Multi-meson exchange in non-Abelian gauge theories

We find a new sum rule for the residue of the J = 0 fixed pole in Compton scattering. The sum rule applies to any target and links the residue to a calculable set of contributions from the resonance region. We discuss proton, neutron, deutron and pion targets and the possibility of discriminating from previous models. The relationship with the parton model is noted and the feasibility of measuring pion photoabsorption cross sections is discussed.     

Structural Insights into Pixatimod (PG545) Inhibition of Heparanase,a Key Enzyme in Cancer and Viral Infections

Pixatimod (PG545), a heparan sulfate (HS) mimetic and anticancer agent currently in clinical trials, is a potent inhibitor of heparanase. Heparanase is an endo-β-glucuronidase that degrades HS in the extracellular matrix and basement membranes and is implicated in numerous pathological processes such as cancer and viral infections, including SARS−CoV-2. To understand how PG545 interacts with heparanase, we firstly carried out a conformational analysis through a combination of NMR experiments and molecular modelling which showed that the reducing end β-D-glucose residue of PG545 adopts a distorted conformation. This was followed by docking and molecular dynamics simulations to study the interactions of PG545 with heparanase, revealing that PG545 is able to block the active site by binding in different conformations, with the cholestanol side-chain making important hydrophobic interactions. While PG545 blocks its natural substrate HS from binding to the active site, small synthetic heparanase substrates are only partially excluded, and thus pentasaccharide or larger substrates are preferred for assaying this class of inhibitor. This study provides new insights for the design of next-generation heparanase inhibitors and substrates.     

Novel polyurethanes from xylan and TDI: Preparation and characterization

Xylan is a hemicellulose, which is found abundantly in nature. In this work, a novel polyurethane was developed involving xylan and tolylene-2,4-diisocyanate (TDI). Polymer synthesis was achieved through conventional heat or microwave-assisted reaction in dimethyl sulfoxide. Because xylan has multiple OH groups on each polymer chain, the TDI/xylan molar ratio had to be adjusted to produce a soluble polymeric product. The reaction products were characterized by ¹³C NMR, FTIR, thermogravimetric analysis, and differential scanning calorimetry. The xylan polyurethane was shown to exhibit improved thermal stability over xylan.     

Self-Assembled Functionalized Graphene Nanoribbons from Carbon Nanotubes

Graphene nanoribbons (GNR) were generated in ethanol solution by unzipping pyrrolidine-functionalized carbon nanotubes under mild conditions. Evaporation of the solvent resulted in regular few-layer stacks of graphene nanoribbons observed by transmission electron microscopy (TEM) and X-ray diffraction. The experimental interlayer distance (0.49–0.56 nm) was confirmed by computer modelling (0.51 nm). Computer modelling showed that the large interlayer spacing (compared with graphite) is due to the presence of the functional groups and depends on their concentration. Stacked nanoribbons were observed to redissolve upon solvent addition. This preparation method could allow the fine-tuning of the interlayer distances by controlling the number and/or the nature of the chemical groups in between the graphene layers.     

Modeling cell dynamics under mobile phone radiation

Perturbations by pulse-modulated microwave radiation from GSM mobile phones on neuron cell membrane gating and calcium oscillations have been suggested as a possible mechanism underlying activation of brain states and electroencephalographic epiphenomena. As the employ of UMTS phones seems to reveal other symptoms, a unified phenomenological framework is needed. In order to explain possible effects of mobile phone radiation on cell oscillations, GSM and UMTS low-frequency envelopes have been detected, recorded and used as input in cell models. Dynamical systems endowed with contiguous regular and chaotic regimes suitable to produce stochastic resonance can both account for the perturbation of the neuro-electrical activity and even for the low intensity of the signal perceived by high sensitive subjects. Neuron models of this kind can be employed as a reductionist hint for the mentioned phenomenology. The Hindmarsh-Rose model exhibits frequency enhancement and regularization phenomena induced by weak GSM and UMTS. More realistic simulations of cell membrane gating and calcium oscillations have been performed with the help of an adaptation of the Chay-Keizer dynamical system. This scheme can explain the suspected subjective sensitivity to mobile phone signals under the thermal threshold, in terms of cell calcium regularity mechanisms. Concerning the two kinds of emission, the stronger occupation of the ELF band of last generation UMTS phones is compensated by lower power emitted.     

The peculiar conformational characteristics of cis-poly(tert-butlacetylene)

Molecular mechanics was applied to investigate the conformational structure of cis-poly(tert-butylacetylene) (c-PTBA). The program CHAMP, adopting a MM2-based force field, was used for a thorough search of the minimum-energy conformers of oligomers, helices and long segments of the c-PTBA chain. The results show that short oligomers are not good models of the polymer, as their preferred conformations are not allowed inside the chain. Segments of 8₃ and 8₅ helices appear as the most probable feature. Junctions between right- and left-handed helices can occur at the cost of 1,7 kcal/mol: such defects may run through the chain, the barrier to their shift being 7,8 kcal/mol. Other conformational defects, having energies in the range of 1–3 kcal/mol above the minimum, form knuckle-joints in the helix, changing the axial direction by 70–80°. The present calculations suggest a disordered model of c-PTBA in solution, made of rather stiff helical segments. Such a picture corresponds to a chain less rigid than proposed by previous computations on substituted polyacetylenes, and seems consistent with the observed physical aging of c-PTBA films. Preliminary packing calculations of right- and left-handed helices lead to density values not much higher than observed, indicating rather low degrees of disorder and free volume in the solid state.     

Recent results on the polymorphic behavior of isotactic polypropylene

The more recent results concerning the polymorphic behavior of isotactic polypropylene (iPP) are presented. Specifically contributions on the recently determined γ phase crystal structure, in which non-parallel chains coexist, and the novel results concerning the β phase are summarized. The new data appear to suggest that the normally occurring α-phase is in general neither the kinetically favored nor the thermodynamically more stable phase but rather it represents the best compromise between these two criteria. The implications and the open questions arising from the new findings with respect to morphology and polymer crystal growth are considered.     

The σ‐donating and π‐accepting properties of ortho‐Si(CH3)3 phosphinine macrocycles

A theoretical investigation of both the ortho‐Si(CH₃)₃ phosphinine and some silacalix[n]phosphinines was performed. The optimized geometries agree well with those reported from X‐ray analysis and other structural studies. The silacalix[n]phosphinine macrocyle is very flexible because of the C Si C bridges. This, in turn, makes possible the formation of strained configurations in solid packed structures. In the silacalix[3]phosphinine, a P P bonding interaction that is presumably responsible for its structural and electronic features seems to exist. The molecular orbital calculations corroborate that both the π‐accepting properties and the σ‐donating capacities of the phosphinine unit may be enhanced by ortho‐Si(CH₃)₃ substitution. These features satisfy the proposal of the synthesizers as regards the production of macrocyclic phosphorus compounds, with good π‐accepting properties and strong σ‐donating capacities, which are sufficiently flexible as to encapsulate metals with coordination spheres of different geometries. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:160–169, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10118