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PubMed1997

Characterization of the interaction of Ca2+ with hydroxy and non-hydroxy fatty acid species of cerebroside sulfate by Fourier transform infrared spectroscopy and molecular modeling.

Ca2+-mediated interactions between the carbohydrate groups of glycolipids, including that of cerebroside sulfate (galactosylceramide I3-sulfate), have recently been implicated as a basis of cell recognition and adhesion. Hydroxylation of the fatty acid of this lipid has an effect on these interactions. Therefore, FT-IR spectroscopy was used to study the interaction of Ca2+ with semisynthetic hydroxy (HFA) and non-hydroxy fatty acid (NFA) species of cerebroside sulfate (CBS). Ca2+ caused partial dehydration of the sulfate group and reduced hydrogen bonding of the sugar hydroxyls of both species. The amide I and II bands of the lipids in the absence of Ca2+ (NH4+ salt forms) suggested that the N-H of the HFA species is involved in a bent intramolecular hydrogen bond, probably with the fatty acid hydroxyl group and the glycosidic oxygen, while that of the NFA species is involved in a linear intermolecular hydrogen bond with the C=O and/or other oxygens. Ca2+ caused a rearrangement of the hydrogen-bonding network in the interfacial region of the HFA species involving the amide group. The results suggested increased hydrogen bonding of the C=O and a shift in hydrogen bonding of the N-H of the Ca2+ salt form of the HFA species from a bent intramolecular hydrogen bond to a linear intermolecular hydrogen bond, probably with the C=O of neighboring molecules, similar to the NFA species. The involvement of the fatty acid alpha-hydroxyl group in the rearranged network was indicated by a reduction in mobility of the alpha-CH group of the HFA species, in contrast to that of the NFA species. Participation of the alpha-OH group in hydrogen-bonding networks in the interfacial region of both the NH4+ and Ca2+ salt forms caused a significant increase in the interchain packing, as evident from correlation field splitting of the HFA-CBS methylene scissoring mode, while this did not occur for the NFA species. The absence of intramolecular hydrogen bonding of the N-H with the glycosidic oxygen for both salt forms of the NFA species and for the Ca2+ salt form of the HFA species may destabilize the "bent shovel", bilayer planar conformation of the sugar and cause it to be in the extended, bilayer perpendicular conformation. Calculations of the three-dimensional interaction energy of Ca2+ with CBS showed strong binding around the sulfate and the surface of galactose facing the bilayer in the bent shovel conformation. Ca2+ binding at this surface would disrupt intra- and intermolecular hydrogen-bonding interactions of the head group, thus accounting for its effect in inducing a transition to the extended conformation.

Binding Sites↗
PubMed2005

Morphology-controlled synthesis of monodisperse ZnO troughs at the air-water interface under mild conditions.

In this work, an interfacial synthetic strategy to produce ZnO troughs at the air-water interface under mild conditions was demonstrated. The as-prepared ZnO trough was monodisperse, single crystalline, and pure. By manipulation of the magnet stirring time, the fabricated ZnO can be manipulated as an independent or intertwined trough. Various preparative parameters, such as initial reactants and their concentrations, had been examined, and the results revealed that troughlike ZnO could be only obtained by adopting zinc acetate and hexamethylenetetramine as precursors at concentrations larger than 0.01 M. The as-obtained ZnO, within the dimension of several micrometers, showed a trough-related photoluminescence emission at 356 nm. Moreover, the evolution process of a ZnO trough after reaction for 5, 30, 60, and 90 min was viewed by X-ray diffraction and field emission scanning electron microscopy characterization, and a possible mechanism was also proposed and discussed, which might help us to further our understanding toward the reaction mechanism at the air-water interface.

Air↗
PubMed2005

Structural characterization and reliable biomechanical assessment of integrative cartilage repair.

Structural and functional characterization of integrative cartilage repair in controlled model systems can play a key role in the development of innovative strategies to improve the long-term outcome of many cartilage repair procedures. In this work, we first developed a method to reproducibly generate geometrically defined disk/ring cartilage composites and to remove outgrown fibrous layers which can encapsulate cartilaginous tissues during culture. We then used the model system to test the hypothesis that such fibrous layers lead to an overestimation of biomechanical parameters of integration at the disk/ring interface. Transmission electron microscopy images of the composites after 6 weeks of culture indicated that collagen fibrils in the fibrous tissue layer were well integrated into the collagen network of the cartilage disk and ring, whereas molecular bridging between opposing disk/ring cartilage surfaces was less pronounced and restricted to regions with narrow interfacial regions (< 2 microm). Stress-strain profiles generated from mechanical push-out tests for composites with the layers removed displayed a single and distinct peak, whereas profiles for composites with the layers left intact consisted of multiple superimposed peaks. As compared to composites with removed layers, composites with intact layers had significantly higher adhesive strengths (161+/-9 vs. 71+/-11 kPa) and adhesion energies (15.0+/-0.7 vs. 2.7+/-0.4 mJ/mm2). By combining structural and functional analyses, we demonstrated that the outgrowing tissue formed during in vitro culture of cartilaginous specimens should be eliminated in order to reliably quantify biomechanical parameters related to integrative cartilage repair.

Adhesiveness↗
PubMed1996

Phospholipase A2 engineering. Deletion of the C-terminus segment changes substrate specificity and uncouples calcium and substrate binding at the zwitterionic interface.

It has been suggested [Dijkstra, B. W., Drenth, J., & Kalk, K. H. (1981) Nature 289, 604-606] that the interfacial binding site of phospholipase A2 (PLA2) involves a large number of residues, including a cluster at the N-terminus and another cluster at the C-terminus. The approaches of multiple mutation and deletion were used to test the roles of the C-terminal residues of bovine pancreatic PLA2 overexpressed in Escherichia coli. A double mutant K120A/K121A and a deletion mutant delta 115-123/ C27A were constructed, and structural and functional analyses were performed on both mutants. The double mutant showed little perturbation in the global structure on the basis of proton NMR and X-ray crystallographic analyses. The proton NMR analysis of the deletion mutant suggested that a few residues at the active site, the hydrophobic channel, and the calcium binding loop are perturbed, but the global conformation is not changed. The mutants were then characterized for catalytic and binding properties by use of various kinetic and spectroscopic methods. The double mutant behaved in a manner similar to that of the wild type (WT) PLA2 in every property examined. The deletion mutant was found to show an interesting change of substrate specificity. The kcat,app of the zwitterionic DC8PC micelles but not the anionic DC8PM micelles decreased by a factor of > 100; however, the activity of DC8PC was restored upon addition of 4 M NaCl. The results of fluorescence spectroscopic studies indicate that the deletion mutant behaves in a manner similar to that of WT in the binding to anionic vesicles and to zwitterionic neutral diluent. Thus, the binding affinity of the enzyme to the interface (the E to E* step) should not be the main cause for the change in substrate specificity. The cause lies at least partially in the binding of substrate or inhibitor to the active site of the enzyme at the interface, i.e., the E* to E*L step, as revealed by the results of equilibrium binding studies. The equilibrium dissociation constants of ligands are generally higher for the deletion mutant (relative to WT) at the zwitterionic interface but not at the anionic interface. The cause for the low affinity of an active site-directed ligand to the active site at the zwitterionic interface could be related to the inability of Ca2+ to enhance ligand binding for the deletion mutant. This is in contrast to the WT PLA2 for which Ca2+ binding enhances binding of the substrate to the active site. Overall, the structural and functional perturbations caused by deleting the C-terminal segment are modest, but the changes in substrate specificity and the uncoupling between substrate and calcium binding are interesting and significant.

Amino Acid Sequence↗
PubMed2003

Low-friction flows of liquid at nanopatterned interfaces.

With the important development of microfluidic systems, miniaturization of flow devices has become a real challenge. Microchannels, however, are characterized by a large surface-to-volume ratio, so that surface properties strongly affect flow resistance in submicrometre devices. We present here results showing that the concerted effect of wetting properties and surface roughness may considerably reduce friction of the fluid past the boundaries. The slippage of the fluid at the channel boundaries is shown to be greatly increased by using surfaces that are patterned on the nanometre scale. This effect occurs in the regime where the surface pattern is partially dewetted, in the spirit of the 'superhydrophobic' effects that have been discovered at macroscopic scales. Our results show for the first time that, in contrast to common belief, surface friction may be reduced by surface roughness. They also open the possibility of a controlled realization of the 'nanobubbles' that have long been suspected to play a role in interfacial slippage.

Friction↗
PubMed1975

Effect of spatial variations in shear on diffusion at the wall of an arterial branch.

The effect of spatially varying shear on transport to the wall of a two-dimensional branch was examined, using oxygen as the test solute and the results of earlier fluid mechanical calculations to provide the shear profiles in a region characterizing the aortic bifurcation. The numerical technique employed allowed both blood-phase and mural resistances to solute uptake to be treated simultaneously and self-consistently. The calculated profiles of wall concentration and mural flux were significantly different from those which would have obtained if the shear had been uniform. The calculations suggest that, even when solute is rapidly taken up from the blood, the occasional high-shear and flow-development sites encountered along the arterial tree prevent the diffusion boundary layer adjacent to the wall from thickening to the point at which nutrition is compromised. The indirect effect of arterial geometry on transport, consequent to its direct effect on the magnitude and the distribution of the relevant hemodynamic variables, was illustrated using the branch area ratio as the geometric parameter. The shapes of the flux and interfacial concentration profiles along the branch wall were markedly dependent on the extent to which wall shear affected intimal permeability.

Animals↗
PubMed1989

Characterization of the L lambda phase in trehalose-stabilized dry membranes by solid-state NMR and X-ray diffraction.

Solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray powder diffraction were used to investigate the mechanism of trehalose (TRE) stabilization of lipid bilayers. Calorimetric investigation of dry TRE-stabilized bilayers reveals a first-order phase transition (L kappa----L lambda) at temperatures similar to the L beta'----(P beta')----L alpha transition of hydrated lipid bilayers. X-ray diffraction studies show that dry mixtures of TRE and 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC) have a lamellar structure with excess crystalline TRE being present. The L kappa phase shows typical gel-phase X-ray diffraction patterns. In contrast, the L lambda-phase diffraction patterns indicate disordered hydrocarbon chains. 2H NMR of specifically 2H chain-labeled DPPC confirmed that the acyl chains are disordered in the L lambda phase over their entire lengths. 2H spectra of the choline headgroup show hindered molecular motions as compared to dry DPPC alone, and 13C spectra of the sn-2-carbonyl show rigid lattice powder patterns indicating very little motion at the headgroup and interfacial regions. Thus, the sugar interacts extensively with the hydrophilic regions of the lipid, from the choline and the phosphate moieties in the headgroup to the glycerol and carbonyls in the interfacial region. We postulate that the sugar and the lipid form an extensive hydrogen-bonded network with the sugar acting as a spacer to expand the distance between lipids in the bilayer. The fluidity of the hydrophobic region in the L lambda phase together with the bilayer stabilization at the headgroup contributes to membrane viability in anhydrobiotic organisms.

Cell Survival↗
PubMed1994

Phosphatidate Kinase, A Novel Enzyme in Phospholipid Metabolism (Characterization of the Enzyme from Suspension-Cultured Catharanthus roseus Cells).

Phosphatidate kinase (adenosine 5[prime]-triphosphate:phosphatidic acid phosphotransferase), a novel enzyme of phospholipid metabolism, was detected recently in the plasma membranes of suspension-cultured Catharanthus roseus cells and purified (J.B. Wissing, H. Behrbohm [1993] Plant Physiol 102: 1243-1249). In the present work the properties of phosphatidate kinase are described. The enzyme showed a pH optimum of 6.1 and an isoelectric point of 4.8, and was rather stable in the presence of its substrates. Although the kinase accepted both ATP and GTP, with Km values of about 12 and 18 [mu]M, respectively, the only lipid substrate was phosphatidic acid; neither lysophosphatidic acid nor any other lipid tested was phosphorylated. With 32P- and 14C-labeled diacylglycerol pyrophosphate, the product of the enzyme, it was shown that the kinase catalyzes a reversible reaction. The activity of the extracted enzyme depended on the presence of surfactants such as Triton X-100 or [beta]-octylglucoside, whereas deoxycholate was strongly inhibitory. Kinetic analysis with Triton X-100/phosphatidate mixed micelles performed according to the "surface dilution" kinetic model showed saturation kinetics with respect to both bulk and surface concentration of phosphatidate. The interfacial Michaelis constant for phosphatidate was determined as 0.6 mol %.

Journal Article↗
PubMed1992

Characterization by infrared spectroscopy of the interaction of a cardiotoxin with phosphatidic acid and with binary mixtures of phosphatidic acid and phosphatidylcholine.

The effect of cardiotoxin IIa from Naja mossambica mossambica, a small basic protein extracted from snake venom, on dimyristoylphosphatidic acid (DMPA) and on equimolar mixtures of DMPA and dimyristoylphosphatidylcholine (DMPC) has been studied by Fourier transform infrared spectroscopy. The interaction of cardiotoxin with DMPA dispersions decreases both the cooperativity of the phase transition of the lipid and the molecular order of the lipid acyl chains in the gel phase. This effect increases with the proportion of the toxin in the complexes and leads to the total abolition of the phase transition of DMPA at a lipid-to-protein molar ratio of 5. Small-angle X-ray results demonstrate that the structure of the lipid-protein complexes is poorly ordered and gives rise to broad diffusion peaks rather than to well-resolved diffraction patterns. Infrared spectra of oriented cardiotoxin-DMPA films show that the protein is not homogeneously oriented with respect to the bilayer surface. The destabilization of the gel-phase structure of DMPA by cardiotoxin also results in a deeper water penetration in the interfacial region of the lipid since more carbonyl ester groups appear to be hydrogen bonded in the presence of the toxin. The infrared results on the phosphate group vibrations also indicate clearly that the basic residues of cardiotoxin interact strongly with the phosphate group of DMPA that becomes partly ionized at a pH as low as 6.5. The results obtained on the interaction of cardiotoxin with an equimolar mixture of DMPA and DMPC clearly demonstrate the ability of this toxin to induce lateral phase separation in this mixture with one phase containing DMPA-rich domains perturbed by cardiotoxin while the second phase is composed of regions enriched in DMPC. Comparison of the results of the current study with those obtained on other basic proteins and polypeptides suggests that charge-induced phase separation occurs only when the charge density on certain regions of the protein structure is high enough to lead to efficient electrostatic interactions with anionic phospholipids. This condition occurs only when the conformation of the protein or polypeptide is well-ordered at the lipid interface.

Cobra Cardiotoxin Proteins↗
PubMedDate not supplied

Mechanisms of surfactant dysfunction in early acute lung injury.

Surfactant dysfunction that occurs during acute lung injury is associated with alterations in phospholipid, total protein, and surfactant apoprotein content. The functional importance of these changes was examined by characterizing the biophysical properties and biochemical composition of lung surfactant from endotoxin-treated guinea pigs (LPS) with acute lung injury. Static and dynamic lung compliance significantly decreased following endotoxin exposure. Lavage fluid demonstrated a neutrophil predominance, and tissue histopathology revealed inflammation consistent with acute lung injury. LPS surfactant isolated by ultracentrifugation had minimum surface tensions of 21 dynes/cm compared to 2 dynes/cm among control samples. Biochemical abnormalities in LPS surfactant included increased total protein, decreased phosphatidylcholine, and increased sphingomyelin, phosphatidylethanolamine, and lysophosphatidylcholine. The addition to normal guinea pig surfactant of butanol extracts precipitated from lavage fluid of LPS animals and containing known amounts of protein caused elevations in minimum surface tensions to > or = 20 dynes/cm at protein to phospholipid ratios equivalent to those observed in LPS surfactant pellets. Addition of equal amounts of precipitate isolated from control animals had no effect on interfacial properties. Furthermore, addition of lysophosphatidylcholine and sphingomyelin to normal surfactant to simulate composition changes observed in LPS surfactant had minimal effect on surface film behavior. The results support the hypothesis that aqueous soluble inhibitors of surfactant are generated within the alveolar compartment during acute inflammation, and that surfactant dysfunction cannot be accounted for on the basis of phospholipid composition changes.

Animals↗
PubMed2000

[A surface reacted layer study of titanium-zirconium alloy after dental casting].

OBJECTIVE: To investigate the influence of the mold temperature on the surface reacted layer of Ti-Zr alloy castings. METHODS: Ti-Zr alloy was casted into a mold which was made of a zircon (ZrO2.SiO2) for inner coating and a phosphate-bonded material for outer investing with a casting machine (China) designed as vacuum, pressure and centrifuge. At three mold temperatures (room temperature, 300 degrees C, 600 degrees C) the Ti-Zr alloy was casted separately. The surface roughness of the castings was calculated by instrument of smooth finish (China). From the surface to the inner part the Knoop hardness and thickness in reacted layer of Ti-Zr alloy casting was measured. The structure of the surface reacted layer was analysed by SEM. Elemental analyses of the interfacial zone of the casting was made by element line scanning observation. RESULTS: The surface roughness of the castings was increased significantly with the mold temperature increasing. At a higher mold temperature the Knoop hardness of the reactive layer was increased. At the three mold temperature the outmost surface was very hard, and microhardness data decreased rapidly where they reached constant values. The thickness was about 85 microns for castings at room temperature and 300 degrees C, 105 microns for castings at 600 degrees C. From the SEM micrograph of the Ti-Zr alloy casting, the surface reacted layer could be divided into three different layers. The first layer was called non-structure layer, which thickness was about 10 microns for room temperature group, 20 microns for 300 degrees C and 25 microns for 600 degrees C. The second layer was characterized by coarse-grained acicular crystal, which thickness was about 50 microns for three mold temperatures. The third layer was Ti-Zr alloy. The element line scanning showed non-structure layer with higher level of element of O, Al, Si and Zr, The higher the mold temperature during casting, the deeper the Si permeating and in the second layer the element Si could also be found. CONCLUSION: The mold temperature is one of the major factors influencing to casting quality. In order to reduce the surface reacted layer of Ti-Zr alloy castings, the lower mold temperature and the investment without Si should be chosen.

Dental Alloys↗
PubMed2000

Infrared spectroscopic characterization of the interaction of cationic lipids with plasmid DNA.

Fourier transform infrared spectroscopy was used to characterize the interaction of the cationic lipids 1,2-dioleoyl-3-trimethylammonium-propane and dioctadecyldimethylammonium bromide with plasmid DNA. The effect of incorporating the neutral colipids cholesterol and dioleoylphosphatidylethanolamine on this interaction was also examined. Additionally, dynamic and phase analysis light scattering were used to monitor the size and zeta potential of the resulting complexes under conditions similar to the Fourier transform infrared measurements. Results suggest that upon interaction of cationic lipids with DNA, the DNA remains in the B form. Distinct changes in the frequency of several infrared bands arising from the DNA bases, however, suggest perturbation of their hydration upon interaction with cationic lipids. A direct interaction of the lipid ammonium headgroup with and dehydration of the DNA phosphate is observed when DNA is complexed with these lipids. Changes in the apolar regions of the lipid bilayer are minimal, whereas the interfacial regions of the membrane show changes in hydration or molecular packing. Incorporation of helper lipids into the cationic membranes results in increased conformational disorder of the apolar region and further dehydration of the interfacial region. Changes in the hydration of the DNA bases were also observed as the molar ratio of helper lipid in the membranes was increased.

Cholesterol↗
PubMed2001

Recombinant expression and characterization of the Candida rugosa lip4 lipase in Pichia pastoris: comparison of glycosylation, activity, and stability.

Although Candida rugosa utilizes a nonuniversal serine codon (CUG) for leucine, it is possible to express lipase genes (LIP) in heterologous systems. After replacing the 19 CUG codons in LIP4 with serine codons by site-directed mutagenesis, a recombinant LIP4 was functionally overexpressed in Pichia pastoris in this study. This recombinant glycosylated lipase was secreted into the culture medium with a high purity of 100 mg/liter in a culture broth. Purified recombinant LIP4 had a molecular mass of 60 kDa, showing a range similar to that of lipase in a commercial preparation. Since LIP4 has only a glycosylation site at position Asn-351, this position may also be the major glycosylation site in C. rugosa lipases. Although the thermal stability of recombinant LIP4 significantly increased from 52 to 58 degrees C after glycosylation, there were no significant differences in the catalytic properties of recombinant glycosylated lipase from P. pastoris and the unglycosylated one from Escherichia coil. These two recombinant LIP4s showed higher esterase activities toward long-chain ester (C16 and C18) and exhibited higher lipase activities toward unsaturated and long-chain lipids. In addition, LIP4 does not show interfacial activation as compared with LIP1 toward lipid substrates of tributyrin and triolein. These observations demonstrated that LIP4 shows distinguished catalytic activities with LIP1 in spite of their high sequence homology.

Candida↗
PubMed1999

Colloidal Protein Solutions as a New Standard Sensor for Adhesive Wettability Measurements.

Wetting experiments are carried out for the investigation of the interfacial interactions that take place when various solid materials are in contact with protein colloidal solutions. These materials are chosen because they are widely used in the field of orthopedic surgery. Contact angles (sessile drop) and surface tension (du Nouy's ring method) are measured in standardized conditions which take into account the various parameters disturbing wetting phenomena. An original cumulative deposits method (CDM) described here permits observation of the evolution of the wettability and allows estimation of the protein adhesion rate. Two families of materials emerge according to their behavior when they are in contact with the protein solution. One set (a chromium cobalt alloy and two polyethylenes) presents no detectable adhesion. The other set (stainless steel, zirconia, alumina, virgin and nitrogen implanted titanium alloy) shows a cumulative adhesion of proteins. We propose to adopt a BSA buffered solution as a referential sensor to characterize solid surfaces. From the CDM, the practical parameter P*, called the adhesive wettability power, can be used in order to analyze and classify biomaterials. Copyright 1999 Academic Press.

Journal Article↗
PubMed2004

Mechanical compression-induced pressure sores in rat hindlimb: muscle stiffness, histology, and computational models.

Pressure sores affecting muscles are severe injuries associated with ischemia, impaired metabolic activity, excessive tissue deformation, and insufficient lymph drainage caused by prolonged and intensive mechanical loads. We hypothesize that mechanical properties of muscle tissue change as a result of exposure to prolonged and intensive loads. Such changes may affect the distribution of stresses in soft tissues under bony prominences and potentially expose additional uninjured regions of muscle tissue to intensified stresses. In this study, we characterized changes in tangent elastic moduli and strain energy densities of rat gracilis muscles exposed to pressure in vivo (11.5, 35, or 70 kPa for 2, 4, or 6 h) and incorporated the abnormal properties that were measured in finite element models of the head, shoulders, pelvis, and heels of a recumbent patient. Using in vitro uniaxial tension testing, we found that tangent elastic moduli of muscles exposed to 35 and 70 kPa were 1.6-fold those of controls (P < 0.05, for strains </=5%) and strain energy densities were 1.4-fold those of controls (P < 0.05, for strains >/=5%). Histological (phosphotungstic acid hematoxylin) evaluation showed that this stiffening accompanied extensive necrotic damage. Incorporating these effects into the finite element models, we were able to show that the increased muscle stiffness in widening regions results in elevated tissue stresses that exacerbate the potential for tissue necrosis. Interfacial pressures could not predict deep muscle (e.g., longissimus or gluteus) stresses and injuring conditions. We conclude that information on internal muscle stresses is required to establish new criteria for pressure sore prevention.

Animals↗
PubMed2005

Electrochemical and spectroscopic investigations of immobilized de novo designed heme proteins on metal electrodes.

On the basis of rational design principles, template-assisted four-helix-bundle proteins that include two histidines for coordinative binding of a heme were synthesized. Spectroscopic and thermodynamic characterization of the proteins in solution reveals the expected bis-histidine coordinated heme configuration. The proteins possess different binding domains on the top surfaces of the bundles to allow for electrostatic, covalent, and hydrophobic binding to metal electrodes. Electrostatic immobilization was achieved for proteins with lysine-rich binding domains (MOP-P) that adsorb to electrodes covered by self-assembled monolayers of mercaptopropionic acid, whereas cysteamine-based monolayers were employed for covalent attachment of proteins with cysteine residues in the binding domain (MOP-C). Immobilized proteins were studied by surface-enhanced resonance Raman (SERR) spectroscopy and electrochemical methods. For all proteins, immobilization causes a decrease in protein stability and a loosening of the helix packing, as reflected by a partial dissociation of a histidine ligand in the ferrous state and very low redox potentials. For the covalently attached MOP-C, the overall interfacial redox process involves the coupling of electron transfer and heme ligand dissociation, which was analyzed by time-resolved SERR spectroscopy. Electron transfer was found to be significantly slower for the mono-histidine-coordinated than for the bis-histidine-coordinated heme. For the latter, the formal heterogeneous electron-transfer rate constant of 13 s(-1) is similar to those reported for natural heme proteins with comparable electron-transfer distances, which indicates that covalently bound synthetic heme proteins provide efficient electronic communication with a metal electrode as a prerequisite for potential biotechnological applications.

Biophysics↗
PubMed2001

Sonochemical Processes and Formation of Gold Nanoparticles within Pores of Mesoporous Silica.

Mesoporous silica with gold nanoparticles inside its pores was prepared by the soaking and ultrasound-induced reduction method. This new composite was characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and high-resolution transmission electron microscopy (HRTEM) techniques. The results showed that nearly spherical-shaped gold nanoparticles, with mean size in diameter of 5.2 nm, are located in the pores, most of which are less than 6 nm in diameter. The ultrasonic irradiation time dependence of optical absorption for the soaked porous solid sample, as suggested by the variation in absorbance at 310 and 544 nm, indicated the reduction of Au (III) ions, and the nucleation and aggregation of gold nanoparticles within pores of mesoporous silica. Additionally, the reaction rates estimated phenomenologically by the absorbance decay at 310 nm for both the porous sample and the corresponding soaking solution presented the enhancement of the sonochemical reduction rate of Au (III) ions within pores of mesoporous silica. It is assumed that the extensive liquid-solid interfacial zones in the pores, due to the high specific surface areas and great porosity of the mesoporous solid, are the major regions where the efficient sonochemical reduction induced by the cavitation takes place. Copyright 2001 Academic Press.

Journal Article↗
PubMed2000

A spectroscopic analysis of thermal stability of the Chromobacterium viscosum lipase.

The thermal stability of the lipase from Chromobacterium viscosum was assessed by deactivation (loss of activity), fluorescence, circular dichroism (CD) and static light scattering (SLS) measurements. Lipase fluorescence emission is dominated by the tryptophyl contribution. An increase in the tyrosyl contribution from 2 to 16% was only observed upon prolonged incubation at 60 degrees C. The effect of temperature on the tryptophyl quantum yield was studied and two activation energies were calculated. Tryptophan residues in the native structure have an activation energy of 1.9 kcal mol(-1) for temperature-dependent non-radiative deactivation of the excited state. A structural change occurs at approximately 66.7 degrees C and the activation energy increases to 10.2 kcal mol(-1). This structural change is not characterized by tryptophan exposure on the surface of the protein. The deactivation and the evolution of structural changes with time after lipase incubation at 60 degrees C were assessed by fluorescence, CD and SLS measurements. CD spectra show that both secondary and tertiary structures remain native-like after incubation at 60 degrees C in spite of the fluorescence changes observed (red-shift from 330 to 336 nm on the trytophyl emission). SLS measurements together with the CD data show that deactivation may be due to protein association between native molecules. Deactivation and the decrease on the fraction of non-associated native lipase evaluated by changes in fluorescence intensity with time, show apparent first order kinetics. According to the rate constants, fluorescence changes precede deactivation pointing to an underestimation of the deactivation. Reactivation upon dilution during the activity assay and substrate-induced reactivation due to lipase interfacial adsorption are possible causes for this underestimation.

Chemical Phenomena↗