Effects of organic solvents on the scission energy of rodlike micelles

The linear viscoelastic response of erucyl bis(hydroxyethyl) methylammonium chloride with added KCl has been studied as a function of temperature and nonpolar solvent addition. The plateau modulus is independent of temperature from 25 to 40 degrees C, in contrast to previous studies with salicylate counterions that showed a plateau modulus increasing with temperature over this range. The average micelle length, L, predicted by the model of Cates, depends experimentally on Escis/kBT, where Escis is the scission energy of the chain and kBT is the Boltzmann constant times the absolute temperature. With ethanol addition, the calculated average contour length, L, decreases by a factor of 4 as ethanol concentration varies from 0 to 1.3 M. This corresponds to an apparent energy for scission, Escis, decrease from 81 +/- 8 to 74 +/- 7 kJ/mol. On the other hand, only 80 mM of hexane is required to cause a decrease in Escis to the same level, and for hexane addition levels above 70 mM a disruption in the plateau modulus indicates the disruption of the rodlike structure. The correspondence between the effect of temperature and the effect of solvent addition allows the development of "solvent/temperature" superposition rules to predict the rheology of these viscoelastic fluids at elevated temperatures.     

Molecular mechanics studies of ketene derivatives and related structures

The MM2 and MM3 force fields have now been parameterized for ketene and its various derivatives. With the addition of the C_sp ? O bond stretching and C?C_sp?O bond bending parameters, calculations were performed on ketene and six substituted ketene compounds. The MM2 results are quite good with only minimal errors in the calculation of C? H bond lengths and H-C-H bond angles. Additionally, C? F bond parameters in MM2 have been re-adjusted to give better results in monofluorinated species, but, unfortunately, resulting in greater error in the polyfluorinated compounds. The results of geometry calculations by MM3 are similar to those obtained by MM2 with the exception of a significant improvement in the geometry of dimethylketene. The MM3 vibrational frequencies calculated in this study are also in good agreement with available experimental and ab initio results with the exception of a few low frequency in-and out-of-plane bending modes. © 1992 by John Wiley & Sons, Inc.     

Room temperature solution synthesis of alkyl-capped tetrahedral shaped silicon nanocrystals

We report the room temperature solution synthesis of alkyl protected silicon nanocrystals. The nanocrystals are of unusually uniform tetrahedral morphology and of a limited size distribution. The nanocrystals were characterized by transmission and scanning electron microscopy as well as atomic force microscopy.     

The synthesis of pyrimido[4,5-c] pyridazines and pyrido[2,3-d]pyrimidines related to toxoflavin and fervenulin

4-Deazatoxoflavin (1,6-dimethyl-1,5,6,7-tetrahydropyrimido[4,5-c]-5,7-pyridazinedione) ( 7 ) and several other 4-deazatoxoflavin and 4-deazafervenulin analogs have been prepared with the required intermediates. 3,5,7-Trimethyl-1,2,3,4-tetrahydropyrido[2,3-d]-2,4-pyrimidinedione ( 19 ) and 8-phenyl-3,5,7-trimethyl-2,3,4,8-tetrahydropyrido[2,3-d]-2,4-pyrimidinedione ( 20 ) were also prepared.     

13C-H coupling constants and maximum overlap orbitals in some three-membered heterocyclic compounds

The criterion of maximum total overlap, modified to permit inclusion of ionic structures, has been employed to determine the hybridized bonding orbitals for ethyleneimine, ethylene oxide, ethylene sulfide, and cyclopropane. ¹³C-H coupling parameters obtained from the NMR spectra of these compounds, which are linear functions of s-character in the appropriate carbon orbitals, served as experimental criteria for adjusting ionic content. The results appear reasonable as judged by calculated HCH angles and the order of heteroatom electronegativities.

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Zusammenfassung Das Kriterium maximaler Überlappung wurde in einer Modifikation, die den Einschluß ionischer Strukturen gestattet, zur Bestimmung der Hybridzustände von Äthylenimin, Äthylenoxyd, Äthylensulfid und Cyclopropan herangezogen. Die ¹³CH-Kopplungskonstanten der NMR-Spektren, die linear vom s-Charakter der entsprechenden Kohlenstoffzustände abhängen, dienten als experimentelles Kriterium für die Bestimmung der Anteils an ionischen Strukturen. Nach den berechneten HCH-Winkeln sowie der Reihenfolge der Elektronegativitäten der Heteroatome zu schließen, scheinen die Ergebnisse befriedigend zu sein.

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Résumé Le critère de recouvrement maximum total, modifié pour permettre l'inclusion de structures ioniques, a été utilisé pour déterminer les orbitales hybrides de l'éthylèneimine, l'oxide d'éthylène, le sulfure d'éthylène et le cyclopropane. Les paramètres de couplage ¹³C-H obtenus des spectres RNM de ces composés, qui sont des fonctions linéaires du caractère s des orbitaux de carbone correspondants, servaient de critère expérimental pour l'ajustage du contenu ionique. Le calcul des angles HCH et l'ordre des électronégativités des hétéroatomes conduisent à conclure que les résultants sont raisonnables.

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We wish to thank the National Institutes of Health for a Grant in partial support of this work. One of us (P. R. Certain) thanks the National Science Foundation for support through the Undergraduate Research Participants program.     

Positron annihilation lifetime evaluation of thermal cycling effects in atactic polystyrene

Positron annihilation lifetime spectroscopy (PALS) was used to investigate the thermal cycling characteristics of two different molecular weight atactic polystyrene resins. Annealed compression molded samples were thermally cycled over the temperature range 253 K to 393 K with PAL spectra taken in 10 K increments upon both cooling and heating. The longest lived component lifetime and intensity, indicative of orthopositronium pick-off, exhibit thermal dependencies that can be interpreted in a manner consistent with anticipated free volume changes associated with structural transitions.     

Multielectron redox chemistry of iron porphyrinogens

Iron octamethylporphyrinogens were prepared and structurally characterized in three different oxidation states in the absence of axial ligands and with sodium or tetrafluoroborate as the only counterions. Under these conditions, the iron- and ligand-based redox chemistry of iron porphyrinogens can be defined. The iron center is easily oxidized by a single electron (E(1/2) = -0.57 V vs NHE in CH(3)CN) when confined within the fully reduced macrocycle. The porphyrinogen ligand also undergoes oxidation but in a single four-electron step (E(p) = +0.77 V vs NHE in CH(3)CN); one of the ligand-based electrons is intercepted for the reduction of Fe(III) to Fe(II) to result in an overall three-electron oxidation process. The oxidation equivalents in the macrocycle are stored in C(alpha)-C(alpha) bonds of spirocyclopropane rings, formed between adjacent pyrroles. EPR, magnetic and Mossbauer measurements, and DFT computations of the redox states of the iron porphyrinogens reveal that the reduced ligand gives rise to iron in intermediate spin states, whereas the fully oxidized ligand possesses a weaker sigma-donor framework, giving rise to high-spin iron. Taken together, the results reported herein establish a metal-macrocycle cooperativity that engenders a multielectron chemistry for iron porphyrinogens that is unavailable to heme cofactors.     

Exo- und endo-Tricarbonyleisenkomplexe von bicyclischen 2,3-Dimethylidenverbindungen

Exo- and endo-Tricarbonyliron Complexes of Bicyclic 2,3-Dimethylidene Compounds. The preparation of exo- and endo-tricarbonyliron complexes (exo- and endo- 5 , -6 , -8 , and 9 ) of 2,3-dimethylidene-5-bicyclo[2.2.1]heptene( 1 ), -bicyclo[2.2.1]-heptane ( 2 ), -5-bicyclo[2.2.2]octene ( 3 ) and -bicyclo[2.2.2]octane ( 4 ) is described. The complexes are obtained by thermal reaction of the bicyclic butadienes with di-ironenneacarbonyl in hexane solution. exo- and endo- 5 are also formed photochemically from ironpentacarbonyl and 1 in pentane solution at ?35°. The structural assignment of exo- and endo -5 and -6 is based on their mass-spectra and on coordination shifts in ¹H- and ¹³C-NMR.-spectra exo- and endo -6 are correlated with exo- and endo -5 , respectively, by hydrogenation. Hydrogenation of the uncomplexed double bond in exo- and endo -5 occurs in both complexes from the exo side as shown by deuteration experiments. The free ligand 1 reacts in the same stereospecific manner.     

Model systems for flavoenzyme activity: flavin-functionalised SAMs as models for probing redox modulation through hydrogen bonding

We report the fabrication of flavin-functionalised self-assembled monolayers upon gold electrodes and their subsequent redox modulation via hydrogen bonding to 2,6-diethylamidopyridine.     

Directed evolution studies with combinatorial libraries of T4 lysozyme mutants

Summary Gene duplication with divergence to new functions has been an important mechanism in protein evolution. However, the questions of how many new functions can arise from a particular ancestral gene and how many mutational steps are typically required to generate new functions have been difficult to approach experimentally. We have addressed these questions using T4 lysozyme as a model system by synthesizing two combinatorial libraries of >10⁷ mutant T4 lysozyme genes: one library with an average of 14 missense mutations spread throughout the gene and one library in which 13 active site residues have been simultaneously randomized. These libraries were placed under selection inlacZ orpheA deficient strains ofE. coli to investigate whether they sample sufficient diversity to contain mutants with acquired -galactosidase or prephenate dehydratase activities. Although neither selection yielded T4 lysozyme mutants with these new activities, a novelE. coli locus was cloned that weakly complements these mutants, allowing them to form 1 mm colonies in 4–6 weeks. This growth rate corresponds to a turnover number of approximately 1000 or 25 min^– for thelacZ orpheA complementation systems, respectively, thus defining the limits of evolved enzymatic activity detectable in these selections. Thus, the strong selective pressure uncovered an unexpected solution to the biochemical blocks, a frequently observed phenomenon in selection experiments. The characterization of this locus will allow its elimination from futureE. coli complementation schemes.