Lanthanide probes

Last updated

Lanthanide probes are a non-invasive [1] analytical tool commonly used for biological and chemical applications. Lanthanides are metal ions which have their 4f energy level filled and generally refer to elements cerium to lutetium in the periodic table. [2] The fluorescence of lanthanide salts is weak because the energy absorption of the metallic ion is low; hence chelated complexes of lanthanides are most commonly used. [3] The term chelate derives from the Greek word for “claw,” and is applied to name ligands, which attach to a metal ion with two or more donor atoms through dative bonds. The fluorescence is most intense when the metal ion has the oxidation state of 3+. Not all lanthanide metals can be used and the most common are: Sm(III), Eu(III), Tb(III), and Dy(III). [3]

Contents

History

EuFOD, an example of a europium complex EuFOD 3D skeletal.png
EuFOD, an example of a europium complex

It has been known since the early 1930s that the salts of certain lanthanides are fluorescent. [4] The reaction of lanthanide salts with nucleic acids was discussed in a number of publications during the 1930s and the 1940s where lanthanum-containing reagents were employed for the fixation of nucleic acid structures. [3] In 1942 complexes of europium, terbium, and samarium were discovered to exhibit unusual luminescence properties when excited by UV light. [3] However, the first staining of biological cells with lanthanides occurred twenty years later when bacterial smears of E. coli were treated with aqueous solutions of a europium complex, which under mercury lamp illumination appeared as bright red spots. [1] Attention to lanthanide probes increased greatly in the mid-1970s when Finnish researchers proposed Eu(III), Sm(III), Tb(III), and Dy(III) polyaminocarboxylates as luminescent sensors in time-resolved luminescent (TRL) immunoassays. [1] Optimization of analytical methods from the 1970s onward for lanthanide chelates and time-resolved luminescence microscopy (TRLM) resulted in the use of lanthanide probes in many scientific, medical and commercial fields. [1]

Techniques

There are two main assaying techniques: heterogeneous and homogeneous. If two lanthanide chelates are used in the analysis one after the other—it is called heterogeneous assaying. [4] The first analyte is linked to a specific binding agent on a solid support such as a polymer and then another reaction couples the first poorly luminescent lanthanide complex with a new better one. [1] [4] This tedious method is used because the second more luminescent compound would not bind without the first analyte already present. Subsequent time resolved detection of the metal-centered luminescent probe yields the desired signal. Antigens, steroids and hormones are routinely assayed with heterogeneous techniques. Homogeneous assays rely on direct coupling of the lanthanide label with an organic acceptor. [1]

The relaxation of excited molecules states often occurs by the emission of light which is called fluorescence. There are two ways of measuring this emitted radiation: as a function of frequency (inverse to wavelength) or time. [4] Conventionally the fluorescence spectrum shows the intensity of fluorescence at different wavelengths, but since lanthanides have relatively long fluorescence decay times (ranging from one microsecond to one millisecond), it is possible to record the fluorescence emission at different decay times from the given excitation energy at time zero. This is called time resolved fluorescence spectroscopy. [5]

Mechanism

Lanthanides can be used because their small size (ionic radius) gives them the ability to replace metal ions inside protein complex such as calcium or nickel. The optical properties of lanthanide ions such as Ln(III) originate in the special features of their electronic [Xe]4fn configurations. [4] These configurations generate many electronic levels, the number of which is given by [14!/n!(14- n)!], translating into 3003 energy levels for Eu(III) and Tb(III). [1]

The energies of these levels are well defined due to the shielding of the 4f orbitals by the filled 5s and 5p sub-shells, [4] and are not very sensitive to the chemical environments in which the lanthanide ions are inserted. Inner-shell 4f-4f transitions span both the visible and near-infrared ranges. [1] They are sharp and easily recognizable. Since these transitions are parity forbidden, the lifetimes of the excited states are long, which allows the use of time resolved spectroscopy, [4] a definitive asset for bioassays and microscopy. The only drawback of f-f transitions are their faint oscillator strengths which may in fact be turned into an advantage. [1]

The energy absorbed by the organic receptor (ligand) is transferred onto Ln(III) excited states, and sharp emission bands originating from the metal ion are detected after rapid internal conversion to the emitting level. [1] The phenomenon is termed sensitization of the metal centered complex (also referred to as antenna effect) and is quite complex. [4] The energy migration path though goes through the long-lived triplet state of the ligand. Ln(III) ions are good quenchers of triplet states so that photobleaching is substantially reduced. The three types of transitions seen for lanthanide probes are: LMCT, 4f-5d, and intraconfigurational 4f-4f. The former two usually occur at energies too high to be relevant for bio-applications. [1] [4]

Applications

Cancer research

Screening tools for the development of new cancer therapies are in high demand worldwide and often require the determination of enzyme kinetics. [1] The high sensitivity of lanthanide luminescence, particularly of time-resolved luminescence has revealed to be an ideal candidate for this purpose. There are several ways of conducting this analysis by the use of fluorogenic enzyme substrates, substrates bearing donor/acceptor groups allowing fluorescence resonance energy transfer (FRET) and immunoassays. For example, guanine nucleotide binding proteins consist of several subunits, one of which comprises those of the Ras subfamily. [1] Ras GTPases act as binary switches by converting guadenosine triphosphate (GTP) into guadenosine diphosphate (GDP). Luminescence of the Tb(III) complex with norfloxacin is sensitive to determine the concentration of phosphate released by the GTP to GDP transformation. [1]

pH probes

Protonation of basic sites in systems comprising a chromophore and a luminescent metal center leads the way for pH sensors. [4] Some initially proposed systems were based on pyridine derivatives but these were not stable in water. [1] More robust sensors have been proposed in which the core is a substituted macrocycle usually bearing phosphinate, carboxylate or four amide coordinating groups. It has been observed that lanthanide luminescent probe emission increases about six-fold when decreasing the pH of the solution from six to two. [1]

Hydrogen peroxide sensor

Hydrogen peroxide can be detected with high sensitivity by the luminescence of lanthanide probes—however only at relatively high pH values. A lanthanide-based analytical procedure was proposed in 2002 based on the finding that the europium complex with various tetracyclines binds hydrogen peroxide forming a luminescent complex. [1]

Estimating molecule size and atom distances

FRET in lanthanide probes is a widely used technique to measure the distance between two points separated by approximately 15–100 Angstrom. [6] Measurements can be done under physiological conditions in vitro with genetically encoded dyes, and often in vivo as well. The technique relies on a distant- dependent transfer of energy from a donor fluorophore to an acceptor dye. Lanthanide probes has been used to study DNA-protein interactions (using a terbium chelate complex) to measure distances in DNA complexes bent by the CAP protein. [6]

Protein conformation

Lanthanide probes have been used to detect conformational changes in proteins. Recently the Shaker potassium ion channel, [6] a voltage-gated channel involved in nerve impulses was measured using this technique. [7] Some scientist also have used lanthanide based luminescence resonance energy transfer (LRET) which is very similar to FRET to study conformational changes in RNA polymerase upon binding to DNA and transcription initiation in prokaryotes. LRET was also used to study the interaction of the proteins dystrophin and actin in muscle cells. Dystrophin is present in the inner muscle cell membrane and is believed to stabilize muscle fibers by binding to actin filaments. Specifically labelled dystrophin with Tb labelled monoclonal antibodies labeled were used. [6]

Virology

Traditional virus diagnostic procedures are being replaced by sensitive immunoassays with lanthanides. The time resolved fluorescence based technique is generally applicable and its performance has also been tested in the assay of viral antigens in clinical specimens. [6]

Medical imaging

Several systems have been proposed which combine MRI capability with lanthanides probes in dual assays. [4] The luminescent probe may for instance serve to localize the MRI contrast agent. [8] This has helped to visualize the delivery of nucleic acids into cultured cells. Lanthanides are not used for their fluorescence but their magnetic qualities. [8] [9]

Biology - Receptor-ligand interactions

Lanthanide probes displays unique fluorescence properties, including long lifetime of fluorescence, large Stokes shift and narrow emission peak. These properties is highly advantageous to develop analytical probes for receptor-ligand interactions. Many lanthanide-based fluorescence studies have been developed for GPCRs, including CXCR1, [10] insulin-like family peptide receptor 2, [11] protease-activated receptor 2, [12] β2-adrenergic receptor [13] and C3a receptor. [14]

Instrumentation

The emitted photons from excited lanthanides are detected by highly sensitive devices and techniques such as single-photon detection. If the lifetime of the excited emitting level is long enough, then time-resolved detection (TRD) can be used to enhance the signal-to-noise ratio. [5] The instrumentation used to perform LRET is relatively simple, although slightly more complex than conventional fluorimeters. The general requirements are a pulsed UV excitation source and time-resolved detection.

Lanthanide probe 1.jpg

Light sources which emit short duration pulses can be divided into the following categories: [3]

The most important factors in the selection of the pulsed light source for are the duration and intensity of the light. [3] Pulsed lasers for the 300 to 500 nm range have now replaced spark caps in fluorescence spectroscopy. There are four general types of pulsing lasers used: lasers with pulsed excitation, lasers with G-switching, mode locked lasers and cavity dumped lasers. Pulsed nitrogen lasers (337 nm) have often been used as an excitation source in time resolved fluorometry. [3]

In time resolved fluorometry the fast photomultiplier tube is the only practical single photon detector. Good single photon resolution is also an advantage in counting photons from long decay fluorescent probes, such as lanthanide chelates. [4]

These commercial instruments are available in the market today: Perkin-Elmer Micro Filter Fluorometer LS-2, Perkin-Elmer Luminescence Spectrometer Model LS 5, and LKB-Wallac Time-Resolved Fluorometer Model 1230. [3]

Ligands

Lanthanide probes' ligands must meet several chemical requirements for the probes to work properly. These qualities are: water solubility, large thermodynamic stability at physiological pHs, kinetic inertness and absorption above 330 nm to minimize destruction of live biological materials. [1]

The chelates which have been studied and utilized to date can be classified into the following groups: [3]

  1. Tris chelates (three ligands)
  2. Tetrakis chelates (four ligands)
  3. Mixed ligand complexes
  4. Complexes with neutral donors
  5. Others such as: phthalate, picrate, and salicylate complexes.

The efficiency of the energy transfer from the ligand to the ion is determined ligand-metal bond. The energy transfer is more efficient when bonded covalently than through ionic bonding. [15] Substituents in the ligand which are of electron-donating such as hydroxy, methoxy and methyl groups increase the fluorescence. [3] The opposite effect is seen when an electron-withdrawing group (such as nitro) is attached. [3] [4] Furthermore, the fluorescence intensity is increased by fluorine substitution to the ligand. The energy transfer to the metal ion increases as the electronegativity of the fluorinated group makes the europium-oxygen bond of a more covalent nature. Increased conjugation by aromatic substituents by replacing phenyl by naphtyl groups is shown to enhance fluorescence. [15]

See also

Related Research Articles

<span class="mw-page-title-main">Europium</span> Chemical element, symbol Eu and atomic number 63

Europium is a chemical element; it has symbol Eu and atomic number 63. Europium is a silvery-white metal of the lanthanide series that reacts readily with air to form a dark oxide coating. It is the most chemically reactive, least dense, and softest of the lanthanide elements. It is soft enough to be cut with a knife. Europium was isolated in 1901 and named after the continent of Europe. Europium usually assumes the oxidation state +3, like other members of the lanthanide series, but compounds having oxidation state +2 are also common. All europium compounds with oxidation state +2 are slightly reducing. Europium has no significant biological role and is relatively non-toxic compared to other heavy metals. Most applications of europium exploit the phosphorescence of europium compounds. Europium is one of the rarest of the rare-earth elements on Earth.

The lanthanide or lanthanoid series of chemical elements comprises at least the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. In the periodic table, they fill the 4f orbitals. Lutetium is also sometimes considered a lanthanide, despite being a d-block element and a transition metal.

<span class="mw-page-title-main">Ligand</span> Ion or molecule that binds to a central metal atom to form a coordination complex

In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's electron pairs, often through Lewis bases. The nature of metal–ligand bonding can range from covalent to ionic. Furthermore, the metal–ligand bond order can range from one to three. Ligands are viewed as Lewis bases, although rare cases are known to involve Lewis acidic "ligands".

<span class="mw-page-title-main">Terbium</span> Chemical element, symbol Tb and atomic number 65

Terbium is a chemical element; it has symbol Tb and atomic number 65. It is a silvery-white, rare earth metal that is malleable, and ductile. The ninth member of the lanthanide series, terbium is a fairly electropositive metal that reacts with water, evolving hydrogen gas. Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite, xenotime and euxenite.

<span class="mw-page-title-main">Praseodymium</span> Chemical element, symbol Pr and atomic number 59

Praseodymium is a chemical element; it has symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered one of the rare-earth metals. It is a soft, silvery, malleable and ductile metal, valued for its magnetic, electrical, chemical, and optical properties. It is too reactive to be found in native form, and pure praseodymium metal slowly develops a green oxide coating when exposed to air.

Plate readers, also known as microplate readers or microplate photometers, are instruments which are used to detect biological, chemical or physical events of samples in microtiter plates. They are widely used in research, drug discovery, bioassay validation, quality control and manufacturing processes in the pharmaceutical and biotechnological industry and academic organizations. Sample reactions can be assayed in 1-1536 well format microtiter plates. The most common microplate format used in academic research laboratories or clinical diagnostic laboratories is 96-well with a typical reaction volume between 100 and 200 µL per well. Higher density microplates are typically used for screening applications, when throughput and assay cost per sample become critical parameters, with a typical assay volume between 5 and 50 µL per well. Common detection modes for microplate assays are absorbance, fluorescence intensity, luminescence, time-resolved fluorescence, and fluorescence polarization.

<span class="mw-page-title-main">Ligand (biochemistry)</span> Substance that forms a complex with a biomolecule

In biochemistry and pharmacology, a ligand is a substance that forms a complex with a biomolecule to serve a biological purpose. The etymology stems from Latin ligare, which means 'to bind'. In protein-ligand binding, the ligand is usually a molecule which produces a signal by binding to a site on a target protein. The binding typically results in a change of conformational isomerism (conformation) of the target protein. In DNA-ligand binding studies, the ligand can be a small molecule, ion, or protein which binds to the DNA double helix. The relationship between ligand and binding partner is a function of charge, hydrophobicity, and molecular structure.

<span class="mw-page-title-main">Gadolinium(III) chloride</span> Chemical compound

Gadolinium(III) chloride, also known as gadolinium trichloride, is GdCl3. It is a colorless, hygroscopic, water-soluble solid. The hexahydrate GdCl3∙6H2O is commonly encountered and is sometimes also called gadolinium trichloride. Gd3+ species are of special interest because the ion has the maximum number of unpaired spins possible, at least for known elements. With seven valence electrons and seven available f-orbitals, all seven electrons are unpaired and symmetrically arranged around the metal. The high magnetism and high symmetry combine to make Gd3+ a useful component in NMR spectroscopy and MRI.

<span class="mw-page-title-main">Quenching (fluorescence)</span> Reduction of light emitted from fluorescent substances

In chemistry, quenching refers to any process which decreases the fluorescent intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisions. As a consequence, quenching is often heavily dependent on pressure and temperature. Molecular oxygen, iodine ions and acrylamide are common chemical quenchers. The chloride ion is a well known quencher for quinine fluorescence. Quenching poses a problem for non-instant spectroscopic methods, such as laser-induced fluorescence.

Surface-enhanced laser desorption/ionization (SELDI) is a soft ionization method in mass spectrometry (MS) used for the analysis of protein mixtures. It is a variation of matrix-assisted laser desorption/ionization (MALDI). In MALDI, the sample is mixed with a matrix material and applied to a metal plate before irradiation by a laser, whereas in SELDI, proteins of interest in a sample become bound to a surface before MS analysis. The sample surface is a key component in the purification, desorption, and ionization of the sample. SELDI is typically used with time-of-flight (TOF) mass spectrometers and is used to detect proteins in tissue samples, blood, urine, or other clinical samples, however, SELDI technology can potentially be used in any application by simply modifying the sample surface.

<span class="mw-page-title-main">Molecular sensor</span>

A molecular sensor or chemosensor is a molecular structure that is used for sensing of an analyte to produce a detectable change or a signal. The action of a chemosensor, relies on an interaction occurring at the molecular level, usually involves the continuous monitoring of the activity of a chemical species in a given matrix such as solution, air, blood, tissue, waste effluents, drinking water, etc. The application of chemosensors is referred to as chemosensing, which is a form of molecular recognition. All chemosensors are designed to contain a signalling moiety and a recognition moiety, that is connected either directly to each other or through a some kind of connector or a spacer. The signalling is often optically based electromagnetic radiation, giving rise to changes in either the ultraviolet and visible absorption or the emission properties of the sensors. Chemosensors may also be electrochemically based. Small molecule sensors are related to chemosensors. These are traditionally, however, considered as being structurally simple molecules and reflect the need to form chelating molecules for complexing ions in analytical chemistry. Chemosensors are synthetic analogues of biosensors, the difference being that biosensors incorporate biological receptors such as antibodies, aptamers or large biopolymers.

<span class="mw-page-title-main">Ken Raymond</span> American inorganic chemist

Kenneth Norman Raymond is a bioinorganic and coordination chemist. He is Chancellor's Professor of Chemistry at the University of California, Berkeley, Professor of the Graduate School, the Director of the Seaborg Center in the Chemical Sciences Division at Lawrence Berkeley National Laboratory, and the President and Chairman of Lumiphore.

<span class="mw-page-title-main">DOTA (chelator)</span> Chemical compound

DOTA (also known as tetraxetan) is an organic compound with the formula (CH2CH2NCH2CO2H)4. The molecule consists of a central 12-membered tetraaza (i.e., containing four nitrogen atoms) ring. DOTA is used as a complexing agent, especially for lanthanide ions. Its complexes have medical applications as contrast agents and cancer treatments.

<span class="mw-page-title-main">Fluorescence in the life sciences</span> Scientific investigative technique

Fluorescence is used in the life sciences generally as a non-destructive way of tracking or analysing biological molecules. Some proteins or small molecules in cells are naturally fluorescent, which is called intrinsic fluorescence or autofluorescence. Alternatively, specific or general proteins, nucleic acids, lipids or small molecules can be "labelled" with an extrinsic fluorophore, a fluorescent dye which can be a small molecule, protein or quantum dot. Several techniques exist to exploit additional properties of fluorophores, such as fluorescence resonance energy transfer, where the energy is passed non-radiatively to a particular neighbouring dye, allowing proximity or protein activation to be detected; another is the change in properties, such as intensity, of certain dyes depending on their environment allowing their use in structural studies.

Time-resolved fluorescence energy transfer (TR-FRET) is the practical combination of time-resolved fluorometry (TRF) with Förster resonance energy transfer (FRET) that offers a powerful tool for drug discovery researchers. TR-FRET combines the low background aspect of TRF with the homogeneous assay format of FRET. The resulting assay provides an increase in flexibility, reliability and sensitivity in addition to higher throughput and fewer false positive/false negative results. FRET involves two fluorophores, a donor and an acceptor. Excitation of the donor by an energy source produces an energy transfer to the acceptor if the two are within a given proximity to each other. The acceptor in turn emits light at its characteristic wavelength.

Nanoclusters are atomically precise, crystalline materials most often existing on the 0-2 nanometer scale. They are often considered kinetically stable intermediates that form during the synthesis of comparatively larger materials such as semiconductor and metallic nanocrystals. The majority of research conducted to study nanoclusters has focused on characterizing their crystal structures and understanding their role in the nucleation and growth mechanisms of larger materials.

Upconverting nanoparticles (UCNPs) are nanoscale particles that exhibit photon upconversion. In photon upconversion, two or more incident photons of relatively low energy are absorbed and converted into one emitted photon with higher energy. Generally, absorption occurs in the infrared, while emission occurs in the visible or ultraviolet regions of the electromagnetic spectrum. UCNPs are usually composed of rare-earth based lanthanide- or actinide-doped transition metals and are of particular interest for their applications in in vivo bio-imaging, bio-sensing, and nanomedicine because of their highly efficient cellular uptake and high optical penetrating power with little background noise in the deep tissue level. They also have potential applications in photovoltaics and security, such as infrared detection of hazardous materials.

The advanced reprocessing of spent nuclear fuel is a potential key to achieve a sustainable nuclear fuel cycle and to tackle the heavy burden of nuclear waste management. In particular, the development of such advanced reprocessing systems may save natural resources, reduce waste inventory and enhance the public acceptance of nuclear energy. This strategy relies on the recycling of major actinides and the transmutation of minor actinides in appropriate reactors. In order to fulfill this objective, selective extracting agents need to be designed and developed by investigating their complexation mechanism.

Curium compounds are compounds containing the element curium (Cm). Curium usually forms compounds in the +3 oxidation state, although compounds with curium in the +4, +5 and +6 oxidation states are also known.

<span class="mw-page-title-main">Europium compounds</span> Chemical compounds

Europium compounds are compounds formed by the lanthanide metal europium (Eu). In these compounds, europium generally exhibits the +3 oxidation state, such as EuCl3, Eu(NO3)3 and Eu(CH3COO)3. Compounds with europium in the +2 oxidation state are also known. The +2 ion of europium is the most stable divalent ion of lanthanide metals in aqueous solution. Many europium compounds fluoresce under ultraviolet light due to the excitation of electrons to higher energy levels. Lipophilic europium complexes often feature acetylacetonate-like ligands, e.g., Eufod.

References

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Bünzli, Jean-Claude G. (12 May 2010). "Lanthanide Luminescence for Biomedical Analyses and Imaging". Chemical Reviews. 110 (5): 2729–2755. doi:10.1021/cr900362e. PMID   20151630.
  2. ↑ House, James (2013). Inorganic chemistry (2nd ed.). Waltham, MA: Elsevier/Academic Press. ISBN   978-0123851109.
  3. 1 2 3 4 5 6 7 8 9 10 11 Soini, Erkki; Lövgren, Timo; Reimer, Charles B. (January 1987). "Time-Resolved Fluorescence of Lanthanide Probes and Applications in Biotechnology". CRC Critical Reviews in Analytical Chemistry. 18 (2): 105–154. doi:10.1080/10408348708542802.
  4. 1 2 3 4 5 6 7 8 9 10 11 12 13 Bünzli, edited by J.-C.G.; Choppin, G.R. (1989). Lanthanide probes in life, chemical, and earth sciences : theory and practice. Amsterdam: Elsevier. ISBN   978-0444881991.{{cite book}}: |first1= has generic name (help)
  5. 1 2 Hemmilä, I.; Laitala, V. (July 2005). "Progress in Lanthanides as Luminescent Probes". Journal of Fluorescence. 15 (4): 529–542. doi:10.1007/s10895-005-2826-6. PMID   16167211. S2CID   9978828.
  6. 1 2 3 4 5 Selvin, Paul R. (June 2002). "Principles and Biophysical Applications of Lanthanide-Based Probes". Annual Review of Biophysics and Biomolecular Structure. 31 (1): 275–302. doi:10.1146/annurev.biophys.31.101101.140927. PMID   11988471.
  7. ↑ Turro, C; Fu, PK; Bradley, PM (2003). "Lanthanide ions as luminescent probes of proteins and nucleic acids". Metal Ions in Biological Systems. 40: 323–53. PMID   12723154.
  8. 1 2 Heffern, Marie C.; Matosziuk, Lauren M.; Meade, Thomas J. (23 April 2014). "Lanthanide Probes for Bioresponsive Imaging". Chemical Reviews. 114 (8): 4496–4539. doi:10.1021/cr400477t. PMC   3999228 . PMID   24328202.
  9. ↑ Aime, Silvio; Fasano, Mauro; Terreno, Enzo (1998). "Lanthanide(III) chelates for NMR biomedical applications". Chemical Society Reviews. 27 (1): 19. doi:10.1039/A827019Z.
  10. ↑ Inglese, J.; Samama, P.; Patel, S.; Burbaum, J.; Stroke, I. L.; Appell, K. C. (1998-02-24). "Chemokine receptor-ligand interactions measured using time-resolved fluorescence". Biochemistry. 37 (8): 2372–2377. doi:10.1021/bi972161u. ISSN   0006-2960. PMID   9485384.
  11. ↑ Shabanpoor, Fazel; Hughes, Richard A.; Bathgate, Ross A. D.; Zhang, Suode; Scanlon, Denis B.; Lin, Feng; Hossain, Mohammed Akhter; Separovic, Frances; Wade, John D. (July 2008). "Solid-phase synthesis of europium-labeled human INSL3 as a novel probe for the study of ligand-receptor interactions". Bioconjugate Chemistry. 19 (7): 1456–1463. doi:10.1021/bc800127p. ISSN   1520-4812. PMID   18529069.
  12. ↑ Hoffman, Justin; Flynn, Andrea N.; Tillu, Dipti V.; Zhang, Zhenyu; Patek, Renata; Price, Theodore J.; Vagner, Josef; Boitano, Scott (2012-10-17). "Lanthanide labeling of a potent protease activated receptor-2 agonist for time-resolved fluorescence analysis". Bioconjugate Chemistry. 23 (10): 2098–2104. doi:10.1021/bc300300q. ISSN   1520-4812. PMC   3556274 . PMID   22994402.
  13. ↑ Martikkala, Eija; Lehmusto, Mirva; Lilja, Minna; Rozwandowicz-Jansen, Anita; Lunden, Jenni; Tomohiro, Takenori; Hänninen, Pekka; Petäjä-Repo, Ulla; Härmä, Harri (2009-09-15). "Cell-based beta2-adrenergic receptor-ligand binding assay using synthesized europium-labeled ligands and time-resolved fluorescence". Analytical Biochemistry. 392 (2): 103–109. doi:10.1016/j.ab.2009.05.022. ISSN   1096-0309. PMID   19464246.
  14. ↑ Dantas de Araujo, Aline; Wu, Chongyang; Wu, Kai-Chen; Reid, Robert C.; Durek, Thomas; Lim, Junxian; Fairlie, David P. (2017-05-31). "Europium-Labeled Synthetic C3a Protein as a Novel Fluorescent Probe for Human Complement C3a Receptor" (PDF). Bioconjugate Chemistry. 28 (6): 1669–1676. doi:10.1021/acs.bioconjchem.7b00132. ISSN   1043-1802. PMID   28562031.
  15. 1 2 Samuel, Amanda P. S.; Xu, Jide; Raymond, Kenneth N. (19 January 2009). "Predicting Efficient Antenna Ligands for Tb(III) Emission". Inorganic Chemistry. 48 (2): 687–698. doi:10.1021/ic801904s. PMID   19138147. S2CID   28774044.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.