Model studija u sintezi epotilona - alternativni pristup sintezi C1-C7 fragmenta

In this model study an alternative synthetic approach to the C1-C7 fragment of epothilones was investigated. Starting froth 4,4-dimethylcyclopentenone, a 7 step reaction sequence afforded the key intermediate 7 in 27%, overall yield. Surprisingly, the attempted deprotection of latent functionalities in 7 failed, indicating the incompatibility of the ethoxyethyl protective group with the reaction conditions employed.U okviru model studije ispitivan je alternativni sintetički pristup C1–C7 fragmentu molekula epotilona. Polazeći od 4,4-dimetilciklopentenona, ključni intermedijer 7 dobijen je u sedam faza, u ukupnom prinosu od 27 %. Deprotekciju latentnih funkcionalnih grupa u molekulu 7 nije bilo moguće izvršiti, zbog iznenađujuće labilnosti etoksietil zaštitne grupe u baznim reakcionim uslovima

Halogenase Genes in Nonribosomal Peptide Synthetase Gene Clusters of Microcystis (Cyanobacteria): Sporadic Distribution and Evolution

Cyanobacteria of the genus Microcystis are known to produce secondary metabolites of large structural diversity by nonribosomal peptide synthetase (NRPS) pathways. For a number of such compounds, halogenated congeners have been reported along with nonhalogenated ones. In the present study, chlorinated cyanopeptolin- and/or aeruginosin-type peptides were detected by mass spectrometry in 17 out of 28 axenic strains of Microcystis. In these strains, a halogenase gene was identified between 2 genes coding for NRPS modules in respective gene clusters, whereas it was consistently absent when the strains produced only nonchlorinated corresponding congeners. Nucleotide sequences were obtained for 12 complete halogenase genes and 14 intermodule regions of gene clusters lacking a halogenase gene or containing only fragments of it. When a halogenase gene was found absent, a specific, identical excision pattern was observed for both synthetase gene clusters in most strains. A phylogenetic analysis including other bacterial halogenases showed that the NRPS-related halogenases of Microcystis form a monophyletic group divided into 2 subgroups, corresponding to either the cyanopeptolin or the aeruginosin peptide synthetases. The distribution of these peptide synthetase gene clusters, among the tested Microcystis strains, was found in relative agreement with their phylogeny reconstructed from 16S–23S rDNA intergenic spacer sequences, whereas the distribution of the associated halogenase genes appears to be sporadic. The presented data suggest that in cyanobacteria these prevalent halogenase genes originated from an ancient horizontal gene transfer followed by duplication in the cyanobacterial lineage. We propose an evolutionary scenario implying repeated gene losses to explain the distribution of halogenase genes in 2 NRPS gene clusters that subsequently defines the seemingly erratic production of halogenated and nonhalogenated aeruginosins and cyanopeptolins among Microcystis strains

Ketone Hydrosilylation with Sugar Silanes Followed by Intramolecular Aglycone Delivery: An Orthogonal Glycosylation Strategy

Gettin' a little sugar—no alcohol required : A procedure for the direct glycosylation of ketones without a hydroxy intermediate enables the site-selective glycosylation of hydroxyketones at the ketone or the alcohol functionality without the use of protecting groups on the aglycone (see scheme). Site selectivity is controlled by the catalyst structure in hydrosilylation and dehydrogenative silylation reactions with sugar silanes. Bn=benzyl.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/63086/1/anie_200901666_sm_miscellaneous_information.pd

Stringency of the 2-His–1-Asp Active-Site Motif in Prolyl 4-Hydroxylase

The non-heme iron(II) dioxygenase family of enzymes contain a common 2-His–1-carboxylate iron-binding motif. These enzymes catalyze a wide variety of oxidative reactions, such as the hydroxylation of aliphatic C–H bonds. Prolyl 4-hydroxylase (P4H) is an α-ketoglutarate-dependent iron(II) dioxygenase that catalyzes the post-translational hydroxylation of proline residues in protocollagen strands, stabilizing the ensuing triple helix. Human P4H residues His412, Asp414, and His483 have been identified as an iron-coordinating 2-His–1-carboxylate motif. Enzymes that catalyze oxidative halogenation do so by a mechanism similar to that of P4H. These halogenases retain the active-site histidine residues, but the carboxylate ligand is replaced with a halide ion. We replaced Asp414 of P4H with alanine (to mimic the active site of a halogenase) and with glycine. These substitutions do not, however, convert P4H into a halogenase. Moreover, the hydroxylase activity of D414A P4H cannot be rescued with small molecules. In addition, rearranging the two His and one Asp residues in the active site eliminates hydroxylase activity. Our results demonstrate a high stringency for the iron-binding residues in the P4H active site. We conclude that P4H, which catalyzes an especially demanding chemical transformation, is recalcitrant to change

Lipopeptides: from self-assembly to bioactivity

This Feature Article discusses several classes of lipopeptide with important biomedical applications as antimicrobial and antifungal agents, in immune therapies and in personal care applications among others. Two main classes of lipopeptide are considered: (i) bacterially-expressed lipopeptides with a cyclic peptide headgroup and (ii) linear lipopeptides (with one or more lipid chains) based on bio-derived and bio-inspired amino acid sequences with current clinical applications. The applications are briefly summarized, and the biophysical characterization of the molecules is reviewed, with a particular focus on self-assembly. For several of these types of biomolecule, the formation of micelles above a critical micelle concentration has been observed while others form bilayer structures, depending on conditions of pH and temperature. As yet, there are few studies on the possible relationship between self-assembly into structures such as micelles and bioactivity of this class of molecule although this is likely to attract further attention

Convergent Site-Selective Carbohydrate-Peptide Ligations With Dehydroalanine and Aziridine -2 -Carboxylic Acid-Containing Peptides

222 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2005.The development of two convergent strategies for site-selective peptide modification and their application to the preparation of alpha-thio analogs of mucin related glycoconjugates is described. Chemoselectivity in these reactions was realized through the incorporation of dehydroalanine and aziridine-2-carboxylic acid residues into peptides and subsequent conjugation of the resulting peptide electrophiles with various thiols. Oligosaccharides incorporating a suitable anomeric thio-functionality were prepared via dehydrative glycosylation and glycolamide-auxiliary controlled sialylation reactions. The efficiency of the dehydroalanine ligation approach was demonstrated by the preparation of thio-isosteres of the four tumor-associated carbohydrate antigens, T N, T, STN, and 2,6-ST, as a pair of diastereoisomers at the newly formed cysteine stereocenter. This 1,4-conjugate addition ligation proceeded in high yield and with retention of alpha-anomeric configuration of the carbohydrate donor. In addition, site- and stereoselective peptide modification of aziridine-2-carboxylic acid-containing peptides was achieved. A solid-phase peptide synthesis methodology that allows for the rapid generation of peptides incorporating the aziridine residue was developed. The resulting peptides were conjugated with various thiol nucleophiles, such as anomeric carbohydrate thiols, farnesyl thiol, and biochemical tags, both in solution and on solid support. This ligation proceeds with the retention of the anomeric configuration of carbohydrate nucleophiles, and (R) configuration at the alpha-carbon of the newly formed cysteine derivative. This strategy, combined with native chemical ligation, provided convergent and rapid access to complex glycoconjugates. The developed convergent peptide modification approaches should allow for the construction of a variety of glycopeptide and lipopeptide conjugates for biological evaluations.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD