On the resolution capabilities and limits of fluorescence lifetime correlation spectroscopy (FLCS) measurements.

Quantitative tests were performed in order to explore the practical limits of FLCS. We demonstrate that: a) FLCS yields precise and correct concentration values from as low as picomolar to micromolar concentrations; b) it is possible to separate four signal components in a single detector setup; c) diffusion times differing only 25% from each other can be resolved by separating a two component mixture based on the different fluorescence lifetimes of both components; d) most of the inherent technical limitations of conventional FCS are easily overcome by FLCS employing a single detector channel confocal detection scheme.

Single molecule genotyping by TIRF microscopy.

As part of a programme to develop a metrological framework for single molecule measurements in biology, we have investigated the applications of single molecule imaging to genomics. Specifically, we have developed a technique for measuring the frequencies of single nucleotide polymorphisms (SNPs) in complex or pooled samples of DNA. We believe that this technique has applications to statistical genotyping-the identification of correlations between SNP frequencies and particular phenotypes-and other areas where it is desirable to track the frequencies of SNPs in complex DNA populations.

Silencing of human ferrochelatase causes abundant protoporphyrin-IX accumulation in colon cancer.

Hemes and heme proteins are vital components of essentially every cell of virtually every eukaryote organism. Previously, we demonstrated accumulation of the heme precursor protoporphyrin-IX (PpIX) in gastrointestinal tumor tissues. To elucidate the mechanisms of PpIX accumulation by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), we studied expression of the relevant enzymes of the heme synthetic pathway. Here, we describe a significant down-regulation of ferrochelatase (FECH) mRNA expression in gastric, colonic, and rectal carcinomas. Accordingly, in an in vitro model of several carcinoma cell lines, ferrochelatase down-regulation and loss of enzymatic activity corresponded with an enhanced PpIX-dependent fluorescence. Direct detection of PpIX in minute amounts was achieved by a specifically developed pulsed solid-state laser dual delay fluorimetry setup. Silencing of FECH using small interfering RNA (siRNA) technology led to a maximum 50-fold increased PpIX accumulation, imageable by a specifically adapted two-photon microscopy unit. Our results show that in malignant tissue a transcriptional down-regulation of FECH occurs, which causes endogenous PpIX accumulation. Furthermore, accumulation of intracellular PpIX because of FECH siRNA silencing provides a small-molecule-based approach to molecular imaging and molecular therapy.

Accurate single-pair Förster resonant energy transfer through combination of pulsed interleaved excitation, time correlated single-photon counting, and fluorescence correlation spectroscopy.

Quantitative distance measurements are difficult to obtain in spite of the strong distance dependency of the energy transfer efficiency. One problem for the interpretation of the Forster resonant energy transfer (FRET) efficiency is the so-called zero-efficiency peak caused by FRET pairs with missing or nonfluorescent acceptors. Other problems occurring are direct excitation of the acceptor, spectral crosstalk, and the determination of the quantum efficiency of the dyes as well as the detector sensitivity. Our approach to overcome these limitations is based on the pulsed-interleaved excitation (PIE) of both the acceptor and the donor molecule. PIE is used to excite the acceptor dye independently of the FRET process and to prove its existence via fluorescence. This technique enables us to differentiate a FRET molecule, even with a very low FRET efficiency, from a molecule with an absent or non-fluorescent acceptor. Crosstalk, direct acceptor excitation, and molecular brightness of acceptor and donor molecules are determined by analyzing the data with fluorescence correlation spectroscopy (FCS). FRET efficiencies of the same data set are also determined by analyzing the lifetimes of the donor fluorophores. The advantages of the PIE-FRET approach are demonstrated on a polyproline assay labeled with Alexa-555 and Alexa-647 as donor and acceptor, respectively.