Craig-Hückel Hybrid Aromatic Metalla-dehydro[11]annulenes Constructed by a Formal [10+1] Cycloaddition Reaction.

Aromatic metalla-annulenes are important aromatic compounds, research into which has been mainly concentrated on metal-benzenes and their lower homologues. Reports on their superior homologs are rare, and this has greatly limited the systematic study of their properties. In this work, a series of osma-dehydro[11]annulenes with good air and thermal stability were prepared in high yields through a simple [10+1] strategy, by incorporating a metal fragment into conjugated ten-carbon chains in a one-pot reaction. They are the first monometallic aromatic metalla-[n]annulenes with the ring size larger than 6, and their Craig-Hückel hybrid aromaticity is supported by various physical and computational parameters. Besides, these complexes show versatile reactivities, not only giving further evidence for their aromaticity, but also demonstrating their physical and chemical properties can easily be regulated. This work enriches the metalla-aromatic chemistry, and provides a new avenue for the synthesis of large metalla-annulenes with different ring sizes.

Mining the genome sequence for novel enzyme activity: characterisation of an unusual member of the hormone-sensitive lipase family of esterases from the genome of Streptomyces coelicolor A3 (2).

The sequence of the estA gene (locus SCO 7131) of S. coelicolor A3 (2) suggested that it might differ in substrate specificity from other characterised members of the hormone-sensitive lipase (HSL) family of lipases and esterases. This difference may be attributed to the unique substitutions within the conserved motifs of the family. There was no homologue with any other lipase or esterase to estA in the chromosome of S. avermitilis or other streptomyces species and the sequence showed differences in the conserved motifs from the characterised members of the HSL family. The gene was cloned and expressed as a His-tagged protein in Escherichia coli. The purified enzyme was an esterase, which hydrolyzed the acetate ester of p-nitrophenol, but had little activity on esters with longer side chains, unlike the other characterised bacterial members of the HSL family which showed optimal activity against caproate (C(6)) esters. Site-directed mutagenesis was used to change two amino acids to the consensus for the HSL family. This increased the activity against butyrate and caproate esters. The changes also affected thermostability: in one case increasing stability and in the other case reducing it. A profile was constructed for the HSL family and used to detect 119 members in the protein database. The location of conserved amino acid motifs in a 3-D homology model of the enzyme identified further members of the family with unusual amino acid replacements.

A cold-active esterase of Streptomyces coelicolor A3(2): from genome sequence to enzyme activity.

The genome sequence of Streptomyces coelicolor A3(2) contains 51 putative lipase and esterase genes mostly of unknown function. The gene estB (locus SCO 6966) was expressed as a His-tagged protein in E. coli. Esterase B was active at low temperatures exerting its maximum activity at 30 degrees C and retaining more than 25% of its activity at 4 degrees C. The optimum pH was 8-8.5. The enzyme was active against short synthetic p-nitrophenylesters (C2-C10) with maximum activity towards the acetate ester (C2). The esterase was tested on 13 series of racemic esters of potential interest for the synthesis of chiral pharmaceutical compounds. 4 of the series were substrates and a modest degree of enantioselectivity was observed (enantiomeric ratios of 1.1-1.9).