Total synthesis of narbonolide and biotransformation to pikromycin

An improved total synthesis of narbonolide and its biotransformation to pikromycin is reported. This total synthesis utilized an intramolecular Nozaki-Hiyama-Kishi coupling that significantly improved macrocyclization yields (90-96%) and allowed for differentiation of the C3- and C5-oxidation states. A pikAI deletion mutant of Streptomyces venezuelae was used to biotransform synthetic narbonolide to pikromycin by glycosylation and oxidation in vivo. This integration of synthetic chemistry and engineered biotransformations holds great promise for the synthesis of novel macrolide analogues of biological interest.     

Pseudogaps in the three-band Hubbard model

Using the strong coupling diagram technique, the energy spectrum of the three-bandHubbard model is investigated. In these calculations, the series in powers of thecopper-oxygen hybridization for the irreducible part is approximated by two lowest-orderterms. For parameters of hole-doped cuprates the calculated energy spectrum consists oflower and upper Hubbard subbands of predominantly copper nature, oxygen bands with someadmixture of copper states and the Zhang-Rice states of mixed nature. The spectrumcontains two pseudogaps, the lower of which separates the Hubbard subband from Zhang-Riceand oxygen bands. The pseudogaps arise due to multiple reabsorption of carriers in stateswith double occupancy of sites by holes or electrons.     

Raman study of NH4CI at high pressures

Abstract

The Raman spectrum of ammonium chloride has been carefully studied in the vicinity of its Λ-transition. Spectra were recorded as a function of temperature under various fixed pressures close to 1.6 kbar, which is the pressure required to make the Λ-transition become second order. The critical exponents derivable from the spectroscopic data seem to be as reliable as those from other types of data, even in the multicritical region.     

Raman and resonance-Raman spectra of CsI/I−3

Raman and resonance-Raman spectra of the I^?₃ ion isolated within CsI crystals have been studied using 647 nm and 488 nm exciting radiation. Sample temperatures between 300 and 20 K have been used. Eleven overtones of the symmetric stretching mode (nν₁) have been observed in the resonance-Raman spectrum excited by the 488 nm Ar⁺ laser line. Bands centred at 153, 170, 264 and 304 cm^?1 have been assigned as ν₃, 2ν₂, ν₁+ν₃ and 2ν₃(Σ⁺) respectively. The remaining structure between the nν₁ lines has been assigned as due to combinations of these lines with the lattice vibrations of the CsI crystal.     

Continuous and discontinuous phase transitions in ammonium halides

Raman spectra of NH₄Cl and NH₄Br have been recorded as functions of temperature and pressure. The λ-type phase transition in NH₄Cl has been studied as (i) a weakly first order. (ii) a tricritical and (iii) a second order transition. A strongly first order transition has been studied in NH₄Br. The analysis of the data has concentrated on the correlation of frequency shift with volume change across the phase change regions. This correlation has been established for the frequencies of the ν₂ and ν₅ Raman modes of NH₄Cl at zero pressure (1st order) 1.6 kbar (tricritical) and 2.8 kbar (2nd order), and the frequencies of the ν₅ Raman mode of NH₄Br at zero pressure (1st order). A single Y (mode Grünelsen parameter) has been shown to describe each frequency shift right through the phase change region once an order-disorder contribution has been introduced at and below the transition temperatures.     

Molecular Dynamics Simulations Guide Chimeragenesis and Engineered Control of Chemoselectivity in Diketopiperazine Dimerases

In the biosynthesis of the tryptophan-linked dimeric diketopiperazines (DKPs), cytochromes P450 selectively couple DKP monomers to generate a variety of intricate and isomeric frameworks. To determine the molecular basis for selectivity of these biocatalysts we obtained a high-resolution crystal structure of selective Csp²−N bond forming dimerase, AspB. Overlay of the AspB structure onto C−C and C−N bond forming homolog NzeB revealed no significant structural variance to explain their divergent chemoselectivities. Molecular dynamics (MD) simulations identified a region of NzeB with increased conformational flexibility relative to AspB, and interchange of this region along with a single active site mutation led to a variant that catalyzes exclusive C−N bond formation. MD simulations also suggest that intermolecular C−C or C−N bond formation results from a change in mechanism, supported experimentally through use of a substrate mimic.     

The acute toxicity of tri-n-butyltin glycocholate

The oral acute toxicity for tri-n-butyltin glycocholate (TBT-GA), a newly synthesized organotin steroid, was determined using Long Evans rats. The compound was suspended in corn oil and adminstered by gavage using standard techniques. Unlike tri-n-butyltin taurocholate, which exhibited two different toxicities, one for the tri-n-butyltin moiety and one for the taurocholic acid moiety, the TBT-GA exhibited a single toxicity, that of the whole molecule. The LD₅₀ value was 213 mg kg^?1 (0.274 mmol kg), which on a millimolar basis is similar to that observed for other tri-n-butyltin compounds. The dead rats exhibited distended stomachs, enlarged ceca, and lesions in the intestinal tract. The actual cause of death could not be positively identified. Animals that survived more than three days also exhibited time- and dose-related atrophy of the thymus gland. With 36% more male than female rats succumbing to TBT-GA, the chemical appears to be more toxic towards male than female rats.     

Nonuniqueness in inverse source and scattering problems

The fields radiated by spherically symmetric time-harmonic sources are used to illustrate how little can be learned about a source from knowledge of the radiated field outside of the source volume. It is shown that even if it is known that the source is spherically symmetric, it is not possible to determine its radial structure. Moreover, even if the radial structure of the source is known apart from a constant amplitude and a finite radius, it is not possible to evaluate those two unknowns independently. These examples are applied to demonstrate explicitly that two methods that have been claimed to produce exact unique solutions to inverse-source and inverse-scattering problems do not yield the claimed results.     

Studies in the tetraarylborates : Part VII. The preparation and reagent properties of some new nitrogen heterocyclic tetraarylboraees — especially sodium tetrakis(l-imidazolyl) borate,a novel gravimetric reagent for hydrogen ion

Three new tetraarylborates containing nitrogen heterocycles have been synthesized. Sodium tetrakis(1-imidazolyl)borate and sodium tetrakis(1-benzimidazolyl)borate were prepared in very high yield by syntheses which were easy to perform in situ. Interesting analytical properties which are very different from the carbocyclic tetraarylborates were noted in both compounds. Precipitate formation oftetrakis(1-imidazolyl)borate with hydrogen and divalent cations was observed. The feasibility of determination of the hydrogen cation by tetrakis(1-imidazolyl)borate was demonstrated.