Dynamics of nonspecific adsorption of insulin to erythrocyte membranes

Molecules may arrive at targets (receptors, enzymes, etc.) localized on a membrane surface by first adsorbing onto the surface and then surface diffusing to the targets. The flux rate of molecules arriving at targets via this mechanism depends on the surface diffusion coefficient of the molecules and, in some circumstances, on the adsorption/desorption kinetics. The technique of total internal reflection with fluorescence recovery after photobleaching (TIR-FRAP) was used here to study these rate parameters of fluorescein-labeled insulin (f-insulin) interacting with erythrocyte ghosts. Ghosts were adhered to polylysine coated slides for TIR illumination. Some ghosts became flattened and unsealed on the polylysine so that both extracellular and cytoplasmic sides of the membrane were openly exposed to the solution. An aluminum thin film between the polylysine and the fused silica of a slide quenched background fluorescence from f-insulin adsorbed directly onto the polylysine. An interference fringe pattern from two intersecting and totally internally reflecting laser beams provided surface-selective excitation with a spatial variation of illumination intensity across a ghost for surface diffusion measurements. Measured characteristic values of desorption rate constants ranged from 0.043 to 270 s^–1. According to a preexisting theoretical model, the largest desorption rate constant in this range would result in some increase in the total flux rate to a perfect sink target due to capture from the surface, provided that the surface diffusion coefficient was about 10^–8 cm²/s. However, based on TIR-FRAP measurements on our system, we estimate that the surface diffusion coefficient is less than about 5×10^–10 cm²/s. The combination of novel techniques presented here may prove valuable to other workers seeking to make diffusive and chemical kinetic rate parameter measurements of biomolecules at biological cell membranes.     

Thermally stimulated desorption of C<Subscript>60</Subscript> and C<Subscript>70</Subscript> fullerenes from rigid-chain polyimide films

Polyimide-fullerene composite thin coatings are investigated using thermal desorption mass spectrometry in the temperature range 20–800°C. It is found that, at temperatures below the temperature of decom-position of the polymer matrix, thermally stimulated desorption of fullerene molecules is limited by the diffusion of fullerene molecules in the matrix. The diffusion coefficients and activation energies of diffusion of C₆₀ and C₇₀ fullerene molecules are determined from the experimental data on thermally stimulated desorption in the framework of several approaches. It is revealed that the diffusion of C₇₀ molecules in the polyimide matrix is more hindered than the diffusion of C₆₀ molecules in the same matrix.     

Measurements and modeling of long range 3He diffusion in the lung using a "slice-washout" method

In healthy lung tissue, pulsed-gradient-spin-echo (PGSE) methods reveal apparent diffusion coefficients (ADC) of the order 0.20 cm2 s(-1); for diffusion times of approximately 2 ms. For these short diffusion times the ADC is only sensitive to structures approximately (2Dt)1/2 approximately 0.6mm in size. Recent work, using magnetic tagging of the longitudinal magnetization has revealed much smaller ADC values for longer length scales. In this work, the in vivo ADC from within the air-spaces, was measured using a new technique. The signal from a series of images was analyzed from a slice that was repeatedly imaged. Diffusion tends to "top-up" the non-renewable polarization within the slice, which leads to a non-exponential decay in image signal. Image data were compared to 1D finite-difference simulations of diffusion to calculate a long range ADC value. The results yield values of the order 0.034 cm2 s(-1), which are nearly an order of magnitude smaller than those reported by PGSE measurements at shorter diffusion times.     

Escape rate of muonium from micelles — As determined by competition kinetics

A competition was established for the reaction of muonium atoms (Mu) between nitrate ions in water and benzene or styrene solubilized in micelles. The nitrate was 3.3-times more efficient at inhibiting muonated free radical formation with benzene than with styrene as the radical-producing solute. Kinetic analysis of this system indicates that Mu emerges from micelles, on average, at least three times during its short (ns) lifetime, these being medium sized micelles carrying on average 3 benzene molecules. So Mu is certainly not trapped, nor even localized. Its escape rate is estimated to be 9×10⁸ s^–1, which is commensurate with an ordinary diffusion time. The results were obtained by determining the yield of muonated free radicals formed within the micelles using muon-level-crossing-resonance spectroscopy.     

The steady state of heterogeneous catalysis, studied by first-principles statistical mechanics

The turnover frequency of the catalytic oxidation of CO at RuO2(110) was calculated as a function of temperature and partial pressures using ab initio statistical mechanics. The underlying energetics of the gas-phase molecules, dissociation, adsorption, surface diffusion, surface chemical reactions, and desorption were obtained by all-electron density-functional theory. The resulting CO2 formation rate [in the full (T,p(CO),p(O2)) space], the movies displaying the atomic motion and reactions over times scales from picoseconds to seconds, and the statistical analyses provide insight into the concerted actions ruling heterogeneous catalysis and open thermodynamic systems in general.     

脂肪醇聚氧乙烯醚在空气-水界面上铺展单分子膜的脱附性质

采用Wilhelmy法测定脂肪醇聚氧乙烯醚在空气-水界面上铺展单分子膜的动态表面压,并由此计算动态脱附动力学参数.实验结果表明,样品铺展膜的脱附初期扩散系数约10-3～10- 4cm2/sec,远远大于溶液表面吸附的扩散系数,铺展膜的初始表面浓度越大,其初期脱附扩散系数也越大.但是从脱附整个过程看,铺展膜的初始表面浓度越大,其平均脱附速率常数越小,脱附过程是属于扩散控制和表面能垒控制共同作用的混合模式.     

Laser desorption jet-cooling of organic molecules

Laser desorption followed by jet-cooling allows wavelength-selective as well as mass-selective detection of molecules desorbed from a surface without fragmentation. The cooling characteristics and detection sensitivity of laser desorption jet-cooling of organic molecules are investigated. From the rotational contour of the electronic origin of the S ₁ S ₀ transition of laser-desorbed anthracene, rotational cooling to 5–10 K is demonstrated. Vibrational cooling is studied for laser-desorbed diphenylamine, a molecule with low-energy vibrations, and a vibrational temperature below 15 K is found. The absolute detection sensitivity is determined for the perylene molecule. Using two-color (1+1) resonance enhanced multi-photon ionization (with a measured ionization efficiency of 0.25) for detection, it is found that one ion is produced in the detection region for every 2×10⁵ perylene molecules evaporated from the desorption laser spot. A two-color (1+1) REMPI spectrum (400 points) of perylene is recorded using only 30 picogram of material.     

Self-motile colloidal particles: from directed propulsion to random walk

The motion of an artificial microscale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion is fully characterized experimentally. It is shown that at short times it has a substantial component of directed motion, with a velocity that depends on the concentration of fuel molecules. At longer times, the motion reverts to a random walk with a substantially enhanced diffusion coefficient. Our results suggest strategies for designing artificial chemotactic systems.     

Incidence of apparent restricted diffusion in three different models of cerebral infarction

High speed magnetic resonance imaging (MRI) and short diffusion times are used to investigate the appearance of restricted diffusion in three different models of cerebral infarction. The models are: the middle cerebral artery occlusion (MCAO) model in the rat, the carotid occlusion model in the gerbil, and the Rose Bengal microvascular occlusion model in the rat. All three were investigated for 16 b-values equally spaced between 10 and 1510 s/mm² using two distinct experiments. In the ct (constant time) experiment, the diffusion time was held constant at 11.7 ms while the b-value was varied with the gradient strength. In the cg (constant gradient) experiment, the gradient strength was held constant and the b-value increased by varying the diffusion time from 4.4 to 11.7 ms. A monoexponential decay of the signal intensity with b-value in the ct experiment accompanied by nonmonoexponential (NME) decay in the cg experiment is indicative of restricted diffusion. As this phenomenon is detectable only at short diffusion times, it cannot be due to restriction by impermeable membranes, and we have thus termed this apparent restriction. For the MCAO model and the carotid occlusion model, apparent restriction was found both inside the infarct territory and in some regions outside it. No definite evidence for restriction was found for the Rose Bengal model, which was, however, only studied from 24 h post-insult.     

Surface diffusion and entrapment of simple molecules on porous silica

Interactions of simple molecules with the surface of porous silica have been investigated using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. A monolayer of water diffuses into pores at temperatures higher than 110 K. Multilayers of water are also incorporated in pores via sequential surface diffusion. In contrast, a methanol monolayer tends to stay on the surface up to 150 K, and carbon dioxide diffuses into pores rather gradually. Results can be explained as the contribution of hydrogen bonds between the adsorbate–substrate and adsorbate–adsorbate interactions. The predominance of the former (latter) might be responsible for single-molecule migration of methanol and carbon-dioxide (collective diffusion of water molecules) on the surface. These molecules are entrapped at higher coordination sites in pores, as revealed from thermal desorption peaks appearing at higher temperatures than those from non-porous silica. However, no significant difference is observed in desorption kinetics of CF₂Cl₂, Kr, CH₄, and N₂ molecules between the porous and non-porous silica substrates.     

Translational and Rotational Motions of Albumin Sensed by a Non-Covalent Associated Porphyrin Under Physiological and Acidic Conditions: A Fluorescence Correlation Spectroscopy and Time Resolved Anisotropy Study

The interaction between a free-base, anionic water-soluble porphyrin, TSPP, and the drug carrier protein, bovine serum albumin (BSA) has been studied by time-resolved fluorescence anisotropy (TRFA) and fluorescence correlation spectroscopy (FCS) at two different pH-values. Both rotational correlation times and translational diffusion times of the fluorescent species indicate that TSPP binding to albumin induces very little conformational changes in the protein under physiological conditions. By contrast, at low pH, a bi-exponential decay is obtained where a short rotational correlation time (phi (int) = 1.2 ns) is obtained, which is likely associated to wobbling movement of the porphyrin in the protein binding site. These physical changes are corroborated by circular dichroism (CD) data which show a 37% loss in the protein helicity upon acidification of the medium. In the presence of excess porphyrin formation of porphyrin J-aggregates is induced, which can be detected by time-resolved fluorescence with short characteristic times. This is also reflected in FCS data by an increase in molecular brightness together with a decrease in the number of fluorescent molecules passing through the detection volume of the sample.     

Associative desorption of adsorbed oxygen promoted by translational energy of incident O2 molecules

Effects of the translational energy of incident oxygen molecules on surface processes from dissociative adsorption to subsequent associative desorption on Pt(1 1 1) have been investigated by means of supersonic molecular beam scattering. The reaction rate of associative desorption increases with the incident translational energy. The enhanced associative desorption can be attributed to nonthermal diffusion of dissociated oxygen, promoted by the translational energy of incoming molecules.     

Simulation of small molecule diffusion using continuous space disordered networks

Disordered networks were created and kinetic Monte Carlo simulations were conducted in order to assess the effects of jump network connectivity on the diffusion coefficient. Off-lattice jump networks were created using reverse Monte Carlo, with an objective function defined by agreement to specified inter-site connectivity, inter-site (jump path) distance and consecutive jump angle distributions. Both Gaussian and Poisson distributions of connectivity were applied, with average connectivities spanning a range appropriate for small molecules within a variety of polymers. Distance and angle distributions were taken from earlier work on jump networks in polypropylene. At short times, anomalous diffusion with a range of exponents n ≥ 0.4 was found over the domain of average connectivities. At longer times, diffusion was normal for connectivities above the percolation threshold, while particles were trapped for lower connectivities. The percolation threshold was slightly higher for Gaussian distributions of connectivity than for Poisson distributions. The diffusion coefficient increased linearly for connectivities well above the threshold, with slightly faster diffusion occurring for Gaussian distributions of connectivity.     

表面羟基辅助甲醛分子在金红石型TiO2(110)表面迁移动力学研究

As the photo-dissociation product of methanol on the TiO₂(110) surface,the diffusion and desorption processes of formaldehyde (HCHO) were investigated by using scanning tunneling microscope (STM) and density functional theory (DFT).The molecular-level images revealed the HCHO molecules could diffuse and desorb on the surface at 80 K under UV laser irradiation.The diffusion was found to be mediated by hydrogen adatoms nearby,which were produced from photodissociation of methanol.Diffusion of HCHO was significantly decreased when there was only one H adatom near the HCHO molecule.Furthermore,single HCHO molecule adsorbed on the bare TiO₂(110) surface was quite stable,little photo-desorption was observed during laser irradiation.The mechanism of hydroxyl groups assisted diffusion of formaldehyde was also investigated using theoretical calculations.     

Diffusion generated T1 and T2 contrast

In MR images of porous organic samples (such as roots or wood) in water media, the sample is often surrounded by a bright ring, with a corresponding decreased T1 value in T1 maps. When the medium is removed, or contrast agents are added, the ring disappears, indicating that the signal does not originate in the outer layers of the sample, but from the medium itself. It can be shown that this "bright ring effect" is only observed when the medium experiences a reduction in T1 when permeating the sample. In order to investigate this effect, a computer model was used to simulate the diffusion of magnetisation between regions that exhibit different relaxation constants. Using this model, the origin of the signal increase was found to be an inflow effect, as diffusion transports relaxed magnetisation from the boundary regions of the sample into the surrounding medium. In the case of the "bright ring" around the plants described above, a mixing of short T1 values from within the sample and long T1 values within the medium occurs, yielding a "transition region" between the two values. There, a signal increase can be observed at T1 weighted images, compared to the signal from the medium beyond this transition region. The width of the transition region is on the order of magnitude of the diffusion displacement that is calculated from the T1 value as diffusion time. In addition to causing the bright ring around the plant samples, this diffusion effect also limits the resolution of the relaxation time maps. This effect is not limited to T1 relaxation but also applies to T2 relaxation. However, at high B0 field strengths such as those used in this study (11.7 T), a T2 effect is not usually observed due to the considerably shorter T2 times in plants (about 50 ms, compared to T1 times of higher than 1 s). Because the diffusion length during this T2 relaxation is short with respect to the resolution of the imaging experiments, no T2 ring effect is seen.     

Surface electrochemistry for the construction of highly sensitive immunoassay system

The determination of acetycholinesterase (AChE) was carried out based on the chemisorption/reductive desorption of thiocholine on a silver electrode surface. Thiocholine, which was produced through the hydrolysis of acetylthiocholine with AChE, chemisorbed on the silver surface during an incubation time; then the silver electrode was transferred into an alkaline solution and the adsorbent was electrochemically desorbed. The increase in the enzymatic activity resulted in the increase of the number of the thiocholine molecules chemisorbed and, accordingly, that of the cathodic current response for the electrochemical desorption. The thiol molecules were accumulated on the electrode surface through the chemisorption, which resulted in an enhanced sensitivity for measuring the enzyme activity. The detection limit for AChE was 0.01 U l⁻¹, i.e., a hundred times lower than that obtained by a conventional amperometric sensor. This method was successfully applied to the highly sensitive measurement of a peptide hormone, B-type natriuretic peptide (BNP), with the use of the enzyme-labeled anti-BNP antibody.     

Swelling/deswelling of polyacrylamide gels in aqueous NaCl solution: Light scattering and macroscopic swelling study

Swelling kinetics of water-swollen polyacrylamide (PAAm) hydrogels (WSG) was investigated in various concentrations of aqueous NaCl by macroscopic swelling measurements. For lower concentration of NaCl, WSG showed exponential swelling whereas at higher concentration of NaCl it underwent deswelling at short times and exponential swelling at long times. From these studies, collective diffusion coefficient, D, of the polymer network and polymer?Csolvent interaction parameter, ??, were calculated and found to decrease with increase in [NaCl]. Collective diffusion coefficients measured from dynamic light scattering (DLS) and that obtained from macroscopic swelling measurements are found to agree well. Measured ensemble-averaged dynamic structure factor f(q,t) for WSG and salt-swollen gels (SSG) showed an initial decay followed by a plateau at long times and it can be described by harmonically bound Brownian particle (HBBP) model. Enhanced scattering intensity at low scattering angles using static light scattering (SLS) measurements revealed the presence of inhomogeneities in PAAm gels. The reasons for increased scattering intensity of SSG over WSG gel and the linear decrease of D with increase in NaCl concentration are explained.     

Desorption of cesium from pyrolytic graphite basal surfaces with strongly non-equilibrium behaviour

A field reversal kinetic study of the desorption of Cs⁺ and Cs from a graphite basal surface (ZYB graphite) gives results that neither follow the Saha-Langmuir equation, nor fulfil the Schottky energy cycle. The non-equilibrium behaviour is due to a lowering of the rate of the ionic desorption, while the neutral desorption at low temperatures is described by normal rate parameters. At high temperatures, the neutral desorption rate constant is dominated by vertical diffusion into the bulk. The unexpected behaviour of the ion desorption is probably coupled to vertical diffusion too. It is suggested that the desorbing ions leave the surface from special positions on the surface, which are reached during the migration out from the bulk.     

Sulphur overlayers on the Au(1 1 0) surface: LEED and TPD study

The adsorption and desorption of sulphur on the clean reconstructed Au(1 1 0)-(1 × 2) surface has been studied by low energy electron diffraction, Auger electron spectroscopy and temperature programmed desorption. The results obtained show a complex behaviour of the S/Au(1 1 0) system during sulphur desorption at different temperatures. Two structures of the stable ordered sulphur overlayer on the Au(1 1 0) surface, p(4 × 2) and c(4 × 4), were found after annealing the S/Au(1 1 0) system at 630 K and 463 K, respectively. The corresponding sulphur coverage for these overlayers was estimated by AES signal intensity analysis of the Au NOO and S LMM Auger lines to be equal to 0.13 ML and 0.2 ML, respectively. Both sulphur structures appear after removing an excess of sulphur, which mainly desorbs at 358 K as determined from TPD spectra. Furthermore, it was not possible to produce the lower coverage p(4 × 2) sulphur structure by annealing the c(4 × 4) surface. In the case of the p(4 × 2) S overlayer on the Au(1 1 0)-(1 × 2) surface it is proposed that the sulphur is attached to “missing row” sites only. The c(4 × 4) S overlayer arises via desorption of S₂ molecules that are formed on the surface due to mobility of sulphur atoms after a prolonged anneal.     

Ascending air bubbles in solutions of surface-active molecules: Influence of desorption kinetics

We present and discuss experiments on bubble rise velocities performed in various solutions of surface-active molecules; we particularly focus our study on the role of bulk-surface exchanges. For proteins, the vanishing desorption limit (irreversible adsorption) can be achieved experimentally and measurements are found in quantitative agreement with the stagnant cap model. The opposite limit of fast exchange rates is obtained by using short alcohols that turn out to slightly influence bubble velocities; a surface remobilization at high concentrations has been evidenced with isopropyl alcohol. Received 16 November 1999 and Received in final form 24 March 2000