Time-temperature superposition and dynamic mechanical behavior of atactic polystyrene

The dynamic mechanical behavior of monodisperse atactic polystyrene (mol. wt. 98,000) has been measured in the frequency range, 10⁻⁵ to 10 Hz and temperature range 359–374K. The time-temperature superposition of the entire data in the frequency range of overlap seems less satisfactory in both the real and imaginary components of the complex shear modulus, G′ and G″, respectively. The lack of adequate superposition becomes pronounced in the tan ϕ (G″/G′) plots. The tan ϕ plots provide a more discerning criteria for the superposition than the G′ or G″ spectra. An analysis based on an earlier model for anelastic deformation shows that of the several changes that may occur in the dynamic mechanical behavior on heating of polystyrene, the predominant ones are both an increase in the size of the microshear domains and the correlations of movement of segments near entanglements. These decrease the contribution to the modulus on heating near T_g so that the time-temperature superposition is vitiated.     

The energy calibration of LEP in the 1993 scan

This report summarizes the procedure for providing the absolute energy calibration of the LEP beams during the energy scan in 1993. The average beam energy around the LEP ring was measured in 25 calibrations with the resonant depolarization technique. The time variation of this average beam energy is well described by a model of the accelerator based on monitored quantities. The absolute calibration of the centre of mass energies of the off-peak points is determined with a precision of 2 parts in 10⁵ resulting in a systematic error on the Z-mass of about 1.4 MeV and on the Z-width of about 1.5 MeV.     

Controlling photochemistry with distinct hydrophobic nanoenvironments

A combination of hydrophobic forces and guest templation drive the assembly of cavitands into molecular capsules. Encapsulated guests such as dibenzyl ketones reside in an essentially dry environment, and upon irradiation, undergo rearrangement processes that are templated by the shape of the 1 nm x 2 nm cavity.     

Nontoxic block copolymer nanospheres: design and characterization

Biodegradable polymers capable of self-assembly into hollow nanospheres of less than 100 nm have significant potential for biotechnology applications such as drug delivery and gene therapy. Here we describe the synthesis of a novel ABA-type triblock copolymer made from a hydrophobic tyrosine-derived core and two hydrophilic poly(ethylene glycol) end groups (poly(ethylene glycol)-block-oligo(desaminotyrosyltyrosine octyl ester suberate)-block-poly(ethylene glycol)). We describe the self-assembly of this triblock copolymer and characterize its particles as 100 nm size vesicular nanospheres. The vesicular nature of these particles was determined by light scattering and electron microscopy. The nanospheres did not exhibit any short-term cytotoxicity toward UMR-106 cells at a concentration up to 2 mg/mL.     

Resorcinarenes as templates: a general strategy for the synthesis of large macrocycles

The concept that resorcinarenes can be used as templates for the synthesis of large macrocycles is introduced. By way of example, previously inaccessible, aromatic crown ethers compounds are synthesized.     

A traceless linker for aliphatic amines that rapidly and quantitatively fragments after reduction

Reduction sensitive linkers (RSLs) have the potential to transform the field of drug delivery due to their ease of use and selective cleavage in intracellular environments. However, despite their compelling attributes, developing reduction sensitive self-immolative linkers for aliphatic amines has been challenging due to their poor leaving group ability and high pK_a values. Here a traceless self-immolative linker composed of a dithiol-ethyl carbonate connected to a benzyl carbamate (DEC) is presented, which can modify aliphatic amines and release them rapidly and quantitatively after disulfide reduction. DEC was able to reversibly modify the lysine residues on CRISPR–Cas9 with either PEG, the cell penetrating peptide Arg₁₀, or donor DNA, and generated Cas9 conjugates with significantly improved biological properties. In particular, Cas9–DEC–PEG was able to diffuse through brain tissue significantly better than unmodified Cas9, making it a more suitable candidate for genome editing in animals. Furthermore, conjugation of Arg₁₀ to Cas9 with DEC was able to generate a self-delivering Cas9 RNP that could edit cells without transfection reagents. Finally, conjugation of donor DNA to Cas9 with DEC increased the homology directed DNA repair (HDR) rate of the Cas9 RNP by 50% in HEK 293T cell line. We anticipate that DEC will have numerous applications in biotechnology, given the ubiquitous presence of aliphatic amines on small molecule and protein therapeutics.

Reduction sensitive linkers (RSLs) have the potential to transform the field of drug delivery due to their ease of use and selective cleavage in intracellular environments.     

Donor‐Substituted Perethynylated Dehydroannulenes and Radiaannulenes: Acetylenic Carbon Sheets Featuring Intense Intramolecular Charge Transfer

In this article, we report the preparation of unprecedented π‐conjugated macrocycles (Fig. 1) by acetylenic scaffolding using modular tetraethynylethene (TEE, 3,4‐diethynylhex‐3‐ene‐1,5‐diyne) building blocks. A novel photochemical access to (Z)‐bisdeprotected TEEs (Scheme 1) enabled the synthesis of the anilino‐substituted perethynylated octadehydro[12]‐ ( 5 ) and dodecadehydro[18]annulenes ( 6 ) (Scheme 2). Following the serendipitous discovery of perethynylated radiaannulenes (Scheme 3) that can be viewed as hybrids between perethynylated dehydroannulenes and expanded radialenes, two series of monocyclic ( 7 – 9 ; Scheme 6) and bicyclic ( 10 and 11 ; Scheme 7) representatives were prepared. Substantial strain in the macrocyclic perimeter of radiaannulene 7 was revealed by X‐ray crystal‐structure analysis (Fig. 2). Nevertheless, mono‐ and bicyclic radiaannulenes are stable at room temperature in air for months. The opto‐electronic properties of both dehydroannulenes and radiaannulenes are substantially enhanced by the introduction of the peripheral anilino donor groups that undergo strong intramolecular charge‐transfer interactions with the electron‐accepting all‐C cores. As a result, the UV/VIS spectra feature intense, bathochromically shifted charge‐transfer bands that disappear upon protonation of the anilino moieties and are fully recovered upon neutralization (Figs. 4–9). A comparison between anilino‐substituted perethynylated dehydroannulenes, expanded radialenes, and radiaannulenes revealed that the efficiency of the intramolecular charge‐transfer interaction strongly depends on the structure of the electron‐accepting all‐C perimeter. Electrochemical investigations (Table) demonstrated that the radiaannulenes are particularly powerful electron acceptors. Thus, bicyclic radiaannulene 11 , which possesses eight peripheral 3,5‐di(tert‐butyl)phenyl substituents, is reversibly reduced at ?0.83 V in THF (vs. Fc⁺/Fc), making it a better electron acceptor than buckminsterfullerene C₆₀ under comparable conditions.     

Synthesis of a linear benzo[3]phenylene-[60]fullerene dyad

The synthesis of a new linear benzo[3]phenylene-[60]fullerene dyad 1 is achieved over 10 steps in 15% overall yield by using an efficient sequence combining a double cobalt(I)-mediated cyclotrimerization with a Bingel reaction.     

Fragmentation study of peptide acetals and aldehydes using in-source collision-induced dissociation

The fragmentation of peptide acetals and peptide diols, corresponding to the hydrated form of the peptide aldehyde, is dominated by the successive losses of two molecules of MeOH and water, respectively. Using model peptides, the fragmentation mechanism, with respect to the loss of methanol and water, was elucidated. The first loss was certainly charge-directed whereas the second probably occurred via the nucleophilic attack of the nitrogen of an amine on the C-terminal carbon leading to a cyclic ion.