Analytical investigation of two-step adsorption kinetics on surfaces

Non-fibril amyloid aggregation at the air/water interface: self-adaptive pathway resulting in a 2D Janus nanofilm

The amyloid states of proteins are implicated in several neurodegenerative diseases and bioadhesion processes. However, the classical amyloid fibrillization mechanism fails to adequately explain the formation of polymorphic aggregates and their adhesion to various surfaces. Herein, we report a non-fibril amyloid aggregation pathway, with disulfide-bond-reduced lysozyme (R-Lyz) as a model protein under quasi-physiological conditions. Very different from classical fibrillization, this pathway begins with the air-water interface (AWI) accelerated oligomerization of unfolded full-length protein, resulting in unique plate-like oligomers with self-adaptive ability, which can adjust their conformations to match various interfaces such as the asymmetric AWI and amyloid-protein film surface. The pathway enables a stepwise packing of the plate-like oligomers into a 2D Janus nanofilm, exhibiting a divergent distribution of hydrophilic/hydrophobic residues on opposite sides of the nanofilm. The resulting Janus nanofilm possesses a top-level Young's modulus (8.3 ± 0.6 GPa) among amyloid-based materials and exhibits adhesive strength two times higher (145 ± 81 kPa) than that of barnacle cement. Furthermore, we found that such an interface-directed pathway exists in several amyloidogenic proteins with a similar self-adaptive 2D-aggregation process, including bovine serum albumin, insulin, fibrinogen, hemoglobin, lactoferrin, and ovalbumin. Thus, our findings on the non-fibril self-adaptive mechanism for amyloid aggregation may shed light on polymorphic amyloid assembly and their adhesions through an alternative pathway.

Response surface optimized extraction of 1-deoxynojirimycin from mulberry leaves (Morus alba L.) and preparative separation with resins

In the present study, the extraction technology and preparative separation of 1-deoxynojirimycin from mulberry leaves were systematically investigated. Four extraction parameters (ethanol concentration, extraction temperature, extraction time and ratio of solvent to sample) were explored by response surface methodology (RSM). The results indicated that the maximal yield of 1-deoxynojirimycin was achieved with an ethanol concentration of 55%, extraction temperature of 80 °C, extraction time of 1.2 h and ratio of solvent to sample of 12:1. The extraction yield under these optimum conditions was found to be 256 mg/100 g dry mulberry leaves. A column packed with a selected resin was used to perform dynamic adsorption and desorption tests to optimize the separation process. The results show that the preparative separation of 1-deoxynojirimycin from mulberry leaves can be easily and effectively done by adopting 732 resin. In conclusion, 732 resin is the most appropriate for the separation of 1-deoxynojirimycin from other components in mulberry leaves extracts, and its adsorption behavior can be described with Langmuir isotherms and a two-step adsorption kinetics model. The recovery and purity of 1-deoxynojirimycin in the final product were 90.51% and 15.3%, respectively.

Unusual two-stage kinetics of ethylene adsorption on Si(100) unraveled by surface optical spectroscopy and Monte Carlo simulation

The adsorption of ethylene on a Si(100)-2×1 surface in an ultrahigh vacuum has been monitored at room temperature by use of real-time surface differential reflectance spectroscopy, which clearly demonstrated that the adsorption follows a two-stage process. About half a monolayer is obtained for 1 L, while the second stage is much slower, yielding the complete monolayer for an exposure of ∼400  L. The kinetics over the full range has been successfully reproduced by a Monte Carlo calculation. The key point of this two-stage adsorption kinetic lies in the reduced adsorption probability (by a factor of several hundreds) on the Si dimers, neighbors of dimers which have already reacted, with respect to the adsorption probability on isolated dimers. This new kind of adsorption kinetics, due to a repulsion between already adsorbed molecules and additional molecules impinging on the surface, makes it a textbook case for a "cooperative" adsorption process.

Analysis of kinetic Langmuir model. Part I: Integrated kinetic Langmuir equation (IKL): a new complete analytical solution of the Langmuir rate equation

In the article, a new integrated kinetic Langmuir equation (IKL) is derived. The IKL equation is a simple and easy to analyze but complete analytical solution of the kinetic Langmuir model. The IKL is compared with the nth-order, mixed 1,2-order, and multiexponential kinetic equations. The impact of both equilibrium coverage θ(eq) and relative equilibrium uptake u(eq) on kinetics is explained. A newly introduced Langmuir batch equilibrium factor f(eq) that is the product of both parameters θ(eq)u(eq) is used to determine the general kinetic behavior. The analysis of the IKL equation allows us to understand fully the Langmuir kinetics and explains its relation with respect to the empirical pseudo-first-order (PFO, i.e., Lagergren), pseudo-second-order (PSO), and mixed 1,2-order kinetic equations, and it shows the conditions of their possible application based on the Langmuir model. The dependence of the initial adsorption rate on the system properties is analyzed and compared to the earlier published approximate equations.

Adsorption behaviors of poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers on solid substrates

Adsorption behaviors of functional poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers were investigated on gold and self-assembled monolayer (SAM) by means of time course attenuated total reflection-surface enhanced infrared absorption and surface plasmon resonance spectroscopies. While 1.5th and 2.5th generation (G1.5 and G2.5) ester-terminated dendrimers were slightly adsorbed on all substrates examined, the adsorption of G2 amine-terminated dendrimer increased in the order dodecanethiol SAM<bare gold<3-mercaptopropionic acid (MPA) SAM. The time course results also revealed that the G2 dendrimer displayed the steep increase of adsorption on the bare gold, followed by the slight progress of adsorption. G2 dendrimers adsorbed on MPA SAM involved their subsequent conformational rearrangement, while the adsorption equilibrium was quickly attained for G3 dendrimer on the MPA SAM. It was supported that G3 dendrimer was adsorbed with the perpendicularly extended structure on the MPA SAM substrate, and G2 dendrimer varied its conformation from the globular structure to the extended one perpendicular to the substrate.

Monitoring of the self-assembled monolayer of 1-hexadecanethiol on a gold surface at nanomolar concentration using a piezo-excited millimeter-sized cantilever sensor

In this paper, we describe a new method of measuring alkanethiol monolayer formation on a gold surface. A gold-coated millimeter-sized rectangular-shaped lead zirconate titanate (PZT) cantilever of dimensions 3.5 x 2 x 0.05 mm, previously shown to detect a picogram level of mass change, was used to measure the adsorption kinetics of 1-hexadecanethiol in ethanol over six orders of concentration range (1 nM to 10 mM) in real time. The flexural mode of cantilever vibration, 45.5 +/- 0.01 kHz, was monitored during the self-assembly. The total resonant frequency change obtained for the 1 nM, 10 nM, 100 nM, 1 microM, 4 mM, 8 mM, and 10 mM thiol concentrations were 116 +/- 2 (n = 2), 225 (n = 1), 270 +/- 10 (n = 2), 440 +/- 10 (n = 2), 900 +/- 10 (n = 2), 900 +/- 10 (n = 2), and 900 +/- 10 (n = 2) Hz, respectively. These results compare favorably to literature results in that the rate of the monolayer formation is concentration-dependent and the exponential change during adsorption follows the reversible first-order Langmuir kinetic model. The rate constants of adsorption and desorption were 0.061 M(-1) s(-1) and 3.61 x 10(-4) s(-1), respectively. The significance of the results is that millimeter-sized PZT cantilevers can be used in real-time for characterizing self-assembly of monolayer formation at nanomolar concentration levels. In addition, at 1 nM, the adsorption was found not to be diffusion limited.