Chiral symmetry breaking yields the I-Au60 perfect golden shell of singular rigidity.

The combination of profound chirality and high symmetry on the nm-scale is unusual and would open exciting avenues, both fundamental and applied. Here we show how the unique electronic structure and bonding of quasi-2D gold makes this possible. We report a chiral symmetry breaking, i.e., the spontaneous formation of a chiral-icosahedral shell (I-Au60) from achiral (Ih) precursor forms, accompanied by a contraction in the Au-Au bonding and hence the radius of this perfect golden sphere, in which all 60 sites are chemically equivalent. This structure, which resembles the most complex of semi-regular (Archimedean) polyhedra (34.5*), may be viewed as an optimal solution to the topological problem: how to close a 60-vertex 2D (triangular) net in 3D. The singular rigidity of the I-Au60 manifests in uniquely discrete structural, vibrational, electronic, and optical signatures, which we report herein as a guide to its experimental detection and ultimately its isolation in material forms.

Direct interaction of a vancomycin derivative with bacterial enzymes involved in cell wall biosynthesis.

BACKGROUND: The glycopeptide antibiotic vancomycin complexes DAla-DAla termini of bacterial cell walls and peptidoglycan precursors and interferes with enzymes involved in murein biosynthesis. Semisynthetic vancomycins incorporating hydrophobic sugar substituents exhibit efficacy against DAla-DLac-containing vancomycin-resistant enterococci, albeit by an undetermined mechanism. Contrasting models that invoke either cooperative dimerization and membrane anchoring or direct inhibition of bacterial transglycosylases have been proposed to explain the bioactivity of these glycopeptides. RESULTS: Affinity chromatography has revealed direct interactions between a semisynthetic hydrophobic vancomycin (DCB-PV), and select Escherichia coli membrane proteins, including at least six enzymes involved in peptidoglycan assembly. The N(4)-vancosamine substituent is critical for protein binding. DCB-PV inhibits transglycosylation in permeabilized E. coli, consistent with the observed binding of the PBP-1B transglycosylase-transpeptidase. CONCLUSIONS: Hydrophobic vancomycins interact directly with a select subset of bacterial membrane proteins, suggesting the existence of discrete protein targets. Transglycosylase inhibition may play a role in the enhanced bioactivity of semisynthetic glycopeptides.

In vivo processing and antibiotic activity of microcin B17 analogs with varying ring content and altered bisheterocyclic sites.

BACKGROUND: The Escherichia coli peptide antibiotic microcin B17 (MccB17) contains four oxazole and four thiazole rings, and inhibits DNA gyrase. The role of individual and tandem pairs of heterocycles in bioactivity has not been determined previously. RESULTS: The two tandem 4,2-bisheterocycles in MccB17 were varied by expression of MccB17 or mutants containing altered sequences at Gly39-Ser40-Cys41 or Gly54-Cys55-Ser56. A mixture of five-nine-ring MccB17 isoforms were separated and quantitated for antibiotic potency. Mutagenesis of the thiazole-oxazole pair significantly affected antibiotic activity compared with the upstream oxazole-thiazole, which might stabilize partially cyclized intermediates against proteolysis. CONCLUSIONS: Enzymatic heterocyclization in native MccB17 occurs distributively. Antibiotic activity correlates with the number of rings and is differentially sensitive to both the location and the identity of the 4,2-tandem heterocycle pairs in MccB17. Such tandem heterocycles might be useful pharmacophores in combinatorial libraries.

Mutational analysis of posttranslational heterocycle biosynthesis in the gyrase inhibitor microcin B17: distance dependence from propeptide and tolerance for substitution in a GSCG cyclizable sequence.

Microcin B17 (MccB17) is a peptidyl antibiotic that is secreted in stationary phase by several strains of Escherichia coli. The antibiotic efficacy of this polypeptide depends on the posttranslational modification of eight cysteine and serine residues to thiazoles and oxazoles, respectively, within the 69 aa McbA structural gene product. Mono- and bisheterocycle formation is mediated by MccB17 synthetase, an enzyme complex composed of three proteins: McbB, -C, and -D. After substrate processing, an N-terminal 26 aa propeptide sequence is cleaved to afford the mature antibiotic. A method for the overexpression and rapid purification of microcin synthetase has been developed using a calmodulin-binding peptide tag. The determinants of substrate recognition and synthetase-mediated heterocycle formation were investigated by a systematic evaluation of 15 McbA1-46 analogues representing minimal substrates containing the first bisheterocyclization site (Gly39-Ser40-Cys41-Gly42) and variants thereof. Each substrate analogue was overexpressed and affinity-purified as fusions to maltose-binding protein, incubated with purified synthetase, and evaluated for processing by Western blots, UV spectroscopy, and mass spectrometry. Insights gained into the process of enzymatic heterocycle formation from cysteine and serine residues are discussed, including the distance dependence of the first cyclized residue from the propeptide and the local sequence context at the cyclizable sites. A model for McbA substrate recognition and processing by MccB17 synthetase is proposed.