11B NMR sensing of d-block metal ions in vitro and in cells based on the carbon-boron bond cleavage of phenylboronic acid-pendant cyclen (cyclen = 1,4,7,10-tetraazacyclododecane)

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

Design, Synthesis, and Biological Evaluation of Boron-Containing Macrocyclic Polyamines and Their Zinc(II) Complexes for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a binary therapeutic method for cancer treatment based on the use of a combination of a cancer-specific drug containing boron-10 (¹⁰B) and thermal neutron irradiation. For successful BNCT, ¹⁰B-containing molecules need to accumulate specifically in cancer cells, because destructive effect of the generated heavy particles is limited basically to boron-containing cells. Herein, we report on the design and synthesis of boron compounds that are functionalized with 9-, 12-, and 15-membered macrocyclic polyamines and their Zn²⁺ complexes. Their cytotoxicity, intracellular uptake activity into cancer cells and normal cells, and BNCT effect are also reported. The experimental data suggest that mono- and/or diprotonated forms of metal-free [12]aneN₄- and [15]aneN₅-type ligands are uptaken into cancer cells, and their complexes with intracellular metals such as Zn²⁺ would induce cell death upon thermal neutron irradiation, possibly via interactions with DNA.

Design and synthesis of boron containing monosaccharides by the hydroboration of d-glucal for use in boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is one of the radiotherapies that involves the use of boron-containing compounds for the treatment of cancer. Boron-10 (¹⁰B) containing compounds that can accumulate in tumor tissue are expected to be suitable agents for BNCT. We report herein on the design and synthesis of some new BNCT agents based on a d-glucose scaffold, since glycoconjugation has been recognized as a useful strategy for the specific targeting of tumors. To introduce a boryl group into a d-glucose scaffold, we focused on the hydroboration of d-glucal derivatives, which have a double bond between the C1 and C2 positions. It was hypothesized that a C-B bond could be introduced at the C2 position of d-glucose by the hydroboration of d-glucal derivatives and that the products could be stabilized by conversion to the corresponding boronic acid ester. To test this hypothesis, we prepared some 2-boryl-1,2-dideoxy-d-glucose derivatives as boron carriers and evaluated their cytotoxicity and cellular uptake activity to cancer cells, especially under hypoxic conditions.

A review of responsive MRI contrast agents: 2005-2014

This review focuses on MRI contrast agents that are responsive to a change in a physiological biomarker. The response mechanisms are dependent on six physicochemical characteristics, including the accessibility of water to the agent, tumbling time, proton exchange rate, electron spin state, MR frequency or superparamagnetism of the agent. These characteristics can be affected by changes in concentrations or activities of enzymes, proteins, nucleic acids, metabolites, or metal ions, or changes in redox state, pH, temperature, or light. A total of 117 examples are presented, including ones that employ nuclei other than (1) H, which attests to the creativity of multidisciplinary research efforts to develop responsive MRI contrast agents.

Theoretical studies on the structural and spectroscopic properties of an iminocoumarin-based probe and its metal complexation: an implication for a fluorescence probe

To understand the sensing behaviors of molecular fluorescent probes, an N,N-di(picolyl)aminoethyl-iminocoumarin probe (L) and its complexation with metal(II) ions (ML, M = Mg, Ca, Zn, Cd and Hg) were examined by relativistic density functional theory (DFT). Four stable conformational isomers (labeled as g1, g2, a1 and a2) for each of them have been optimized, except for CaL having only three without the g2 isomer. All of these structures have been confirmed by frequency calculations. In the aqueous solution, the a2 isomer of the L probe was calculated to be the most stable, while the g1 isomer turns out to be energetically favorable upon binding with metal ions. At these isomeric geometries, the experimentally obtained absorption was well reproduced by calculations of time-dependent DFT (TD-DFT) and a conductor-like polarized continuum model (CPCM). A slight red-shifting from L (508 nm) to ML (516-528 nm) was found. This is due to the metal affinity that stabilizes the LUMOs of ML greater than the HOMOs. Singlet excited-state structures of L and ML (M = Zn, Cd and Hg) were fully optimized using the TD-DFT approach, giving more relaxed geometries than their respective ground-state ones. Their fluorescent emissions in the aqueous solution were calculated to be 543 and 551-560 nm, respectively, agreeing with experimental values of 543 nm for L and 558 nm for ZnL. The present study also presents theoretical support for a sensing mechanism of photo-induced charge transfer of the L probe that was proposed in the previous experiment.

Molecular recognition with boronic acids-applications in chemical biology

Small molecules have long been used for the selective recognition of a wide range of analytes. The ability of these chemical receptors to recognise and bind to specific targets mimics certain biological processes (such as protein-substrate interactions) and has therefore attracted recent interest. Due to the abundance of biological molecules possessing polyhydroxy motifs, boronic acids-which form five-membered boronate esters with diols-have become increasingly popular in the synthesis of small chemical receptors. Their targets include biological materials and natural products including phosphatidylinositol bisphosphate, saccharides and polysaccharides, nucleic acids, metal ions and the neurotransmitter dopamine. This review will focus on the many ways in which small chemical receptors based on boronic acids have been used as biochemical tools for various purposes, including sensing and detection of analytes, interference in signalling pathways, enzyme inhibition and cell delivery systems. The most recent developments in each area will be highlighted.

Dynamic cyclen-metal complexes for molecular sensing and chirality signaling

Structural dynamism plays important roles in artificial and biological systems, because it controls structures and functions of various molecules and assemblies. In this review, molecular recognition and self-assembling behavior of dynamic armed cyclen-metal complexes are discussed at the molecular and supramolecular levels. These metal complexes provide useful platforms for molecular receptors, supramolecules, and molecular assemblies that can respond rapidly to guest molecules and environments. Since armed cyclens have many structural and geometrical variations, they form a wide variety of metal complexes having specific sensing and signaling functions. The Lewis acidity of the metal cations plays an essential role in anion binding and in hydrolytic catalysis of phosphate esters. Characteristic luminescence and magnetic properties of lanthanides also enable techniques for effective bio-imaging. They also serve as chiral building blocks for self-assembled architectures, which offer chirality integration effective for chirality sensing and signaling at the supramolecular level.