https://pubs.rsc.org/en/content/articlepdf/2019/NP/C8NP00092A

Marine natural products.

We have used the pH-induced self-assembly of a peptide-amphiphile to make a nanostructured fibrous scaffold reminiscent of extracellular matrix. The design of this peptide-amphiphile allows the nanofibers to be reversibly cross-linked to enhance or decrease their structural integrity. After cross-linking, the fibers are able to direct mineralization of hydroxyapatite to form a composite material in which the crystallographic c axes of hydroxyapatite are aligned with the long axes of the fibers. This alignment is the same as that observed between collagen fibrils and hydroxyapatite crystals in bone.

http://science.sciencemag.org/content/sci/294/5547/1684.full.pdf

Self-assembly and mineralization of peptide-amphiphile nanofibers

Mesoporous organic-inorganic hybrid materials, a new class of materials characterized by large specific surface areas and pore sizes between 2 and 15 nm, have been obtained through the coupling of inorganic and organic components by template synthesis. The incorporation of functionalities can be achieved in three ways: by subsequent attachment of organic components onto a pure silica matrix (grafting), by simultaneous reaction of condensable inorganic silica species and silylated organic compounds (co-condensation, one-pot synthesis), and by the use of bissilylated organic precursors that lead to periodic mesoporous organosilicas (PMOs). This Review gives an overview of the preparation, properties, and potential applications of these materials in the areas of catalysis, sorption, chromatography, and the construction of systems for controlled release of active compounds, as well as molecular switches, with the main focus being on PMOs.

Silica-based mesoporous organic-inorganic hybrid materials.

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

For present purposes, a protein-bound metal site consists of one or more metal ions and all protein side chain and exogenous bridging and terminal ligands that define the first coordination sphere of each metal ion. Such sites can be classified into five basic types with the indicated functions: (1) structural -- configuration (in part) of protein tertiary and/or quaternary structure; (2) storage -- uptake, binding, and release of metals in soluble form: (3) electron transfer -- uptake, release, and storage of electrons; (4) dioxygen binding -- metal-O{sub 2} coordination and decoordination; and (5) catalytic -- substrate binding, activation, and turnover. The authors present here a classification and structure/function analysis of native metal sites based on these functions, where 5 is an extensive class subdivided by the type of reaction catalyzed. Within this purview, coverage of the various site types is extensive, but not exhaustive. The purpose of this exposition is to present examples of all types of sites and to relate, insofar as is currently feasible, the structure and function of selected types. The authors largely confine their considerations to the sites themselves, with due recognition that these site features are coupled to protein structure at all levels. In themore » next section, the coordination chemistry of metalloprotein sites and the unique properties of a protein as a ligand are briefly summarized. Structure/function relationships are systematically explored and tabulations of structurally defined sites presented. Finally, future directions in bioinorganic research in the context of metal site chemistry are considered. 620 refs.« less

/pdf/structural-and-functional-aspects-of-metal-sites-in-biology-a6d65lev8n.pdf

Structural and Functional Aspects of Metal Sites in Biology

In physiological fluids and seawater, adhesion of synthetic polymers to solid surfaces is severely limited by high salt, pH, and hydration, yet these conditions have not deterred the evolution of effective adhesion by mussels. Mussel foot proteins provide insights about adhesive adaptations: Notably, the abundance and proximity of catecholic Dopa (3,4-dihydroxyphenylalanine) and lysine residues hint at a synergistic interplay in adhesion. Certain siderophores—bacterial iron chelators—consist of paired catechol and lysine functionalities, thereby providing a convenient experimental platform to explore molecular synergies in bioadhesion. These siderophores and synthetic analogs exhibit robust adhesion energies (E(ad) ≥-15 millijoules per square meter) to mica in saline pH 3.5 to 7.5 and resist oxidation. The adjacent catechol-lysine placement provides a "one-two punch," whereby lysine evicts hydrated cations from the mineral surface, allowing catechol binding to underlying oxides.

/pdf/adaptive-synergy-between-catechol-and-lysine-promotes-wet-2tk2bclzp7.pdf

Adaptive synergy between catechol and lysine promotes wet adhesion by surface salt displacement

Vanadium Peroxide Complexes

Dissolved-iron availability plays a critical role in controlling phytoplankton growth in the oceans1,2 The dissolved iron is overwhelmingly (∼99%) bound to organic ligands with a very high affinity for iron3,4,5, but the origin, chemical identity and biological availability of this organically complexed Fe is largely unknown6 The release into sea water of complexes that strongly chelate iron could result from the inducible iron-uptake systems of prokaryotes (siderophore complexes)7,8,9 or by processes such as zooplankton-mediated degradation and release of intracellular material (porphyrin complexes) Here we compare the uptake of siderophore- and porphyrin-complexed 55Fe by phytoplankton, using both cultured organisms and natural assemblages Eukaryotic phytoplankton efficiently assimilate porphyrin-complexed iron, but this iron source is relatively unavailable to prokaryotic picoplankton (cyanobacteria) In contrast, iron bound to a variety of siderophores is relatively more available to cyanobacteria than to eukaryotes, suggesting that the two plankton groups exhibit fundamentally different iron-uptake strategies Prokaryotes utilize iron complexed to either endogenous7,8,9 or exogenous siderophores9, whereas eukaryotes may rely on a ferrireductase system10,11 that preferentially accesses iron chelated by tetradentate porphyrins, rather than by hexadentate siderophores Competition between prokaryotes and eukaryotes for organically-bound iron may therefore depend on the chemical nature of available iron complexes, with consequences for ecological niche separation, plankton community size-structure and carbon export in low-iron waters

Competition among marine phytoplankton for different chelated iron species

Iron is a limiting nutrient for primary production in large areas of the oceans. Dissolved iron(III) in the upper oceans occurs almost entirely in the form of complexes with strong organic ligands presumed to be of biological origin. Although the importance of organic ligands to aquatic iron cycling is becoming clear, the mechanism by which they are involved in this process remains uncertain. Here we report observations of photochemical reactions involving Fe(III) bound to siderophores--high-affinity iron(III) ligands produced by bacteria to facilitate iron acquisition. We show that photolysis of Fe(III)-siderophore complexes leads to the formation of lower-affinity Fe(III) ligands and the reduction of Fe(III), increasing the availability of siderophore-bound iron for uptake by planktonic assemblages. These photochemical reactions are mediated by the alpha-hydroxy acid moiety, a group which has generally been found to be present in the marine siderophores that have been characterized. We suggest that Fe(III)-binding ligands can enhance the photolytic production of reactive iron species in the euphotic zone and so influence iron availability in aquatic systems.

Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands.

The vast majority of bacteria require iron for growth.1,2 Iron is an essential element required for key biological processes including amino acid synthesis, oxygen transport, respiration, nitrogen fixation, methanogenesis, the citric acid cycle, photosynthesis and DNA biosynthesis. However, obtaining iron presents challenges for the majority of microorganisms. While iron is the fourth most abundant transition metal in the Earth's crust, the insolubility of iron(III) [Ksp of Fe(OH)3 = 10-39] at physiological pH in aerobic environments severely limits the availability of this essential nutrient. Pathogenic and marine bacteria face similar challenges for obtaining iron because both live in very low iron environments. Bacteria typically require micromolar levels of total iron for growth, yet the iron concentration in the surface waters of the oceans is only 0.01-2 nM.3-7 In humans cellular iron is also very low and is sequestered by lactoferrin, transferrin, and ferritin as a primary defense mechanism at the onset of infection.8 Given its cellular importance, it is not surprising that microbes have evolved multiple pathways designed to extract iron from their surrounding environments, tailored to the molecular constraints of the iron pool (Figure 1).



Figure 1

Microbial (Gram negative) iron uptake pathways.



In this review the general pathways by which bacteria acquire iron are considered first as an overview to illustrate the singular importance of iron for microbial growth. The focus of this review is on siderophore-mediated iron uptake, particularly structural characteristics of marine siderophores and the reactivity that these characteristics confer. Relatively little is known about marine microbial iron transport compared to that for terrestrial and pathogenic microbes, yet comparison of the structures and reactivity may hint at the biological advantage that these structural traits confer to marine microbes and very possibly provide insights to siderophore-mediated iron uptake in some pathogens.

/pdf/microbial-iron-acquisition-marine-and-terrestrial-5403ee3fct.pdf

Alison Butler

Papers

Adaptive synergy between catechol and lysine promotes wet adhesion by surface salt displacement

Vanadium Peroxide Complexes

Competition among marine phytoplankton for different chelated iron species

Photochemical cycling of iron in the surface ocean mediated by microbial iron(III)-binding ligands.

Microbial Iron Acquisition: Marine and Terrestrial Siderophores