50 Years of Metal Ion Complexing Agents : From Basics to Business
posted by Department of Chemistry for HKU and Public
Event Type: Public Lecture/Forum/Seminar/Workshop/Conference/Symposium
Event Nature: Science & Technology
Speaker: Professor Kenneth N. Raymond
Department of Chemistry, University of California, Berkeley, USA
Selective complexation of metal ions is a matter of fundamental science and of practical application. Nature provides many examples of selectivity in metal complexation and transport that can be used to design selective chelators. However, the lanthanides and actinides, because of their variable coordination number, highly ionic nature and lability, provide a particular challenge to the chemist. The lanthanides and actinides are oxophiles and so selective and strong chelators for them should contain oxygen donors.
In a 40 year program at LBNL, highly selective actinide sequestering agents were developed using the siderophore hypothesis.1 This is based on the fact that the toxicity of Pu(IV) is due to its biochemical similarity to Fe(III). Modeled after the siderophore enterobactin, actinide sequestering agents are composed of catecholate and HOPO chelating subunits attached to various molecular backbones via amide linkages, to match the coordination environment of specific actinide ions. Some of these compounds, such as those shown (with the coordinating atoms in red), are being developed for clinical use in the case of human contamination by the actinides. 2,3
A large family of multidentate sequestering agents based on three types of ligand groups has been developed. These groups are shown at right. In each case the wavy line denotes a point of attachment to a skeletal group of a larger molecule. Remarkably, these groups are often very effective antenna ligands for excitation of the f element center. This was first found for the IAM complexes of Tb(III) (discussed later). Chiral derivatives of the IAM family of ligands are able to generate circularly polarized luminescence from their metal complexes. Circularly polarized luminescence (CPL) is a rarely used spectroscopy that discriminates between luminescent chiral complexes. Using several different chiral chelating ligands, the world’s first actinide Circularly Polarized Luminescence (CPL) spectra with curium (III) complexes were reported.4
Highly luminescent Ln(III) complexes (with Ln = Tb, Eu) for applications in biotechnology have been developed and will be briefly described. In particular, the 2-hydroxyisophthalamide (IAM) chelate for Tb(III) exhibits highly efficient emission (Φtotal ≈ 60%), large extinction coefficients (εmax >20,000 M-1 cm-1), and long luminescence lifetimes (τH2O > 2.45 ms) at dilute concentrations in standard biological buffers. Tri-macrocyclic Tb(III) complexes in this class (shown at right) display long-
term stability, with little if any change in their spectral properties (including lifetime, quantum yield, and emission spectrum) over time or in different chemical environments.
Functionalized derivatives with terminal amine, carboxylate, and N-hydroxysuccinimide groups suitable for derivatization and protein bioconjugation have also been developed and are in use commercially for human, veterinary and forensic diagnostic assays as well as new drug development.7
However, the remarkable properties of these compounds begs the question of why they are so efficient. That is the subject of our current work, which is based on the Berkley campus, but continues collaboration with Dr. David Shuh and coworkers at LBNL, with a focus on a fundamental open problem:what is the energy transfer mechanism? We have begun with a series of europium complexes that show a high variability of quantum yield in their Eu(L)2 complexes. Wehave published the preliminary experiments, which show the feasibility of this approach. 8 Now we seek to experimentally identify the dominant energy transfer process that enables ligand-to-metal energy transfer in a set of high-quantum-yield lanthanide complexes. Second, we intend to continue ongoing studies of the effects of bonding and coordination on the effici
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