Tip-induced dynamic characterization of multilayers and variable morphologies of cytochrome c molecules on highly oriented pyrolytic graphite revealed by in-situ scanning tunnelling microscopy

The dynamic states of cytochrome c multilayers on electrochemically pretreated highly oriented pyrolytic graphite (HOPG) have been studied by in-situ scanning tunnelling microscopy (STM) under potential control of both the tip and the substrate in cytochrome c and phosphate buffer solution. The dynamic characterization of cytochrome c multilayers and relatively stable adsorbed single cytochrome c molecules scattered on HOPG imply that physically adsorbed multilayers were more easily influenced by the STM tip than those of chemically adsorbed single molecules. In-situ STM images of chemically adsorbed cytochrome c molecules with discernible internal structures on HOPG revealed that morphologies of cytochrome c molecules also suffered tip influence; possible tip-sample-substrate interactions have been discussed.

Viologen-thiol self-assembled monolayers for immobilized horseradish peroxidase at gold electrode surface

A stable, well-behaved self-assembled monolayer (SAM) of viologen-functionalized thiol was used to immobilize and electrically connect horseradish peroxidase (HRP) at gold electrode. Viologen groups in SAMs facilitated the electron transfer from the electrode to the protein active site so that HRP exhibited a quasi-reversible redox behavior. HRP adsorbed in the SAMs is very stable, and close to a monolayer with the surface coverage of 6.5 x 10(-11) mol/cm(2). The normal potential of HRP is -580 mV vs Ag/AgCl corresponding to ferri/ferro active center and the standard electron transfer rate constant is 3.41 s(-1) in 0.1 M phosphate buffer solution (pH 7.1). This approach shows a great promise for designing enzyme electrodes with other redox proteins and practical use in tailoring a variety of amperometric biosensor devices. Copyright (C) 1997 Elsevier Science Ltd.

Direct electron transfer to cytochrome c oxidase in self-assembled monolayers on gold electrodes

The monolayer of cytochrome c oxidase maintaining physiological activity and attached covalently to the self-assembled monolayers of 3-mercaptopropionic acid (MPA) on a gold electrode was obtained. The results of cyclic voltammetry show that direct electron transfer between cytochrome c oxidase and the electrode surface is a fast and diffusionless process. MPA has a dual role as both electrode modifier and the bridging molecule which: keeps cytochrome c oxidase at an appropriate orientation without denaturation and enables direct electron transfer between the protein and the modified electrode. Immobilized cytochrome c oxidase exhibits biphasic phenomena between the concentration of the electrolyte and the normal potentials; meanwhile its electrochemical behavior is also influenced by the buffer components. The quasi-reversible electron transfer process of cytochrome c oxidase with formal potential 385 mV vs. SHE in 5mM phosphate buffer solution (pH 6.4) corresponds to the redox reaction of cyt a(3) in cytochrome c oxidase, and the heterogeneous electron transfer rate constant obtained is 1.56 s(-1). By cyclic voltammetry measurements, it was observed that oxidation and reduction of cytochrome c in solution were catalyzed by the immobilized cytochrome c oxidase. This cytochrome c oxidase/MPA/Au system provides a good mimetic model to study the physiological functions of membrane-associated enzymes and hopefully to build a third-generation biosensor without using a mediator.

Interaction of brilliant cresyl blue and methylene green with DNA studied by spectrophotometric and voltammetric methods

The binding behavior of two cationic dyes, brilliant cresyl blue (BCB) and methylene green (MG) to calf thymus DNA was studied by spectrophotometric and voltammetric methods. A red shift of the adsorption spectra and hypochromism accompany the binding of BCB and MG to calf thymus DNA. In 5 x 10(-2) mol dm(-3) NaCl, 5 x 10(-3) mol dm(-3) tris-HCl pH 6.87 buffer solution, the apparent binding constants are: K-BCB+ 3.0 x 10(4)M(-1) (N = 4.13) and K-MG+ = 8.8 x 10(4)M(-1) (n = 4.44). Electrochemical studies show that the formal potentials shift negatively upon addition of DNA, indicating that the oxidized forms of the dyes have stronger affinity to DNA than the reduced ones. K-BCB+/K-BCBH and K-MG+/K-MGH are evaluated to be 10.39 and 7.04. respectively. Our investigation suggests that the two cationic dyes interact with DNA predominantly via electrostatic interaction.

In situ electrochemical scanning tunnelling microscopy investigation of structure for horseradish peroxidase and its electrocatalytic property

Immobilization of protein molecules is a fundamental problem for scanning tunnelling microscopy (STM) measurements with high resolution. In this paper, an electrochemical method has been proved to be an effective way to fix native horseradish peroxidase (HRP) as well as inactivated HRP from electrolyte onto a highly oriented pyrolytic graphite (HOPG) surface. This preparation is suitable for both ex situ and in situ electrochemical STM (ECSTM) measurements. In situ STM has been successfully employed to observe totally different structures of HRP in three typical cases: (1) in situ ECSTM reveals an oval-shaped pattern for a single molecule in neutral buffer solution, which is in good agreement with the dimension determined as 6.2 x 4.3 x 1.2. nm(3) by ex situ STM for native HRP; (2) in situ ECSTM shows that the adsorbed HRP molecules on HOPG in a denatured environment exhibit swelling globes at the beginning and then change into a V-shaped pattern after 30 min; (3) in situ ECSTM reveals a black hole in every ellipsoidal sphere for inactivated HRP in strong alkali solution. The cyclic voltammetry results indicate that the adsorbed native HRP can directly catalyse the reduction of hydrogen peroxide, demonstrating that a direct electron transfer reduction occurred between the enzyme and HOPG electrode, whereas the corresponding cyclic voltammograms for denatured HRP and inactivated HRP adsorbed on HOPG electrodes indicate a lack of ability to catalyse H2O2 reduction, which confirms that the HRP molecules lost their biological activity. Obviously, electrochemical results powerfully support in situ STM observations.

Calculation of the nonlinear optical coefficient of the NdAl3(BO3)(4) crystal

For the first time, we present the calculation of the nonlinear optical coefficient of the NdAl3(BO3)(4) (NAB) crystal from a systematic and quantitative standpoint. Based on the dielectric theory of complex crystals and the Levine bond charge model, the method of calculation of the second-order nonlinear optical tensor coefficients of complex crystals has been given systematically. The chemical bond parameters and linear and nonlinear susceptibilities of the NAB crystal have been calculated in detail, and the calculated value of d(11)(NAB) is -5.81 x 10(-9) esu, which agrees with the measured value of 4.06 x 10(-9) esu.

Effect of structures of modifiers on electrochemistry of di-mu-oxo dimanganese 2,2'-bipyridine complex

The gold electrodes modified with 2-picolinic acid , nicotinic acid, iso-nicotinic or thiophene were prepared using membrane transfer method, The electrochemistry of di-mu-oxodimanganese 2,2'-bipyridine complex was studied in the acetic acid buffer solution at different modified gold electrodes, It was found that the modifiers which can promote the electrochemical reaction of the complex should be of at least two functional groups, One group can be bound to the electrode surface and the other can form electron transfer pathway between the modifier and the complex through sal; bridge or hydrogen bond, In addition, the mechanism of the electrochemical reaction was discussed.

Ac voltammetric and square wave voltammetric study of the self-assembled monolayers of thiol-functionalized viologen derivative

Electroactive self-assembled monolayers (SAMs) containing viologen group are formed through the adsorption of thiol-functionalized viologen compound CH3(CH2)(9)V2+(CH2)(8)SH, where V2+ is N,N'-dialkylbipyridinium (i.e. a viologen group), onto gold electrodes from methanol/water solution and its electrochemical behavior is investigated ty Ac voltammetry and square wave voltammetry, which have the high sensitivity against background charging. The viologen SAM formed is a sub-monolayer and the normal potentials corresponding to the two successive one-electron transfer processes of the active centers (viologen) are -360 mV and -750 mV (vs. Ag/AgCl) in 0.1 mol/L phosphate buffer solutions (pH 6.96) respectively, and the standard electron transfer rate constant is 9.0 s(-1). The electrochemical behavior of this SAM in various solutions has been preliminarily discussed.

STUDY ON ELECTROCATALYTIC OXIDATION OF NADH BY FERROCENE DERIVATIVES AND DETERMINATION OF CATALYTIC RATE-CONSTANT AT MICRODISK ELECTRODE

The electrocatalytic oxidation of NADH by ferrocene derivatives and the influence of complexation with beta-cyclodextrin (beta-CD) were investigated at a microdisk electrode in a buffer solution. The cyclic voltammetric behavior of the ferrocene derivatives on the microdisk electrode was used to determine the electron-transfer rate constant from NADH to the ferricinium species. The heterogeneous rate constants and the diffusion coefficient of ferrocene derivatives were determined with the microdisk electrode. The effect of temperature and pH on the electrocatalytic oxidation of NADH were assessed.

INTERACTIONS OF RARE-EARTH LENS WITH BOVINE BLOOD CU(ZN)-SUPEROXIDE DISMUTASE

The interactions of lanthanium trichloride and terbium trichloride with bovine blood Cu (Zn)-superoxide dismutase [Cu(Zn)-SOD] in the aqueous solution of hexamethylenetetrarnine buffer (pH = 6.3) have been studied by using fluorescece, CD and ESR spectra. The results indicated that rare earth ions were coordinated to the carboxyl groups of acidic amino acid residues which were far from active center of the Cu(Zn)-SOD molecule and only lightly disturbed the secondary structure of the enzyme protien, and made the coordination structure of enzyme-bound CU2+ come from the rhombchedron to the axial shape at 77 K and the activity of Cu(Zn)-SOD enzyme was not nearly changed at room temperature.

Fluorescence spectroscopy studies on the action of rare earths with bovine serum albumin in multimolecular system

In this paper. the interaction of tripositive terbium ions (Tb3+) with bovine serum albumin (BSA) has been investigated in the presence of some alkaline earth metals and citric acid (Cit) by using fluorescence spectroscopy. The results show that Tb-31. BSA and Cit can form ternary complex BSA . Tb-2. Cit(4) in mu =0.1 mol/l NaCl. pH6.3 hexamethylenetetramine buffer. Other tare earths are able to compete for the same binding site in BSA with Tb3+-. The sequence of con,petition is Eu3+>Pr3+>Yb3+>Gd3+>La3+>Ca2+ and Mg2+ cannot replace Tb bound to BSA.