Metal-dependent self-assembly of a microbial surfactant

Metallosurfactant aggregates: Structures, properties, and potentials for multifarious applications

Metallosurfactants offer important scientific and technological advances due to their novel interfacial properties. As a special class of structures formed by the integration of metal ions into amphiphilic surfactant molecules, these metal-based amphiphilic molecules possess both organometallic and surface chemistries. This review critically examines the structural transitions of metallosurfactants from micelle to vesicle upon metal coordination. The properties of a metallosurfactant can be changed by tuning the coordination between the metal ions and surfactants. The self-assembled behavior of surfactants can be controlled by selecting transition-metal ions that enhance their catalytic efficiency in environmental applications by applying a hydrogen evolution reaction or oxygen evolution reaction. We present the different scattering techniques available to examine the properties of metallosurfactants (e.g., size, shape, structure, and aggregation behavior). The utility of metallosurfactants in catalysis, the synthesis of nanoparticles, and biomedical applications (involving diagnostics and therapeutics) is also explored.

Interplay between Supramolecular and Coordination Interactions in Synthetic Amphiphiles: Triggering Metal Starvation and Anchorage onto MRSA Cell Surface

The present work highlights the implications of supramolecular interaction and metal coordination on the self-assembly behavior and bactericidal potential of salicaldehyde-(C1) and napthaldehyde-based (C2) amphiphiles against methicillin-resistant Staphylococcus aureus (MRSA). LB trough and atomic force microscope (AFM) analysis indicated the propensity of the amphiphiles to form a monolayer as well as spherical aggregates, with the critical micelle concentration (CMC) for C2 (7.0 μM) being lower than C1 (18.5 μM) in water. Formation of an amphiphile-metal complex was evidenced by ESI-MS, FTIR, FETEM-EDX, and ITC analysis. Growth of S. aureus MRSA 100 cells was remarkably impaired in the presence of 5.0 μM C1 or 20 μM C2 as compared to free cells or cells grown in the presence of equivalent levels of amphiphile-metal complexes, suggesting that the amphiphiles perhaps sequester metal and induce metal starvation in MRSA. C1 and C2 rendered superior membrane damage in MRSA and were less toxic to human embryonic kidney (HEK 293) cells as compared to their metal complexes. C1 and C2 rendered a dose-dependent inhibition of S. aureus biofilm formation, while revival of biofilm upon Zn(II) addition suggested that zinc starvation rendered by the amphiphiles may induce biofilm inhibition. C1 imposed a concentration-dependent metal starvation response in MRSA as there was an upregulation of the cntL gene and downregulation of cntA gene, which are involved in synthesis of the zincophore staphylopine (Stp) and transport of the Stp-Zn complex, respectively. ITC analysis revealed that binding of C1 and C2 to staphylococcal lipoteichoic acid (LTA) was stronger than the corresponding Zn(II) complexes, which perhaps accounted for the higher bactericidal potency of the amphiphiles. The study provides a fundamental understanding on how the chemistry-driven multimodal interaction of the amphiphile translates into growth inhibition and metal starvation in MRSA and advances the idea of combating drug resistance in pathogenic bacteria through amphiphiles, which are pluri-active.

An overview of siderophores for iron acquisition in microorganisms living in the extreme

Siderophores are iron-chelating molecules produced by microbes when intracellular iron concentrations are low. Low iron triggers a cascade of gene activation, allowing the cell to survive due to the synthesis of important proteins involved in siderophore synthesis and transport. Generally, siderophores are classified by their functional groups as catecholates, hydroxamates and hydroxycarboxylates. Although other chemical structural modifications and functional groups can be found. The functional groups participate in the iron-chelating process when the ferri-siderophore complex is formed. Classified as acidophiles, alkaliphiles, halophiles, thermophiles, psychrophiles, piezophiles, extremophiles have particular iron requirements depending on the environmental conditions in where they grow. Most of the work done in siderophore production by extremophiles is based in siderophore concentration and/or genomic studies determining the presence of siderophore synthesis and transport genes. Siderophores produced by extremophiles are not well known and more work needs to be done to elucidate chemical structures and their role in microorganism survival and metal cycling in extreme environments.

Vesicle self-assembly of amphiphilic siderophores produced by bacterial isolates from Soap Lake, Washington

Soap Lake, located in Washington State, is a meromictic soda lake that was the subject of a prior National Science Foundation funded Microbial Observatory. Several organisms inhabiting this lake have been identified as producers of siderophores that are unique in structure. Two isolates found to be of the species Halomonas, SL01 and SL28, were found to produce suites of amphiphilic siderophores consisting of a peptidic head-group, which binds iron appended to fatty acid moieties of various lengths. The ability for siderophores to self-assemble into vesicles was determined for three suites of amphiphilic siderophores of unique structure (two from SL01 and one from SL28). These siderophores resemble the amphiphilic aquachelin siderophores produced by Halomonas aquamarina strain DS40M3, a marine bacterium. Vesicle self-assembly studies were performed by dynamic light scattering and epifluorescence microscopy. The addition of ferric iron (Fe3+) at different equivalents, where an equivalence of iron is defined as equal to the molarity of the siderophore, demonstrated vesicle formation. This was suggested by both dynamic light scattering and epifluorescence microscopy. Bacteria thriving under saline and alkaline conditions are capable of producing unique siderophores that self-assemble in micelles and vesicles due to ferric iron chelation.

Lanthanide-Dipicolinic Acid Coordination Driven Micelles with Enhanced Stability and Tunable Function

Lanthanide-incorporated polymer micelles have been prepared driven by the lanthanide-dipicolinic acid (Ln-DPA) coordination. The terdentate DPA ligand is grafted to the PVP block of a diblock copolymer poly(4-vinylpyridine)-b-poly(ethylene oxide) (P4VP48-b-PEO193). Upon addition of Eu(III) ions to a solution of the DPA16-g-P4VP48-b-PEO193 block copolymer, intermolecular cross-links form and the ligand-carrying blocks assemble, leading to the formation of micelles, stabilized by the hydrophilic PEO blocks. The DPA exhibits a dual function in this study. First, the chelate group strongly coordinates to Eu(III) in a three to one ratio, and leads to high stability of the formed micelles, as proven by light scattering and luminescence spectroscopy. Second, DPA acts as an antenna that transfers energy to the Eu(III) ion and dramatically enhances the luminescence emission. The Eu(III) emission is moreover most sensitive for local environment and allows to shine light on the internal structure of this class of self-assembled 36 nm size soft nanoparticles. With the same strategy gadolinium(III) can be incorporated providing micelles which show enhanced magnetic relaxation rates. Micelles capping a mixture of Eu(III) and Gd(III) show both enhanced luminescence emission and magnetic relaxation rates, and the functions can be tuned by regulating the mixing ratio of Eu(III) and Gd(III), showing great potential for developing multimodal imaging agents for diagnostic as well as therapeutic applications.

Self-assembly of surfactin in aqueous solution: role of divalent counterions

Myriad applications of surfactin in environmental and biomedical field prompt understanding the self-assembly behaviour of surfactin in aqueous solution as well as its interaction with counterions. Effect of four divalent counterions namely, Ni(2+), Zn(2+), Cd(2+), and Ca(2+) on the self-assembly of the surfactin, a biosurfactant isolated from Bacillus subtilis YB7 is studied by fluorescence spectroscopy, dynamic light scattering, optical and electron microscopic studies. The critical micelle concentration (CMC) and aggregation number (Nagg) of surfactin are 96.76 ± 15.49 μM and 101.12 ± 2.53, respectively. The degree of counterion association increases as its ionic radius decreases. Ni(2+) exhibits the highest and Ca(2+) the least degree of counterion association. Addition of counterion reduces the size of the microstructures, aggregation number (Nagg) and zeta potential. The reduction in the zeta potential indicates the neutralization of the negative charges on the electrical double layer of the microstructures. Differential interference contrast (DIC) and transmission electron microscopic (TEM) images of surfactin show the presence of vesicles and large aggregates including giant vesicles. On the addition of Ca(2+), fusion of vesicles into large aggregates is predominantly observed. Ni(2+) induces the transition of large spherical vesicles into small spherical, worm-like vesicles and multicompartment-like structures (vesosome). Such structures are the evidences for metal ion coordinated intervesicular interactions. This study reveals that the self-assembly process of surfactin can be controlled by the addition of metal ions according to the requirements.

Balance of coordination and hydrophobic interaction in the formation of bilayers in metal-coordinated surfactant mixtures

Metal-ligand coordination and hydrophobic interaction are two significant driving forces in the aggregation of mixtures of M(n+) surfactants and alkyldimethylamine oxide (CnDMAO) in aqueous solutions. The coordinated systems exhibit rich aggregation behavior. This study investigated the effect of M(n+) ions (Zn(2+), Ca(2+), Ba(2+), Al(3+), Fe(3+), La(3+), Eu(3+), and Tb(3+)) and hydrophobic chains (hydrocarbon and fluorocarbon) on the formation of metal-coordinated bilayers. We found that fluorocarbon chains and branched hydrocarbon chains are preferable to the corresponding linear hydrocarbon chains for the formation of an Lα phase. Moreover, Lα phases formed by fluorocarbon chains exhibited higher viscoelasticity than ones formed by the hydrocarbons, and the bilayers formed by branched chains were rather flexible, revealing obvious undulation. The construction of bilayers was also strongly affected by metal ions due to their variable coordination ability with CnDMAO. Our results contribute to the understanding of the formation of metal-coordinated bilayers, which is driven by the interplay of noncovalent forces.

Self-assembled aggregates originated from the balance of hydrogen-bonding, electrostatic, and hydrophobic interactions

Rich phase behavior was observed in salt-free cationic and anionic (catanionic) mixtures of a double-tailed surfactant, di(2-ethylhexyl)phosphoric acid (abbreviated as DEHPA), and tetradecyldimethylamine oxide (C(14)DMAO) in water. At a fixed C(14)DMAO concentration, phase transition from L(1) phase to L(α) phase occurs with increasing amounts of DEHPA. Moreover, in the L(α) phase, with the increase in DEHPA concentration, a gradual transition process from vesicle phase (L(αv)) to stacked lamellar phase (L(αl)) was determined by cryo- and FF-TEM observations combining with (2)H NMR measurements. The rheological data show that the viscosity increases with DEHPA amounts for L(αv) phase samples because of the increase in vesicle density. At a certain molar ratio of DEHPA to C(14)DMAO, i.e., 80:250, the samples are with the highest viscoelasticity, indicating the existence of densely packed vesicles. While for L(αl) phase samples, with increasing DEHPA amount, a decrease of bilayer curvature was induced, leading to a decrease of viscosity obviously. Compared with general catanionic surfactant mxitures, in addition to the electrostatic interaction of ion pairs, the transition of the microstructures is also ascribed to the formation of the hydrogen bonding (-N(+)-O-H···O-N-) between C(14)DMAO molecules and protonated C(14)DMAOH(+), which induces the growth of aggregates and the decrease of aggregate curvatures.

Metallosurfactants of bioinorganic interest: Coordination-induced self assembly

This review covers selected surfactant ligands that undergo a change in aggregate morphology upon coordination of a metal ion, with a particular focus on coordination-induced micelle-to-vesicle transitions. The surfactants include microbially produced amphiphilic siderophores, as well as synthetic amphiphilic ligands. The mechanism of the metal-induced phase change is considered in light of the coordination chemistry of the metal ions, the nature of the ligands, and changes in molecular geometry that result from metal coordination. Of particular interest are biologically produced amphiphiles that coordinate transition metal ions and amphiphilic ligands of relevance to bioinorganic chemistry.

SANS investigation of the photosynthetic machinery of Chloroflexus aurantiacus

Green photosynthetic bacteria harvest light and perform photosynthesis in low-light environments, and contain specialized antenna complexes to adapt to this condition. We performed small-angle neutron scattering (SANS) studies to obtain structural information about the photosynthetic apparatus, including the peripheral light-harvesting chlorosome complex, the integral membrane light-harvesting B808-866 complex, and the reaction center (RC) in the thermophilic green phototrophic bacterium Chloroflexus aurantiacus. Using contrast variation in SANS measurements, we found that the B808-866 complex is wrapped around the RC in Cfx. aurantiacus, and the overall size and conformation of the B808-866 complex of Cfx. aurantiacus is roughly comparable to the LH1 antenna complex of the purple bacteria. A similar size of the isolated B808-866 complex was suggested by dynamic light scattering measurements, and a smaller size of the RC of Cfx. aurantiacus compared to the RC of the purple bacteria was observed. Further, our SANS measurements indicate that the chlorosome is a lipid body with a rod-like shape, and that the self-assembly of bacteriochlorophylls, the major component of the chlorosome, is lipid-like. Finally, two populations of chlorosome particles are suggested in our SANS measurements.

Iron(III)-siderophore coordination chemistry: Reactivity of marine siderophores

Two remarkable features of many siderophores produced by oceanic bacteria are the prevalence of an α-hydroxy-carboxylic acid functionality either in the form of the amino acid β-hydroxy aspartic acid or in the form of citric acid, as well as the predominance of amphiphilic siderophores. This review will provide an overview of the photoreactivity that takes place when siderophores containing β-hydroxy aspartic acid and citric acid are coordinated to iron(III). This photoreactivity raises questions about the role of this photochemistry in microbial iron acquisition as well as upper-ocean iron cycling. The self-assembly of amphiphilic siderophores and the coordination-induced phase-change of the micelle-to-vesicle transformation will also be reviewed. The distinctive photosensitive and self-assembly properties of marine siderophores hint at possibly new microbial iron acquisition mechanisms.

New surfactant phosphine ligands and platinum(II) metallosurfactants. Influence of metal coordination on the critical micelle concentration and aggregation properties

We have prepared the first platinum(II) metallosurfactants from a new family of linear surfactant phosphines Ph(2)P(CH(2))(n)SO(3)Na {1 (n = 2), 2 (n = 6), and 3 (n = 10)}, which were synthesized by reaction between the halosulfonates X(CH(2))(n)SO(3)Na and sodium diphenylphosphide. The metallosurfactants cis-[PtCl(2)L(2)] (L = 1-3) were obtained after reaction between the phosphines and PtCl(2) in dimethylsulfoxide. All compounds were fully characterized by the usual methods {NMR ((1)H, (13)C, (31)P, (195)Pt), IR, MS-ESI and HRMS}. By exploring the surfactant properties of phosphines 1-3 and their respective platinum metallosurfactants cis-[PtCl(2)L(2)] (L = 1-3) through surface tension measurements, dynamic light scattering spectroscopy, and cryo-TEM microscopy, we were able to analyze the influence of the metal coordination on the critical micelle concentration (cmc) and the aggregation properties. The cmc values of platinum metallosurfactants were considerably lower than those obtained for the free phosphines 1-3. This behavior could be understood by an analogy between the structure of cis-[PtCl(2)L(2)] complexes and bolaform surfactants. The calculated values of area per molecule also showed different tendencies between 1-3 and cis-[PtCl(2)L(2)] complexes, which could be explained on the basis of the possible conformations of these compounds in the air-water interface. The study of aggregates by dynamic light scattering spectroscopy and cryo-TEM microscopy showed the formation of spherical disperse medium size vesicles in all cases. However, substantial differences were observed between the three free phosphines (the population of micellar aggregates increased with long chain length) and also between phosphines and their respective metallosurfactants.

Ferric stability constants of representative marine siderophores: marinobactins, aquachelins, and petrobactin

The coordination of iron(III) to the marine amphiphilic marinobactin and aquachelin siderophores, as well as to petrobactin, an unusual 3,4-dihydroxybenzoyl siderophore is reported. Potentiometric titrations were performed on the apo siderophore to determine the ligand pK(a) values, as well as the complex formed with addition of 1 equiv of Fe(III). The log K(ML) values for Fe(III)-marinobactin-E and Fe(III)-aquachelin-C are 31.80 and 31.4, respectively, consistent with the similar coordination environment in each complex, while log K(ML) for Fe(III)-petrobactin is estimated to be about 43. The pK(a) of the beta-hydroxyaspartyl hydroxyl group was determined to be 10.8 by (1)H NMR titration. (13)C NMR and IR spectroscopy were used to investigate Ga(III) coordination to the marinobactins. The coordination-induced shifts (CIS) in the (13)C NMR spectrum of Ga(III)-marinobactin-C compared to apo-marinobactin-C indicates that the hydroxamate groups are coordinated to Ga(III); however, the lack of CISs for the carbons of the beta-hydroxyamide group suggests this moiety is not coordinated in the Ga(III) complex. Differences in the IR spectrum of Ga(III)-marinobactin-C and Fe(III)-marinobactin-C in the 1600-1700 cm(-1) region also corroborates Fe(III) is coordinated to the beta-hydroxyamide moiety, whereas Ga(III) is not coordinated.

XAS study of a metal-induced phase transition by a microbial surfactant

The metal-induced micelle-to-vesicle phase change that the ferric complex of the microbially produced amphiphile, marinobactin E (M(E)), undergoes has been investigated by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). Marinobactin E is one member of the suite of siderophores, marinobactins A-E, that are used by the source bacterium to facilitate iron acquisition. Fe(III)-M(E) undergoes a micelle-to-multilamellar vesicle transition in the presence of Cd(II) and Zn(II). XRD measurements indicate the interlamellar repeat distance of the Cd(II)- and Zn(II)-induced multilamellar vesicles is approximately 5.3 nm. XAS spectra of the sedimented Cd(II)- and Zn(II)-induced multilamellar vesicles suggests hexadentate coordination of Cd(II) and Zn(II) consisting of two monodentate carboxylate ligands and four water ligands. This coordination environment supports the hypothesis that Cd(II) and Zn(II) bridge the terminal carboxylate moiety of two Fe(III)-M(E) headgroups, pulling the headgroups together in an arrangement that favors vesicle formation over the formation of micelles. XAS spectra of the Fe(III) center in the sedimented Cd(II)- and Zn(II)-induced vesicles confirm the anticipated six-coordinate geometry of Fe(III) by the M(E) headgroup via the two hydroxamate groups and the alpha-hydroxy amide moiety.