Detecting beta-amyloid aggregation from time-resolved emission spectra.

The aggregation of beta-amyloids is one of the key processes responsible for the development of Alzheimer's disease. Early molecular-level detection of beta-amyloid oligomers may help in early diagnosis and in the development of new intervention therapies. Our previous studies on the changes in beta-amyloid's single tyrosine intrinsic fluorescence response during aggregation demonstrated a four-exponential fluorescence intensity decay, and the ratio of the pre-exponential factors indicated the extent of the aggregation in the early stages of the process before the beta-sheets were formed. Here we present a complementary approach based on the time-resolved emission spectra (TRES) of amyloid's tyrosine excited at 279 nm and fluorescence in the window 240-450 nm. TRES have been used to demonstrate sturctural changes occuring on the nanosecond time scale after excitation which has significant advantages over using steady-state spectra. We demonstrate this by resolving the fluorescent species and revealing that beta-amyloid's monomers show very fast dielectric relaxation, and its oligomers display a substantial spectral shift due to dielectric relaxation, which gradually decreases when the oligomers become larger.

Glucose-dependent changes in NAD(P)H-related fluorescence lifetime of adipocytes and fibroblasts in vitro: potential for non-invasive glucose sensing in diabetes mellitus.

The aim of this study was to test the hypothesis that glucose can be monitored non-invasively by measuring NAD(P)H-related fluorescence lifetime of cells in an in vitro cell culture model. Autofluorescence decay functions were measured in 3T3-L1 adipocytes by time-correlated single-photon counting (excitation 370nm, emission 420-480nm). Free NADH had a two-exponential decay but cell autofluorescence fitted best to a three-exponential decay. Addition of 30mM glucose caused a 29% increase in autofluorescence intensity, a significantly shortened mean lifetime (from 7.23 to 6.73ns), and an increase in the relative amplitude and fractional intensity of the short-lifetime component at the expense of the two longer-lifetime components. Similar effects were seen with rotenone, an agent that maximizes mitochondrial NADH. 3T3-L1 fibroblasts stained with the fluorescent mitochondrial marker, rhodamine 123 showed a 16% quenching of fluorescence intensity when exposed to 30mM glucose, and an increase in the relative amplitude and fractional intensity of the short lifetime at the expense of the longer lifetime component. We conclude that, though the effect size is relatively small, glucose can be measured non-invasively in cells by monitoring changes in the lifetimes of cell autofluorescence or of a dye marker of mitochondrial metabolism. Further investigation and development of fluorescence intensity and lifetime sensing is therefore indicated for possible non-invasive metabolic monitoring in human diabetes.

Fluorescence nanotomography using resonance energy transfer: demonstration with a protein-sugar complex.

A new approach to structural sensing, fluorescence resonance energy transfer nanotomography, which interprets fluorescence decay measurement in terms of site density analysis of molecular distributions, has been applied to a glucose sensor based on competitive binding with malachite green labelled dextran to the sugar binding protein concanavalin A labelled with allophycocyanin. Opportunities for structural sensing in clinical medicine are highlighted.

Molecular distribution sensing in a fluorescence resonance energy transfer based affinity assay for glucose.

A newly developed method for determining molecular distribution functions is applied to a widely researched glucose affinity sensor. The reduction in fluorescence resonance energy transfer (FRET) to a malachite green (MG)-dextran complex from allophycocyanin (APC) bound to concanavalin A (ConA) due to displacement of the complex by glucose from ConA provides the basis of the assay. The higher sensitivity and specificity of a new approach to fluorescence decay analysis, over the methods based on conventional Forster-type models, is demonstrated and critical parameters in competitive binding FRET sensing derived.

Near-infrared fluorescence lifetime assay for serum glucose based on allophycocyanin-labeled concanavalin A.

We describe an assay scheme for glucose based on fluorescence resonance energy transfer (FRET) between concanavalin A (con A), labeled with the near-infrared fluorescent protein allophycocyanin (APC) as donor, and dextran labeled with malachite green (MG) as acceptor. Glucose competitively displaces dextran-MG and leads to reduction in FRET, assessed by time-domain fluorescence lifetime measurements using time-correlated single-photon counting. The assay is operative in the glucose concentration range 2.5-30 mM, and therefore suitable for use in monitoring diabetes control. Albumin and serum inhibit FRET but the interference can be prevented by removal of high molecular weight substances by membrane filters. APC shows promise for incorporation in an implanted glucose sensor which can be interrogated from outside the body.

A time-resolved near-infrared fluorescence assay for glucose: opportunities for trans-dermal sensing.

We report a time-resolved near-infrared fluorescence assay for glucose detection that incorporates pulsed diode laser excitation. Reduction in fluorescence resonance energy transfer to a malachite green-Dextran complex from allophycocyanin bound to concanavalin A (ConA) due to displacement of the complex by glucose from ConA provides the basis of the assay. The fluorescence quenching kinetics are analysed and discussed in detail. The change in fluorescence decay kinetics in the presence of glucose is found from dimensionality studies to be brought about by a change in the distribution of malachite green-Dextran acceptors. Glucose concentrations are measured in solution to within +/- 10% over the range 0-30 mM.

Metal Ion Quenching Kinetics of DTDCI in Viscous Solution and Nafion Membranes: Model System for Near Infrared Fluorescence Sensing.

Fo¨rster type energy transfer offers opportunities as a sensitive and frequently selective method of monitoring the concentration of analytes in medical sensing applications. However, the fluorescence quenching kinetics observed in microheterogeneous media such as tissue are likely to be less than ideal. In the present article, the quenching kinetics of the carbocyanine dye DTDCI by transition metal ions in solutions and in the microheterogeneous polymer Nafion (a registered trademark of Dupont Corporation) are reported and compared with a view to understanding the complex fluorescence kinetics likely to be encountered in biological media. © 1998 Society of Photo-Optical Instrumentation Engineers.