Hae-Jo Kim

Tunable Heptamethine–Azo Dye Conjugate as an NIR Fluorescent Probe for the Selective Detection of Mitochondrial Glutathione over Cysteine and Homocysteine

Although a lot of mitochondria-targeting biothiol probes have been developed and applied to cellular imaging through thiol-induced disulfide cleavage or Michael addition reactions, relatively few probes assess mitochondrial GSH with high selectivity over Cys and Hcy and with NIR fluorescence capable of noninvasive imaging in biological samples. In order to monitor mitochondrial GSH with low background autofluorescence, we designed a heptamethine-azo conjugate as an NIR fluorescent probe by introducing a tunable lipophilic cation unit as the biomarker for mitochondria and a nitroazo group as the GSH-selective reaction site as well as the fluorescence quencher. The probe exhibited a dramatic off-on NIR fluorescence response toward GSH with high selectivity over other amino acids including Cys and Hcy. Further application to cellular imaging indicated that the probe was highly responsive to the changes of mitochondrial GSH in cells.

Fluorescence Turn-On Sensor for Cyanide Based on a Cobalt(II)−Coumarinylsalen Complex

A Co(II)-salen based fluorescent sensor (1.Co) that can selectively recognize cyanide anions in 1:2 binding stoichiometry over other anions has been developed. 1.Co displayed fluorescence enhancement upon the addition of cyanide owing to the interruption of photoinduced electron transfer from the coumarin fluorophore to the cobalt(II) ion. A general regression method was developed to calculate the binding constants in the 1:2 binding system, through which the 1:2 binding between 1.Co and cyanide anions was estimated to be in the range of micromolar dissociation constants.

Ratiometric Fluorescence Imaging of Cellular Glutathione

A fluorescent probe (1) with a hydrogen bond was designed for the detection of GSH. The probe exhibited a rapid and ratiometric fluorescence response to GSH through a Michael reaction and allowed us to obtain clear cellular images for GSH.

Highly activated Michael acceptor by an intramolecular hydrogen bond as a fluorescence turn-on probe for cyanide

An activated Michael acceptor type of probe by an intramolecular hydrogen bond has shown a selective fluorescence turn-on response to cyanide through a conjugated addition of the nucleophilic anion to the enone probe with a 1300-fold increase in its fluorescence intensity.

Highly Stereospecific Generation of Helical Chirality by Imprinting with Amino Acids : A Universal Sensor for Amino Acid Enantiopurity

Generation of helical and axial chirality has been the topic of much interest in biology and chemistry. Controlling axial chirality with stereogenic-center-based chirality has shown to be useful for determining the absolute configurations or enantiopurities of chiral acids, alcohols, and natural or unnatural amino acids, and also for stereoselective synthesis of unnatural amino acids. However, it has been a challenge to control axial chirality from stereogenic-center-based chirality with a high degree of stereospecificity. In biology, amino acid chirality (the l form) is used for the stereospecific generation of helical chirality (for example, the right-handed a helix of proteins). The energy difference between the rightand left-handed a-helical peptides is small per amino acid residue, and more than twenty l amino acids are required to favor the right-handed a helix. Finding the minimal structural requirement of a receptor for generating helical chirality from amino acid chirality may provide interesting insights into stereospecific folding and stereoselective recognition of molecules. Herein we report how helical chirality can be imprinted onto 2,2’-dihydroxybenzophenone (1) in a highly stereospecific manner with a single amino acid (Scheme 1). A signaling group can also be attached to the receptor 2 for general sensing of amino acid enantiopurity. Compound 1 is readily available commercially. It has axial or helical chirality and exists as an equal mixture of rapidly interconverting P and M forms (P-1 and M-1 in Scheme 1a). Receptor 1 resembles [4]helicene in that they are both helical with four consecutive six-membered rings (including hydrogen bonds in 1). Amines react with 1 to form imines within minutes at ambient temperatures (Scheme 1b). Under the same conditions, it takes weeks to form imines with benzophenone. Thus the two hydroxy groups in 1 greatly activate the carbonyl group towards nucleophilic attack through double H bonding. If the tetramethylammonium salt of alanine (0.1m) is added to a solution of 1 (0.1m) in a protic solvent, such as CD3OD (Scheme 1c), two sets of signals are detected in the H NMR spectrum (Figure 1a). The two compounds in Scheme 1c are diastereomeric and are expected to give distinct NMR signals. However, if an aprotic solvent, such as CD3CN is used (Scheme 1b), a remarkably clean H NMR spectrum results with just one set of signals (Figure 1b). Computation can be used to help explain the high stereospecificity for imine formation in an aprotic solvent such as CD3CN. Density functional theory computation (DFT; B3LYP at the 6-31G* level) shows that the global minimum-energy structure of the imine 1-A-G formed between anionic l-alanine and 1 has two internal H bonds (Figure 2a). One internal H bond in 1-A-G is a resonanceScheme 1. Generating helical chirality in 2,2’-dihydroxybenzophenone 1 with a single amino acid. A= l-alanine, G=global minimum, L= local minimum. See text for details.

Tight binding and fluorescent sensing of oxalate in water.

A dinuclear copper complex that binds tightly and selectively to oxalate over other dicarboxylates like malonate, succinate, and glutarate has been developed. This receptor can be used for fluorescent detection of oxalate in water at physicological pH by chemosensing ensemble approach. Crystal structure of oxalate bound to the receptor together with molecular mechanics and DFT computations provide insights into the tight and selective binding of the anion by the receptor.

Sugar recognition by C3-symmetric oxazoline hosts

C3-Symmetric tris(oxazoline) derivatives (2S,2R) were designed to complex sugars and alcohols in nonpolar organic solvent through complementary intermolecular hydrogen-bonding interactions. 1H NMR titration demonstrated that the C3-symmetric hosts were capable of anomer-selective recognition for n-octyl-d-glucopyranoside. 1H NMR data revealed intermolecular hydrogen bonds between 2S and glucopyranosides.

Amino acid recognition of pyridine bis(oxazoline)–copper(II) complex in aqueous solvent

Abstract Enantioselective recognition of amino acids has been studied with C2-symmetric chiral pyridine bis(oxazoline)–copper(II) complexes at physiological pH condition. UV–visible titration revealed strong binding of submillimolar dissociation constant between pyridine bis(oxazoline)–copper(II) complex and amino acids in aqueous solution. Moderate selectivity of up to 2:1 between d - and l -amino acids was achieved. The enantiomers were baseline resolved by capillary electrophoresis, using the bis( l -lysine)–copper(II) complex as a chiral selector.

Mitochondria-Targeting Chromogenic and Fluorescence Turn-On Probe for the Selective Detection of Cysteine by Caged Oxazolidinoindocyanine

We report a chromogenic and fluorescence turn-on probe based on crotonoyl ester-functionalized oxazolidinoindole for the selective detection of cysteine in neutral buffer. The probe rapidly formed indocyanophenolate through the Michael addition and a subsequent cyclization reaction of cysteine, inducing both a dramatic bathochromic shift (>130 nm) and a large fluorescence turn-on response (F/F0 12) in the UV-vis and fluorescence spectra and affording a micromolar limit of detection (LOD = 5.0 μM) of cysteine in HEPES buffer. When cysteine was added, the probe exhibited a dual optical change with strong green fluorescence and dramatic red color by the oxazolidinoindole-to-hydroxyethylindolium transformation. Further cellular application of the probe was successfully performed for the mitochondrial imaging of HeLa cells.

Ratiometric fluorescence probes based on a Michael acceptor type of coumarin and their application for the multichannel imaging of in vivo glutathione

A series of Michael acceptors based on a coumarin moiety, were developed as fluorescent probes for ratiometric detection of in vivo glutathione. The α,β-unsaturated Michael acceptors were transformed into non-conjugated molecules through the Michael addition of biothiols. The resulting UV-vis and fluorescence spectra of the probes revealed characteristic ratiometric responses, which were successfully applied for the multichannel imaging of in vivo glutathione.