High-resolution X-ray structure of an acyl-enzyme species for the class D OXA-10 beta-lactamase

Beta-lactamases are resistance enzymes for beta-lactam antibiotics. These enzymes hydrolyze the beta-lactam moieties of these antibiotics, rendering them inactive. Of the four classes of known beta-lactamases, the enzymes of class D are the least understood. We report herein the high-resolution (1.9 A) crystal structure of the class D OXA-10 beta-lactamase inhibited by a penicillanate derivative. The structure provides evidence that the carboxylated Lys-70 (a carbamate) is intimately involved in the mechanism of the enzyme.     

Synthetic peptidoglycan substrates for penicillin-binding protein 5 of Gram-negative bacteria

The major constituent of the bacterial cell wall, peptidoglycan, is comprised of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) with an appended peptide. Penicillin-binding proteins (PBPs) are involved in the final stages of bacterial cell wall assembly. Two activities for PBPs are the cross-linking of the cell wall, carried out by dd-transpeptidases, and the dd-peptidase activity, that removes the terminal d-Ala residue from peptidoglycan. The dd-peptidase activity moderates the extent of the cell wall cross-linking. There exists a balance between the two activities that is critical for the well-being of bacterial cells. We have cloned and purified PBP5 of Escherichia coli. The membrane anchor of this protein was removed, and the enzyme was obtained as a soluble protein. Two fragments of the polymeric cell wall of Gram-negative bacteria (compounds 5 and 6) were synthesized. These molecules served as substrates for PBP5. The products of the reactions of PBP5 and compounds 5 and 6 were isolated and were shown to be d-Ala and the fragments of the substrates minus the terminal d-Ala. The kinetic parameters for these enzymic reactions were evaluated. PBP5 would appear to have the potential for turnover of as many as 1.4 million peptidoglycan strands within a single doubling time (i.e., generation) of E. coli.     

A practical synthesis of nitrocefin

Nitrocefin is a key reagent for high and low throughput assays of the activities of penicillin-binding proteins (PBPs) and beta-lactamases, the former used for discovery of antibiotics and the latter for inhibitors of resistance determinants for beta-lactam antibiotics. This compound is commercially available but is prohibitively expensive because of the circuitous routes to its synthesis. We describe herein a three-step synthesis of nitrocefin that gives an overall yield of 44%. This is a practical route to the synthesis of this key reagent for drug discovery.     

Comparative study of monomer droplet nucleation in the seeded batch and semibatch miniemulsion polymerisation of styrene

Monomer droplet nucleation in the seeded miniemulsion polymerisation of styrene under monomer-flooded and monomer-starved conditions was studied. The miniemulsion feeds were added to the reactor either batchwise or semibatchwise. The droplets preserved longer under flooded conditions. As a result, the batch operation led to a larger number of particles (N_p) than the semibatch operation. For the miniemulsion droplets containing predissolved polymer, the final N_p was independent of the way that the feed was added to the reactor and was equivalent to the number of monomer droplets in the original miniemulsion feed. The size distribution of the final latexes, however, was influenced by the operation type. For the batch operation, the rate of polymerisation (R_p) with the miniemulsion feeds was higher than that with the conventional monomer emulsion feed because of the monomer droplet nucleation. But for the semibatch operation, the opposite was true because of R_p controlled by the rate of monomer diffusion from rather stable miniemulsion droplets to the growing polymer particles.     

Computational investigation of irreversible inactivation of the zinc-dependent protease carboxypeptidase A

Zinc proteases are ubiquitous and the zinc ion plays a central function in the catalytic mechanism of these enzymes. A novel class of mechanism-based inhibitors takes advantage of the zinc ion chemistry in carboxypeptidase A (CPA) to promote covalent attachment of an inhibitor to the carboxylate of Glu-270, resulting in irreversible inhibition of the enzyme. The effect of the active site zinc ion on irreversible inactivation of CPA was probed by molecular orbital (MO) calculations on a series of active site models and the Cl(-) + CH(3)Cl S(N)2 reaction fragment. Point charge models representing the active site reproduced energetics from full MO calculations at 12.0 A separation between the zinc and the central carbon of the S(N)2 reaction, but at 5.0 A polarization played an important role in moderating barrier suppression. ONIOM MO/MO calculations that included the residues within 10 A of the active site zinc suggest that about 75% of the barrier suppression arises from the zinc ion and its ligands. A model of the pre-reactive complex of the 2-benzyl-3-iodopropanoate inactivator with CPA was constructed from the X-ray structure of l-phenyl lactate bound in the active site of the enzyme. The model was fully solvated and minimized by using the AMBER force field to generate the starting structure for the ONIOM QM/MM calculations. Optimization of this structure led to the barrierless S(N)2 displacement of the iodide of the inhibitor by Glu-270, assisted by interaction of the zinc ion with the leaving group. The resulting product is in good agreement with the X-ray structure of the covalently modified enzyme obtained by irreversible inhibition of CPA by 2-benzyl-3-iodopropanoate.     

Method for accurate experimental determination of singlet and triplet exciton diffusion between thermally activated delayed fluorescence molecules

Understanding triplet exciton diffusion between organic thermally activated delayed fluorescence (TADF) molecules is a challenge due to the unique cycling between singlet and triplet states in these molecules. Although prompt emission quenching allows the singlet exciton diffusion properties to be determined, analogous analysis of the delayed emission quenching does not yield accurate estimations of the triplet diffusion length (because the diffusion of singlet excitons regenerated after reverse-intersystem crossing needs to be accounted for). Herein, we demonstrate how singlet and triplet diffusion lengths can be accurately determined from accessible experimental data, namely the integral prompt and delayed fluorescence. In the benchmark materials 4CzIPN and 4TCzBN, we show that the singlet diffusion lengths are (9.1 ± 0.2) and (12.8 ± 0.3) nm, whereas the triplet diffusion lengths are negligible, and certainly less than 1.0 and 1.2 nm, respectively. Theory confirms that the lack of overlap between the shielded lowest unoccupied molecular orbitals (LUMOs) hinders triplet motion between TADF chromophores in such molecular architectures. Although this cause for the suppression of triplet motion does not occur in molecular architectures that rely on electron resonance effects (e.g. DiKTa), we find that triplet diffusion is still negligible when such molecules are dispersed in a matrix material at a concentration sufficiently low to suppress aggregation. The novel and accurate method of understanding triplet diffusion in TADF molecules will allow accurate physical modeling of OLED emitter layers (especially those based on TADF donors and fluorescent acceptors).

A method for measuring triplet diffusion between TADF molecules is presented, and implications of limited triplet diffusion for OLEDs discussed.     

Determination of Copper by Adsorptive Stripping Voltammetry in the Presence of Calcein Blue

An electrochemical adsorptive stripping approach is presented for the trace measurement of copper in some real samples. The method is based on the reduction of Cu²⁺ at pH 5.5 calcein blue (CB) containing solution at ?250 mV (vs. Ag/AgCl), adsorption of Cu? CB complex on hanging mercury drop electrode (HMDE) and the voltammetric determination by further reduction to Cu⁺ at HMDE. Experimental optimum conditions were determined in the fundamental studies. At the experimental optimum conditions the adsorbed complex of Cu²⁺ and calcein blue gives a well defined cathodic stripping peak current at ?0.135 V, which has been used for the determination of copper in the concentration range of 0.02 to 15 ng/mL with accumulation time of 90 s. The relative standard deviation (RSD) for the determination of 0.5 and 6.0 ng mL^?1 were 2.60 and 1.94% respectively. (n=10). The method has been applied to the analysis of copper in analytical reagent grade salts and tap water, mineral water and drug samples with satisfactory results.     

Muropeptides in Pseudomonas aeruginosa and their Role as Elicitors of β‐Lactam‐Antibiotic Resistance

Muropeptides are a group of bacterial natural products generated from the cell wall in the course of its turnover. These compounds are cell‐wall recycling intermediates and are also involved in signaling within the bacterium. However, the identity of these signaling molecules remains elusive. The identification and characterization of 20 muropeptides from Pseudomonas aeruginosa is described. The least abundant of these metabolites is present at 100 and the most abundant at 55,000 molecules per bacterium. Analysis of these muropeptides under conditions of induction of resistance to a β‐lactam antibiotic identified two signaling muropeptides (N‐acetylglucosamine‐1,6‐anhydro‐N‐acetylmuramyl pentapeptide and 1,6‐anhydro‐N‐acetylmuramyl pentapeptide). Authentic synthetic samples of these metabolites were shown to activate expression of β‐lactamase in the absence of any β‐lactam antibiotic, thus indicating that they serve as chemical signals in this complex biochemical pathway.     

Co-opting the cell wall in fighting methicillin-resistant Staphylococcus aureus: potent inhibition of PBP 2a by two anti-MRSA beta-lactam antibiotics

Methicillin-resistant Staphylococcus aureus (MRSA) is a global bacterial scourge that has become resistant to many classes of antibiotics, and treatment options for MRSA infections are limited. The cause of MRSA resistance to all commercially available beta-lactam antibiotics is the acquisition of the gene mecA, which encodes penicillin-binding protein 2a (PBP 2a). PBP 2a is a transpeptidase, which in contrast to the other transpeptidases of S. aureus does not experience inhibition by beta-lactam antibiotics. The lack of inhibition is due to a closed conformation for the active site for PBP 2a, which opens up only in the course of the catalytic function of the protein. Here we show that two new anti-MRSA antibiotics now undergoing clinical trials, ceftaroline and ME1036, are able to inhibit PBP 2a effectively, a process that is enhanced in the presence of a cell wall structural surrogate. It is likely that in the course of bacterial growth the occupancy of the allosteric site for the cell wall is co-opted by these antibiotics, and under these conditions the second-order rate constant for the encounter of the antibiotic and PBP 2a approaches the clinically useful value of 10(4)-10(5) M-1 s-1. These compounds are potent inhibitors of PBP 2a as well as PBPs from other species, and have potential as therapeutic agents for treatment of serious infections by MRSA and other resistant bacterial pathogens.     

Synthesis,photocrosslinking characteristics,and biocompatibility evaluation of N‐vinyl pyrrolidone/polycaprolactone fumarate biomaterials using a new proton scavenger

Photocrosslinkable and biodegradable polymeric networks were formulated based on N‐vinyl pyrrolidone‐poly(ε‐caprolactone fumarate) (NVP/PCLF) compositions for hard tissue engineering applications using a new proton scavenger, propylene oxide (PO). The synthesized macromers were obtained as a white clear paste with no colorization. The obtained macromers were thoroughly characterized using spectroscopic (NMR and FT‐IR) and chromatographic (gel permeation chromatography (GPC)) techniques. Photocrosslinking of the PCLF/NVP compositions was achieved using camphorquinone and dimethyl toluidine (DMT) as a photoinitiator system. The cytocompatibility of the macromers and their corresponding networks were evaluated via MTT assay. Characterization of the networks indicated the importance of NVP content on the network properties. Sol fraction studies indicated that more than 90% of the PCLFs were crosslinked over the studied range of PCLF/NVP compositions; however most of the NVP above a stoichiometric ratio of one NVP to fumerate unit remained unreacted. It was also found that in the concentrations more than 10% NVP, the unreacted NVP monomer neither participated in the crosslinking reaction nor homopolymerized to poly(vinyl pyrrolidone) (PVP). The elastic modulus (G′) and estimated molecular weight between crosslinks also confirmed that at the higher NVP content the PCLF photocrosslinking was hindered. Copyright © 2008 John Wiley & Sons, Ltd.