Mechanistic studies on the tyrosinase-catalyzed formation of the anachelin chromophore

Polydopamine, harness of the antibacterial potentials-A review

Antibiotic resistance is one of the major causes of morbidity and mortality, triggered by the adhesion of microbes and to some extent the formation of biofilms. This condition has been quite challenging in the health and industrial sector. Conditions and processes required to foil these infectious and resistance are of much concern. The synthesis of PDA material, inspired by the Mytilus edulis foot protein (MEFP)5 possesses unique characteristics that allow for, adhesion, photothermal therapy, synergistic effects with other materials, biocompatibility process, etc. Therefore, their usage holds great potential for dealing with both the infectious nature and the antibiotic resistance processes. Hence, this review provides an overview of the mechanism involved in accomplishing and eradicating bacteria, the recently harnessed antibacterial effect of the PDA through other properties they possess, a way forward in tapping the benefit embedded in the PDA, and the future perspective.

Structural and Biosynthetic Analysis of the Fabrubactins, Unusual Siderophores from Agrobacterium fabrum Strain C58

Siderophores are iron-chelating molecules produced by microorganisms and plants to acquire exogenous iron. Siderophore biosynthetic enzymology often produces elaborate and unique molecules through unusual reactions to enable specific recognition by the producing organisms. Herein, we report the structure of two siderophore analogs from Agrobacterium fabrum strain C58, which we named fabrubactin (FBN) A and FBN B. Additionally, we characterized the substrate specificities of the NRPS and PKS components. The structures suggest unique Favorskii-like rearrangements of the molecular backbone that we propose are catalyzed by the flavin-dependent monooxygenase, FbnE. FBN A and B contain a 1,1-dimethyl-3-amino-1,2,3,4-tetrahydro-7,8-dihydroxy-quinolin (Dmaq) moiety previously seen only in the anachelin cyanobacterial siderophores. We provide evidence that Dmaq is derived from l-DOPA and propose a mechanism for the formation of the mature Dmaq moiety. Our bioinformatic analyses suggest that FBN A and B and the anachelins belong to a large and diverse siderophore family widespread throughout the Rhizobium/Agrobacterium group, α-proteobacteria, and cyanobacteria.

An alternative approach to the synthesis of the three fragments of anachelin H

The synthesis of the fully protected peptide, polyketide and alkaloid fragments of anachelin H is presented. The peptide fragment was prepared using a liquid phase peptide synthesis; the polyketide fragment was synthetized using a cross metathesis and an intramolecular oxa-Michael reaction as the key steps to introduce the desired stereochemistry; finally, the alkaloid fragment was obtained by an oxidative cyclization of a catechol derivative using potassium ferricyanide. The synthesis of all fragments was based on the use of natural amino acids as sources of asymmetry. The independent synthesis of the three fragments should allow more efficient biological studies on the fragments instead of the whole natural product. Experiments to illustrate the coupling of fragments and the effectiveness of the convergent strategy are also described.

Enantioselective transfer hydrogenation, a key step for the synthesis of 3-aminotetrahydroquinolines

An enantioselective transfer hydrogenation has been successfully achieved to furnish 3-aminotetrahydroquinolines. The reaction was conducted in the presence of Hantzsch dihydropyridine and a catalytic amount of chiral phosphoric acid under mild conditions.

Poly(ethylene glycol) adlayers immobilized to metal oxide substrates through catechol derivatives: influence of assembly conditions on formation and stability

We have investigated five different poly(ethylene glycol) (PEG, 5 kDa) catechol derivatives in terms of their spontaneous surface assembly from aqueous solution, adlayer stability, and resistance to nonspecific blood serum adsorption as a function of the type of catechol-based anchor, assembly conditions (temperature, pH), and type of substrate (SiO(2), TiO(2), Nb(2)O(5)). Variable-angle spectroscopic ellipsometry (VASE) was used for layer thickness evaluation, X-ray photoelectron spectroscopy (XPS) for layer composition, and ultraviolet-visible optical spectroscopy (UV-vis) for cloud point determination. Polymer surface coverage was influenced by the type of catechol anchor, type of the substrate, as well as pH and temperature (T) of the assembly solution. Furthermore, it was found to be highest for T close to the cloud point (T(CP)) and pH of the assembly solution close to pK(a1) (dissociation constant of the first catechol hydroxy group) of the polymer and to the isoelectric point (IEP) of the substrate. T(CP) turned out to depend on not only the ionic strength of the assembly solution, but also the type of catechol derivative and pH. PEG-coating dry thickness above 10 A correlated with low serum adsorption. We therefore conclude that optimum coating protocols for catechol-based polymer assembly at metal oxide interfaces have to take into account specific physicochemical properties of the polymer, anchor, and substrate.

Surface modifications based on the cyanobacterial siderophore anachelin: from structure to functional biomaterials design

This review describes the design, synthesis and evaluation of novel catechol based anchors for surface modification. The anachelin chromophore, the catecholate fragment of the siderophore anachelin from the cyanobacterium Anabaena cylindrica, allows for the immobilization of polyethylene glycol (PEG) on titania and glass surfaces thus rendering them protein resistant and antifouling. It is proposed that catecholate siderophores constitute a class of natural products useful for surface modification similar to dihydroxyphenylalanine and dopamine derived compounds found in mussel adhesive proteins. Second-generation dopamine derivatives featuring a quaternary ammonium group were found to be equally efficient in generating antifouling surfaces. The anachelin chromophore, merged via a PEG linker to the glycopeptide antibiotic vancomycin, allowed for the generation of antimicrobial surfaces through an operationally simple dip-and-rinse procedure. This approach offers an option for the prevention of nosocomial infections through antimicrobial implants, catheters and stents. Consequences for the mild generation of functional biomaterials are discussed and novel strategies for the immobilization of complex natural products, proteins and DNA on surfaces are presented.

Aerucyclamides A and B: isolation and synthesis of toxic ribosomal heterocyclic peptides from the cyanobacterium Microcystis aeruginosa PCC 7806

Two new modified hexacyclopeptides, aerucyclamides A and B, were isolated from the toxic freshwater cyanobacterium Microcystis aeruginosa PCC 7806. The constitution was assigned by spectroscopic methods, and the configuration determined by chemical degradation and analysis by Marfey's method combined with chemical synthesis. Synthetic aerucyclamide B was obtained through oxidation of aerucyclamide A (MnO2, benzene). The aerucyclamides were found to be toxic to the freshwater crustacean Thamnocephalus platyurus, exhibiting LC50 values for congeners A and B of 30.5 and 33.8 microM, respectively.

Biomimetic Total Synthesis and Antimicrobial Evaluation of Anachelin H

The first biomimetic total synthesis of the iron chelator anachelin H isolated from the cyanobacterium Anabaena cylindrica is reported. A first generation approach delivered one enantiomeric series of the polyketide fragment. Comparison of the 1H NMR data suggested the relative configuration of this anachelin fragment. The relative and absolute configuration of anachelin H was then established by total synthesis. A second generation approach involved the enzymatic conversion of N,N-dimethyltyramine to the anachelin chromophore. It was demonstrated that the enzyme tyrosinase is activated by the product during this reaction, the anachelin chromophore can serve as a tyrosinase activator. Anachelin H was evaluated against a panel of eleven bacterial and fungal pathogens, and moderate antibiotic activity (32 microg/mL) against Moraxella catarrhalis was found.

Synthesis and evaluation of the bis-nor-anachelin chromophore as potential cyanobacterial ligand

The cyanobacterial metabolite anachelin, postulated to serve as a biological ligand to Fe (siderophore), is composed of a fascinating blend of polyketide, peptide, and alkaloid building blocks. In particular, the latter consists of a N,N-dimethyltetrahydroquinolinium fragment, of which the biosynthesis is unknown. To investigate the role of this permanently positively charged fragment, we developed a synthesis of both the anachelin chromophore and its bis-nor derivative lacking the N,N-dimethyl groups starting from suitably protected nitro-DOPA in six and five steps, respectively, and in 50-64% overall yield. Both compounds were then compared for their chemical behavior toward oxidation. It was found that the bis-nor-anachelin chromophore is readily oxidized in solution in the presence of air, with a clear dependence of the rate of oxidation on the pH value. In addition, we could demonstrate that the enzyme tyrosinase, postulated to serve as key catechol oxidase in the biosynthesis of anachelin, also oxidized the bis-nor-hydroquinonamine derivative. Last, Fe(III) was shown to be an effective oxidant for the bis-nor-anachelin chromophore, resulting in all cases in the corresponding aminoquinone. In stark contrast, the anachelin chromophore resisted oxidation under various conditions surveyed (i.e., mediated by air, by tyrosinase, and by Fe(III)). In particular, Fe(III) was readily complexed by the anachelin chromophore, and the resulting complexes were characterized. In conclusion, these experiments demonstrate that the bis-nor-anachelin chromophore is unlikely to serve as cyanobacterial ligand, due to its instability toward oxidation. Moreover, the permanent quaternary ammonium group in anachelin renders the alkaloid chromophore much more stable against oxidation and thus results in its use as ligand for Fe(III).