Publications

We present mid-infrared (MIR) spectroscopy of a Type II-plateau supernova, SN\n2004dj, obtained with the Spitzer Space Telescope, spanning 106--1393 d after\nexplosion. MIR photometry plus optical/near-IR observations are also reported.\nAn early-time MIR excess is attributed to emission from non-silicate dust\nformed within a cool dense shell (CDS). Most of the CDS dust condensed between\n50 d and 165 d, reaching a mass of 0.3 x 10^{-5} Msun. Throughout the\nobservations much of the longer wavelength (>10 microns) part of the continuum\nis explained as an IR echo from interstellar dust. The MIR excess strengthened\nat later times. We show that this was due to thermal emission from warm,\nnon-silicate dust formed in the ejecta. Using optical/near-IR line-profiles and\nthe MIR continua, we show that the dust was distributed as a disk whose radius\nappeared to be slowly shrinking. The disk radius may correspond to a grain\ndestruction zone caused by a reverse shock which also heated the dust. The\ndust-disk lay nearly face-on, had high opacities in the optical/near-IR\nregions, but remained optically thin in the MIR over much of the period\nstudied. Assuming a uniform dust density, the ejecta dust mass by 996 d was 0.5\n+/- 0.1) x 10^{-4} Msun, and exceeded 10^{-4}Msun by 1393 d. For a dust density\nrising toward the center the limit is higher. Nevertheless, this study suggests\nthat the amount of freshly-synthesized dust in the SN 2004dj ejecta is\nconsistent with that found from previous studies, and adds further weight to\nthe claim that such events could not have been major contributors to the cosmic\ndust budget.\n

Recent spectroscopic evidence implicating a binuclear iron site at the reaction center of fatty acyl desaturases suggested to us that certain fatty acyl hydroxylases may share significant amino acid sequence similarity with desaturases. To test this theory, we prepared a cDNA library from developing endosperm of the castor-oil plant (Ricinus communis L.) and obtained partial nucleotide sequences for 468 anonymous clones that were not expressed at high levels in leaves, a tissue deficient in 12-hydroxyoleic acid. This resulted in the identification of several cDNA clones encoding a polypeptide of 387 amino acids with a predicted molecular weight of 44,407 and with approximately 67% sequence homology to microsomal oleate desaturase from Arabidopsis. Expression of a full-length clone under control of the cauliflower mosaic virus 35S promoter in transgenic tobacco resulted in the accumulation of low levels of 12-hydroxyoleic acid in seeds, indicating that the clone encodes the castor oleate hydroxylase. These results suggest that fatty acyl desaturases and hydroxylases share similar reaction mechanisms and provide an example of enzyme evolution.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The reaction of one equiv of 1-alkynylphosphines, R2PC≡CR′ (R = Et, iPr, or Ph and R′ = Ph or Mes), with Cp2Zr(pyr)(η2-Me3SiC≡CSiMe3) resulted in formation of monoalkyne complexes. In the case where R = Et, iPr, or Ph and R′ = Ph, a "ligand free" zirconacyclopropene complex is produced. These complexes are stabilized by intermolecular donation of the phosphorus lone-pair in the dimeric complexes [Cp2Zr(η2-R2PC≡CPh)]2 (R = Et, iPr, or Ph). However, with R = Ph and R′ = Mes, the zirconocyclopropene-pyridine complex Cp2Zr(pyr)(η2-Ph2PC≡CMes) is formed. Homocoupling of the 1-alkynylphosphines was demonstrated by reaction of a second equiv of Ph2PC≡CPh with [Cp2Zr(η2-Ph2PC≡CPh)]2 to give the diphosphinozirconacyclopentadiene Cp2Zr[2,5-(Ph2P)2-3,4-Ph2C4] with high regioselectivity (77%). The zirconacyclopropene complexes also react with one equiv of PhC≡CPh or EtC≡CEt to give zirconacyclopentadienes in which the phosphino substituent preferentially adopts the 2-position (α) of the zirconacyclopentadiene ring. These unsymmetrical zirconacyclopentadienes undergo substitution of the R2PCCR′ moiety with the less bulky alkynes PhC≡CPh or EtC≡CEt. The substituents on the 1-alkynylphosphines significantly influence the rates of alkyne substitution such that sterically more demanding substituents in either the α- (-PiPr2) or β- (-Mes) position of the zirconacycle lead to faster exchange. The α-phosphinozirconacyclopentadienes were readily converted to 1-phosphinobutadienes via reaction with benzoic acid. The zirconacyclopentadiene Cp2Zr[2-Ph2P-3,4,5-Ph3C4] was converted to the corresponding thiophene oxide by the oxo-transfer reaction with sulfur dioxide. In the case of ((1Z,3E)-3-ethyl-2-phenylhexa-1,3-dienyl)diphenylphosphine and (3,4,5-triphenylthiophen-2-yl oxide)diphenylphosphine, the molecules were isolated as their phosphine oxides. Reactions of the zirconacyclopropene complexes [Cp2Zr(η2-Ph2PC≡CPh)]2 and Cp2Zr(pyr)(η2-Ph2PC≡CMes) with the diyne (F5C6)C≡C−1,4-C6H4−C≡C(C6F5) gave bis(zirconacycle)s terminated with phosphino groups. These bis(zirconacycle)s were converted to the corresponding phosphino-terminated oligomers by protonolysis with hydrochloric acid. In addition, the Ph2PCCMes moiety of Cp2Zr[2-Ph2P-3-Mes-4-(C6F5)C4]−1,4-C6H4−Cp2Zr[2-Ph2P-3-Mes-4-(C6F5)C4] was exchanged with PhC≡CPh to give the phenylene(zirconacyclopentadiene) Cp2Zr[2,3-Ph2-4-(C6F5)C4]−1,4-C6H4−Cp2Zr[2,3-Ph2-4-(C6F5)C4].

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Fatty alcohols in the C12-C18 range are used in personal care products, lubricants, and potentially biofuels. These compounds can be produced from the fatty acid pathway by a fatty acid reductase (FAR), yet yields from the preferred industrial host Saccharomyces cerevisiae remain under 2% of the theoretical maximum from glucose. Here we improved titer and yield of fatty alcohols using an approach involving quantitative analysis of protein levels and metabolic flux, engineering enzyme level and localization, pull-push-block engineering of carbon flux, and cofactor balancing. We compared four heterologous FARs, finding highest activity and endoplasmic reticulum localization from a Mus musculus FAR. After screening an additional twenty-one single-gene edits, we identified increasing FAR expression; deleting competing reactions encoded by DGA1, HFD1, and ADH6; overexpressing a mutant acetyl-CoA carboxylase; limiting NADPH and carbon usage by the glutamate dehydrogenase encoded by GDH1; and overexpressing the Δ9-desaturase encoded by OLE1 as successful strategies to improve titer. Our final strain produced 1.2g/L fatty alcohols in shake flasks, and 6.0g/L in fed-batch fermentation, corresponding to ~ 20% of the maximum theoretical yield from glucose, the highest titers and yields reported to date in S. cerevisiae. We further demonstrate high-level production from lignocellulosic feedstocks derived from ionic-liquid treated switchgrass and sorghum, reaching 0.7g/L in shake flasks. Altogether, our work represents progress towards efficient and renewable microbial production of fatty acid-derived products.