Studies on the interaction of Nile red with horseradish peroxidase in solution

Application of Fluorescence Techniques to Characterise the Preparation of Protein-Containing Sol-Gel Derived Hosts for use as Catalytic Media

In this work we collate and review the usage that we have made of fluorescence techniques employed to follow the sol to gel transition and aging in different tetraethylorthosilicate based materials. The sol-gel method allows porous glasslike of good optical quality to be produced at relatively low (ambient) temperatures, which facilitates the incorporation of a range of molecules; from laser dyes to biomolecules. Here the use of “common” viscosity (DASPMI) and polarity (Nile red) sensitive fluorescence probes to monitor the host manufacture is made. Nile red was also used to label two catalytically active proteins (cytochrome c and subtilisin Carlsberg). These were incorporated into the different host media and the dye used to ascertain changes in protein conformation, both upon incorporation and at the end of an aging period. Complementary measurements of catalytic activity were performed. The probe emission was monitored via steady state and time-resolved fluorescence techniques and comparison made with the catalytic activity measurements to elucidate the amount of accessible and active protein. Overall it was found that the hosts became stable after an aging period approaching 20 days and that the major influence on the catalytic reaction rates was that of host mediated mass transport.

Protein aggregation: pathways, induction factors and analysis

Control and analysis of protein aggregation is an increasing challenge to pharmaceutical research and development. Due to the nature of protein interactions, protein aggregation may occur at various points throughout the lifetime of a protein and may be of different quantity and quality such as size, shape, morphology. It is therefore important to understand the interactions, causes and analyses of such aggregates in order to control protein aggregation to enable successful products. This review gives a short outline of currently discussed pathways and induction methods for protein aggregation and describes currently employed set of analytical techniques and emerging technologies for aggregate detection, characterization and quantification. A major challenge for the analysis of protein aggregates is that no single analytical method exists to cover the entire size range or type of aggregates which may appear. Each analytical method not only shows its specific advantages but also has its limitations. The limits of detection and the possibility of creating artifacts through sample preparation by inducing or destroying aggregates need to be considered with each method used. Therefore, it may also be advisable to carefully compare analytical results of orthogonal methods for similar size ranges to evaluate method performance.

Spectral resolution of a second binding site for Nile Red on cytochrome P4503A4

Nile Red is sequentially metabolized by cytochrome P4503A4 to the N-monoethyl and N-desethyl products, which typifies the metabolism of many amine containing drugs. Sequential metabolism of a single substrate results in complex kinetics that confound predictive models of drug clearance. As a fluorescent model for drugs which undergo sequential metabolism, Nile Red provides the opportunity to monitor drug-CYP interactions wherein the fluorescent properties of Nile Red could, in principle, be exploited to determine individual rate and equilibrium constants for the individual reactions. Previously, it was shown that Nile Red binds at the active site and fluoresces (K(D) approximately 50nM) with maximum emission at approximately 620nm, but it was unclear whether a red-shifted emission, at approximately 660nm, consisted of only free Nile Red or Nile Red bound at a second site on the protein. Here, equilibrium binding studies, including 'reverse titrations' spanning low ratios of CYP3A4/Nile Red, indicate two binding sites for Nile Red with a contribution to the 'red emission' greater than can be accounted for by free Nile Red. Singular value decomposition affords basis spectra for both spectral components and fits well to the experimentally determined concentration dependence of Nile Red emission. In addition, the red spectral component, with an apparent K(D)=2.2muM, is selectively eliminated by titration with the known allosteric effectors of CYP3A4, alpha-napthoflavone and testosterone. Furthermore, the double mutant L2311F/D214E, previously demonstrated to perturb a peripheral allosteric site, lacks the red-emitting Nile Red binding site, but retains the blue-emitting site. Together these data indicate that a second Nile Red site competes with other effectors of CYP3A4 at a site that results in Nile Red emission at 660nm.

Fluorescence probe techniques to monitor protein adsorption-induced conformation changes on biodegradable polymers

The study of protein adsorption and any associated conformational changes on interaction with biomaterials is of great importance in the area of implants and tissue constructs. This study aimed to evaluate some fluorescent techniques to probe protein conformation on a selection of biodegradable polymers currently under investigation for biomedical applications. Because of the fluorescence emanating from the polymers, the use of monitoring intrinsic protein fluorescence was precluded. A highly solvatochromic fluorescent dye, Nile red, and a well-known protein label, fluorescein isothiocyanate, were employed to study the adsorption of serum albumin to polycaprolactone and to some extent also to two starch-containing polymer blends (SPCL and SEVA-C). A variety of fluorescence techniques, steady state, time resolved, and imaging were employed. Nile red was found to leach from the protein, while fluorescein isothiocyanate proved useful in elucidating a conformational change in the protein and the observation of protein aggregates adsorbed to the polymer surface. These effects were seen by making use of the phenomenon of energy migration between the fluorescent tags to monitor interprobe distance and the use of fluorescence lifetime imaging to ascertain the surface packing of the protein on polymer.

Diffusion in a Sol−Gel-Derived Medium with a View toward Biosensor Applications

Time-resolved fluorescence anisotropy and fluorescence recovery after photobleaching were applied to study the diffusion of dyes and a fluorescence-labeled enzyme in a sol-gel-derived medium. This type of medium exhibits attractive properties such as robustness, low processing temperature, high porosity, large internal surface area, and can act as protective immobilization media for biologically active molecules. This makes it a suitable candidate for biosensor applications. The glasslike nature and good optical quality allows for light addressable entities to be incorporated and accessed using spectroscopy. This type of matrix, once formed, can be anything from an ordered gel to a robust glassy block depending on the aging process. In this work we apply confocal microscopy and time-resolved fluorescence techniques to study both rotational and lateral diffusion with aging time within a silica sol-gel derived monolith. An enzyme, horseradish peroxidase, was labeled with Alexa Fluor 488 and rotation related to both the enzyme and the probe monitored during the matrix aging process. Diffusion coefficients of between ca. 0.5 x 10(-7) and 4 x 10(-7) cm2 s(-1) were obtained from preliminary FRAP measurements of fluorescein and correlated to differences in the catalytic activity of HRP incorporated in the monolith.

Effect of the labelling ratio on the photophysics of fluorescein isothiocyanate (FITC) conjugated to bovine serum albumin

The non-linearity of the fluorescence emission on increasing the probe to protein ratio has long been regarded as problematic and has lead to the development of dyes to overcome this effect. One of the causes of this non-linear response can be ascribed to the overlap of the label's own absorption and emission spectra. At higher labelling ratios, this affords the possibility of a reasonably efficient energy migration pathway, thus reducing the observed quantum yield of the dye. In this work we study the photophysics of fluorescein isothiocyanate (FITC) when conjugated to bovine serum albumin (BSA) at different labelling ratios (in the range FITC : BSA 1 : 17-15 : 1) using both steady state and time-resolved fluorescence techniques where on going from under labelled to over labelled samples a decrease in the initial (and steady state) anisotropy is observed, accompanied by an increase in the complexity of the decay kinetics and a decrease in the average lifetime. The band structure, elucidated by synchronous scan fluorescence spectroscopy, is also found to change on increased labelling. These results can be applied to the study of protein conformation and were confirmed by the analysis of denaturing BSA using urea.

Use of fluorescence to monitor the incorporation of horseradish peroxidase into a sol–gel derived medium

The solvatochromic dye nile red has been employed to monitor the incorporation of an enzyme (horseradish peroxidase) into a sol-gel derived medium. The fluorescence spectrum of the dye, when incorporated into the enzyme, was analysed as the sum of Gaussian component spectra and relative changes between these component spectra were monitored upon encapsulation of the dye-enzyme system within the host matrix. Activity of the confined enzyme was verified and the effect of temperature was also investigated, through the examination of nile red fluorescence in the sol-gel derived matrix, where a stabilising effect was noted.