Polyoxometalates: Very large clusters - Nanoscale magnets

Artificial photosynthesis: molecular systems for catalytic water oxidation.

The manganese and calcium ions of photosynthetic oxygen evolution.

The essential involvement of manganese in photosynthetic water oxidation was implicit in the observation by Pirson in 1937 that plants and algae deprived of Mn in their growth medium lost the ability to evolve O{sub 2}. Addition of this essential element to the growth medium resulted in the restoration of water oxidation within 30 min. There is increased interest in the study of Mn in biological chemistry and dioxygen metabolism in the last two decades with the discovery of several Mn redox enzymes. The list of enzymes where Mn is required for redox activity includes a Mn superoxide dismutase, a binuclear Mn-containing catalase, a binuclear Mn-containing ribonucleotide reductase, a proposed binuclear Mn site in thiosulfate oxidase, a Mn peroxidase that is capable of oxidative degradation of lignin, and perhaps the most complex and important, the tetranuclear Mn-containing oxygen-evolving complex in photosystem II (Mn-OEC). Mn is well suited for the redox role with accessible oxidation states of II, III, and IV, and possibly V: oxidation states that have all been proposed to explain the mechanisms of the Mn redox enzymes.

Manganese Cluster in Photosynthesis: Where Plants Oxidize Water to Dioxygen

Molecular imaging is the visualization, characterization and measurement of biological processes at the molecular and cellular levels in humans and other living systems. Molecular imaging agents are probes used to visualize, characterize and measure biological processes in living systems. These two definitions were put forth by the Sociey of Nuclear Medicine (SNM) in 2007 as a way to capture the interdisciplinary nature of this relatively new field. The emergence of molecular imaging as a scientific discipline is a result of advances in chemistry, biology, physics and engineering, and the application of imaging probes and technologies has reshaped the philosophy of drug discovery in the pharmaceutical sciences by providing more cost effective ways to evaluate the efficacy of a drug candidate and allowing pharmaceutical companies to reduce the time it takes to introduce new therapeutics to the marketplace. Finally the impact of molecular imaging on clinical medicine has been extensive since it allows a physician to diagnose a patient’s illness, prescribe treatment and monitor the efficacy of that treatment non-invasively.

Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) were the first molecular imaging modalities used clinically. SPECT requires the use of a contrast agent labeled with a gamma emitting radionuclide, which should have an ideal gamma energy of 100-250 keV. These gamma rays are recorded by the detectors of a dedicated gamma camera or SPECT instrument and after signal processing can be converted into an image indentifying the localization of the radiotracer. PET requires the injected radiopharmaceutical to be labeled with a positron emitting radionuclide. As the radionuclide decays it ejects a positron from its nucleus which travels a short distance before being annihilated with an electron to release two 511 keV gamma rays 180° apart that are detected by the PET scanner (Figure 1). After sufficient acquisition time the data are reconstructed using computer based algorithms to yield images of the radiotracer’s location within the organism. When compared to SPECT, PET has greater advantages with respect to sensitivity and resolution and has been gaining in clinical popularity, with the number of PET-based studies expected to reach 3.2 million by 2010.1 While SPECT and PET technology has been around for decades, its use remained limited because of the limited availability of relevant isotopes which had to be produced in nuclear reactors or particle accelerators. However, the introduction of the small biomedical cyclotron, the self-contained radionuclide generator and the dedicated small animal or clinical SPECT and PET scanners to hospitals and research facilities has increased the demand for SPECT and PET isotopes.



Figure 1

Cartoon depicting the fundamental principle of Positron Emission Tomography (PET). As the targeting group interacts with the cell surface receptor, the positron emitting radio-metal decays by ejecting β+ particles from its nucleus. After traveling ...



Traditional PET isotopes such as 18F, 15O, 13N and 11C have been developed for incorporation into small molecules, but due to their often lengthy radio-syntheses, short half-lives and rapid clearance, only early time points were available for imaging, leaving the investigation of biological processes, which occur over the duration of hours or days, difficult to explore. With the continuing development of biological targeting agents such as proteins, peptides, antibodies and nanoparticles, which demonstrate a range of biological half-lives, a need arose to produce new radionuclides with half-lives complementary with their biological properties. As a result, the production and radiochemistry of radiometals such as Zr, Y, In, Ga and Cu have been investigated as radionuclide labels for biomolecules since they have the potential to combine their favorable decay characteristics with the biological characteristics of the targeting molecule to become a useful radiopharmaceutical (Tables ​(Tables11 and ​and22).2



Table 1

Gamma- and Beta-Emitting Radiometals





Table 2

Positron-Emitting Radiometals



The number of papers published describing the production or use of these radiometals continues to expand rapidly, and in recognition of this fact, the authors have attempted to present a comprehensive review of this literature as it relates to the production, ligand development and radiopharmaceutical applications of radiometals (excluding 99mTc) since 1999. While numerous reviews have appeared describing certain aspects of the production, coordination chemistry or application of these radiometals,2-18 very few exhaustive reviews have been published.10,12 Additionally, this review has been written to be used as an individual resource or as a companion resource to the review written by Anderson and Welch in 1999.12 Together, they provide a literature survey spanning 50 years of scientific discovery. To accomplish this goal, this review has been organized into three sections: the first section discusses the coordination chemistry of the metal ions Zr, Y, In, Ga and Cu and their chelators in the context of radiopharmaceutical development; the second section describes the methods used to produce Zr, Y, In, Ga and Cu radioisotopes; and the final section describes the application of these radiometals in diagnostic imaging and radiotherapy.

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Coordinating Radiometals of Copper, Gallium, Indium, Yttrium and Zirconium for PET and SPECT Imaging of Disease

Synthesis, crystal structures, reactivity, and magnetochemistry of a series of binuclear complexes of manganese(II), -(III), and -(IV) of biological relevance. The crystal structure of [L'MnIV(.mu.-O)3MnIVL'](PF6)2.H2O containing an unprecedented short Mn.cntdot..cntdot..cntdot.Mn distance of 2.296 .ANG.

Light-driven oxidation of water to molecular oxygen in photosystem II (PS II) has been shown to occur at an active site comprising four manganese ions. There are five kinetically resolved intermediates in the catalytic cycle which have been designated S{sub 0} to S{sub 4}. S{sub 0} represents the most reduced and S{sub 4} the most oxidized level. Here the authors report the synthesis and x-ray structure of the first ({mu}-peroxo)dimanganese(IV) complex, (L{sub 2}Mn{sub 2}({mu}-O){sub 2}({mu}-O{sub 2}))(ClO{sub 4}){sub 2} (1) (L = 1,4,7-trimethyl-1,4,7-triazacyclononane), as a model for part of the S{sub 4} {yields} S{sub 0} transformation in PS II.

[L2Mn2(.mu.-O)2(.mu.-O2)](ClO4)2. The first binuclear (.mu.-peroxo)dimanganese(IV) complex (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). A model for the S4 .fwdarw. S0 transformation in the oxygen-evolving complex in photosynthesis

1,4,7-Triazacyclononane-N,N',N''-triacetate (TCTA), a new hexadentate ligand for divalent and trivalent metal ions. Crystal structures of [CrIII(TCTA)], [FeIII(TCTA)], and Na[CuII(TCTA)].bul.2NaBr.bul.8H2O

The complexes [L2MnIII2(µ-O)(µ-MeCO2)2](ClO4)2, (1), and [L′2MnIII2(µ-O)(µ-MeCO2)2](ClO4)2·H2O, (2), where L and L′ represent the cyclic triamines 1,4,7-triazcyclononane and (N,N′,N″-trimethyl-1,4,7-triazacyclononane, have been synthesized from Mn(MeCO2)3·2H2O in water–ethanol mixtures; characterization by X-ray crystallography showed that both complexes contain a µ-oxo-di-µ-acetato-metal(III) core, as found in hemerythrin, and therefore these complexes may have bioinorganic significance.

Assembly and structural characterization of binuclear µ-oxo-di-µ-acetato bridged complexes of manganese(III). Analogues of the di-iron(III) centre in hemerythrin

Dinuclear Manganese(II,III,IV) Model Complexes for the Active Center of the Metalloprotein Photosystem II: Synthesis, Magnetism, and Crystal Structure of [LMnIII(μ‐O)(μ‐CH3CO2)2MnIVL][ClO4]3 (L = N,N′N″‐Trimethyl‐1,4,7‐triazacyclononane)

Ursula Bossek

Papers

[L2Mn2(.mu.-O)2(.mu.-O2)](ClO4)2. The first binuclear (.mu.-peroxo)dimanganese(IV) complex (L = 1,4,7-trimethyl-1,4,7-triazacyclononane). A model for the S4 .fwdarw. S0 transformation in the oxygen-evolving complex in photosynthesis

1,4,7-Triazacyclononane-N,N',N''-triacetate (TCTA), a new hexadentate ligand for divalent and trivalent metal ions. Crystal structures of [CrIII(TCTA)], [FeIII(TCTA)], and Na[CuII(TCTA)].bul.2NaBr.bul.8H2O

Assembly and structural characterization of binuclear µ-oxo-di-µ-acetato bridged complexes of manganese(III). Analogues of the di-iron(III) centre in hemerythrin

Dinuclear Manganese(II,III,IV) Model Complexes for the Active Center of the Metalloprotein Photosystem II: Synthesis, Magnetism, and Crystal Structure of [LMnIII(μ‐O)(μ‐CH3CO2)2MnIVL][ClO4]3 (L = N,N′N″‐Trimethyl‐1,4,7‐triazacyclononane)