Post-PCR detection of nucleic acids using metalloporphyrin labels and time-resolved fluorescence

Probing DNA hybridization in homogeneous solution and at interfaces via measurement of the intrinsic fluorescence decay time of a single label

The hybridization of DNA oligomers including molecular beacons can be detected by measurement of either the decay time or the intensity of a single fluorescent label attached to the end of the respective oligonucleotide. The method works both in solution and solid phase and can distinguish between fully complementary and mismatch sequences as demonstrated for a 15-mer oligonucleotide and a 25-mer molecular beacon. The fluorescence lifetime method is advantageous in (a) requiring a single label (and therefore a single labeling step) only; and (b), being based on measurement of a self-referenced magnitude that is hardly affected by parameters such as fluctuations in light intensity that make measurement of intensity more prone to interferences.

Homogeneous time-resolved fluorescence assays for the detection of activity and inhibition of phosphatase enzymes employing phosphorescently labeled peptide substrates

A rapid, homogenous, antibody-free assay for phosphatase enzymes was developed using the phosphorescent platinum (II)-coproporphyrin label (PtCP) and time-resolved fluorescent detection. An internally quenched decameric peptide substrate containing a phospho-tyrosine residue, labeled with PtCP-maleimide and dabcyl-NHS at its termini was designed. Phosphatase catalysed dephosphorylation of the substrate resulted in a minor increase in PtCP signal, while subsequent cleavage by chymotrypsin at the dephosphorylated Tyr-Leu site provided a 3.5 fold enhancement of PtCP phosphorescence. This phosphorescence phosphatase enhancement assay was optimized to a 96 well plate format with detection on a commercial TR-F plate reader, and applied to measure the activity and inhibition of alkaline phosphatase, recombinant human CD45, and tyrosine phosphatases in Jurkat cell lysates within 40 min. Parameters of these enzymatic reactions such as Km's, limits of detection (L.O.D's) and IC50 values for the non-specific inhibitor sodium orthovanadate were also determined.

Analysis of close proximity quenching of phosphorescent metalloporphyrin labels in oligonucleotide structures

Quenching of phosphorescent platinum(II) and palladium(II) coproporphyrin (MeCP) labelled oligonucleotides was investigated. Strong hybridization-specific quenching was observed in duplex DNA structures with a variety of quenchers and with two identical porphyrin labels when in close proximity. Classical resonance energy transfer mechanism was ruled out, since quenching did not correlate with spectral overlaps and lifetime changes were insignificant. Quenching of MeCP by the free quenchers in solution revealed that porphyrin-porphyrin quenching is predominantly static while other dyes quench dynamically. The results suggest that the quenching in DNA duplex proceeds via direct contact.

Characterization of the coupling of quantum dots and immunoglobulin antibodies

Water-soluble quantum dots (QDs) were used to label goat anti-human immunoglobulin antibodies (Abs), and the labeling process was characterized by column purification. The QDs obtained in organic solvent were modified with mercaptoacetic acid (MAA) and became water-soluble. These water-soluble QDs were linked to the antibodies using the coupling reagents ethyl-3-(dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS). The linking process was shown to be effective by ultra-filter centrifugation and column purification. After comparing the quantities of Abs and water-soluble QDs involved in the linking reaction via column purification, it was found that a molar Abs:QD ratio of >1.2 resulted in most of the water-soluble QDs becoming covalently linked to the Abs. The circular dichroism (CD) spectra of Abs and QD-Ab conjugates were very similar to each other, indicating that the secondary structure of Abs remained largely intact after the conjugation. Finally, antigen (Ag)-antibody (Ab) recognition reactions perfomed on the surface of a glass slide showed that the conjugate retained the activity of Abs. This work lends support to the idea of linking biomolecules to QDs, and thus should aid the application of QDs to the life sciences.

A fluorescence enhancement-based sensor using glycosylated metalloporphyrin as a recognition element for levamisole assay

A fluorescence sensor based on the supermolecular recognition by glycosylated metalloporphyrin for levamisole (LEV) assay is reported. For the preparation of a LEV-sensitive active material, 5, 10, 15, 20-tetrakis[2-(2, 3, 4, 6-tetraacetyl-beta-D-glucopyranosyl)-1-O-phenyl] porphyrin and its metal complexes were synthesized and used in an optode membrane prepared by including glycosylated metalloporphyrin in chitosan matrice. The immobilized glycosylated metalloporphyrin is shown to be weakly fluorescent as a result of the inhibiting of the electron tansfer by central metal. The fluorescence enhancement of the metalloporphyrin modified optode membrane by LEV is based on the complexation with the central metal moiety of metalloporphyrin and weakening the inhibiting of the electron tansfer for metalloporphyrin. The glycosylated metalloporphyrin/chitosan optode membrane showed excellent selectivity toward LEV with respect to a number of interferents and exhibited stable response. The calibration graph obtained with the proposed sensor was linear over the range of 1.3x10(-5)-3.5x10(-7)ML(-1), with a detection limit of 3.5x10(-7)ML(-1) for LEV. The prepared sensor is applied for the determination of LEV in pharmaceutical preparations and the results agreed with the values obtained by the pharmacopoeia method.

Emerging Applications of Phosphorescent Metalloporphyrins

The subject of phosphorescent metalloporphyrins is reviewed, focusing mainly on the development and application of Pt- and Pd-porphyrins. A summary of their general chemical and photophysical properties, and guidelines for rational design of the phosphorescent labels, bioconjugates and probes is given. Examples of different detection formats and particular bio-analytical applications developed in recent years are presented. The potential of phosphorescent porphyrin label methodology is discussed and compared to that of the long-decay fluorescent lanthanide chelates and other common fluorophores.