Molecular two-point recognition of fructosyl valine and fructosyl glycyl histidine in water by fluorescent Zn(II)-terpyridine complexes bearing boronic acids.

Selective recognition of fructosyl amino acids in water by arylboronic acid-based receptors is a central field of modern supramolecular chemistry that impacts biological and medicinal chemistry. Fructosyl valine (FV) and fructosyl glycyl histidine (FGH) occur as N-terminal moieties of human glycated hemoglobin; therefore, the molecular design of biomimetic receptors is an attractive, but very challenging goal. Herein, we report three novel cationic Zn-terpyridine complexes bearing a fluorescent N-quinolinium nucleus covalently linked to three different isomers of strongly acidified phenylboronic acids (ortho-, 2Zn; meta-, 3Zn and para-, 4Zn) for the optical recognition of FV, FGH and comparative analytes (D-fructose, Gly, Val and His) in pure water at physiological pH. The complexes were designed to act as fluorescent receptors using a cooperative action of boric acid and a metal chelate. Complex 3Zn was found to display the most acidic -B(OH)2 group (pKa = 6.98) and exceptionally tight affinity for FV (K = 1.43 × 105 M-1) with a strong quenching analytical response in the micromolar concentration range. The addition of fructose and the other amino acids only induced moderate optical changes. On the basis of several spectroscopic tools (1H, 11B NMR, UV-Vis, and fluorescence titrations), ESI mass spectrometry, X-ray crystal structure, and DFT calculations, the interaction mode between 3Zn and FV is proposed in a 1 : 1 model through a cooperative two-point recognition involving a sp3 boronate-diol esterification with simultaneous coordination bonding of the carboxylate group of Val to the Zn atom. Fluorescence quenching is attributed to a static complexation photoinduced electron transfer mechanism as evidenced by lifetime experiments. The addition of FGH to 3Zn notably enhanced its emission intensity with micromolar affinity, but with a lower apparent binding constant than that observed for FV. FGH interacts with 3Zn through boronate-diol complexation and coordination of the imidazole ring of His. DFT-optimized structures of complexes 3Zn-FV and 3Zn-FGH show a picture of binding which shows that the Zn-complex has a suitable (B⋯Zn) distance to the two-point recognition with these analytes. Molecular recognition of fructosyl amino acids by transition-metal-based receptors has not been explored until now.

Water-soluble fluorescent chemosensor for sorbitol based on a dicationic diboronic receptor. Crystal structure and spectroscopic studies.

Selective recognition of saccharides by phenylboronic dyes capable of functioning in aqueous conditions is a central topic of modern supramolecular chemistry that impacts analytical sciences and biological chemistry. Herein, a new dicationic diboronic acid structure 11 was synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptor for six saccharides in pure water at pH = 7.4. This dicationic receptor 11 has been designed particularly to respond to sorbitol and involves two convergent and strongly acidified phenyl boronic acids, with a pKa of 6.6, that operate as binding sites. The addition of sorbitol in the micromolar concentration range to receptor 11 induces strong fluorescence change, but in the presence of fructose, mannitol, glucose, lactose and sucrose, only moderate optical changes are observed. This change in emission is attributed to a static complexation photoinduced electron transfer mechanism as evidenced by lifetime experiments and different spectroscopic tools. The diboronic receptor has a high affinity/selectivity to sorbitol (K = 31 800 M-1) over other saccharides including common interfering species such as mannitol and fructose. The results based on 1H, 11B NMR spectroscopy, high-resolution mass spectrometry and density functional theory calculations, support that sorbitol is efficiently bound to 11 in a 1 : 1 mode involving a chelating diboronate-sorbitol complexation. Since the experimental B⋯B distance (5.3 Å) in 11 is very close to the calculated distance from the DFT-optimized complex with sorbitol, the efficient binding is attributed to strong acidification and preorganization of boronic acids. These results highlight the usefulness of a new diboronic acid receptor with a strong ability for fluorescent recognition of sorbitol in physiological conditions.

Selective Luminescent Chemosensing of Chloride Based on a Cyclometalated Platinum(II) Complex in Water: Crystal Structures, Spectroscopic Studies, Extraction, and Bioimaging.

Selective anion sensing by luminescent chemosensors capable of operating in aqueous conditions is a central field of modern supramolecular chemistry that impacts analytical and biological chemistry. A cationic cyclometalated [Pt(N^C^N)NCCH3]OTf complex, 1 [N^C^N = 1,3-bis(1-(p-tolyl)-benzimidazol-2'-yl)benzene, OTf = triflate], was prepared, structurally described by single-crystal X-ray diffraction and studied in-depth as a luminescent chemosensor for anions in aqueous phase and solid state. A series of related neutral [Pt(N^C^N)X] complexes (X = Cl, 2; CN, 3 and I, 4) were formed readily upon treatment of 1 with the respective NaX salt in aqueous media and were described structurally by X-ray diffraction. Complex 1 is hydrostable with phosphorescent green emission originated by intraligand transitions, and [dyz(Pt) → π*(N^C^N)] charge transfer transitions, as evidenced by TD-DFT calculations and lifetime. Additions of halides, pseudohalides, oxyanions, and dicarboxylates to a neutral aqueous solution of 1 modified its green emission intensity with a pronounced affinity (K = 1.5 × 105 M-1) and turn-on signal toward Cl- within the micromolar concentration range. Pt complex 1 is two orders of magnitude more selective for Cl- than the other halides, CN- and basic oxyanions. Such Cl- affinity for a metal-based chemosensor in aqueous media is still rare. On the basis of X-ray crystallographic analysis and multiple spectroscopic tools (NMR, UV-vis, luminescence, MS, lifetimes) the origin of this selectivity hinges on the cooperative three-point recognition involving one coordination bond (Pt-Cl) and two convergent short C-H···Cl- contacts. This strong affinity and efficient optical response can be utilized in quantitative Cl- sensing in real samples and solid-liquid extractions. Additionally, chloro-Pt complex, 2 may be relevant to bioimaging as a marker for cell nuclei, as revealed by its emission within living cells and intracellular distribution by confocal microscopic studies. These results demonstrate the usefulness of the new water-stable luminescent Pt-N^C^N complexes as effective analytical tools in anion sensing and extraction agents.

Fluorescence Sensing of Monosaccharides by Bis-boronic Acids Derived from Quinolinium Dicarboxamides: Structural and Spectroscopic Studies.

Three new diboronic acid-substituted bisquinolinium salts were synthesized, structurally described by single-crystal X-ray diffraction, and studied in-depth as fluorescent receptors for six monosaccharides and two open-chain polyols in water at physiological pH. The dicationic pyridine-2,6-dicarboxamide-based receptors contain two N-quinolinium rings as the fluorescent units covalently linked to three different isomers of phenylboronic acid (ortho, 2; meta, 3; and para, 4) as chelating binding sites for polyols. Additions of glucose/fructose in the micromolar concentration range to receptors 2 and 3 induce significant fluorescence changes, but in the presence of arabinose, galactose, mannose, and xylose, only modest optical changes are observed. This optical change is attributed to a static photoinduced electron transfer mechanism. The meta-diboronic receptor 3 exhibited a high affinity/selectivity toward glucose (K = 3800 M-1) over other monosaccharides including common interfering species such as fructose and mannitol. Based on multiple spectroscopic tools, electrospray ionization high-resolution mass spectrometry, crystal structures, and density functional theory calculations, the binding mode between 3 and glucose is proposed as a 1:1 complex with the glucofuranose form involving a cooperative chelating diboronate binding. These results demonstrate the usefulness of a new set of cationic fluorescent diboronic acid receptors with a strong ability for optical recognition of glucose in the sub-millimolar concentration range.

Recent Advances in Luminescent Recognition and Chemosensing of Iodide in Water.

This Minireview covers the latest developments of chemosensors based on transition-metal receptors and organic fluorophores with specific binding sites for the luminescent detection and recognition of iodide in aqueous media and real samples. In all selected examples within the last decade (made-post 2010), the iodide sensing and recognition is probed by monitoring real-time changes of the fluorescence or phosphorescence properties of the chemosensors. This review highlights effective strategies to iodide sensing from a structural approach where the iodide recognition/sensing process, through supramolecular interactions as coordination bonds, hydrogen bonds, halogen bonds and electrostatic interactions, is transduced into an optical change easily measurable. The selective iodide sensing is an active field of research with global interest due to the importance of iodide in biological, medicinal, industrial, environmental and chemical processes.

A sensitive photoluminescent chemosensor for cyanide in water based on a zinc coordination polymer bearing ditert-butyl-bipyridine.

Sensitive and direct sensing of cyanide in buffered aqueous solutions at pH = 7.0 by three new blue photoluminescent zinc-1,4-cyclohexanedicarboxylato coordination polymers bearing di-alkyl-2,2'-bipyridines has been achieved. Specifically, a Zn-polymer with the general formula: {[Zn2(H2O)2(e,a-cis-1,4-chdc)2(4,4'-dtbb)2]·7H2O}n, (1,4-chdc = 1,4-cyclohexanedicarboxylato and 4,4'-dtbb = 4,4'-ditert-butyl-2,2'-bipyridine) has been synthesized in high yield and studied as a luminescent chemosensor for halides, pseudohalides and a series of oxyanions in neutral water. CN- ions can be quantitatively detected by this polymer based on complete quenching (λem = 434 nm) in the sub-micromolar concentration range with a pronounced selectivity over common anions such as acetate, bromide and iodide. The quenching response (KSV = 9.7(±0.2) × 104 M-1) by the addition of CN- was also observed in the presence of typical interfering anions with a very low detection limit of 0.9 µmol L-1 in buffered water at pH = 7.0. On the basis of the crystal structure and solid state CPMAS 13C-NMR correlation and 1H NMR, IR-ATR, MS-ESI(+) and SEM-EDS experiments, the optical change is attributed to the efficient release of its corresponding ditert-butyl-bipyridine, with the simultaneous formation of a zinc cyanide complex. The CPMAS 13C-NMR spectrum of the coordination polymer is consistent with the symmetry of the crystal structure. The use of flexible coordination polymers as fluorescent sensors for fast and selective detection of cyanide ions in pure aqueous solutions has been unexplored until now.