Search

Selective recognition and sensing of catecholamine-based neurotransmitters by fluorescent synthetic receptors capable of operating in pure water is a central topic of modern supramolecular chemistry that impacts biological and analytical chemistry. Despite advances achieved in the recognition of some neurotransmitters such as dopamine, little effort has been invested in the optical recognition of other neurotransmitters of paramount importance in biochemistry and medicinal chemistry such as the drug L-dihydroxy-phenylalanine (levodopa). Herein, a cationic Cu(II)-terpyridine complex bearing an intramolecular fluorescent quinolinium ring covalently linked to phenylboronic acid (CuL1) was synthesized, structurally described by single-crystal X-ray diffraction and studied in-depth as a fluorescent receptor for neurotransmitters in water. The complex CuL1 was designed to act as a receptor for levodopa through two Lewis acids of different natures (Cu(II) and B atoms) as cooperative binding points. The receptor CuL1 was found to have a strongly acidified -B(OH) 2 group (p K a = 6.2) and exceptionally high affinity for levodopa ( K = 4.8 × 10 6 M -1 ) with selectivity over other related neurotransmitters such as dopamine, epinephrine, norepinephrine and nucleosides in the micromolar concentration range at physiological pH. Such levodopa affinity/selectivity for a boronic acid-based receptor in water is still rare. On the basis of spectroscopic tools ( 11 B NMR, UV-vis, EPR, and fluorescence), high-resolution ESI-MS, crystal structure, and DFT calculations, the interaction mode of CuL1 with levodopa is proposed in a 1 : 1 model using two-point recognition involving a boronate-catechol esterification and a coordination bond Cu(II)-carboxylate. Furthermore, a visual sensing ensemble was constructed using CuL1 and the commercial fluorescent dye eosin Y. Levodopa is efficiently detected by the displacement of the eosin Y bound to the Cu(II)-receptor, monitoring its green emission. The use of Cu(II)-boronate complexes for fast and selective neurotransmitter sensing was unexplored until now.

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 (p K a = 6.98) and exceptionally tight affinity for FV ( K = 1.43 × 10 5 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 ( 1 H, 11 B 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 sp 3 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.

The new dicationic pyridine-2,6-dicarboxamide-based compound 1 bearing two N -alkylquinolinium units was synthesized, structurally determined by single-crystal X-ray diffraction, and studied in-depth as a fluorescent receptor for nucleotides and inorganic phosphorylated anions in pure water. The addition of nucleotides to 1 at pH = 7.0 quenches its blue emission with a selective affinity towards adenosine 5'-triphosphate (ATP) and guanosine 5'-tripohosphate (GTP) over other nucleotides such CTP, UTP, ADP, AMP, dicarboxylates and inorganic anions. On the basis of the spectroscopic tools ( 1 H, 31 P NMR, UV-vis, fluorescence), MS measurements and DFT calculations, receptor 1 binds ATP with high affinity (log  K = 5.04) through the simultaneous formation of strong hydrogen bonds and π-π interactions between the adenosine fragment and quinolinium ring with binding energy calculated in 8.7 kcal mol -1 . High affinity for ATP/GTP is attributed to the high acidity of amides and preorganized rigid structure of 1. Receptor 1 is an order of magnitude more selective for ATP than GTP. An efficient photoinduced electron transfer quenching mechanism with simultaneous receptor-ATP complexation in both the excited and ground states is proposed. Additionally, multiple spectroscopic studies and molecular dynamics simulations showed that 1 can intercalate into DNA base pairs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Biological catecholamines such as l-DOPA and dopamine play vital physiological roles in the brain and are chemical indicators of human diseases. A new range of fluorescent Zn(ii)-terpyridine complexes are described and studied in-depth as chemosensors for catecholamine-based neurotransmitters and nucleosides in pure water. The new Zn-terpyridine-based chemosensors contain a cationic N-isoquinolinium nucleus as the optical indicator covalently linked to three different isomers of strongly acidified phenylboronic acids (ortho-, 2.Zn; meta-, 3.Zn and para-, 4.Zn, substituted derivatives) as catechol binding sites. The addition of l-DOPA, dopamine, epinephrine, l-tyrosine and nucleosides to Zn(ii)-boronic acid chemosensors at physiological pH quenches their blue emission with a pronounced selectivity and an unprecedented high affinity towards l-DOPA (log K = 6.01). This efficient response by l-DOPA was also observed in the presence of coexisting species in blood plasma and urine with a detection limit of 3.0 μmol L -1 . A photoinduced electron transfer quenching mechanism with simultaneous chemosensor - l-DOPA complexation in both the excited and ground states is proposed. The fluorescence experimental observations show that the 2.Zn·eosin-Y adduct can be used as a selective naked-eye chemosensing ensemble for l-DOPA with a fast turn-on fluorescent response and color change from blue to green under UV light at the micromolar level. On the basis of multiple spectroscopic techniques ( 1 H, 11 B NMR, UV-Vis, and fluorescence), MS-ESI experiments, crystal structures, and DFT calculations, the binding mode between Zn(ii)-chemosensors and l-DOPA is proposed in a 1 : 1 model through a cooperative two-point recognition involving the reversible esterification of the boronic acid moiety with the aromatic diol fragment of l-DOPA together with the coordination of the carboxylate anion to the Zn(ii) atom with strong electrostatic contribution.

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: {[Zn 2 (H 2 O) 2 (e,a-cis-1,4-chdc) 2 (4,4'-dtbb) 2 ]·7H 2 O} 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 (λ em = 434 nm) in the sub-micromolar concentration range with a pronounced selectivity over common anions such as acetate, bromide and iodide. The quenching response (K SV = 9.7(±0.2) × 10 4 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 13 C-NMR correlation and 1 H 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 13 C-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.

Three Ni(ii)-POCOP pincer complexes [NiCl{C 6 H 2 -4-OH-2,6-(OPPh 2 ) 2 }], 1; [NiCl{C 6 H 2 -4-OH-2,6-(OPtBu 2 ) 2 }], 2 and [NiCl{C 6 H 2 -4-OH-2,6-(OPiPr 2 ) 2 }], 3 were studied as bifunctional molecular sensors for inorganic anions and acetate. In CH 3 CN, fluoride generates a bathochromic shift with a colorimetric change for 1-3 with a simultaneous fluorescence turn on, this optical effect is based on deprotonation of the para-hydroxy group of the POCOP ligand. On the other hand, in a neutral aqueous solution of 80 vol% CH 3 CN, additions of cyanide produce a distinct change of color by forming very stable complexes with the nickel-based receptors 1-3 with log K a in the range of 4.38-5.03 M -1 and pronounced selectivity over other common anions such as iodide, phosphate, and acetate. Additionally, bromide shows a modest spectral change and affinity, but lower than those observed for cyanide. On the basis of 1 H NMR experiments, UV-vis titrations, ESI-MS experiments, and the crystal structure of the neutral bromo complex of 1, it is proposed that the colorimetric change involves an exchange of chloride by CN - on the Ni(ii) atom. The Ni(ii)-based sensor 1 allows the fluorescent selective detection of fluoride with a limit of 5.66 μmol L -1 and colorimetric sensing of cyanide in aqueous medium in the micromolar concentration range.