Synthesis of isomaltooligosaccharides and oligodextrans in a recycle membrane bioreactor by the combined use of dextransucrase and dextranase

A recycle ultrafiltration membrane reactor was used to develop a continuous synthesis process for the production of isomaltooligosaccharides (IMO) from sucrose, using the enzymes dextransucrase and dextranase. A variety of membranes were tested and the parameters affecting reactor stability, productivity, and product molecular weight distribution were investigated. Enzyme inactivation in the reactor was reduced with the use of a non-ionic surfactant but its use had severe adverse effects on the membrane pore size and porosity. During continuous isomaltooligosaccharide synthesis, dextransucrase inactivation was shown to occur as a result of the dextranase activity and it was dependent mainly on the substrate availability in the reactor and the hydrolytic activity of dextranase. Substrate and dextranase concentrations (50-200 mg/mL(-1) and 10-30 U/mL(-1), respectively) affected permeate fluxes, reactor productivity, and product average molecular weight. The oligodextrans and isomaltooligosaccharides formed had molecular weights lower than in batch synthesis reactions but they largely consisted of oligosaccharides with a degree of polymerization (DP) greater than 5, depending on the synthesis conditions. No significant rejection of the sugars formed was shown by the membranes and permeate flux was dependent on tangential flow velocity. (C) 2004 Wiley Periodicals, Inc.

Selective separation of beta-lactoglobulin from sweet whey using CGAs generated from the cationic surfactant CTAB

The selective separation of whey proteins was studied using colloidal gas aphrons generated from the cationic surfactant cetyl trimethyl ammonium bromide (CTAB). From the titration curves obtained by zeta potential measurements of individual whey proteins, it was expected to selectively adsorb the major whey proteins, i.e., bovine serum albumin, alpha-lactalbumin, and beta-lactoglobulin to the aphrons and elute the remaining proteins (lactoferrin and lactoperoxidase) in the liquid phase. A number of process parameters including pH, ionic strength, and mass ratio of surfactant to protein (M-CTAB/M-TP) were varied in order to evaluate their effect on protein separation. Under optimum conditions (2 mmol/l CTAB, M-CTAB/M-TP = 0.26-0.35, pH 8, and ionic strength = 0.018 mol/l), 80-90% beta-lactoglobulin was removed from the liquid phase as a precipitate, while about 75% lactoferrin and lactoperoxidase, 80% bovine serum albumin, 95% immunoglobulin, and 65% alpha-lactalbumin were recovered in the liquid fraction. Mechanistic studies using zeta potential measurements and fluorescence spectroscopy proved that electrostatic interactions modulate only partially the selectivity of protein separation, as proteins with similar surface charges do not separate to the same extent between the two phases. The selectivity of recovery of beta-lactoglobulin probably occurs in two steps: the first being the selective interaction of the protein with opposite-charged surfactant molecules by means of electrostatic interactions, which leads to denaturation of the protein and subsequent formation and precipitation of the CTAB-beta-lactoglobulin complex. This is followed by the separation of CTAB-beta-lactoglobulin aggregates from the bulk liquid by flotation in the aphron phase. In this way, CGAs act as carriers which facilitate the removal of protein precipitate. (c) 2005 Wiley Periodicals, Inc.

Structure of single-wall peptide nanotubes: in situ flow aligning X-ray diffraction

The structure of single wall peptide nanotubes is presented for the model surfactant-like peptide A6K. Capillary flow alignment of a sample in the nematic phase at high concentration in water leads to oriented X-ray diffraction patterns. Analysis of these, accompanied by molecular dynamics simulations, suggests the favourable self-assembly of antiparallel peptide dimers into beta-sheet ribbons that wrap helically to form the nanotube wall.

Marked influence of the bridging carbonyl ligands on the photo- and electrochemistry of the clusters [Ru-3(CO)(8)(mu-CO)(2)-(alpha-diimine)] (alpha-diimine = 2,2 '-bipyridine, 4,4 '-dimethyl-2,2 '-bipyridine and 2,2 '-bipyrimidine)

Bonding, photochemical and electrochemical properties of the clusters [Ru-3(CO)(8)(mu-CO)(2)(alpha-diimine)] (alpha-diimine=2,2'-bipyridine (1), 4,4'-dimethyl-2,2'-bipyridine (2) and 2,2'-bipyrimidine (3)) are strongly influenced by the presence of bridging carbonyl ligands. Irradiation at 471 nm initially results in the population of a sigma(Ru-3)pi*(alpha-diimine) excited state. From this state, fast decay takes place to the optically hardly directly accessible pi(Ru/mu-CO) pi*(alpha-diimine) lowest excited state. These assignments agree with theoretical (TD-DFT) results, resonance Raman and picosecond time-resolved infrared spectra. The involvement of the bridging carbonyl ligands in the electron transfer increases the energetic barrier for the formation of open-structure photoproducts such as biradicals and zwitterions. Zwitterions were therefore only obtained in strongly coordinating media such as pyridine at 250 K. The bridging carbonyl ligands also stabilize the radical anions produced upon one-electron reduction of the clusters [Ru-3(CO)(8)(mu-CO)(2)(alpha-diimine)] and observed with cyclic voltammetry, EPR and IR spectroelectrochemistry (for alpha-diimine=2,2'-bipyrimidine). In contrast, open-triangle intermediates formed along the reduction path to [Ru(CO)(2)(alpha-diimine)](n) and [Ru-2(CO)(8)](2-) are more reactive than their triosmium analogues.

Reaction of a phospholipid monolayer with gas-phase ozone at the air-water interface: measurement of surface excess and surface pressure in real time

The reaction between gas-phase ozone and monolayers of the unsaturated lipid 1-palmitoy1-2-oleoyl-sn-glycero-3-phosphocholine, POPC, on aqueous solutions has been studied in real time using neutron reflection and surface pressure measurements. The reaction between ozone and lung surfactant, which contains POPC, leads to decreased pulmonary function, but little is known shout the changes that occur to the interfacial material as a result of oxidation. The results reveal that the initial reaction of ozone with POPC leads to a rapid increase in surface pressure followed by a slow decrease to very low values. The neutron reflection measurements, performed on an isotopologue of POPC with a selectively deuterated palmitoyl strand, reveal that the reaction leads to loss of this strand from the air-water interface. suggesting either solubilization of the product lipid or degradation of the palmitoyl strand by a reactive species. Reactions of H-1-POPC on D2O reveal that the headgroup region of the lipids in aqueous solution is not dramatically perturbed by the reaction of POPC monolayers with ozone supporting degradation of the palmitoyl strand rather than solubilization. The results are consistent with the reaction of ozone with the oleoyl strand of POPC at the air water interface leading to the formation of OH radicals. the highly reactive OH radicals produced can then go on to react with the saturated palmitoyl strands leading to the formation or oxidized lipids with shorter alkyl tails.

Puroindoline-a, a lipid binding protein from common wheat, spontaneously forms prolate protein micelles in solution

The self-assembly in solution of puroindoline-a (Pin-a), an amphiphilic lipid binding protein from common wheat, was investigated by small angle neutron scattering, dynamic light scattering and size exclusion chromatography. Pin-a was found to form monodisperse prolate ellipsoidal micelles with a major axial radius of 112 +/- 4.5 A ˚ and minor axial radius of 40.4 +/- 0.18 A ˚ . These protein micelles were formed by the spontaneous self-assembly of 38 Pin-a molecules in solution and were stable over a wide pH range (3.5–11) and at elevated temperatures (20–65 degC). Pin-a micelles could be disrupted upon addition of the non-ionic surfactant dodecyl-b-maltoside, suggesting that the protein self-assembly is driven by hydrophobic forces, consisting of intermolecular interactions between Trp residues located within a well-defined Trp-rich domain of Pin-a.

Coupling thin layer electrochemistry with epifluorescence microscopy: an expedient way of investigating electrofluorochromism of organic dyes

The first example of thin layer electrochemistry coupled to epifluorescence microscopy in the total internal reflectance mode is described and applied to the investigation of electrochemically modulated fluorescence of an organic dye (chloromethoxytetrazine) in solution. This technique allows to generate full redox switch of fluorescence when converting reversibly the dye into its anion radical, as well as to record the spectral features of the electrogenerated species. Recording simultaneously fluorescence intensity and lifetime along with coulombic charge as a function of the electrode potential will lead to a deep insight into the redox quenching mechanism.

Modulating self-assembly of a nanotape-forming peptide amphiphile with an oppositely charged surfactant

A peptide amphiphile (PA) C16-KTTKS, containing a pentapeptide headgroup based on a sequence from procollagen I attached to a hexadecyl lipid chain, self-assembles into extended nanotapes in aqueous solution. The tapes are based on bilayer structures, with a 5.2 nm spacing. Here, we investigate the effect of addition of the oppositely charged anionic surfactant sodium dodecyl sulfate (SDS) via AFM, electron microscopic methods, small-angle X-ray scattering and X-ray diffraction among other methods. We show that addition of SDS leads to a transition from tapes to fibrils, via intermediate states that include twisted ribbons. Addition of SDS is also shown to enhance the development of remarkable lateral ‘‘stripes’’ on the nanostructures, which have a 4 nm periodicity. This is ascribed to counterion condensation. The transition in the nanostructure leads to changes in macroscopic properties, in particular a transition from sol to gel is noted on increasing SDS (with a further reentrant transition to sol on further increase of SDS concentration). Formation of a gel may be useful in applications of this PA in skincare applications and we show that this can be controlled via development of a network of fine stranded fibrils.

Separation of astaxanthin from cells of Phaffia rhodozyma using colloidal gas aphrons (CGA) in a flotation column

The aim of this study is to investigate the separation of astaxanthin from the cells of Phaffia rhodozyma using colloidal gas aphrons (CGA), which are surfactant stabilized microbubbles, in a flotation column. It was reported in previous studies that optimum recoveries are achieved at conditions that favor electrostatic interactions. Therefore, in this study, CGA generated from the cationic surfactant hexadecyl trimethyl ammonium bromide (CTAB) were applied to suspensions of cells pretreated with NaOH. The different operation modes (batch or continuous) and the effect of volumetric ratio of CGA to feed, initial concentration of feed, operating height, and flow rate of CGA on the separation of astaxanthin were investigated. The volumetric ratio was found to have a significant effect on the separation of astaxanthin for both batch and continuous experiments. Additionally, the effect of homogenization of the cells on the purity of the recovered fractions was investigated, showing that the homogenization resulted in increased purity. Moreover, different concentrations of surfactant were used for the generation of CGA for the recovery of astaxanthin on batch mode; it was found that recoveries up to 98% could be achieved using CGA generated from a CTAB solution 0.8 mM, which is below the CTAB critical micellar concentration (CMC). These results offer important information for the scale-up of the separation of astaxanthin from the cells of P. rhodozyma using CGA.

Recovery of gallic acid with colloidal gas aphrons generated from a cationic surfactant

In a previous study we have demonstrated that gallic acid (GA) in its anionic form can be recovered from aqueous solutions using colloidal gas aphrons (CGA) generated from the cationic surfactant cetyltrimethylammonium bromide (CTAB). The aim of the present work is to get a better understanding of the separation mechanism in order to determine the optimum operating conditions to maximise the recovery of GA while preserving its antioxidant properties. Zeta potential measurements were carried out to characterise the surface charge of GA, CTAB and their mixtures at three different pH conditions (both in buffers and in aqueous solutions). GA interacted strongly with CTAB at pH higher than its pKa 3.14 where it is ionised and negatively charged. However, at pH higher than 7 GA becomes oxidised and loses its antioxidant power. GA recovery was mainly affected by pH, ionic strength, surfactant/GA molar ratio, mixing conditions and contact time. Scale-up of the separation using a flotation column resulted in both higher recovery and reproducibility. Preliminary experiments with grape marc extracts confirmed the potential application of this separation for the recovery of polyphenols from complex feedstocks

2D hexagonal mesoporous platinum films exhibiting biaxial, in-plane pore alignment

The synthesis of 2D hexagonal mesoporous platinum films with biaxial, in-plane pore alignment is demonstrated by electrodeposition through an aligned lyotropic liquid crystal templating phase. Shear force is used to align a hexagonal lyotropic liquid crystalline templating phase of an inexpensive and a commercially available surfactant, C16EO10, at the surface of an electrode. Electrodeposition and subsequent characterisation of the films produced shows that the orientation and alignment of the phase is transferred to the deposited material. Transmission electron microscopy confirms the expected nanostructure of the films, whilst transmission and grazing incidence small angle X-ray scattering analysis confirms biaxial, in plane alignment of the pore structure. In addition further electrochemical studies in dilute sulfuric acid and methanol show that the pores are accessible to electrolyte solution as indicated by a large current flow; the modified electrode therefore has a high surface area, that catalyses methanol oxidation, and the pores have a very large aspect ratio (of theoretical maximum 2 × 105). Films with such aligned mesoporosity will advance the field of nanotechnology where the control of pore structure is paramount. The method reported is sufficiently generic to be used to control the structure and order of many materials, thus increasing the potential for the development of a wide range of novel electronic and optical devices.

1,1′-Bis(diphenylphosphino)ferrocene bridging two mono(cyclopentadienyl) cobalt moieties: synthesis, structure, electrochemistry and DFT studies

Reaction of [Co(eta(5)-C5H5)(CO)(2)], 1, with 1,1'-bis(diphenylphosphino)ferrocene (dppf) yields the new trinuclear complex [Co(eta(5)-C5H5)(CO)](2)(mu-dppf), 2, which was structurally characterised by single crystal X-ray diffraction and showed two Co(eta(5)-C5H5)(CO) moieties covalently linked by a dppf bridge. Electrochemical studies in dichloromethane revealed that both Co(I) and Fe(II) in the precursors were oxidized to Co(II)/Co(III) and Fe(III), respectively. On the other hand, in 2 the two first oxidation waves were assigned to Co, the Fe(II) centre requiring a higher potential than in free dppf. DFT calculations showed that the HOMOs of 2 were localised in the Co fragments, owing to the destabilisation of the Co(eta(5)-C5H5)(CO) orbitals after binding dppf.

Self-assembled arginine-coated peptide nanosheets in water

The surfactant-like peptide (Ala)6(Arg) is found to self-assemble into 3 nm-thick sheets in aqueous solution. Scanning transmission electron microscopy measurements of mass per unit area indicate a layer structure based on antiparallel dimers. At higher concentration the sheets wrap into unprecedented ultrathin helical ribbon and nanotube architectures.

Interaction between a cationic surfactant-like peptide and lipid vesicles and its relationship to antimicrobial activity

We investigate the properties of an antimicrobial surfactant-like peptide (Ala)6(Arg), A6R, containing a cationic headgroup. The interaction of this peptide with zwitterionic (DPPC) lipid vesicles is investigated using a range of microscopic, X-ray scattering, spectroscopic, and calorimetric methods. The β-sheet structure adopted by A6R is disrupted in the presence of DPPC. A strong effect on the small-angle X-ray scattering profile is observed: the Bragg peaks from the DPPC bilayers in the vesicle walls are eliminated in the presence of A6R and only bilayer form factor peaks are observed. All of these observations point to the interaction of A6R with DPPC bilayers. These studies provide insight into interactions between a model cationic peptide and vesicles, relevant to understanding the action of antimicrobial peptides on lipid membranes. Notably, peptide A6R exhibits antimicrobial activity without membrane lysis.

Tuning chelation by the surfactant-like peptide A6H using predetermined pH values

We examine the self-assembly of a peptide A6H comprising a hexa-alanine sequence A6 with a histidine (H) “head group”, which chelates Zn2+ cations. We study the self assembly of A6H and binding of Zn2+ ions in ZnCl2 solutions, under acidic and neutral conditions. A6H self-assembles into nanotapes held together by a β-sheet structure in acidic aqueous solutions. By dissolving A6H in acidic ZnCl2 solutions, the carbonyl oxygen atoms in A6H chelate the Zn2+ ions and allow for β-sheet formation at lower concentrations, consequently reducing the onset concentration for nanotape formation. A6H mixed with water or ZnCl2 solutions under neutral conditions produces short sheets or pseudocrystalline tapes, respectively. The imidazole ring of A6H chelates Zn2+ ions in neutral solutions. The internal structure of nanosheets and pseudocrystalline sheets in neutral solutions is similar to the internal structure of A6H nanotapes in acidic solutions. Our results show that it is possible to induce dramatic changes in the self-assembly and chelation sites of A6H by changing the pH of the solution. However, it is likely that the amphiphilic nature of A6H determines the internal structure of the self-assembled aggregates independent from changes in chelation.