Synthesis of discodermolide intermediates from engineered polyketides

Key intermediates used in Smith’s total synthesis of discodermolide were synthesized from an engineered polyketide made via precursor feeding to genetically modified polyketide producing bacteria.     

High yielding synthesis of 3a-hydroxypyrrolo[2,3-b]indoline dipeptide methyl esters: synthons for expedient introduction of the hydroxypyrroloindoline moiety into larger peptide-based natural products and for the creation of tryptathionine bridges

This work describes a rapid and high yielding oxidation of 14 tryptophanylated amino acid methyl esters to the corresponding 3a-hydroxypyrrolo[2,3-b]indoline (Hpi) amino acids with generally facile separation of syn-cis and anti-cis diastereomers. Structural X-ray diffraction data are presented for both diastereomers of Tr-Hpi-Gly-OMe, which allow for a putative assignment of the other 13 pairs of diastereomers reported herein, based on correlations with 1H NMR chemical shifts. Selective and high yielding deprotection at either the N or C terminus is described, allowing the Hpi motif to be introduced efficiently into potential targets with minimal protecting group manipulation. Two tripeptides containing Hpi and cysteine were prepared and treated with acid in the Savige-Fontana reaction to produce a cyclic tryptathionine linkage, characteristic of both amatoxins and phallotoxins.     

Rate constant and activation energy measurement for the reaction of atomic hydrogen with thiocyanate and azide in aqueous solution

Arrhenius parameters for the reaction of hydrogen atoms with azide and thiocyanate in aqueous solution have been determined using electron pulse radiolysis and electron paramagnetic resonance free induction decay attenuation measurements. Absolute values for SCN-, N3(-), and HN3 were well-described over the temperature range of 9-81 degrees C by the equations log k5 = (12.03 +/- 0.12) - [(21.05 +/- 0.66 kJ mol(-1))/2.303RT], log k10 = (12.75 +/- 0.21) - [(18.43 +/- 1.22 kJ mol(-1))/2.303RT], and log k15 = (11.59 +/- 0.12) - [(21.44 +/- 0.69 kJ mol(-1))/2.303RT], corresponding to room temperature (22 degrees C) rate constants of (2.07 +/- 0.03) x 10(8), (3.15 +/- 0.08) x 10(9), and (6.31 +/- 0.05) x 10(7) M(-1) s(-1) and activation energies for these chemicals of 21.05 +/- 0.66, 18.4 +/- 1.2, and 21.44 +/- 0.69 kJ mol(-1), respectively. The similarity of these three measured activation energies, taken together with the available information on reaction products, suggests a similar reaction mechanism, which is proposed to be an initial hydrogen atom adduct formation in these molecules, followed by single bond breakage.     

TRYPTOPHAN FLUORESCENCE LIFETIMES AS A FUNCTION OF EXCITATION WAVELENGTH*

Abstract— –Fluorescence decay times of aqueous dilute solutions (?20 µM) of L-tryptophan have been determined using the phase shift technique as well as single photon-counting coupled with synchrotron radiation (ACO at Orsay and SPEAR at Stanford). Decay times were obtained as a function of the excitation wavelength (in the spectral region 220–320 nm) monitoring emission of λ> 320 nm (in certain specified cases, λ> 360 nm). We have found that, at neutral pH and 20°C. fluorescence decays are single exponentials and independent of the excitation wavelength; under these conditions we find τ= 3.1 ± 0.1 ns.     

Pyrolysis of poly-α-acetoxystyrene,a polymer derived from polystyrene and poly(vinyl acetate)

The pyrolysis of poly-α-acetoxystyrene was investigated by using two methods thermogravimetry, and a reactor working under dynamic conditions coupled with a chromatograph.Thermogravimetry was used firstly with a constant heating rate and secondly under isothermal conditions. Two different degradation steps for the polymer were found, the first between 140 and 200°C and the second above 220°C. The kinetics of the overall pyrolysis were studied for the first thermal degradation step. The degradation order is zero when the weight loss is less than 10%, and unity when the weight loss is between 10 and 32%. The activation energy is about 40 kcal/mole for either order. The results obtained by using dynamic and isothermal thermogravimetry are in good agreement.Volatile products were identified and analysed by using a reactor working under dynamic conditions coupled with a chromatograph. The decomposition is similar to that of poly(vinyl acetate) and different to that of known substituted polystyrenes. Acetic acid was the main component of the volatile products (90–95%), the others being the monomer and acetophenone.     

Addition of p-azido-L-phenylalanine to the genetic code of Escherichia coli

We report the selection of a new orthogonal aminoacyl tRNA synthetase/tRNA pair for the in vivo incorporation of a photocrosslinker, p-azido-l-phenylalanine, into proteins in response to the amber codon, TAG. The amino acid is incorporated in good yield with high fidelity and can be used to crosslink interacting proteins.     

Rheology of binary colloidal structures assembled via specific biological cross-linking

The selectivity and range of energies offered by specific biological interactions serve as valuable tools for engineering the assembly of colloidal particles into novel materials. In this investigation, high affinity biological interactions between biotin-coated "A" particles (RA = 0.475 microm) and streptavidin-coated "B" particles (RB = 2.75 microm) drive the self-assembly of a series of binary colloidal structures, from colloidal micelles (a large B particle coated by smaller A particles) to elongated chain microstructures (alternating A and B particles), as the relative number of small (A) to large (B) particles (2 < or = NA/NB < or = 200) is decreased at a low total volume fraction (10(-4) < or = phiT < or = 10(-3)). At a significantly higher total volume fraction (phiT > or = 10(-1)) and a low number ratio (NA/NB = 2), the rheological behavior of volume-filling particle networks connected by streptavidin-biotin bonds is characterized. The apparent viscosity (eta) as a function of the shear rate gamma, measured for networks at phiT = 0.1 and 0.2, exhibits shear-rate-dependent flow behavior, and both the apparent viscosity and the extent of shear thinning increase upon an increase of a factor of 2 in the total volume fraction. Micrographs taken before and after shearing show a structural breakdown of the flocculated binary particle network into smaller flocs, and ultimately a fluidlike suspension, with increasing shear rate. Rheological measurements provide further proof that suspension microstructure is governed by specific biomolecular interactions, as control experiments in which the streptavidin molecules on particles were blocked displayed Newtonian flow behavior. This investigation represents the first attempt at measuring the rheology of colloidal suspensions where assembly is driven by biomolecular cross-linking.     

The rovibrational analysis of the high-resolution IR spectrum of CD2HF below 1200 cm: an interacting tetrad of vibrational levels

The spectrum of CD₂HF was measured by high-resolution interferometric Fourier-transform IR (FTIR) spectroscopy (apodised instrumental band with:0.004 cm⁻¹ fwhm) between 800 and 1200 cm⁻¹ covering the four lowest fundamentals. A complete rotational analysis using a semi-automatic assignment procedure yields accurate band centres (ν₉: 912.2028 cm⁻¹, ν₆:964.4994 cm⁻¹, ν₅: 1050.5104 cm⁻¹, ν₄: 1093.8632 cm⁻¹) and a complete set of first-order Coriolis coupling constants. The most important couplings occur between ν₉ and ν₆ (ξ_a= 1.069 cm⁻¹, ξ_c= −0.3535 cm⁻¹) and between ν₅ and ν₄ (ξ_b= −0.80606 cm⁻¹). The analysis was guided by and compared with results from our ab initio calculations for Coriolis constants and transition moments using CADPAC at TZP/MP2 level.     

The biosynthesis of quadrone and terrecyclic acid

Feedings of [1-¹³C]- and [1,2-¹³C₂]acetate Aspergillus terreus gave quadrone and terrecyclic acid which were analyzed by ¹³C NMR. The pattern of ¹³C-enrichments and couplings is consistent with the formation of 1 and 2 by cyclization of farnesyl pyrophosphate.