New two-step sequence involving a hetero-Diels-Alder and a nonphenolic oxidative coupling reaction: a convergent access to analogs of steganacin

A new family of analogs of steganacin, an important antimitotic compound, was accessed. It takes advantage of a completely stereoselective sequence of two key steps. The central dihydropyrane core is built by a highly diastereoselective and facially controlled hetero-Diels-Alder reaction. It is followed by a nonphenolic biaryl oxidative coupling with a complete atropo-stereoselectivity. It leads to a quick way to form cyclic biaryl lignans.     

Hydrogen Bonding as a Supramolecular Tool for Robust OFET Devices

In the present study, we demonstrated the effect of hydrogen bonding in the semiconducting behaviour of a small molecule used in organic field-effect transistors (OFETs). For this study, the highly soluble dumbbell-shaped molecule, Boc-TATDPP based on a Boc-protected thiophene-diketopyrrolopyrrole (DPP) and triazatruxene (TAT) moieties was used. The two Boc groups of the molecule were removed by annealing at 200 °C, which created a strong hydrogen-bonded network of NH-TATDPP supported by additional π–π stacking. These were characterised by thermogravimetric analysis (TGA), UV/Vis and IR spectroscopy, XRD and high-resolution (HR)-TEM measurements. FETs were fabricated with the semiconducting channel made of Boc-TATDPP and NH-TATDPP separately. It is worth mentioning that the Boc-TATDPP film can be cast from solution and then annealed to get the other systems with NH-TATDPP. More importantly, NH-TATDPP showed significantly higher hole mobilities compared to Boc-TATDPP. Interestingly, the high hole mobility in the case of NH-TATDPP was unaffected upon blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). Thus, this robust hydrogen-bonded supramolecular network is likely to be useful in designing efficient and stable organic optoelectronic devices.     

Squalyl Crown Ether Self‐Assembled Conjugates: An Example of Highly Selective Artificial K+ Channels

The natural KcsA K⁺ channel, one of the best‐characterized biological pore structures, conducts K⁺ cations at high rates while excluding Na⁺ cations. The KcsA K⁺ channel is of primordial inspiration for the design of artificial channels. Important progress in improving conduction activity and K⁺/Na⁺ selectivity has been achieved with artificial ion‐channel systems. However, simple artificial systems exhibiting K⁺/Na⁺ selectivity and mimicking the biofunctions of the KcsA K⁺ channel are unknown. Herein, an artificial ion channel formed by H‐bonded stacks of squalyl crown ethers, in which K⁺ conduction is highly preferred to Na⁺ conduction, is reported. The K⁺‐channel behavior is interpreted as arising from discreet stacks of dimers resulting in the formation of oligomeric channels, in which transport of cations occurs through macrocycles mixed with dimeric carriers undergoing dynamic exchange within the bilayer membrane. The present highly K⁺‐selective macrocyclic channel can be regarded as a biomimetic alternative to the KcsA channel.     

Tailoring of Polarizing Agents in the bTurea Series for Cross‐Effect Dynamic Nuclear Polarization in Aqueous Media

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors ? (¹H) in cross‐effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP‐optimized glycerol/water matrix (“DNP juice”) have been studied. We observe that ? (¹H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e–e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e–e distance or too long a T_1e can dramatically limit ? (¹H). Our study also shows that the molecular structure of AMUPol is not optimal and its ? (¹H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better ? (¹H) than AMUPol itself (by a factor of ca. 1.2).     

Effects of perfluorocarbon gases on the size and stability characteristics of phospholipid-coated microbubbles: osmotic effect versus interfacial film stabilization

Micrometer-sized bubbles coated with phospholipids are used as contrast agents for ultrasound imaging and have potential for oxygen, drug, and gene delivery and as therapeutic devices. An internal perfluorocarbon (FC) gas is generally used to stabilize them osmotically. We report here on the effects of three relatively heavy FCs, perfluorohexane (F-hexane), perfluorodiglyme (F-diglyme ), and perfluorotriglyme (F-triglyme), on the size and stability characteristics of microbubbles coated with a soft shell of dimyristoylphosphatidylcholine (DMPC) and on the surface tension and compressibility of DMPC monolayers. Monomodal populations of small bubbles (~1.3 ± 0.2 μm in radius, polydispersivity index ~8%) were prepared by sonication, followed by centrifugal fractionation. The mean microbubble size, size distribution, and stability were determined by acoustical attenuation measurements, static light scattering, and optical microscopy. The half-lives of F-hexane- and F-diglyme-stabilized bubbles (149 ± 8 and 134 ± 3 min, respectively) were about 2 times longer than with the heavier F-triglyme (76 ± 7 min) and 4-5 times longer than with air (34 ± 3 min). Remarkably, the bubbles are smaller than the minimal size values calculated assuming that the bubbles are stabilized osmotically by the insoluble FC gases. Particularly striking is that bubbles 2 orders of magnitude smaller than the calculated collapse radius can be prepared with F-triglyme, while its very low vapor pressure prohibits any osmotic effect. The interface between an aqueous DMPC dispersion and air, or air (or N(2)) saturated with the FCs, was investigated by tensiometry and by Langmuir monolayer compressions. Remarkably, after 3 h, the tensions at the interface between an aqueous DMPC dispersion (0.5 mmol L(-1)) and air were lowered from ~50 ± 1 to ~37 ± 1 mN m(-1) when F-hexane and F-diglyme were present and to ~40 ± 1 mN m(-1) for F-triglyme. Also noteworthy, the adsorption kinetics of DMPC at the interface, as obtained by dynamic tensiometry, were accelerated up to 3-fold when the FC gases were present. The compression isotherms show that all these FC gases significantly increase the surface pressure (from ~0 to ~10 mN m(-1)) at large molecular areas (70 ?(2)), implying their incorporation into the DMPC monolayer. All three FC gases increase the monolayer's collapse pressures significantly (~61 ± 2 mN m(-1)) as compared to air (~54 ± 2 mN m(-1)), providing for interfacial tensions as low as ~11 mN m(-1) (vs ~18 mN m(-1) in their absence). The FC gases increase the compressibility of the DMPC monolayer by 20-50%. These results establish that, besides their osmotic effect, FC gases contribute to bubble stabilization by decreasing the DMPC interfacial tension, hence reducing the Laplace pressure. This contribution, although significant, still does not suffice to explain the large discrepancy observed between calculated and experimental bubble half-lives. The case of F-triglyme, which has no osmotic effect, indicates that its effects on the DMPC shell (increased collapse pressure, decreased interfacial tension, and increased compressibility) contribute to bubble stabilization. F-hexane and F-diglyme provided both the smallest mean bubble sizes and the longest bubble half-lives.     

Triaryl‐1,3,5‐triazinane‐2,4,6‐triones (Isocyanurates) Peripherally Functionalized by Donor Groups: Synthesis and Study of Their Linear and Nonlinear Optical Properties

The linear optical (LO) and nonlinear optical (NLO) properties of a series of isocyanurates functionalized by donor arms at the periphery are reported herein. These octupolar derivatives were obtained in a straightforward way from commercial isocyanate derivatives and were fully characterized. Although several of these compounds are known, those that exhibited the largest NLO activities are all new compounds. In terms of second‐order activity, several of these derivatives exhibit remarkable activity/transparency tradeoffs. In terms of third‐order activity, the longer derivatives with the stronger donor groups (X=NH₂, NMe₂, or NPh₂) were shown to possess significant two‐photon absorption cross sections. These strongly luminescent derivatives exhibit two‐photon absorption cross sections up to 410 GM. DFT computations were also conducted to unravel their electronic structures and to rationalize their NLO properties. To our knowledge, the present study is the first concerned with the nonlinear optical properties of these original cyclotrimers.     

Multi-scale structuration of glasses: Observations of phase separation and nanoscale heterogeneities in glasses by Z-contrast scanning electron transmission microscopy

Chemical fluctuations are probed in glasses obtained with different thermal histories using scanning transmission electron microscopy in high angle annular dark field mode. Direct imaging of the glass structure is obtained at the sub-nanometer scale with Z-contrast. Macroscopic glass-in-glass separation is probed in a slowly quenched melt where chemical resolution allows the determination of the regions associated with Zr/Zn atoms. In a quickly cooled glass, exempt of macroscopic phase separation, a segregation of Zr/Zn atoms is still evidenced but on a different length scale, suggesting either the beginnings of glass separation or intrinsic features of the glass structure. Glass inhomogeneities must be taken into account to have a quantitative evaluation of nucleation processes. These “nano”-heterogeneities can be associated with the nucleation of zirconia phases, giving important clues to understand the nucleation pathways and the structural role of nucleating agents in aluminosilicate glasses.     

Sensitivity of multi-parametric MRI to the compressive state of the isolated intervertebral discs

Objective

Magnetic resonance imaging (MRI) offers great potential as a sensitive and noninvasive technique for describing the alterations in mechanical properties, as shown in vitro on intervertebral disc (IVD) or cartilage tissues. However, in vivo, the IVD is submitted to complex loading stimuli. Thus, the present question focuses on the influence of the mechanical loading during an MRI acquisition on the relaxation times, magnetization transfer and diffusion parameters within the IVD.

Methods

An apparatus allowing the compression of isolated IVDs was designed and manufactured in acrylonitrile butadiene styrene. IVDs were dissected from fresh young bovine tail, measured for their thickness and submitted to compression just before the MRI acquisition. Six discs received 0% (platen positioned at the initial disc thickness), 5% (platen positioned at 95% of the initial disc thickness), 10%, 20% and 40% deformation. The MRI parameters were compared between the loading states using mean and standard deviation for T1 and T2, and matrix subtraction for Magnetization Transfer, fractional anisotropy and apparent diffusion coefficient.

Results

The compression of the IVD did not lead to any significant change of the MRI parameters, except for the diffusion that decreased in the direction of the compressive stress.

Discussion

This experimental in vitro study shows that multi-parametric MRI on isolated discs in vitro is not sensitive to compression or to the partial confined relaxation that followed the compression.