An augmented ribbon model of protein structure

In this paper, work on a new model,an augmented ribbon model, for describing a number of features of primary, secondary, and super-secondary protein structure in a qualitative but mathematically rigorous way is discussed. The structural features that can be treated by the model include connectivity, directionality, chirality, orientation, and proximity which, in many cases, are difficult to deal with using more traditional structural representations (e.g. wireframe, ball-and-stick, or space-filling CPK). In practice, the information encoded in the augmented ribbon model is represented by a labelled, directed graph (digraph) which provides an efficient means for storing and analyzing the information on computers. This opens the way to computer-based analyses of proteins and for the application of similarity methods that have been shown to be quite useful in treating small molecules.     

Mechanisms of antioxidant action: Transformations of 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl (HTMPO) in polypropylene during processing

When 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (HTMPO) is processed in polypropylene in a closed mixer, almost 50% is converted to other products during the first few minutes whilst the applied torque in the mixer is high. There is associated formation of unsaturation and this fact, in conjunction with the almost complete regeneration of nitroxyl within five minutes, suggests that the corresponding hydroxylamine (HTMPOH), which can be qualitatively identified, is the major transformation product. A study of the UV stability of PP films fabricated from polymer processed for varying times shows that UV stability is related to the quantity of the redox couple (HTMPO + HTMPOH) remaining in the polymer. This is considerably reduced by severe processing. The redox capable has almost no thermal antioxidant (oven aging) activity.     

Excited-state proton-coupled electron transfer within ion pairs

The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.

Electrostatic ion pairs provide a general method to study excited-state proton-coupled electron transfer. A PT_aET_b mechanism is identified for the ES-PCET oxidation of salicylate within photoexcited cationic ruthenium–salicylate ion pairs.     

Mikrobestimmung des Fluors in organischen Substanzen

Zusammenfassung Es wurde ein Mikroverfahren zur Bestimmung von organisch gebundenem Fluor ausgearbeitet. Die Substanz wird mit Kalium im Glasröhrchen aufgeschlossen. Die Röhrchen werden vorher durch Auskochen mit Salzsäure blindwertfrei gemacht. Die Titration des Fluoridions kann durch geeignete Maßnahmen direkt in der Aufschlußlösung erfolgen, so daß die zeitraubende Abdestillation des Fluorids erspart wird. Die Titration selbst wird in bekannter Weise mittels Thoriumnitrat durchgeführt. Als Indikatoren dienen Alizarinrot S, dem Wasserblau oder Chinablau zugesetzt ist. ZurpH-Einstellung dient ein Glycin-Perchlorsäure-Puffer.

---

Summary A method was developed for the determination of organically bound fluorine. The sample is decomposed by heating with potassium in a small glass tube. The tubes are treated ahead of time with hot hydrochloric acid to render them blank-free. The titration of the fluoride ion can be conducted directly in the decomposition solution if proper precautions are taken. The time-consuming distilling off of the fluoride is thus avoided. The titration itself is made in the usual manner by thorium nitrate. The indicator is alizarin red S, to which is-added water blue or China blue. A glycerol-perchloric acid buffer is used to-adjust the p_h.

---

Résumé Mise au point d'un procédé de microdosage du fluor dans les composés-organiques. La substance est attaquée par le potassium dans de petits tubes de verre. Ceux-ci sont préalablement traités dans l'acide chlorhydrique bouillant pour éviter toute correction d'essai à blanc. Le titrage de l'ion fluorure peut être exécuté directement dans la solution provenant de l'attaque sous réserve de respecter des conditions appropriées; on évite ainsi la longue séparation du fluorure par distillation. Le titrage proprement dit est effectué de la façon habituelle à l'aide de nitrate de thorium en présence d'un indicateur constitué par du rouge d'alizarine S auquel on a ajouté du bleu soluble (Wasserblau ou Chinablau). On opère àpH constant à l'aide du tampon glycocolle — acide perchlorique.

     

A new total synthesis of (±)-oestrone

A conceptually new total synthesis of oestrone, based on a novel cascade of radical cyclisations from the iodo aryl vinylcyclopropane 2, via 10, 15, 16 and 17, leading to the intermediate trans,anti,trans-oestradiol derivative 11 in one step, is described. Oxidation of 11, followed by demethylation of the resulting aryl methyl ether 18 then gives (±)-oestrone 1.     

Structural effects in electron transfer reactions: comparative interfacial electrochemical kinetics for cis- versus trans-dioxorhenium(V)(bi)pyridine oxidation

The comparative interfacial oxidation kinetics of the approximate structural isomers trans-(O)₂Re^V(py)⁺₄ and cis-(O)₂Re^V(bpy)(py)⁺₂ (py, pyridine; bpy, 2,2′-bipyridine) have been assessed in aqueous solution via conventional cyclic voltammetry at a highly ordered pyrolytic graphite (HOPG) electrode. HOPG was employed because of its known propensity to diminish interfacial electron transfer (ET) rates (by ca. three to four orders of magnitude) and because of a probable lack of importance of kinetic work terms (diffuse double-layer corrections). Measured rates for the trans complex exceed those for the cis by about a factor of 3. Expressed as an effective activation Gibbs energy difference ΔG^*, this corresponds to a cis-trans difference of ca. 3 kJ mol⁻¹. The actual vibrational barriers to ET have determined from a combination of published X-ray structural results (trans complex) and new resonance Raman results (cis complex). The values are 0.6 kJ mol ⁻¹ for the trans oxidation and 4.4 kJ mol⁻¹ for the cis oxidation (i.e. close to the barrier difference inferred from rate measurements). Further analysis shows that most of the barrier difference is associated with displacement of a (predominantly) Re-N(bpy) stretching mode found only in the cis system. Differences in metal-oxo displacements (cis > trans) are also implicated.     

Synthesis of an s-butyldibenz[a,h]acridine. Alkyl migration in the bernthsen reaction

The Bernthsen reaction between N-1-naphthyl-2-naphthylamine and 2-methylbutanoic acid and its anhydride at 200–230° for seven hours gives a low yield of 12- or 13-s-butyldibenz[a,h]acridine, instead of the expected 14-isomer. The parent molecule dibenz[a,h]acridine results from the same reaction conducted at 270° for thirteen hours. It is suggested that alkyl migration may have occurred in some other cases where the Bernthsen reaction was reported to yield 14-alkyldibenz[a,h]acridines.