The Role of Aromaticity,Hybridization, Electrostatics,and Covalency in Resonance-Assisted Hydrogen Bonds of Adenine–Thymine (AT) Base Pairs and Their Mimics

Invited for this issues cover are Dr. Célia Fonseca Guerra from the VU University of Amsterdam and her collaborators at the University of Girona. The cover picture shows H-bonds in the adenine–thymine Watson–Crick base pair. An essential part of these H-bonds is their covalent component arising from donor–acceptor interactions between N or O lone pairs and the N−H antibonding σ* acceptor orbital. This charge-transfer interaction is represented by green figures walking on the pedestrian crossing, connecting the bases. This covalent component is the reason why H-bonds between DNA and/or unsaturated model bases are significantly stronger than those between analogous saturated bases. This contrasts sharply with the classical picture of predominantly electrostatic H-bonds which is not only incomplete in terms of a proper bonding mechanism, but also fails to explain the trend in stability. For more details, see the Full Paper on p. 318 ff.     

Not Carbon s–p Hybridization,but Coordination Number Determines C−H and C−C Bond Length

A fundamental and ubiquitous phenomenon in chemistry is the contraction of both C−H and C−C bonds as the carbon atoms involved vary, in s–p hybridization, along sp³ to sp² to sp. Our quantum chemical bonding analyses based on Kohn–Sham molecular orbital theory show that the generally accepted rationale behind this trend is incorrect. Inspection of the molecular orbitals and their corresponding orbital overlaps reveals that the above-mentioned shortening in C−H and C−C bonds is not determined by an increasing amount of s-character at the carbon atom in these bonds. Instead, we establish that this structural trend is caused by a diminishing steric (Pauli) repulsion between substituents around the pertinent carbon atom, as the coordination number decreases along sp³ to sp² to sp.     

Absorption and Distribution of Toltrazuril and Toltrazuril Sulfone in Plasma,Intestinal Tissues and Content of Piglets after Oral or Intramuscular Administration

Piglet coccidiosis due to Cystoisospora suis is a major cause of diarrhea and poor growth worldwide. It can effectively be controlled by application of toltrazuril (TZ), and oral formulations have been licensed for many years. Recently, the first parenteral formulation containing TZ in combination with iron (gleptoferron) was registered in the EU for the prevention of coccidiosis and iron deficiency anemia, conditions in suckling piglets requiring routine preventive measures. This study evaluated the absorption and distribution of TZ and its main metabolite, toltrazuril sulfone (TZ-SO₂), in blood and intestinal tissues after single oral (20 mg/kg) or single intramuscular (45 mg/piglet) application of TZ. Fifty-six piglets were randomly allocated to the two treatment groups. Animals were sacrificed 1-, 5-, 13-, and 24-days post-treatment and TZ and TZ-SO₂ levels were determined in blood, jejunal tissue, ileal tissue, and mixed jejunal and ileal content (IC) by high performance liquid chromatography (HPLC). Intramuscular application resulted in significantly higher and more sustained concentrations of both compounds in plasma, intestinal tissue, and IC. Higher concentrations after oral dosing were only observed one day after application of TZ in jejunum and IC. Toltrazuril was quickly metabolized to TZ-SO₂ with maximum concentrations on day 13 for both applications. Remarkably, TZ and TZ-SO₂ accumulated in the jejunum, the primary predilection site of C. suis, independently of the administration route, which is key to their antiparasitic effect.     

Crystalline orientation and molecular transport properties in nanoporous syndiotactic polystyrene films

The orientation of the crystalline δ nanoporous phase in syndiotactic polystyrene films, obtained by different procedures, have been characterized. For both solution cast and biaxially stretched films a high degree of uniplanar orientation, corresponding to the tendency of the ac crystallographic planes, to be parallel to the film plane has been observed and rationalized. According to molecular dynamics simulations of diffusion of small molecules into the δ nanoporous phase, this uniplanar orientation would minimize the molecular diffusivity through the nanoporous crystalline phase.     

1,2‐dichloroethane conformation and molecular organization in syndiotactic polystyrene gels

The conformational equilibrium of 1,2‐dichloroethane (DCE) in syndiotactic polystyrene (sPS)/DCE gels has been investigated by using Fourier transform infrared spectroscopy. Results show that the fraction of DCE in the trans conformation (X_T) increases with increasing polymer concentration. From these X_T values, by assuming that the crystalline phase in the gel presents the clathrate structure, the amount of DCE included in the crystalline phase and the fraction of polymer included in the crystalline phase were evaluated as a function of the gel composition.     

Molecular mechanics and mechanisms of regulation of the stereospecificity in Ziegler-Natta catalysis

The model catalytic sites, proposed in our group for the homogeneous and heterogeneous stereospecific Ziegler- Natta polymerizations of olefins, are reviewed. For all the homogeneous metallocene-based as well as for the heterogeneous catalytic models, a common mechanism of enantioselectivity is indicated by the study of the non-bonded interactions. This mechanism of enantioselectivity, which involves a chiral orientation of the growing chain, is in agreement with a large number of experimental results for these catalytic systems. The model sites for the homogeneous isospecific polymerization of propene are also able to account for the observed enantioselectivities in the regioirregular placements. Recent calculations relative to a peculiar catalytic model site, which should present a polymerization mechanism involving a regular back-skip of the chain, to the starting position after each monomer insertion, are shortly reviewed. The relevance of this polymerization mechanism to the comprehension of the stereospecificity of some homogeneous and heterogeneous catalytic systems is also briefly discussed.     

Influence of Electrical,Chemical and Morphological Properties of Inorganic/Organic Interface on Light Emitting Devices Performance

studies on the influence of chemical and physical treatments on the properties of indium–tin oxide (ITO) thin films are reported. The ITO films are utilized as transparent anodes of organic light-emitting diodes (OLEDs) incorporating poly(9,9-dihexyl-9H-fluorene-2,7-diyl) (PF6) as the hole transporter material and 8-hydroxyquinoline aluminum salt (Alq3) as emitter material. Chemical (HCl, piranha solutions), thermal (vacuum annealing), physical treatments (oxygen plasma, UV ozone) and combined treatments are studied. First, ITO layers with different treatments are characterized by using four point probe method, contact angle measurement, X-Ray diffraction (XRD), surface profilometer, scanning electron microscopy (SEM), UV-Vis-IR transmittance. Later, electrical and optical properties of OLEDs with treated ITO as anode are extensively investigated.     

Charge Transfer and Environment Effects Responsible for Characteristics of DNA Base Pairing

A hitherto unresolved discrepancy between theory and experiment is unraveled. Charge transfer and the influence of the environment in the crystal are vital for understanding the nature and for reproducing the structure of hydrogen bonds in DNA base pairs. The introduction of water molecules and a sodium counterion into the theoretical model (see picture) deforms the geometry of AT and GC in such a way that excellent agreement with the experimental structures is obtained.     

Stereoregular polyamides entirely based on tartaric acid

Two new stereoregular polyamides, namely PTA‐LL and PTA‐LD, derived from (2S,3S)‐(−)‐2,3‐dimethoxy‐1,4‐butanediamine and (2R,3R)‐(+)‐ or (2S,3S)‐(−)‐2,3‐dimethoxybutanedioic acid are described. The two diastereoisomeric polymers contain two pairs of stereocenters in the main chain of the repeating unit, one in the diamine and the other in the diacid counterpart. L and D refer to the configuration of the tartaric acid unit from which they proceed. These polyamides were prepared by polycondensation in solution and were fully characterized by elemental analysis, DSC, and IR/NMR spectroscopies. Number average molecular weights around 30,000 were estimated by GPC and viscosimetry. Both compounds are soluble in water and display large optical activity. CD and ¹H‐NMR measurements in chloroform solution suggested the presence of definite secondary structures in this solvent. Solid samples were found to crystallize upon annealing and the crystal structure of both polyamides was investigated by X‐ray diffraction. PTA‐LL crystallized in an orthorhombic lattice in the space group P22₁2₁ whereas PTA‐LD seemed to adopt a P1 triclinic structure. In both cases the polymer chain appears to be in a folded conformation more contracted than in the common γ‐form of conventional nylons. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 983–993, 1999