Jens-Peter Knemeyer

Self-quenching DNA probes based on dye dimerization for identification of mycobacteria

This paper presents new self-quenching DNA probes that exploit the efficient fluorescence quenching by the formation of dye dimers. The probes consist of a hairpin-structured oligonucleotide that is labeled with two identical fluorescence dyes that are able to form non-fluorescent H-type dimers while the hairpin is closed. We used the oxazine derivative MR 121 that has a sufficient dimerization tendency and can be excited by a pulsed diode laser emitting at 635 nm. Upon hybridization to the target DNA, the dyes are separated and a 12-fold increase of the fluorescence intensity occurs. The probe was used for the specific detection of Mycobacterium xenopi in a model system. Specific target DNA and a control target, differing by six nucleotides were amplified by polymerase chain reaction (PCR). A confocal fluorescence microscope was used to observe the fluorescence bursts of individual probe molecules in the presence of target PCR product and controls. By experiment and by the respective simulation we demonstrated that the secondary structure of the target DNA hinders the hybridization to the DNA probe at room temperature. Based on these data a successful hybridization procedure was developed and allowing the detection of nanomolar concentrations of Mycobacterium xenopi specific target at room temperature, using single-molecule detection techniques.

Recent patents on self-quenching DNA probes.

In this review, we report on patents concerning self-quenching DNA probes for assaying DNA during or after amplification as well as for direct assaying DNA or RNA, for example in living cells. Usually the probes consist of fluorescently labeled oligonucleotides whose fluorescence is quenched in the absence of the matching target DNA. Thereby the fluorescence quenching is based on fluorescence resonance energy transfer (FRET), photoinduced electron transfer (PET), or electronically interactions between dye and quencher. However, upon hybridization to the target or after the degradation during a PCR, the fluorescence of the dye is restored. Although the presented probes were originally developed for use in homogeneous assay formats, most of them are also appropriate to improve surface-based assay methods. In particular we describe patents for self-quenching primers, self-quenching probes for TaqMan assays, probes based on G-quartets, Molecular Beacons, Smart Probes, and Pleiades Probes.

Probing conformational dynamics by photoinduced electron transfer

We demonstrate how photoinduced electron transfer (PET) reactions can be successfully applied to monitor conformational dynamics in individual biopolymers. Single-pair fluorescence resonance energy transfer (FRET) experiments are ideally suited to study conformational dynamics occurring on the nanometer scale, e.g. during protein folding or unfolding. In contrast, conformational dynamics with functional significance, for example occurring in enzymes at work, often appear on much smaller spatial scales of up to several Angströms. Our results demonstrate that selective PET-reactions between fluorophores and amino acids or DNA nucleotides represent a versatile tool to measure small-scale conformational dynamics in biopolymers on a wide range of time scales, extending from nanoseconds to seconds, at the single-molecule level under equilibrium conditions. That is, the monitoring of conformational dynamics of biopolymers with temporal resolutions comparable to those within reach using new techniques of molecular dynamic simulations. We present data about structural changes of single biomolecules like DNA hairpins and peptides by using quenching electron transfer reactions between guanosine or tryptophan residues in close proximity to fluorescent dyes. Furthermore, we demonstrate that the strong distance dependence of charge separation reactions on the sub-nanometer scale can be used to develop conformationally flexible PET-biosensors. These sensors enable the detection of specific target molecules in the sub-picomolar range and allow one to follow their molecular binding dynamics with temporal resolution.

Novel fluorescently labeled enzyme substrates for the sensitive detection of HIV-protease

In this paper we applied the efficient fluorescence quenching of the red-absorbing oxazine derivative MR121 by the amino acid tryptophan to develop a new fluorescence based enzyme assay that can be used for detection of exopeptidases and endopeptidases. Therefore, we developed peptide substrates labeled with only one chromophore, which is quenched by a neighbored tryptophan residue via photoinduced electron transfer. The specific cleavage site for the target enzyme is located between the chromophore and the tryptophan residue. After digestion of the substrate the contact formation between tryptophan and fluorescent dye is precluded and a significant increase in fluorescence intensity occurs. To demonstrate the new assay technique for exopeptidases, a substrate for the Carboxypeptidase A was designed and a detection limit below the picomolar range (~10-13 M) was achieved with standard fluorescence spectrometry. The primary objective was the detection of the HIV-protease, which is an endopeptidase digesting substrates containing seven specific amino acids in the cleavage site. We designed a substrate, which enables the detection of 10-9 M HIV-protease, whereas the continuous monitoring of the fluorescence signal also allows kinetic studies.

„Zwerge“ aus dem Minilabor

Die Nanotechnologie in all ihren Facetten wird fur die moderne Gesellschaft immer wichtiger und gilt als eine der Schlusseltechnologien des 21. Jahrhunderts. In diesem Artikel werden schulgerechte Synthesen von Gold- und Silber-Nanopartikeln mit Hilfe eines Minilabors vorgestellt, die im auserschulischen Lernort, im Schulerpraktikum oder auch im Rahmen groserer Projekte eigenstandig durchgefuhrt werden konnen. Auserdem wird ein schulerzentriertes Konzept fur eine Nanotechnologie-Unterrichtseinheit vorgestellt, welches auf den Erwerb von den in den Bildungsplanen geforderten Schlusselkompetenzen zielt und den Schulerinnen und Schulern einen Einblick in den Wissenschaftsalltag und in wissenschaftliche Arbeits- und Vorgehensweisen geben soll. Nanotechnology and its broad range of applications are becoming more and more important for modern society. It is said to be one of the key technologies of the current century. This article shows ways of implementing nanotechnology in schools. The presented syntheses of gold and silver nanoparticles were carried out in a miniaturized lab and adapted for use in class. They are appropriate to be carried out by the pupils themselves in school or in science centers, in brief or in longer projects. Furthermore, a student-centered teaching concept for a complete nanotechnology unit focused on key qualifications as claimed by the national curricula is presented. Students can learn about scientific work methods and get an insight view of scientific daily routine.

A new assay for single nucleotide polymorphism analysis based on displacement reactions in PNA–DNA double helices

In this article, we describe a new method for single nucleotide polymorphism analysis using the displacement reaction of the DNA in a PNA–DNA double helix by the target DNA. Thereby, the probe consists of a TMR-labeled PNA and a Cy5-labeled DNA forming a FRET system. Due to the displacement of the labeled DNA strand by the target DNA, the FRET is invalidated and the fluorescence of the donor dye (TMR) increases. Investigations of the exchange reaction show that increasing salt concentration and temperature accelerate the exchange rate.

Self-quenching DNA probes based on aggregation of fluorescent dyes

Here we present a novel class of self-quenching, double-labeled DNA probes based on the formation of non fluorescent H-type dye dimers. We therefore investigated the aggregation behavior of the red-absorbing oxazine derivative MR121 and found a dimerization constant of about 3000 M-1. This dye was successfully used to develop hairpin-structured as well as linear self-quenching DNA probes that report the presence of the target DNA by an increase of the fluorescence intensity by a factor of 3 to 12. Generally fluorescence quenching of the hairpin-structure probes is more efficient compared to the linear probes, whereas the kinetic of the fluorescence increase is significantly slower. The new probes were used for the identification of different mycobacteria and their antibiotic resistant species. As a test system a probe for the identification of a DNA sequence specific for the Mycobacterium xenopi was synthesized differing from the sequence of the Mycobacterium fortuitum by 6 nucleotides. Furthermore we developed a method for the discrimination between the sequences of the wild type and an antibiotic resistant species of Mycobacterium tuberculosis. Both sequences differ by just 2 nucleotides and were detected specifically by the use of competing olignonucleotides.