Crystal and molecular structure of reductiomycin

Conflicting results concerning the structure of the antitumor agent reductiomycin have been resolved. When the imino group and the heterocyclic oxygen atom of a previous x-ray study are interchanged, and this structure refined, it is found that: (i) a structure with satisfactory hydrogen bonding results, (ii) there are no unacceptably small nonbonded intermolecular distances, and (iii) agreement with two recent structure formulations based on various kinds of chemical and spectral evidence is achieved.Camille and Henry Dreyfus Teacher Scholar, 1978–1983; National Institutes of Health (National Cancer Institute) Career Development Awardee, 1980–1985.     

On the bernstein conjecture in approximation theory

WithE _2n (|x|) denoting the error of best uniform approximation to |x| by polynomials of degree at most 2n on the interval [?1, +1], the famous Russian mathematician S. Bernstein in 1914 established the existence of a positive constantβ for which lim 2nE _2n (|x|)=β.n→∞ Moreover, by means of numerical calculations, Bernstein determined, in the same paper, the following upper and lower bounds forβ: 0.278<β<0.286. Now, the average of these bounds is 0.282, which, as Bernstein noted as a “curious coincidence,” is very close to 1/(2√π)=0.2820947917... This observation has over the years become known as the Bernstein Conjecture: Isβ=1/(2√π)? We show here that the Bernstein conjecture isfalse. In addition, we determine rigorous upper and lower bounds forβ, and by means of the Richardson extrapolation procedure, estimateβ to approximately 50 decimal places.     

The spectrophotometric determination of uranium by the 1-pyrrolidinecarbodithioate method

Summary A spectrophotometric method for the determination of uranyl ion is proposed which is based on the use of ammonium 1-pyrrolidinecarbodithioate as the chromogenic reagent. Conformity to Beer's law was observed for the range between 0 to 50 ppm of uranium when spectrophotometric measurements are made at 385 nm.

---

Zusammenfassung Ein spektrophotometrisches Verfahren zur Uranylbestimmung mit Ammonium-1-pyrrolidindithiocarbamat als Farbreagens wurde vorgeschlagen. Zwischen 0 und 50 ppm ist das Beersche Gesetz bei 385 nm erfüllt.

     

Dipole sums and intermolecular interaction coefficients derived from refractive index data

The old problem of using refractive index measurements to obtain estimates of dipole sums and interaction coefficients is reconsidered. It is found that a consistent numerical procedure for fitting the measured values to a two-term representation of the frequency dependent polarizability enables satisfactory values to be obtained.     

Synthesis and properties of butyl rubber–poly(ethylene oxide) graft copolymers with high PEO content

Butyl rubber‐poly(ethylene oxide) (PEO) graft copolymers with high PEO content (40–83 wt %) were synthesized by the functionalization and activation of the double bond moiety of butyl rubber containing high (7 mol %) isoprene content and subsequent reaction with PEO of different molecular weights from 750 to 5000 g/mol. The properties of these copolymers, along with other butyl rubber‐PEO graft copolymers were studied in films and in aqueous solution. Despite the high PEO content, films of the copolymers were quite stable in water with respect to mass loss and were capable of releasing an encapsulated probe molecule in a manner that was dependent on the PEO content. At high PEO content they were resistant to the adhesion and growth of C2C12 cells. Despite the resistance of films to dissolution, it was possible to prepare nanosized aqueous assemblies via a THF‐water exchange process and the sizes of the assemblies were tuned by their method of preparation. The assemblies were also able to encapsulate a probe molecule and were found to be nontoxic in vitro. Combined, this set of properties makes these new amphiphilic copolymers promising for a wide range of potential applications. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3383–3394     

A general, convergent strategy for the construction of indolizidine alkaloids: total syntheses of (-)-indolizidine 223AB and alkaloid (-)-205B

N-Toluenesulfonyl aziridines comprise effective second electrophiles in the solvent controlled three-component linchpin union of silyl dithianes for the stereocontrolled convergent elaboration of protected 1,5-amino alcohols. This tactic, in conjunction with a one-flask sequential cyclization, constitutes an effective general strategy for the construction of indolizidine and related alkaloids, illustrated here with the total syntheses of (-)-indolizidine 223AB (1) and alkaloid (-)-205B (2).     

Catechol: A minimal scaffold for non-peptide peptidomimetics of the i + 1 and i + 2 positions of the beta-turn of somatostatin

The design, synthesis, and evaluation of a series of catechol-based non-peptide peptidomimetics of the peptide hormone somatostatin have been achieved. These ligands comprise the simplest known non-peptide mimetics of the i + 1 and i + 2 positions of the somatostatin beta-turn. Incorporation of an additional side chain to include the i position of the beta-turn induces a selective 9-fold affinity enhancement at the sst2 receptor.     

Acyl radical addition to pyridine: multiorbital interactions

The addition of the acetyl radical at the various positions in both pyridine and the pyridinium ion has been investigated using DFT calculations. Additions at the 2-, 3- and 4-positions in these systems are associated with simultaneous SOMO→π* and π→SOMO interactions, with the former interaction dominating in the case of pyridine, and that latter in the case of pyridinium. Simultaneous SOMO→π*, LP_N→SOMO and LP_N→π*_co interactions are predicted for the addition at the nitrogen atom in pyridine. The energy barrier for attack at the nitrogen atom in pyridine is calculated to be 54 kJ mol⁻¹ at the BHandHLYP/6-311G(d,p) level of theory, some 6 kJ mol⁻¹ lower than for the analogous attack at any other atom in pyridine, or at any position in the pyridinium ion. Multiorbital interactions are responsible this preference, resulting in an unusual motion vector in the transition state for attack at the nitrogen atom in pyridine.     

Maximum entropy approach for deducing amino Acid interactions in proteins

We present a maximum entropy approach for inferring amino acid interactions in proteins subject to constraints pertaining to the mean numbers of various types of equilibrium contacts for a given sequence or a set of sequences. We have carried out several kinds of tests for a two-dimensional lattice model with just two types of amino acids with very promising results. We also show that the method works very well even when the mean numbers of contacts are not known and therefore can be applied to real proteins.