Lanthanide chelates also

Lanthanide chelates also can be used in FRET applications with other fluorescent probes and labels (Figure 9.51). In this application, the time-resolved (TR) nature of lanthanide luminescent measurements can be combined with the ability to tune the emission characteristics through energy transfer to an organic fluor (Comley, 2006). TR-FRET, as it is called, is a powerful method to develop rapid assays with low background fluorescence and high sensitivity, which can equal the detection capability of enzyme assays (Selvin, 2000). 

Different lanthanide metals also produce different emission spectrums and different intensities of luminescence at their emission maximums. Therefore, the relative sensitivity of time-resolved fluorescence also is dependent on the particular lanthanide element complexed in the chelate. The most popular metals along with the order of brightness for lanthanide chelate fluorescence are europium(III) > terbium(III) > samarium(III) > dysprosium(III). For instance, Huhtinen et al. (2005) found that lanthanide chelate nanoparticles used in the detection of human prostate antigen produced relative signals for detection using europium, terbium, samarium, and dysprosium of approximately 1.0 0.67 0.16 0.01, respectively. The emission... 

Organic fluorescent dyes with the appropriate spectral properties also can be paired with lanthanide chelates in FRET systems. For instance, many rhodamine dyes and the cyanine dye Cy5 have ideal excitation wavelengths for receiving energy from a nearby europium chelate. The LeadSeeker assay system from GE Healthcare incorporates various Cy5-labeled antibodies for developing specific analyte assays. In addition, if using a terbium chelate as the donor, then a Cy3 fluorescent dye can be used in assays as the acceptor. 

When used with europium or terbium ions, a carbostyril-based lanthanide chelate can be excited at 340 nm and provide sharp characteristic emission bands for transfer of energy to the appropriate acceptor fluor. Similar to the TMT chelator described previously, luminescence from terbium FRET signals well with Cy3 dyes and luminescence from europium can be used with APC or Cy5 dyes. Other fluorescent dyes that have similar excitation and emission ranges to these also can be used as acceptors in TR-FRET assays. For instance, terbium chelates can... 

Other fluorescent probes also may be used to label (strept)avidin molecules for detection of biotinylated targeting molecules. Chapter 9 reviews many additional fluorescent labels, such as quantum dots, lanthanide chelates, and cyanine dye derivatives, all of which may be used in similar protocols to create detection conjugates for (strept)avidin-biotin-based assays. 

Water exchange on cationic lanthanide chelates can also be influenced by the nature of the counter-anions (170,171). Anions like halides, sulfate, nitrate, acetate, and fluoroacetate impose different order on the second coordination shell around the chelate by influencing the hydrogen bond network. Anions with a high charge density like CU and S04 can break up the hydrogen bond network between water molecules around the metal center and by that, slow down the water exchange rate of the inner shell water molecule (171). 

Lanthanide chelates can be also packed inside a nanoscale polymer shell to produce a particulate fluorescent label. Commercially available Eu chelate dyed nanoparticles... 

Certain metal complexes endowed with weak donor groups, such as ( -C5Fl5)2TiCl2 and (C6H5)3SnCl, also show shifts in their spectra in the presence of Ln(fod)3 chelates. In this case it appears likely that the halide ion associates with the lanthanide chelate to create an anionic species Eu(fod)3X that then forms an ion pair with the metal cation. A review article on the utilization of lanthanide shift reagents for the study of metal complexes has been published. ... 

Achiral lanthanide chelates can also be added to CSAs such as arylperfluoroalkyl-carbinols, the ethyl ester of 3,5-dinitrobenzoyl-L-leucine (25) , the 3,5-dinitrobenzoyl derivative of 1-phenylethylamine, Af-(l-(l-naphthyl)ethyl)trifluoroacetamide (26) and a series of l-(l-naphthyl)ethyl urea derivatives of amino acids (27) to enhance the enantiomeric discrimination. With sulfoxide or lactone substrates , the europium ion preferentially associates with the substrate in the bulk solution. Provided the enantiomers have different association constants with the CSA, the isomer that shows the weaker association with the CSA shows the larger lanthanide-induced shifts. Low concentrations of lanthanide relative to the substrate and CSA lead to enhancements of enantiomeric discrimination in the NMR spectrum. If the concentration of lanthanide is too high, binding of the substrate to the lanthanide strips the substrate from the chiral solvating agent and diminishes the chiral discrimination in the NMR spectrum. 

Geometrical isomerism is potentially possible among many of the lanthanide chelates. The 1,3-diketone chelates, the species derived from the aminepolycarboxylic acids, the tropolonates, and many other complex derivatives are asymmetric and, thus, potentially capable of exhibiting optical isomerism. That the only resolutions reported have been limited to some tris (diketone) compounds (35) may refiect more the tendency of these ionically-bonded species to racemize rapidly in polar environments than in the absence of asymmetry. The existence of more than a single modification of each of several tetrakis(di-keto)europium(III) compounds may refiect asymmetry also (1). 

The use of ZnCh and (3-diketonate complexes of lanthanide metals give predominately endo-type addition products and CF diastereofacial selectivity. The CF stereoselectivity seems to prevail in cases using ZnCh as a catalyst and chiral aldehydes that contain groups that do not chelate to the metal. The selectivity clearly decreases when aldehydes that can form chelates are used. The lanthanide catalysts also exhibit strong CF-type selectivity with nonchelating chiral aldehydes. This trend seems to hold with most chelating aldehydes however, ACF products have been reported when a-alkoxy aldehydes are used in the reaction.56 The stereochemical consequences of using lanthanide catalysts with a-alkoxy aldehydes are therefore not predictable. 

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