer rheology and constitutes an obstacle to further development.To address this challenge, we present the largest study to date into the rheological properties of native silk feedstocks from Bombyx mori larvae. A combination of shear and oscillatory measurements were used to examine in detail the relationships between concentration, low shear viscosity, relaxation times, complex modulus and estimates of the molecular weights between entanglements. The results from this highly detailed survey will provide a sound basis for further experimental or theoretical work and lay the foundations for future bio-inspired processing of proteins.

Structural Study of Mg-Based Metal–Organic Frameworks by X-ray Diffraction, 1H, 13C, and 25Mg Solid-State NMR Spectroscopy, and First-Principles Calculations

Understanding the formation principles, structure, and stability of magnesium-based metal–organic frameworks is a difficult task that requires application of a set of experimental and theoretical methods. In this work, three polycrystalline Mg–benzene–1,3,5-tricarboxylates, differing in the dimensionality of their frameworks, were studied by X-ray diffraction (XRD), solid-state nuclear magnetic resonance (NMR) spectroscopy, and first-principles calculations based on the density functional theory (DFT). XRD provided information on the average periodic structures of the three materials. Natural abundance 25Mg, 13C, and 1H magic-angle spinning NMR spectra confirmed the proposed structural models, elucidated hydrogen bonds within two of the materials, and detected the strong effect of the anisotropy of the bulk magnetic susceptibility in one. The calculations of 13C and 25Mg chemical shifts and 25Mg quadrupolar coupling parameters with the gauge-including projector-augmented wave approach enabled assignment of carbon and magnesium NMR signals to carbon and magnesium sites within the three metal–organic framework structures. The detected 13C and 25Mg NMR parameters could not be simply related to the geometry of the environments of these nuclei. DFT-based calculation of formation energies of the materials enabled the prediction of the thermodynamical stability of their structures.

A comparison of the C‒H bond dissociation enthalpies of sulfur-containing fused heterocyclic compounds to the C‒H bond dissociation enthalpies in other heterocycles

The C‒H bond dissociation enthalpies (BDEs) of 26 N, O, S-containing mono-heterocyclic compounds were assessed by the composite high-level ab initio methods G4 and CBS-QB3. In addition, the C‒H BDEs of 32 heterocyclic compounds were evaluated by 13 density functional theory methods. The BMK method showed the lowest root mean square error of 7.2 kJ/mol and the correlation coefficient (R2) was 0.9874 after being compared with the experimental values. Subsequently, we used this method to study the C‒H BDE values of the different positions as well as the substituent effects on benzene and heterocycles in sulfur-containing fused heterocyclic compounds. Both the natural charge distributions of benzo[b]thiophene and benzothiazole and natural spin densities of benzo[b]thiophene derivative radicals were conducted by NBO analysis for further understanding the essence of the C‒H bond.

A mathematical perspective on metastable wetting

In this paper we investigate the dynamical behavior of an interface or polymer, in interaction with a distant attractive substrate. The interface is modeled by the graph of a nearest neighbor path with non-negative integer coordinates, and the equilibrium measure associates to each path \eta a probability proportional to \lambda^{H(\eta)} where \lambda is non-negative and H(\eta) is the number of contacts between \eta and the substrate at zero. The dynamics is the natural spin flip dynamics associated to this equilibrium measure. We let the distance to the substrate at both polymer ends be equal to aN, where 0 2/(1-2a) the mixing time is exponential in N and the system relaxes in an exponential manner which is typical of metastability.

Synthesis and characterization of rhenium(I) complexes based on O, N, N coordinating ligands: DFT/TDDFT studies on the electronic structures and spectral properties

Mononuclear rhenium(I) complexes having fac-[Re(CO)3]+ moiety of general formula fac-[Re(CO)3(L)] have been synthesized in excellent yield by reacting [Re(CO)5Cl] with HL1 and HL2 in a ratio of 1:1 in methanol. Here L− is deprotonated form of 1-[(quinolin-8-ylimino)methyl]naphthalene-2-ol (HL1) and 1-[(quinolin-8-yl-diazenyl)]naphthalene-2-ol (HL2). The elemental analysis and ESI mass spectroscopic measurements ensure the formations of desired complexes. Molecular structures of fac-[Re(CO)3(L1)] and fac-[Re(CO)3(L2)] were confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques. A remarkable change in optical properties is observed in case of fac-[Re(CO)3(L2)] over fac-[Re(CO)3(L1)] where the ligands bearing a NN function (HL2) instead of coordinated CN function (HL1). The ground and excited-state geometries, NMR, absorption, and phosphorescence properties of Re(I) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveals the nature of excitations. The lowest lying triplet excited is associated with the 3MLCT excited state. The emission like transition is consistent with the strong 3MLCT character.

Spider silk gut: development and characterization of a novel strong spider silk fiber.

Spider silk fibers were produced through an alternative processing route that differs widely from natural spinning. The process follows a procedure traditionally used to obtain fibers directly from the glands of silkworms and requires exposure to an acid environment and subsequent stretching. The microstructure and mechanical behavior of the so-called spider silk gut fibers can be tailored to concur with those observed in naturally spun spider silk, except for effects related with the much larger cross-sectional area of the former. In particular spider silk gut has a proper ground state to which the material can revert independently from its previous loading history by supercontraction. A larger cross-sectional area implies that spider silk gut outperforms the natural material in terms of the loads that the fiber can sustain. This property suggests that it could substitute conventional spider silk fibers in some intended uses, such as sutures and scaffolds in tissue engineering.

A new spin on entanglement entropy

We argue that the usual notions of thermodynamic and entanglement entropy have novel analogs in the context of higher spin theories. In particular, the Wald and Ryu-Takayanagi formulas have natural higher spin extensions that we work out and study. On the CFT side, just as standard entanglement entropy in CFT_2 can be computed from twist field correlators, we demonstrate that by introducing corresponding operators carrying higher spin charge we can precisely reproduce our results from the bulk. We also show that the first law for entanglement entropy implies the linearized field equations for the metric and higher spin fields, generalizing recent work on deriving the linearized Einstein equations from the first law.

Dragline silk: a fiber assembled with low-molecular-weight cysteine-rich proteins.

Dragline silk has been proposed to contain two main protein constituents, MaSp1 and MaSp2. However, the mechanical properties of synthetic spider silks spun from recombinant MaSp1 and MaSp2 proteins have yet to approach natural fibers, implying the natural spinning dope is missing critical factors. Here we report the discovery of novel molecular constituents within the spinning dope that are extruded into dragline silk. Protein studies of the liquid spinning dope from the major ampullate gland, coupled with the analysis of dragline silk fibers using mass spectrometry, demonstrate the presence of a new family of low-molecular-weight cysteine-rich proteins (CRPs) that colocalize with the MA fibroins. Expression of the CRP family members is linked to dragline silk production, specifically MaSp1 and MaSp2 mRNA synthesis. Biochemical data support that CRP molecules are secreted into the spinning dope and assembled into macromolecular complexes via disulfide bond linkages. Sequence analysis supports that CRP molecules share similarities to members that belong to the cystine slipknot superfamily, suggesting that these factors may have evolved to increase fiber toughness by serving as molecular hubs that dissipate large amounts of energy under stress. Collectively, our findings provide molecular details about the components of dragline silk, providing new insight that will advance materials development of synthetic spider silk for industrial applications.

Spin-dependent dwell time through ferromagnetic graphene barrier

We investigated the dwell time of electrons tunneling through a ferromagnetic (FM) graphene barrier. The results show that the spin polarization can be efficiently controlled by the barrier width, barrier height, and the incident electron energy. Furthermore, it is found that electrons with different spin orientations will spend different times through the barrier. The difference of the dwell time between spin-up and spin-down electrons arises from the exchange splitting, which is induced by the FM strip. Study results indicate that a ferromagnetic graphene barrier can cause a nature spin filter mechanism in the time domain.

Rashba spin–orbit effect on dwell time in graphene asymmetrical barrier

We investigated the dwell time of electrons tunneling through an asymmetric barrier in monolayer graphene with the Rashba spin–orbit interaction (RSOI). The results show that the dwell time oscillates by increasing the barrier width, RSOI strength, and the bias voltage. In addition, when an external electric field is present, the spin polarization can be observed, whereas for the RSOI alone it is zero. Furthermore, it is found that electrons with different spin orientations will spend different times through the barrier. The difference of the dwell time between spin-up and spin-down electrons arises from the Rashba spin–orbit interaction. It can be concluded that Rashba spin–orbit effect can cause a natural spin filter mechanism in the time domain.

Mononuclear rhenium(i) complexes incorporating 2-(arylazo)phenyl benzyl thioethers: synthesis, structure, spectral, DFT and TDDFT studies

The stoichiometric reaction of [Re(CO)5Cl] with 2-(arylazo)phenyl benzyl thioethers [ArNNC6H4SCH2Ph: Ar = Ph, 4-chlorophenyl, 4-nitrophenyl] (L1–L3) in a ratio of 1 : 1 in dry toluene under refluxing condition afforded the Re(I) complexes of general formula fac-[Re(L)(CO)3Cl] in excellent yields. The ligands used in this present work are 2-(phenylazo)phenyl benzyl thioether (L1), 2-(4-chlorophenylazo)phenyl benzyl thioether (L2) and 2-(4-nitrophenylazo)phenyl benzyl thioether (L3). The elemental analysis and ESI mass spectroscopic measurements ensure the formation of desired complexes. The crystal structure of the complexes [Re(L1)(CO)3Cl], 1 and [Re(L2)(CO)3Cl], 2 were determined by X-ray diffractometry study. The molecular structures observed in the solid state were retained in the solution (1H and 13C NMR spectra). The ground and excited-state geometries, NMR, absorption, and luminescent properties of three Re(I) complexes were examined by DFT and TDDFT methods. The introduction of an electron withdrawing group at the para position of the phenyl ring attached to the NN bond varies the frontier molecular orbital energies, compositions and optical properties of these molecules significantly. The lowest lying triplet excited is associated with an admixture of the 3MLCT and 3ILCT excited state having a cis conformation of the azoaryl moiety. The emission like transition having mixed 3MLCT and 3ILCT nature was characterized by natural transition orbital (NTO) and spin density difference map analysis. The presence of the electron withdrawing NO2 group leads to the low quantum yield through enhanced non-radiative deactivation. The cis orientation at the lowest lying triplet excited state (T1) may imply some kind of photoinduced isomerization in the ligand frame and as a result there may be more than one emitting species in the solution and hence bi-exponential decay nature was observed for all the complexes.

Magnetic resonance properties of actinyl carbonate complexes and plutonyl(VI)-tris-nitrate.

Electronic structures and magnetic properties of actinyl ions AnO2(n+) (An = U, Np, and Pu) and the equatorially coordinated carbonate complexes [UO2(CO3)3](5–), [NpO2(CO3)3](4–), and [PuO2(CO3)3](4–) are investigated by ab initio quantum chemical calculations. The complex [PuO2(NO3)3](−) is also included because of experimentally available g-factors and for comparison with a previous study of [NpO2(NO3)3](−) (Chem.—Eur. J. 2014, 20, 7994-8011). The results are rationalized with the help of crystal-field (CF)-type models with parameters extracted from the ab initio calculations, and with the help of natural orbitals and natural spin orbitals contributing to the magnetic properties and the unpaired spin distribution, generated from the spin–orbit wave functions. These orbitals resemble textbooklike representations of the actinide 5f orbitals. Calculated paramagnetic susceptibilities are used to estimate dipolar 13C chemical shifts for the carbonate ligands. Their signs and order of magnitude are compared to paramagnetic effects observed experimentally in NMR spectra. The results indicate that the experimental spectra are also influenced by contact shifts.

Left-handed optical radiation torque

The generation of a left-handed torque that acts in the opposite direction to light's natural spin angular momentum is reported. The effect is achieved by sending circularly polarized light into an azimuthally patterned birefringent glass disk. Optical forces and torques are two mechanical degrees of freedom available to manipulate matter, and form the basis of optical tweezing strategies1,2. In contrast to the Keplerian intuition that objects should be pushed downstream an incident photon flux, the concept of ‘negative’ optical forces has recently been described3,4 and has triggered many developments5,6,7,8,9,10,11,12,13,14. Here, we report on the counterintuitive angular analogue of negative optical forces by demonstrating that circularly polarized Gaussian light beams give rise to torque with opposite sign to that of the incident optical angular momentum. Such a ‘left-handed’ mechanical effect is demonstrated by the use of an inhomogeneous and anisotropic transparent macroscopic medium. Practical difficulties associated with the direct observation of optically induced spinning of a macroscopic object are circumvented via the rotational Doppler effect15,16. These results shed light on spin–orbit optomechanics and equip the left-handed optomechanical toolbox with angular features.

Effects of environment parameters on sol-gel transition and dry-spinnability of regenerated silk fibroin aqueous solution

Protein concentration, pH, the types and concentrations of metallic ions, and extensional flow are thought to be important environment parameters affecting the natural spinning process. In this study, we investigate the effects of the types and concentrations of metallic ions (Ca2+, Mg2+, and K+ ions), pH, and silk fibroin concentration on the sol-gel transition and the rheological behavior of a regenerated silk fibroin (RSF) aqueous solution. The results show that with an increase in the silk fibroin concentration, the weak acidic RSF aqueous solutions containing Mg2+ or Ca2+ ions undergo a phase transition to a weak gel state. Moreover, the rheological characterization of RSF aqueous solutions shows a dramatic change, and their apparent viscosities increase by almost three orders of magnitude and approach the apparent viscosity of the native dope in the silkworm gland. By using conventional pressure equipment, we investigate the dry-spinnability of weak gels. Further, we observe that the as-spun fibers exhibit a smooth surface and have inferior mechanical properties. The structure of the as-spun fibers is predominantly in a random coil or Silk I conformation.

Theoretical investigation on the mechanism of FeCl3-catalysed cross-coupling reaction of alcohols with alkenes

The mechanism of the FeCl3-catalysed cross-coupling reaction of alcohols with alkenes has been investigated using the density functional theory. All calculations were performed in liquid phase. The structures of intermediates and transition states are computed and analysed in detail. The calculations show that the entire catalytic cycle consists in three steps: (1) H-abstraction, (2) free-radical addition, and (3) hydrogen transfer. The rate-limiting step in the whole catalytic cycle is the hydrogen-abstraction step. Only the quartet potential surface is likely to play an essential role in this cross-coupling reaction. The alpha-C(sp3)–H bond of the phenylpropanol is cleaved in a homolytic fashion. Moreover, we justify the change of the oxidation state of iron along the overall reaction pathway on the basis of the computed natural population atomic charges and spin densities. Our calculated results are consistent with and provide a reliable interpretation for the experimental observations that suggest the cross-coupling reaction occurs through a radical mechanism.

Realizing three-dimensional artificial spin ice by stacking planar nano-arrays

Artificial spin ice is a frustrated magnetic two-dimensional nano-material, recently employed to study variety of tailor-designed unusual collective behaviours. Recently proposed extensions to three dimensions are based on self-assembly techniques and allow little control over geometry and disorder. We present a viable design for the realization of a three-dimensional artificial spin ice with the same level of precision and control allowed by lithographic nano-fabrication of the popular two-dimensional case. Our geometry is based on layering already available two-dimensional artificial spin ice and leads to an arrangement of ice-rule-frustrated units which is topologically equivalent to that of the tetrahedra in a pyrochlore lattice. Consequently, we show, it exhibits a genuine ice phase and its excitations are, as in natural spin ice materials, magnetic monopoles interacting via Coulomb law.

Charge distribution in Mn(salen) complexes

The charge and spin distribution in manganese-salen complexes were analyzed using different basis sets and density functionals. Five population analysis methods [Mulliken, Löwdin, Natural population analysis (NPA), atoms in molecules (AIM), and CHelpG] were used to characterize the charge distribution. Results show that NPA and AIM were the only methods capable of giving charges with the correct sign for all cases under study. According to the analysis of the natural charge and spin distributions, the salen ligand shows a complex behavior, counteracting the effect of the chloro and oxo ligands on the metal center. Furthermore, the presence of a chloride counter ion increases the oxo-radical character of Oxo-Mn(salen) complexes, which may play an important role in the rationalization of the catalytic properties of Mn(salen) complexes. © 2014 Wiley Periodicals, Inc.

Synthesis, structure and spectroscopic properties of Re(i) complexes incorporating 5-arylazo-8-hydroxyquinoline: a density functional theory/time-dependent density functional theory investigation

Dinuclear rhenium(I) complexes having a fac-[Re(CO)3](+) moiety of general formula fac-[Re2(CO)6(hq)2] have been synthesized in excellent yield by reacting [Re(CO)5Cl] with Hhq in a ratio of 1 : 1 in toluene in an argon atmosphere. Here hq(-) is the deprotonated form of 5-phenylazo-8-hydroxyquinoline (Hhq(1)), 5-(2-naphthylazo)-8-hydroxyquinoline (Hhq(2)) and 5-(2-fluorineazo)-8-hydroxyquinoline (Hhq(3)). The reaction of synthesized dinuclear complexes with imidazole (Im) and N-methylimidazole (N-MeIm) in dry dichloromethane under argon atmosphere afforded the mononuclear complexes of general formula fac-[Re(CO)3(hq)(Im)] and fac-[Re(CO)3(hq)(N-MeIm)] respectively in high yield. The elemental analysis and ESI mass spectroscopic measurements confirm the formation of the desired complexes. Molecular structures of fac-[Re(CO)3(hq(1))(Im)] and fac-[Re(CO)3(hq(1))(N-MeIm)] were confirmed by single-crystal X-ray diffraction. The complexes were also characterized by different spectroscopic techniques. The complexes displayed bathochromically shifted intramolecular charge transfer (CT) bands as compared to complexes with unsubstituted 8-hydroxyquinoline complexes. The ground and excited-state geometries, NMR, absorption, and phosphorescence properties of nine Re(i) complexes were examined by DFT and TDDFT methods. The natural transition orbital (NTO) and spin density difference map analysis reveals the nature of excitations. The lowest lying triplet excited is associated with the (3)IL excited state (ligand-localized) having a cis conformation of the pendant arylazo moiety. The emission-like transition is consistent with the strong (3)ILCT character.

Computational Study on the Intramolecular Carbene-CO Coupling in M(CH2)(CO)3 Radicals (M = Co, Rh, Ir)

The intramolecular carbene-carbonyl coupling has been investigated for the simple M(CH2)(CO)3 (M = Co, Rh, Ir) radical complexes at the DFT PBEPBE/TZVP level of theory. The coupling is predicted to be very fast for the cobalt-containing system, but it is still feasible for the systems based on the other two metals. The back-way reaction, that is, the conversion of the ketene complex into carbonyl-carbene complex, cannot be excluded from the Ir-containing system in CH2Cl2, and it is even favored in gas phase. The intermolecular ketene formation by the addition of external CO onto the CH2 moiety is the favored pathway for the Ir-complex. The Laplacian distribution, as well as the natural spin density distribution of all the species, being involved in the reaction, gives explanation for the significant difference between the nature of the Co-complex and the Rh- and Ir-systems.

Shotgun proteomic analysis of the Bombyx mori anterior silk gland: An insight into the biosynthetic fiber spinning process

The Bombyx mori anterior silk gland (ASG) is a natural fiber manipulator for the material provided by the middle and posterior silk glands. In view of the significant role of the ASG in the liquid-crystal spinning process, a shotgun proteomics approach was taken to study the relationship between the function of proteins in the silkworm ASG and the spinning mechanism. A total of 1132 proteins with 7647 unique peptides were identified in the ASG dataset including some involved in the cuticle, ion transportation, energy metabolism, and apoptosis. Two putative cuticle-specific proteins were highly and specifically expressed in the ASG; therefore, the ASG dataset could provide clues for comprehensive understanding of the natural silk spinning mechanism in the silkworm. All MS data have been deposited in the ProteomeXchange with identifier PXD000090.