DNA double-helix-mediated long-range electron transfer

A theoretical analysis based upon large-scale self-consistent Hartree-Fock calculations at a semiempirical quantum theory level (CNDO/S) is performed to investigate long-range electron transfer in a donor-DNA-acceptor molecule, where the donor and acceptor moieties are tethered to the DNA. The π-stacked base pairs are found to dominate the long-range electronic coupling. Despite the π-electron mediated coupling, the exponential distance decay constant of the electron transfer rate is ∼ 1.2–1.6 Å⁻¹, values typical of electron transfer proteins. The calculated long-range electron transfer rate of the order of 10⁶ s⁻¹ for a metal-to-metal distance of 21 Å is found to be in agreement with kinetic measurements by Meade and Kayyem. Based on the current analysis, the π-electrons dominate the long-range electronic coupling interactions in DNA, but they do not lead to one-dimensional molecular wire-like properties. © 1996 John Wiley & Sons, Inc.     

Colossal Anisotropic Thermal Expansion through Coupling Spin Crossover and Rhombus Deformation in a Hexanuclear {FeIII4FeII2} Compound

Colossal and anisotropic thermal expansion is a key function for microscale or nanoscale actuators in material science. Herein, we present a hexanuclear compound of [(Tp*)Fe^III(CN)₃]₄[Fe^II(Ppmp)]₂⋅2 CH₃OH ( 1 , Tp*=hydrotris(3,5-dimethyl-pyrazol-1-yl)borate and Ppmp=2-[3-(2′-pyridyl)pyrazol-1-ylmethyl]pyridine), which has a rhombic core structure abbreviated as {Fe^III₂Fe^II₂}. Magnetic susceptibility measurements and single-crystal X-ray diffraction analyses revealed that 1 underwent thermally-induced spin transition with the thermal hysteresis. The Fe^II site in 1 behaved as a spin crossover (SCO) unit, and significant deformation of its octahedron was observed during the spin transition process. Moreover, the distortion of the Fe^II centers actuated anisotropic deformation of the rhombic {Fe^III₂Fe^II₂} core, which was spread over the whole crystal through the subsequent molecular rearrangements, leading to the colossal anisotropic thermal expansion. Our results provide a rational strategy for realizing the colossal anisotropic thermal expansion and shape memory effects by tuning the magnetic bistability.     

Photoassociative spectroscopy of pure long-range molecules

We have experimentally studied the “pure long-range” state, 0 _g ^? , of³⁹K₂ for the first time by high resolution photoassociative spectroscopy of ultracold potassium atoms. Well-resolved vibrational levels have yielded molecular constants for this state. Analytical expressions for the potential energy curves of the two pure long-range states, 0 _g ^? and 1 _u , are obtained. All the available theoretical and experimental molecular constants of the three special long-range potentials with extrema for all the alkali metals are summarized.     

Effects of concentration of nonionic surfactant and molecular weight of polymers on the morphology of anisotropic polystyrene/poly(methyl methacrylate) composite particles prepared by solvent evaporation method

The effects of the concentration of polyoxyethylene octylphenyl ether (OP-10) as a nonionic surfactant and the molecular weight of polymers (polystyrene (PS) and poly(methyl methacrylate) (PMMA)) on the morphology of anisotropic PS/PMMA composite particles were investigated. In the case of polymers with lower molecular weight (M _w ≈ 6.0 × 10⁴ g/mol), the PS/PMMA composite particles have dimple, via acorn, to hemispherical shapes along with the increase of the OP-10 concentration. On the other hand, when the polymers have higher molecular weight (M _w ≈ 3.3 × 10⁵ g/mol), the morphology of PS/PMMA composite particles changed from dimple, via hemispherical, to snowman-like structure while the concentration of OP-10 was increased. Furthermore, thermodynamic analysis was first simply made by spreading coefficients, and the results indicated that both the concentration of OP-10 aqueous solution and the molecular weight of polymers were very important to the final morphology of anisotropic composite particles.     

Rheological properties of anisotropic poly(para-benzamide) solutions

A study has been made of the viscous properties of poly(para-benzamide) (PBA) solutions in dimethyl acetamide, which undergo a transition from an isotopic to an anisotropic (liquid-crystal) state at a definite concentration C^*. The polymer solutions behave in many respects (as regards the concentration and temperature dependence of viscosity, etc.) like solutions of low molecular weight compounds forming a liquid crystal phase, although the transitions are less pronounced in the polymer solutions owing to their polydispersity. It is shown that the viscometric method, being extremely sensitive to C^*, is convenient for determining phase diagrams of anisotropic polymer solutions. The values of C^* as related to the molecular weight of PBA have been determined, and a general criterion for transition from isotropic to anisotropic solutions established; the latter has the form (CM?)^* ≈ 1.3 × 10⁵ at 20°C. This criterion is in line with the condition for the formation of the liquid-crystal structure in a dispersion of rodlike particles as proposed by Flory. Generalized concentration dependences of viscosity have been plotted by reducing concentration to C^* and viscosity, to the maximum viscosity at the phase transition point. In investigating the flow properties of PBA solutions we revealed the existence of a yield point in the range of low shear stresses, and an intersection of the flow curves of solutions of different concentration at high shear stresses, which excludes a generalized representation of the flow curves in reduced ordinary-type coordinates.     

DNA Responds to Ionizing Radiation as an Insulator,Not as a “Molecular Wire”

The yields of radicals trapped on DNA, measured by EPR spectroscopy of oligodeoxyribonucleotide crystals (the EPR spectrum of a single crystal of d(CCCTAGGG) is shown), are found to be very high (0.7 μmol J⁻¹) and insensitive to long-range (>10⁶ base pairs) versus short-range stacking (8 base pairs) of the bases. These observations are evidence that DNA at 4 K has the properties of an insulator and argue against DNA acting as a “molecular wire”.     

Controlling nanocrystal superlattice symmetry and shape-anisotropic interactions through variable ligand surface coverage

The assembly of colloidal nanocrystals (NCs) into superstructures with long-range translational and orientational order is sensitive to the molecular interactions between ligands bound to the NC surface. We illustrate how ligand coverage on colloidal PbS NCs can be exploited as a tunable parameter to direct the self-assembly of superlattices with predefined symmetry. We show that PbS NCs with dense ligand coverage assemble into face-centered cubic (fcc) superlattices whereas NCs with sparse ligand coverage assemble into body-centered cubic (bcc) superlattices which also exhibit orientational ordering of NCs in their lattice sites. Surface chemistry characterization combined with density functional theory calculations suggest that the loss of ligands occurs preferentially on {100} than on reconstructed {111} NC facets. The resulting anisotropic ligand distribution amplifies the role of NC shape in the assembly and leads to the formation of superlattices with translational and orientational order.     

Carbon-13 NMR investigation of 2,5-diaryl-1,4-dithiins

The ¹³C NMR spectra of eight 2,5-diaryl-1,4-dithiins were recorded and signals were assigned. A linear correlation was observed between the electronegativity of the substituent groups on C-10,10′ and the chemical shifts of C-10,10′ after applying corrections for the magnetic anisotropic effect of the substituents. A Hammett correlation was found between the ¹³C chemical shifts of C-3,6 and C-7,7′ and the σ_p⁺ parameter associated with the substituents on C-10,10′. Extended electronic interaction between the π system of the aryl group and the π system of the dithiin ring was suggested by the observance of an alternating behavior in the magnitude of the substituent effects on the ¹³C shifts of C-2,5 and C-3,6. An alternating effect was also noted in the magnitude of the long-range ¹³C? F coupling constants for these same carbon signals in 2,5-(10,10′-difluoro)diphenyl-1,4-dithiin.     

Measurement of long-range cross-correlation rates using a combination of single- and multiple-quantum NMR spectroscopy in one experiment

A method is described to determine long-range cross-correlations between the modulations of an anisotropic chemical shift (e.g., of a C' carbonyl carbon in a protein) and the fluctuations of a weak long-range dipolar interaction (e.g., in cross-correlation between the same C' carbonyl and the H(N) proton of the neighboring amide group). Such long-range correlations are difficult to measure because the corresponding long-range scalar couplings are so small that Redfield's secular approximation is often violated. The method, which combines features of single- and double-quantum NMR spectroscopy, allows one to cancel the effects of dominant short-range dipolar interactions (e.g., between the CSA of the amide nitrogen N and the dipolar coupling to its attached proton H(N)) and is designed so that the secular approximation is rescued even if the scalar coupling between the long-range dipolar coupling partners is very small. The cross-correlation rates thus determined in ubiquitin cover a wide range because of local motions and variations of the CSA tensors.     

Cyano-bridged terbium(III)-chromium(III) bimetallic quasi-one-dimensional assembly exhibiting long-range magnetic ordering

A new cyano-bridged Tb(III)-Cr(III) heterometallic complex [Tb(H(2)O)(2)(DMF)(4){Cr(CN)(6)}]·H(2)O (DMF = dimethylformamide) (1), assembled from paramagnetic hexacyanochromium(III) [Cr(CN)(6)](3-) building block and highly anisotropic terbium(III) ion has been prepared and structurally and magnetically characterized. Complex 1 shows one-dimensional (1D) zig-zag chain-like structural motif which is further extended into three-dimensional network through hydrogen-bonding interactions. The long-range magnetic ordering observed in complex 1, which is possibly due to interchain magnetic dipolar interactions, illuminates that this complex is a molecule-based magnet with critical temperature of about 5 K. This higher critical temperature among those of Ln(III)-Cr(III) heterometallic complexes exhibiting long-range magnetic ordering is probably due to the introduction of highly anisotropic terbium(III) ion.     

Nanostructuring of Rare-earth-based Single-Molecule Magnets as Long-range Ordered Arrays in the Framework of Organic Metal Halide Perovskites

The nanostructuring of single-molecule magnets (SMMs) on substrates, in nanotubes and periodic frameworks is highly desired for the future magnetic recording devices. However, the ability to organize SMMs into long-range ordered arrays in these systems is still lacking. Here, we report the incorporation of magnetic (RECl₂(H₂O)₆)⁺ (RE=rare earths) molecular groups into the framework of an organic metal halide perovskite (OMHP)—(H₂dabco)CsCl₃. Intriguingly, we show the incorporated rare-earth groups self-organized into long-range ordered arrays that uniformly and periodically distributed in the A sites of OMHP. The ordered (RECl₂(H₂O)₆)⁺ groups serve as SMMs in the perovskite frameworks, exhibiting large effective magnetic moment, moderate magnetic anisotropy and two-step relaxation behavior. With the additional merit of great structural flexibility and multifunction of OMHPs, the preparation of the first SMMs@OMHP magnetic materials furthers the development of molecular spintronics.     

A reassignment of the 13C-NMR spectrum of the alkaloid ajmaline through the use of two-dimensional nmr techniques

The reassignment of the ¹³C chemical nmr spectrum of the alkaloid, ajmaline, is reported. An earlier assignment based on analogy to other alkaloids and deductions from molecular orbital theory was found to be partially in error. The present study uses broadband decoupled (both dimensions) proton-carbon shift correlation and long-range proton-carbon chemical shift correlation techniques. These experiments, in conjunction with earlier work by others on the assignment of the proton spectra, suffice to now unequivocably assign the ¹³C nmr spectrum of the title compound.     

15N-NMR Spectroscopy—New Methods and Applications [New Analytical Methods (28)]

The nitrogen nucleus is the third most important probe (after ¹H and ¹³C) for structural investigations of organic and bioorganic molecules by NMR spectroscopy. For a long time, however, the insufficient sensitivity and low natural abundance of the ¹⁵N isotope hampered detection of the ¹⁵N nucleus, and the quadrupolar ¹⁴N nucleus proved unsuitable for the study of larger molecules with several nonequivalent nitrogen atoms. The advent of new techniques, such as pulse sequences and polarization transfer, in conjunction with the use of high-field magnets and large-sample probe heads largely solved the detection problem. As a result, the last few years have seen a dramatic development of ¹⁵N-NMR spectroscopy as a versatile method for studying molecular structure, both in isotropic (liquid) and anisotropic (solid) phases. The scope of chemical applications extends from inorganic, organometallic, and organic chemistry to biochemistry and molecular biology, and includes the study of reactive intermediates, biopolymers, enzyme-inhibitor complexes, and nitrogen metabolism. Two-dimensional NMR techniques offer additional possibilities for detailed studies of biological systems.     

Long-range 1H-15N couplings and 15N chemical shift assignments of the bis-indole anticancer drug navelbine®

The recently reported application of gradient-enhanced HMQC for the observation of long-range ¹H-¹⁵N heteronuclear couplings at natural abundance has been extended to the bis-indole anticancer drug Navelbine®. Responses correlating protons to N16′ (δ ¹⁵N 138.2 ppm)in the velbanamine subunit and N1 and N9 (δ ¹⁵N 66.0 and 55.3 ppm, respectively) in the vindoline subunit were observable within about 4 hours of data acquisition. The single response to N6′ (δ ^I5N 43.0 ppm), in contrast, was very weak requiring a weekend data acquisition to even be observed. The difficulty in observing correlations to N6′ is presumably a function of molecular mobility in the eight-membered azocine ring.     

Crystallization in thin liquid films induced by shear

It is known that the thin-film structure of confined fluids and solids can be changed when the confining surfaces are sheared. Positional and orientational short- or long-range reordering can occur that often have no bulk counterparts. These multilayer, monolayer, or even sub-monolayer effects are important for understanding adhesion and friction processes, but they have proved difficult to measure, partly due to a lack of experimental techniques and partly to their apparent subtle dependence on many experimental parameters. Here we report the use of shear measurements and "optical absorption spectroscopy" in the surface forces apparatus to measure a shear-induced phase transition of an anisotropic (dye) molecule confined between two shearing mica surfaces in aqueous solution. Our studies on the shear-induced ordering and friction forces of highly anisotropic cyanine dye molecules in thin water films show only a weak effect of molecular anisotropy on shear-induced ordering, friction forces, and the onset of shear-induced crystallization, although dramatic changes do occur when the confined molecules ultimately crystallize.     

Crystalline Supramolecular Nanofibers Based on Dehydrobenzoannulene Derivatives

Supramolecular nanofibers (SNFs) composed of low‐molecular‐weight π‐conjugated molecules exhibit attractive optical and electrical properties and are expected to be the next optoelectronic materials. In this work, five crystalline SNFs have been constructed from three dehydrobenzoannulene (DBA) derivatives. The DBAs were designed to assemble in one dimension in a strategy based on anisotropic crystal growth. The crystallinity of the SNFs allowed the molecular arrangements in the SNFs to be determined. Therefore the mechanism of construction and correlations between the molecular arrangements and optical and electrical properties could be considered. The results clearly indicate that the properties of the SNFs are affected by the chemical structures and molecular arrangements. Moreover, one of the SNFs exhibits a high charge‐carrier mobility (Σμ=0.61 cm² V^?1 s^?1) because of its crystallinity and appropriate molecular arrangement. This systematic experimental study based on a proposed strategy has provided information for improving the electrical properties of SNFs. This strategy will lead to highly functional SNFs.     

Anisotropic Change in the Magnetic Susceptibility of a Dynamic Single Crystal of a Cobalt(II) Complex

Atypically anisotropic and large changes in magnetic susceptibility, along with a change in crystalline shape, were observed in a Co^II complex at near room temperature. This was achieved by combining oxalate molecules, acting as rotor, and a Co^II ion with unquenched orbital angular momentum. A thermally controlled 90° rotation of the oxalate counter anion triggered a symmetry‐breaking ferroelastic phase transition, accompanied by contraction–expansion behavior (ca. 4.5 %) along the long axis of a rod‐like single crystal. The molecular rotation induced a minute variation in the coordination geometry around the Co^II ion, resulting in an abrupt decrease and a remarkable increase in magnetic susceptibility along the direction perpendicular and parallel to the long axis of the crystal, respectively. Theoretical calculations suggested that such an unusual anisotropic change in magnetic susceptibility was due to a substantial reorientation of magnetic anisotropy induced by slight disruption in the ideal D ₃ coordination environment of the complex cation.     

Ab initio potential energy curves and transition dipole moments for the interaction of a ground state He with Na(3s–3p)

Potential energy curves of the states X ²Σ⁺, B (1)²Σ⁺ and A (1)²Π of the NaHe molecule have been calculated accurately in a large range of internuclear distances R from SA-CASSCF-MRCI calculations, using molecular orbitals expanded in cc-pV5Z basis sets. Transition dipole moments have also been calculated for the X–B, X–A and A–B transitions, in the same range of R. Their long-range behaviour have been considered.     

Mg2+‐Dependent High Mechanical Anisotropy of Three‐Way‐Junction pRNA as Revealed by Single‐Molecule Force Spectroscopy

Mechanical anisotropy is ubiquitous in biological tissues but is hard to reproduce in synthetic biomaterials. Developing molecular building blocks with anisotropic mechanical response is the key towards engineering anisotropic biomaterials. The three‐way‐junction (3WJ) pRNA, derived from ϕ 29 DNA packaging motor, shows strong mechanical anisotropy upon Mg²⁺ binding. In the absence of Mg²⁺, 3WJ‐pRNA is mechanically weak without noticeable mechanical anisotropy. In the presence of Mg²⁺, the unfolding forces can differ by more than 4‐fold along different pulling directions, ranging from about 47 pN to about 219 pN. Mechanical anisotropy of 3WJ‐pRNA stems from pulling direction dependent cooperativity for the rupture of two Mg²⁺ binding sites, which is a novel mechanism for the mechanical anisotropy of biomacromolecules. It is anticipated that 3WJ‐pRNA can be used as a key element for the construction of biomaterials with controllable mechanical anisotropy.