Monocycloplatinated Solvento Complex Displays Turn-on Ratiometric

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Monocycloplatinated Solvento Complex Displays Turn-on Ratiometric Phosphorescence Responses to Histamine Gyurim Park,† Seungyeon Yu,† Sinheui Kim,† Yoonseo Nah,† Ahjeong Son,‡ and Youngmin You*,† †

Division of Chemical Engineering and Materials Science and ‡Department of Environmental Science and Engineering, Ewha Womans University, Seoul 03760, Republic of Korea

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S Supporting Information *

ABSTRACT: The study of biological histamine (HA) requires probes capable of ratiometric photoluminescence detection of HA. We discovered that a monocycloplatinated complex having two solvento ligands ([Pt(2(2-naphthyl)quinolinate)(NCCH3)2]ClO4) could produce ratiometric phosphorescence responses to HA in aerated aqueous solutions buffered to pH 7.4. The HA response was characterized with a hypsochromic shift of an emission peak wavelength from 635 to 567 nm. The corresponding phosphorescence intensity ratio (i.e., I567 nm/I635 nm) increased from 0.26 to 1.90. Spectroscopic and spectrometric investigations indicated an occurrence of spontaneous displacement of the labile CH3CN ligands with HA. An independently prepared HA adduct supported this notion. The ratiometric phosphorescence responses to HA were highly tolerant to other biological stimuli, including changes in pH and the presence of biometals and biological Lewis bases such as amino acids, nucleosides, biothiols, neurotransmitters, and small molecular metabolites. Of note was the high selectivity toward HA over common biological ligands, including histidine, cysteine, and homocysteine, which was ascribed to tighter HA binding. Our phosphorescence measurements employing Boc-protected derivatives of HA suggested that the bis-chelate motif involving imidazolyl and terminal amino groups was crucial for eliciting the ratiometric phosphorescence signaling. Finally, the bioimaging utility of the HA probe was validated using RAW 264.7 macrophages that were exogenously supplemented with HA or stimulated with thapsigargin to enrich intracellular HA. Ratiometric phosphorescence imaging microscopy experiments demonstrated the ability of the probe for monitoring intracellular HA uptake. In addition, photoluminescence lifetime imaging microscopy techniques could be applied for visualization of HA within the RAW 264.7 cells, because the HA binding elongated the photoluminescence lifetime. Our study demonstrated the promising utility of inner-sphere interactions of phosphorescent Pt(II) complexes for detection of biological HA.

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INTRODUCTION Histamine (β-imidazolylethylamine, HA hereafter) belongs to a family of biological amines. HA is endogenously produced from L-histidine through the catalytic action of L-histidine decarboxylase and is stored within the granules of mast cells and basophils.1,2 Basal levels of HA in blood plasma fall within several nanomolar concentrations,3 but some pathological conditions increase HA levels.4−7 For example, an abrupt increase in the HA concentration serves as an unambiguous hallmark of inflammation. Rich HA pools in synaptic vesicles were also reported to be involved in neurotransmission.8 However, the lack of proper tools has retarded elucidation of the pathophysiological actions mediated by HA. Several methods exist for the detection of HA. The majority of these methods are based on instrumental techniques involving gas chromatography,9 high-performance liquid chromatography,10 nanofluidics,11 microchip capillary electrophoresis,12 electrochemical flow immunoassays,13 bioelectrocatalysis,14 and enzymatic radiolabeling.15 These techniques are incapable of monitoring biological HA levels in a noninvasive and real© XXXX American Chemical Society

time manner. Fluorescence detection using molecular probes is ideal for HA bioimaging, as it affords a high spatiotemporal resolution and a large dynamic range. A few fluorescent probes of HA have been developed thus far on the basis of fluorophores that display fluorescence responses to HA through dosimetric reactions, including amidation,16 imine formation,17 axial binding at a Zn(II) protoporphyrin,18 and displacement of paramagnetic Ni(II) or Co(II) ions.19−21 A phosphorescent nanoprobe was also developed by exploiting the HA-triggered enzymatic depletion of O2.22 Note that these HA probes displayed monotonic increases or decreases in photoluminescence intensities upon interacting with HA. The bioimaging utility of such turn-off signaling suffers from a low signal fidelity due to instrumental drift or an inhomogeneous distribution of probes at the location of interest. Another disadvantage of using fluorescence probes is that the HA responses may be contaminated by background noise due to Received: September 14, 2018

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DOI: 10.1021/acs.inorgchem.8b02612 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

endogenous HA. To the best of our knowledge, this is the first report of a ratiometric emission probe of HA.

autofluorescence. Therefore, it is appealing to create a novel HA probe system that could provide ratiometric signals with longlived emission. One promising approach to achieving this goal would be to employ room-temperature-phosphorescent probes involving late transition metals, such as Ir(III) and Pt(II). The complexes exhibit phosphorescence lifetimes as long as sub-microseconds or microseconds, permitting temporal discrimination of their phosphorescence signals with short-lived autofluorescence. Phosphorescence properties of the complexes are also amenable to bioimaging applications, with respect to quantum yields and excitation and emission wavelengths.23−36 If labile ligands are introduced to these complexes, they can readily be displaced with strongly coordinating analytes to produce phosphorescence responses. The effectiveness of this displacement assay has been examined with solvento complexes of Ir(III) for phosphorescence staining of subcellular organelles and biological molecules. For example, Li and co-workers reported that a bis-cyclometalated Ir(III) complex having two aqua ligands could selectively stain dead cells.37 The staining action was ascribed to displacement of the labile aqua ligands with Lewis basic species in nuclei in dead cells. Earlier studies by the groups of Wong, Li, and Fei have also established the strong propensity of bis-cyclometalated solvento complexes of Ir(III) toward displacement reactions with histidine.38−40 Phosphorescence changes resulting from the displacement reactions served as a convenient indication of the presence of free histidine and bis(histidine) residues. Motivated by the previous studies, we designed a monocycloplatinated complex having two solvento ligands. Cycloplatinated complexes are versatile platforms for biosensing applications, as their phosphorescence properties are suitable for microscopic experiments and are sensitive to inner-sphere interactions.33,35,41−46 We envisioned that bidentate HA could displace the labile solvento ligands, thereby producing HAselective phosphorescence signals. The ligand lability of Pt(II) complexes was exploited for sensing guanosine 5′-monophosphate, although the displacement did not produce phosphorescence responses.47 Chan and co-workers employed a mononuclear Pt(II) complex having a Pac-Man-type cyclometalating ligand and a labile chloro ligand for phosphorescence turn-off detection of cysteine.48 Note that HA biosensing is unprecedented. Herein, we report the synthesis, sensing behavior, and bioimaging applications of a phosphorescent probe of HA (PtGR) (Scheme 1). PtGR was found to produce turn-on ratiometric phosphorescence responses to HA in aqueous buffered solutions or in RAW 264.7 macrophages exogenously supplemented with HA or stimulated to produce

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RESULTS AND DISCUSSION Design and Histamine Displacement Behaviors of Probe. The molecular construct of PtGR was based on a squareplanar mono-cyclometalated bis(solvento) complex of Pt(II). A low-band-gap-energy ligand, 2-(2-naphthyl)quinoline, was chosen to enable phosphorescence responses in the redemission region. PtGR was prepared through a three-step synthesis. First, the Pd(0)-catalyzed Suzuki−Miyaura reaction between 2-bromoquinoline and 2-naphthalene boronic acid furnished the 2-(2-naphthyl)quinoline ligand. Then, a μ-chlorobridged dinuclear Pt(II) complex precursor of the ligand (i.e., [Pt2(μ-Cl)2(2-(2-naphthyl)quinolinate)2]) was synthesized using K2PtCl4. Finally, the isolated [Pt2(μ-Cl)2(2-(2-naphthyl)quinolinate)2] was treated with AgClO4 in acetonitrile to yield PtGR (i.e., [Pt(2-(2-naphthyl)quinolinate)(NCCH3)2]ClO4). Synthetic details and structural characterization data are summarized in Experimental Details. Ten micromolar PtGR displayed a red phosphorescence emission with a peak wavelength (λem) of 635 nm in an airequilibrated aqueous solution buffered to pH 7.4 (25 mM piperazine-N,N′-bis(2-ethanesulfonic acid) (PIPES), 100 mM KCl) under photoexcitation at a wavelength (λex) of 374 nm. The addition of HA to the PtGR solution elicited a hypsochromic shift in the phosphorescence emission (λem 635 nm → 567 nm) (Figure 1a). The corresponding phosphorescence intensity ratio of 567 nm over 635 nm (i.e., I567 nm/ I635 nm) increased from 0.26 (no HA) to 1.90 (40 μM HA). The phosphorescence response to HA entailed a small hypsochromic shift in the UV−vis absorption spectra (Δν = 850 cm−1), an observation suggestive of the creation of a HA displacement product (Figure 1b). In order to validate this hypothesis, an HA adduct (i.e., [Pt(2-(2-naphthyl)quinolinate)(HA)]ClO 4 (PtHA)) was synthesized independently (see Experimental Details for the synthetic details and structural identification data). As shown in Figure 1, the phosphorescence and UV−vis absorption spectra of PtHA (blue curves) were identical with those obtained from PtGR in the presence of HA (red curves), which provided an unambiguous indication of the formation of PtHA. The spectral coincidence pointed to the occurrence of an HA displacement reaction. Further spectroscopic investigations were performed to monitor the HA displacement reaction. Figure 2a displays the 1 H NMR spectra (300 MHz; DMSO-d6/D2O 3/1, v/v) of 2.0 mM PtGR recorded with increasing HA concentration. The addition of HA evoked a significant upfield shift in the 8H quinoline peak (i.e., Ha → Ha′ in Figure 2a) and a downfield shift in the 5H imidazole peak (i.e., Hb → Hb′ in Figure 2a). The latter shift was consistent with the lowered electron density of imidazole in HA due to the Pt coordination. The aromatic proton signatures were also identical with those observed for PtHA (Figure 2a). The electrospray ionization mass spectrum (positive mode) of an incubated solution of 100 μM PtGR and 1.0 mM HA indicated the formation of PtHA (m/z 560.14) (Figure 2b). In addition, the displacement reaction was further supported by the disappearance of the stretching bands of CN and C−N bonds in the solvento ligand (ν̃ 2340, 2324, 2296 cm−1) in the ATR FT−IR spectra (Figure S1 in the Supporting Information). Taken together, the results unequivocally attest to the displacement of the solvento ligands by HA.

Scheme 1. Histamine Displacement Assay of a Monocycloplatinated Complex (PtGR)

B


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Figure 1. Photophysical responses to HA. (a) Photoluminescence and (b) UV−vis absorption spectra of 10 μM PtGR in an aqueous buffered solution (pH 7.4; 25 mM PIPES, 100 mM KCl) recorded on increasing the concentration of HA (0−40 μM). Blue curves are the spectra of independently synthesized PtHA (see the main text). The inset photo shows the phosphorescence emission of PtGR and PtHA solutions (10 μM).

Figure 2. Ligand displacement with HA. (a) 1H NMR spectra (300 MHz, DMSO-d6/D2O 3/1, v/v) of 2.0 mM PtGR measured on increasing the concentration of HA. The 1H NMR spectra of 2.0 mM HA and 2.0 mM PtHA are also included for comparison. See the chemical equation at the top for the peak assignments. (b) Mass spectrum (ESI, positive) of the HA adduct (PtHA, m/z 560.14). Black and red bars indicate the observed and predicted isotope distributions, respectively, of PtHA.

N,N-trans configuration of the quinolinyl nitrogen and the imidazolyl nitrogen, and the other had an N,N-cis configuration of the two nitrogens. Quantum chemical calculations for the optimized geometries of the two structures were performed at the B3LYP level of theory. The agreement between the simulated electronic absorption spectrum of the N,N-trans

Repeated attempts to grow single crystals of PtGR and PtHA were unsuccessful. The coordination geometry in PtHA was thus explored by performing quantum chemical calculations based on the crystal structure of an antitumor drug, cis[Pt(HA)Cl2].49 Two possible coordination structures of PtHA were subjected to geometry optimization: one structure had an C


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Inorganic Chemistry Table 1. Photophysical Data for PtGR in the Absence or Presence of HAa λabs (nm)b PtGR PtGR + HAj

383 371

ε at λabs (104 M−1 cm−1)c 1.0 0.83

λem (nm)d

PLQYe i

635 567

0.003/0.007 0.005/0.15i

τobs (ns)f

kr (105 s−1)g

knr (105 s−1)h

21 435

1.2 0.11

480 23

Conditions: 10 or 50 μM PtGR in an air-equilibrated aqueous solution buffered to pH 7.4 (25 mM PIPES, 100 mM KCl, 1 vol % DMSO). Absorption peak wavelength. cMolar absorbance at the peak wavelengths. dPhotoluminescence peak wavelength. Excitation wavelength 385 nm. e Photoluminescence quantum yields determined relative to that of [Ru(bpy)3]Cl2 (PLQY = 0.055).50 fPhotoluminescence lifetime obtained after picosecond pulsed laser excitation at 377 nm (temporal resolution 6.4 ns; observation wavelength 567 nm). A biexponential decay model was used. g Radiative rate constant: kr = PLQY/τobs. hNonradiative rate constant: knr = (1 − PLQY)/τobs. iDeaerated toluene. j1.0 mM HA. a

b

Figure 3. Phosphorescence HA selectivity over biological competing Lewis bases. Ratios of the phosphorescence intensities at 567 nm over 635 nm of 10 μM PtGR (pH 7.4; 25 mM PIPES, 100 mM KCl) were recorded in the absence (light gray bars) and presence (dark gray bars) of competing species (amino acids, biothiols, nucleosides 1.0 mM; other competing species 100 μM) and after the subsequent addition of 1.0 mM HA (black bars). Phosphorescence spectra are collected in Figure S8 in the Supporting Information.

μM PtGR toward 10 μM HA (Figure S6 in the Supporting Information). In addition, the HA sensing capability was retained over a broad pH range, 3.2−8.6 (Figure S7 in the Supporting Information). Importantly, the HA sensing ability was not affected by the presence of other competing biological Lewis bases. As summarized in Figure 3, 1.0 mM amino acids (alanine, arginine, asparagine, glutamate, glycine, isoleucine, leucine, lysine, proline, serine, threonine, tryptophan, and valine), 1.0 mM biothiols (GSH and DTT), 100 μM small molecular neurotransmitters (GABA, dopamine, acetylcholine, epinephrine, serotonin, and myo-inositol), 100 μM metabolites (AMP, cAMP, GDP, DTP, ADP, and ATP), and 1.0 mM nucleosides (adenine, cytidine, and adenosine) did not alter the phosphorescence signaling due to HA binding. Several thiolcontaining molecules, such as cysteine, homocysteine, and DTT, appeared to perturb the ratiometric phosphorescence signaling at high concentrations (i.e., 1.0 mM). However, their effects on the HA sensing were minimal because these species interacted weakly with PtGR (vide infra). In addition, the thiol-containing molecules elicited phosphorescence turn-off signals, which contrasted with the turn-on ratiometric response to HA (see Figure S8 in the Supporting Information for the spectra). Although guanosine produced phosphorescence ratiometric responses similar to the case of HA, its exclusive localization within nuclei would abrogate false signaling. Of note is the higher selectivity of PtGR toward HA over histidine (His). Since some solvento complexes of Ir(III) were reported to display strong propensity toward His binding, the ability of PtGR for selective detection of HA in the presence of His is valuable.37−40 In order to quantify the HA binding strength of PtGR, we determined the dissociation constants

geometry and the experimental UV−vis absorption spectrum of PtHA was found (Figure S2 in the Supporting Information). This result supported the notion that PtHA possessed an N,Ntrans configuration, as shown in Scheme 1. Our TD-B3LYP calculations also reproduced the observed absorption peaks at ∼380 nm, which had a dominant π−π* transition localized within the 2-(2-naphthyl)quinolinate ligand. Therefore, the observed hypsochromic shifts in the optical spectra (Figure 1) cannot be ascribed to electronic transitions involving HA orbitals. Presumably, stabilization of the dπ orbital upon HA coordination may be responsible for the responses. Histamine Sensing Properties. The phosphorescent response by the HA displacement was very rapid. A bimolecular rate constant for the displacement reaction under pseudo-firstorder conditions was determined to be as large as 6.2 × 107 M−1 s−1 (Figure S3 in the Supporting Information). The phosphorescence spectra of PtGR and PtHA were invariant to changes in the concentration, which excluded any intermolecular interactions such as probe aggregation or excimer formation as potential origins for the phosphorescence responses (Figure S4 in the Supporting Information). In addition, solvatochromic shifts in the phosphorescence emission were barely observed (Figure S5 in the Supporting Information). Collectively, these results support the claim that the turn-on ratiometric phosphorescence responses were due to the displacement reaction with HA. Photophysical data for PtGR in the absence and presence of HA are compiled in Table 1. The phosphorescence HA response was tolerant to other biological stimuli. The presence of biometals, including 1.0 mM FeCl2, 1.0 mM NiCl2, 1.0 mM CuCl2, and 1.0 mM ZnCl2, did not perturb the ratiometric phosphorescence responses of 10 D


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Kd(cysteine) > 0.57 μM and Kd(homocysteine) > 0.97 μM (Figure S11 in the Supporting Information). Taken together, the analyses revealed that PtGR could bind HA more strongly than cysteine and homocysteine, as well as His. The next question was what structural factors governed the ratiometric phosphorescence responses. To address this question, we synthesized control molecules of HA and examined phosphorescence responses of PtGR toward these molecules (see Figure 5). The imidazolyl and terminal amino groups in HA were protected with tert-butoxycarbonyl moieties to yield Nα,Nτ-bis(tert-butoxycarbonyl)histamine (bis(Boc)-HA). Selective deprotection of one tert-butoxycarbonyl group in boiling water furnished Nα-tert-butoxycarbonylhistamine (Boc-HA). As shown in Figure 5, monotonic increases in the phosphorescence intensities of PtGR were produced by the addition of 1.0 mM imidazole, 1.0 mM Boc-HA, or 1.0 mM bis(Boc)-HA. The phosphorescence turn-on ratios were in the order imidazole < bis(Boc)-HA < Boc-HA. Note that the control molecules did not produce phosphorescence ratiometric responses. The subsequent addition of HA into the solution mixtures provoked ratiometric signaling. These results suggested that the bischelate motif involving the imidazolyl and terminal amino groups is pivotal for eliciting phosphorescence turn-on ratiometric responses. This notion is consistent with the ratiometric but weaker responses to His (Figure S10 in the Supporting Information), because the terminal amino group in His is prone to protonation due to the presence of an anionic carboxylate group. Visualization of Intracellular Histamine. Having investigated the HA sensing behavior, we next demonstrated HA bioimaging using PtGR. RAW 264.7 macrophages were selected, because the cells can accumulate HA from cell media.19,54 An MTT cell proliferation assay demonstrated that PtGR was not significantly toxic to RAW 264.7 macrophages up to a concentration of 500 μM PtGR until 40 min of incubation (Figure S12 in the Supporting Information). Incubation for a period longer than 1 h initiated cell death. The cytotoxicity may be linked to the release of acetonitrile. Thus, RAW 264.7 macrophages were treated with 100 μM PtGR for 20 min to permit microscopic visualization. Note that the feed concentration was higher than those typically employed for molecular probes (greater than several micromolar), although cells were viable throughout our imaging experiments that took less than 30 min. Inductively coupled plasma mass spectrometry measurements after cell lysis revealed intracellular accumulation of Pt species to concentrations of 380 ± 6 nM under this condition (100 μM PtGR, 20 min; Figure S13 in the Supporting Information). Coincubation of the cells with fluorescent organelle-specific stains revealed delocalization of PtGR over several subcellular entities, including endoplasmic reticulum (colocalization coefficient 0.98), plasma membranes (colocalization coefficient 0.60), and mitochondria (colocalization coefficient 0.54) (Figure S14 in the Supporting Information). In contrast, PtGR did not localize within nuclei and lysosomes. Since HA is present throughout the entire cytoplasm in a granular form,55 the delocalization of the probe may be beneficial for HA bioimaging. Ratiometric phosphorescence visualization of intracellular HA was performed by monitoring emission signals through 567 and 634 nm channels in order to maximize the intensity ratio (i.e., I567 nm/I634 nm). As shown in Figure 6, the 567 nm channel image of RAW 264.7 cells pretreated with PtGR was barely visible, but an exogenous supply of 1.0 mM HA enhanced the

(K ds). The phosphorescence Job plot indicated a 1:1 stoichiometry between PtGR and HA (Figure S9 in the Supporting Information). The dissociation constant of HA (Kd(HA)) was thus determined based on a 1:1 equilibrium model between PtGR and free HA: PtGR + HA free V PtHA + 2CH3CN

Since an I567 nm/I635 nm value depended only on equilibrium concentrations of PtGR and PtHA, it can be expressed as eq 1: I567 nm/I635 nm = α1[PtHA] + α2[PtGR]

(1)

In eq 1, α1 and α2 correspond to the I567 nm/I635 nm values per unit molar concentrations of PtHA and PtGR, respectively, and were determined from the phosphorescence spectra. Mass balances for the equilibrium concentrations of PtGR, PtHA, and free HA yielded eqs 2 and 3: I567 nm/I635 nm = {α1[HA]free + α2Kd(HA)} /{[HA]free + Kd(HA)} × 10

(2)

[HA]2free + (10 + Kd(HA) − [HA]total )[HA]free + Kd(HA)[HA]total = 0

(3)

I567 nm/I635 nm values were determined experimentally for 10 μM PtGR on increasing the concentration of HA (Figure 4).

Figure 4. HA (circles) and histidine (His; triangles) titration isotherms plotting the ratios of phosphorescence intensities at 567 nm over 635 nm. The red curves correspond to nonlinear least-squares fits to eq 2.

Iterations involving (i) a nonlinear least-squares fit of the HA titration data of I567 nm/I635 nm to eq 2 and (ii) solutions of the quadratic equation (3) were performed, following the method we reported previously.51−53 It should be mentioned that this method enabled only an estimation of a lower bound of Kd. The iterations returned Kd(HA) > 1.2 pM (Figure 4). The dissociation constant of His (Kd(His)) was also determined, and the value was greater than 1.2 μM (see Figure S10 in the Supporting Information for the spectra). Obviously, the lower bound of Kd(HA) is 6 orders of magnitude smaller than that of Kd(His), indicating much stronger binding with HA. The Kd values of cysteine and homocysteine were estimated similarly using integrated phosphorescence intensities, because they elicited phosphorescence turn-off responses. The lower bounds of Kd were determined to be in the sub-micromolar range: E


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Figure 5. Phosphorescence responses of 10 μM PtGR to 1.0 mM control molecules: (a) imidazole, (b) bis(Boc)-HA, and (c) Boc-HA. Phosphorescence spectra were recorded in the absence (dotted black lines) and presence (solid black lines) of the control molecule and after the subsequent addition of 1.0 mM HA (red lines). Conditions: λex 374 nm; air-equilibrated aqueous solutions buffered to pH 7.4 (25 mM PIPES, 100 mM KCl). Shown at the bottom are the plausible Pt coordination motifs. Note that the coordination strength of bis-chelate motifs increases from left to right.

Figure 6. Ratiometric phosphorescence visualization of intracellular HA in RAW 264.7 macrophages treated with PtGR. The treated cells were viable throughout the imaging experiments. (a) Cells without pretreatment. (b) Cells pretreated with 1.0 mM HA (2 h). (c) Cells pretreated with 300 nM thapsigargin (20 h). Phosphorescence signals were acquired through a 567 nm emission channel (top green panels) and a 634 nm emission channel (middle red channel). The bottom panels show ratio images calculated by dividing the 567 nm channel image by the 634 nm channel image (i.e., I567 nm/I634 nm). Scale bar: 50 μm. Condition: λex 405 nm. (d) Corresponding I567 nm/I634 nm values. Error bars were determined from five cells treated independently, and statistical analyses were performed using a two-tailed Student’s t test at **P ≤ 0.01.

phosphorescence intensity (top green images). In contrast, the 634 nm images were relatively unaffected by the HA treatment and served as internal references. Ratiometric images con-

structed by dividing the 567 nm image by the 634 nm image clearly indicated HA enrichment (I567 nm/I634 nm = 0.18 ± 0.015 (before HA treatment) → 0.37 ± 0.012 (after HA treatment), F


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Inorganic Chemistry

Figure 7. Ratiometric phosphorescence monitoring of the endogenous generation of HA in RAW 264.7 macrophages stimulated with 300 nM thapsigargin. Phosphorescence images of the incubated cells were acquired through 567 and 634 nm channels under photoexcitation at a wavelength of 405 nm at incubation times of (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 4 h, and (e) 20 h. Ratiometric images were produced by dividing the 567 nm image by the 634 nm image. Scale bar: 50 μm. (f) Corresponding phosphorescence intensity ratios (I567 nm/I634 nm) as a function of the incubation time of thapsigargin. Error bars were determined from the results of three different cells.

Figure 8. Photoluminescence lifetime imaging micrographs obtained from RAW 264.7 cells treated with (a) 100 μM PtGR, (b) 100 μM PtGR + 1.0 mM HA, and (c) 100 μM PtGR + 300 nM thapsigargin. Indicated at the bottom are the average photoluminescence lifetimes (τavg) calculated using a nonlinear least-squares fit to a triexponential decay function. See Figure S17 and Table S1 in the Supporting Information for the fit results. Scale bar: 20 μm. Conditions: 40× ; excitation, λex 375 nm pulsed laser (∼30 ps pulse width and ∼2 μW power); band-pass filter, λem 500−600 nm; 200 × 200 pixels; acquisition time 1.0 ms pixel−1.

Figure 5c, we observed ratiometric phosphorescence signals (I567 nm/I634 nm = 0.35 ± 0.022) that were analogous to those obtained under an exogenous HA supply. Again, this I567 nm/ I634 nm value falls in the free HA concentration range ∼1 pM. As another demonstration of the utility of PtGR, the ratiometric visualization of HA was used to monitor the progress of the de novo synthesis of HA in RAW 264.7 cells. As shown in Figure 7, the I567 nm/I634 nm value increased rapidly after the treatment with 300 nM thapsigargin and reached a plateau after 4 h. It is thus concluded that a thapsigargin treatment longer than 4 h may be unnecessary. As a final demonstration of our probe, we performed photoluminescence lifetime imaging experiments. The phosphorescence lifetime (τobs, λem 567 nm) of 50 μM PtGR was 21 ns in an air-equilibrated aqueous buffer devoid of HA (pH 7.4, 25 mM PIPES, 100 mM KCl) (Figure S16 in the Supporting Information). The addition of HA resulted in a 1 order of

although the response was smaller than that obtained in the buffer solution. The ratiometric imaging enabled an estimation of an intracellular HA level after exogenous supply. The I567 nm/ I634 nm value of 0.37 for the HA-pretreated cells corresponded to a free HA concentration of ∼1 pM, according to the HA titration isotherm shown in Figure 4. Although the I567 nm/I634 nm results established in buffers cannot be directly equated to the values recorded in the cellular milieu, this result may be suggestive of poor uptake efficiencies of extracellular HA by RAW 264.7 cells. It has been established that thapsigargin inhibits the activity of Ca2+-ATPase, triggering the de novo synthesis of HA in RAW 264.7 macrophages.17,56 The endogenously produced HA was visualized by pretreating the RAW 264.7 cells with 300 nM thapsigargin (20 h), followed by PtGR. The validity of this approach was confirmed by observing fluorescence turn-on signaling by o-phthaldialdehyde, an established HA fluorescence marker (Figure S15 in the Supporting Information). As shown in G


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Inorganic Chemistry magnitude elongation of τobs (435 ns). This observation motivated us to visualize intracellular HA in RAW 264.7 cells using photoluminescence lifetime imaging microscopy (λex 375 nm (picosecond pulsed laser); λem 500−600 nm; acquisition time 1.0 ms pixel−1; 200 × 200 pixels). As expected, the photoluminescence lifetime micrographs changed significantly upon increasing intracellular HA levels through an exogenous supply or thapsigargin treatments (Figure 8). A nonlinear leastsquares fit of the micrographs to a triexponential decay model revealed increases in the weighted average of τobs by a factor of 2 (Figure S17 and Table S1 in the Supporting Information). The increases were consistent with the phosphorescence lifetime changes obtained in aqueous solutions.

simulate the ionic strength of the biological milieu. Trace metal contamination was removed by treating the buffered solution with Chelex resin (Bio-Rad). Stock solutions of PtGR were dissolved in DMSO (Sigma-Aldrich, biotech grade) to 10 mM. 1H and 13C{1H} NMR spectra were collected with Bruker Ultrashield 500 and 300 plus NMR spectrometers or a Varian Unity-Inova (500 MHz) spectrometer. Chemical shifts were referenced to (CH3)4Si. Electrospray ionization mass spectra (positive mode) were recorded with an Agilent 6120 DW LC/MSD instrument. High-resolution mass spectra (positive mode, FAB, m-NBA) were obtained by employing a JEOL JMS-600W mass spectrometer. Synthesis of 2-(2-Naphthyl)quinoline. 2-Bromoquinoline (2.00 g, 9.61 mmol) and naphthalen-2-yl boronic acid (2.81 g, 16.3 mmol) were placed in a 250 mL one-necked round-bottom flask equipped with a magnetic stir bar. A 100 mL portion of THF was delivered into the flask, after which 50 mL of 2.0 N K2CO3(aq) was added to the stirred THF solution. The reaction mixture was refluxed for 20 h under an Ar atmosphere. The cooled reaction mixture was poured onto 100 mL of water, and the crude product was extracted with CH2Cl2 (3 × 100 mL). The organic layer was dried over anhydrous MgSO4 and concentrated under a reduced pressure. Silica gel column chromatography was performed on increasing the polarity of the eluent from 100% hexane to CH2Cl2/hexane 1/1 (v/v). Rf = 0.35 (CH2Cl2/hexane 1/1, v/v). A white solid was obtained in 79% yield. 1H NMR (300 MHz, CDCl3) δ (ppm): 7.54 (m, 3H), 7.76 (td, J = 6.9, 1.5 Hz, 1H), 7.88 (m, 2H), 8.01 (m, 2H), 8.05 (d, J = 8.7 Hz, 1H), 8.22 (d, J = 8.4 Hz, 1H), 8.28 (d, J = 8.7 Hz, 1H), 8.38 (dd, J = 8.4, 1.8 Hz, 1H), 8.62 (d, J = 1.2 Hz, 1H). 13 C{1H} NMR (126 MHz, CD2Cl2) δ (ppm): 119.43, 125.51, 126.88, 126.94, 127.33, 127.49, 127.84, 128.11, 128.21, 128.95, 129.31, 130.15, 130.25, 134.05, 134.45, 137.31, 137.39, 148.91, 157.28. HR MS (FAB, m-NBA): calcd for C19H14N ([M + H]+), 256.1126; found, 256.1122. Synthesis of [Pt2(μ-Cl)2(2-(2-naphthyl)quinolinate)2]. A μchloro dinuclear Pt complex having 2-(2-naphthyl)quinolinate ligands ([Pt2(μ-Cl)2(2-(2-naphthyl)quinolinate)2]) was prepared following a previous method.57 2-(2-Naphthyl)quinoline (0.500 g, 2.22 mmol) was dissolved in glacial acetic acid (20 mL) in a 50 mL one-necked roundbottom flask. An aqueous solution (∼2.0 mL) of K2PtCl4 (0.920 g, 2.22 mmol) was added slowly to the stirred reaction mixture at 80 °C. The resulting mixture was heated at 120 °C for an additional 24 h under an Ar atmosphere. Yellow precipitates were formed upon cooling to room temperature, which were collected by filtration and washed thoroughly with methanol. The resulting filter cake was recovered, dried in vacuo, and subjected to the next step without further purification. Synthesis of [Pt(2-(2-naphthyl)quinolinate)(NCCH3)2]ClO4 (PtGR). A previously reported method was employed for the synthesis of PtGR.58 [Pt2(μ-Cl)2(2-(2-naphthyl)quinolinate)2] (0.30 g, 0.30 mmol) was dissolved in 80 mL of anhydrous CH3CN. AgClO4 (0.12 mg, 0.60 mmol) was added to the stirred suspension. After it was stirred at room temperature for 20 h in the dark, the reaction solution was filtered through Celite. The filtrate was concentrated under reduced pressure. The addition of 50 mL of ether to 5 mL of the filtrate gave an orange powder in 60% yield. 1H NMR (300 MHz, DMSO-d6) δ (ppm): 2.08 (s, 6H), 7.50 (m, 3H), 7.73 (m, 3H), 7.93 (d, J = 7.2 Hz, 1H), 8.07 (d, J = 8.1 Hz, 1H), 8.54 (d, J = 9.0 Hz, 1H), 8.62 (d, J = 2.1 Hz, 1H), 8.78 (d, J = 8.7 Hz, 1H), 8.99 (d, J = 8.7 Hz, 1H). 13C{1H} NMR (126 MHz, DMSO-d6) δ (ppm): 1.14, 2.86, 117.37, 118.07, 125.33, 126.55, 127.34, 127.40, 127.84, 127.86, 128.09, 128.73, 129.42, 131.05, 131.22, 133.96, 135.43, 141.65, 145.08, 145.92, 165.91. LR MS (ESI, positive mode): calcd for C21H15N2Pt ([M]+), 531.1; found, 531.1. Synthesis of [Pt(2-(2-naphthyl)quinolinate)(HA)]ClO 4 (PtHA). [Pt2(μ-Cl)2(2-(2-naphthyl)quinolinate)2] (0.085 g, 0.09 mmol) and histamine (0.020 g, 0.18 mmol) were dissolved in a mixture of CH2Cl2 and CH3OH (3/1, v/v) in a 25 mL one-necked round-bottom flask equipped with a magnetic stir bar. AgClO4 (0.037 mg, 0.18 mmol) was added to the stirred suspension. After it was stirred at room temperature for 4 h under an Ar atmosphere, the reaction solution was filtered through Celite. The filtrate was concentrated under reduced pressure. Further purification by preparatory TLC (Sigma-Aldrich) was carried out employing CH2Cl2/CH3OH 9/1 (v/ v) as an eluent. An orange powder was obtained in 39% yield. Rf = 0.36.

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SUMMARY We discovered that a monocycloplatinated complex having two solvento ligands (PtGR) produced turn-on ratiometric phosphorescence responses to HA. The responses were due to spontaneous displacement of the solvento ligands with HA. Spectroscopic investigations employing an independently prepared HA adduct (PtHA) convincingly supported this claim. The phosphorescence HA responses were tolerant to other biological stimuli, including pH changes and the presence of biometals, amino acids, nucleosides, biothiols, neurotransmitters, and metabolites. In particular, the phosphorescence probe was very selective to HA over histidine due to very tight binding. HA bioimaging utility was evaluated with RAW 264.7 macrophages that were supplemented exogenously with HA or stimulated with thapsigargin to enrich HA. Visualization of intracellular HA was successfully demonstrated through microscopic techniques involving dual-channel ratiometric imaging microscopy and photoluminescence lifetime imaging microscopy. Our probe displayed unique advantages for HA bioimaging, as it enabled estimations of intracellular HA levels.

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EXPERIMENTAL DETAILS

Materials and General Methods. Commercially available chemicals were used as received unless otherwise stated. PiperazineN,N′-bis(2-ethanesulfonic acid) (PIPES, >99.0%), potassium chloride (puratonic grade), histamine (>97.0%), histamine dihydrochloride (>99.0%), potassium tetrachloroplatinate(II) (98.0%), 2-bromoquinoline (>97.0%), naphthalen-2-yl boronic acid (>95.0%), tetrakis(triphenylphosphine)palladium(0) (>99.0%), 4-aminobutyric acid (>99.0%), dopamine hydrochloride (>99.0%), acetylcholine chloride (>99.0%), (−)-epinephrine (>99.0%), L-glutamic acid (>99.0%), adenosine 5′-monophosphate disodium salt (>99.0%), adenosine 3′,5′-cyclic monophosphate (>98.5%), adenosine 5′-diphosphate sodium salt (>95.0%), guanosine 5′-diphosphate sodium salt (>96.0%), guanosine 5′-triphosphate sodium salt hydrate (>95.0%), adenosine 5′-triphosphate disodium salt hydrate (>99%), adenine (>99.0%), cytidine (>99.0%), guanosine (>98.0%), DL-dithiothreitol (>99.0%), and L-tryptophan (>98.0%) were purchased from SigmaAldrich and used as received. Adenosine (>99.0%), DL-alanine (>98.5%), DL-asparagine monohydrate (>99.0%), L-aspartic acid (>99.0%), L-cysteine (>98.5%), glycine (>99.0%), DL-isoleucine (>98.0%), DL-leucine (>98.0%), L-(+)-lysine monohydrochloride (>98.0%), DL-methionine (>99.0%), DL-tyrosine (>98.0%), L-valine (>98.0%), DL-phenylalanine (>98.0%), DL-proline (>99.0%), L-serine (>99.0%), DL-threonine (>98.0%), DL-glutamine (>98.0%), and DLarginine hydrochloride (>97.0%) were purchased from Tokyo Chemical Industry and used as received. PIPES-buffered aqueous solutions (25 mM) were prepared in Milli-Q grade water from a Milli-Q Direct 16 system (18.2 MΩ cm; Merck KGaA) and by adjusting the pH to 7.4 with standard KOH (45 wt %, Sigma-Aldrich) and HCl (1.0 N, Fluka) solutions. KCl (100 mM) was included in the PIPES solution to H


Article

Inorganic Chemistry H NMR (300 MHz, DMSO-d6) δ (ppm): 2.73 (m, 2H), 2.98 (m, 2H), 6.29 (s, 2H), 7.27 (s, 1H), 7.34 (s, 1H), 7.47 (m, 2H), 7.53 (m, 1H), 7.62 (m, 2H), 7.69 (s, 1H), 7.77 (d, J = 7.8 Hz, 1H), 7.94 (d, J = 7.8 Hz, 1H), 8.13 (d, J = 8.1 Hz, 1H), 8.51 (d, J = 8.7 Hz, 1H), 8.66 (s, 1H), 8.82 (d, J = 8.7 Hz, 1H), 12.73 (s, 1H). 13C{1H} NMR (126 MHz, DMSO-d6) δ (ppm): 27.20, 41.82, 113.60, 118.65, 125.38, 126.67, 126.95, 127.11, 127.94, 128.51, 129.34, 129.71, 130.25, 131.52, 134.19, 137.04, 139.82, 140.85, 146.85, 147.26. LR MS (ESI, positive mode): calcd for C24H21N4Pt ([M]+), 560.1; found, 560.4. Synthesis of Nα,Nτ-Bis(tert-butoxycarbonyl)histamine (bis(Boc)-HA). The method reported by Kuwahara and co-workers was employed for the synthesis of bis(Boc)-HA.59 Histamine dihydrochloride (1.00 g, 5.43 mmol) was placed in a 100 mL two-necked round-bottom flask equipped with a magnetic stir bar. Triethylamine (1.65 g, 16.3 mmol) diluted in 20 mL of dry CH3OH was placed in the flask at room temperature, and the reaction mixture was stirred for 1/2 h under an Ar atmosphere. Di-tert-butyl dicarbonate (2.37 g, 10.9 mmol) was delivered into the mixture, which was stirred for an additional 1 h. The reaction mixture was diluted in 100 mL of water, and the crude product was extracted with CH2Cl2 (3 × 100 mL). The organic layer was washed with brine, dried over anhydrous Mg2SO4, and concentrated under reduced pressure. Silica gel column chromatography was performed with increasing polarity of the eluent from CH2Cl2 to CH2Cl2/CH3OH 49/1 (v/v). Rf = 0.40 (CH2Cl2/ CH3OH 49/1, v/v). A white solid was obtained in 88% yield. 1H NMR (300 MHz, CDCl3) δ (ppm): 1.43 (m, 9H), 1.62 (m, 9H), 2.73 (t, J = 6.6 Hz, 2H), 3.43 (m, 2H), 4.97 (s, 1H), 7.14 (d, J = 1.2 Hz, 1H), 8.01 (d, J = 1.2 Hz, 1H). 13C{1H} NMR (126 MHz, CD2Cl2) δ (ppm): 28.15, 28.60, 40.06, 79.30, 85.61, 85.83, 129.96, 137.05, 141.27, 147.27, 156.16. LR MS (ESI, positive mode): calcd for C15H26N3O4 ([M + H]+), 312.2; found, 312.1. Synthesis of Nα-tert-Butoxycarbonylhistamine (Boc-HA). The previous method established by Qu and co-workers was employed for the synthesis of Boc-HA.60 In a 100 mL one-necked round-bottom flask containing 50 mL of water was placed bis(Boc)-HA (0.500 g, 1.61 mmol). The reaction mixture was refluxed for 1 h. The solution was cooled to room temperature, and the crude product was extracted with EtOAc (3 × 100 mL) and dried over anhydrous MgSO4. The concentrated solution was subjected to purification by silica gel column chromatography with CH2Cl2/CH3OH 9/1 (v/v) as eluent. A white solid was obtained in 43% yield. Rf = 0.60 (CH2Cl2/CH3OH 9/1, v/v). 1 H NMR (300 MHz, CDCl3) δ (ppm): 1.44 (s, 9H), 2.81 (t, J = 6.6 Hz, 2H), 3.44 (m, 2H), 5.00 (s, 1H), 6.84 (s, 1H), 7.59 (s, 1H). 13C{1H} NMR (126 MHz, CD2Cl2) δ (ppm): 28.64, 40.60, 79.53, 117.18, 135.16, 156.51. LR MS (ESI, positive mode): calcd for C10H18N3O2 ([M + H]+), 212.1; found, 212.1. Steady-State UV−Vis Absorption Measurements. UV−vis absorption spectra were collected on an Agilent Cary 300 spectrophotometer at 298 K. A 10 μM solution was used for the measurements, unless otherwise stated. Typically, 3 μL from the 10 mM stock solution (DMSO) of PtGR was diluted in 3.0 mL of DMSO or an aqueous solution buffered to pH 7.4 (25 mM PIPES, 100 mM KCl) to a 10 μM concentration. Steady-State Phosphorescence Measurements. Phosphorescence spectra were obtained using a Photon Technology International Quanta Master 400 scanning spectrofluorometer at 298 K. The 10 μM solution used for the steady-state UV−vis absorption measurements was employed. The solution was placed in a quartz cell (Hellma, beam path length 1.0 cm) for phosphorescence measurements. All of the phosphorescence sensing experiments were performed in airequilibrated solutions (DMSO and aqueous buffers). The excitation wavelength for PtGR was 374 nm. The phosphorescence spectra were recorded in the emission range 500−700 nm. Determination of Phosphorescence Lifetimes. PtGR (50 μM) in an air-equilibrated aqueous solution buffered to pH 7.4 (25 mM PIPES, 100 mM KCl) was employed. Phosphorescence decay traces were acquired on the basis of time-correlated single-photon-counting (TCSPC) techniques using a PicoQuant FluoTime 200 instrument after pulsed laser excitation at 377 nm (pulse duration 6.4 ns). Transient photon signals were collected at λobs 567 or 635 nm through 1

an automated motorized monochromator. Typically, acquisition was terminated when the accumulated photon count reached 103. Phosphorescence decay traces were analyzed using mono- or multiexponential decay models embedded in OriginLab OriginPro 2018 software. In the case of multiphasic decay, average phosphorescence lifetime (τobs) values were calculated from the relationship τobs = ∑Aiτi2/∑Aiτi, where Ai and τi are the pre-exponential factor and the time constant, respectively. Determination of Photoluminescence Quantum Yields. The photoluminescence quantum yields (PLQYs) were determined relatively, following the equation PLQY = PLQYref(I/Iref)(Aref/A)(n/ nref)2, where A, I, and n are the absorbance at the excitation wavelength, the integrated phosphorescence intensity, and the refractive index of the solvent, respectively. [Ru(bpy)3]Cl2 (PLQY = 0.055, H2O; λex 455 nm) was used as the reference material.50 Phosphorescence spectra were integrated in the region 500−850 nm for both PtGR and [Ru(bpy)3]Cl2. Phosphorescence Histamine Titration. Stock solutions of 10 mM PtGR and 1.0 mM HA were prepared in DMSO (Sigma-Aldrich, biotech grade). PtGR (10 μM) was prepared by diluting 3 μL of the PtGR stock solution in 3.0 mL of an air-equilibrated aqueous solution buffered to pH 7.4 (25 mM PIPES, 100 mM KCl). Phosphorescence spectra of the PtGR solution were taken (λex 374 nm) with the continuous addition of 3 μL of 1.0 mM HA. The final concentration of HA was 40 μM. The HA titration experiment was performed in triplicate. The average values and standard deviations of the three experiments were employed for the determination of the dissociation constant for HA (i.e., Kd(HA)). Determination of the Bimolecular Rate Constant for the Reaction between PtGR and Histamine. Stock solutions of 10 mM PtGR and 1.0 mM HA were prepared in DMSO (Sigma-Aldrich, biotech grade). A 3 μL portion of the 10 mM PtGR was added to an airequilibrated aqueous solution (pH 7.4; 25 mM PIPES, 100 mM KCl) in a quartz cuvette (Hellma, beam path length 1.0 cm). Phosphorescence intensities at λobs 567 nm were monitored under continuous excitation at λex 374 nm. A 3, 6, 15, 30, or 60 μL portion of 1.0 mM HA (i.e., 1, 2, 5, 10, or 20 μM) was quickly added to the stirred solution of PtGR, and the temporal changes in the phosphorescence intensity at 567 nm were recorded for 1400 s. Monitoring HA Displacement Employing 1H NMR Spectroscopy. Stock solutions of PtGR and HA (20 mM) were prepared in DMSO-d6. 1H NMR spectra were recorded on a Bruker 300 plus NMR spectrometer, with DMSO-d6/D2O 3/1 (v/v) as the solvent (total volume 1.0 mL). 1H NMR spectra were taken for 2.0 mM PtGR on increasing the concentration of HA (0, 2, and 8.0 mM). ATR FT-IR Measurements. ATR FT-IR measurements were performed for powdery samples of PtGR, HA, and PtHA, with an Agilent Cary 630 FT−IR spectrometer. Evaluations of HA Selectivity. Stock solutions of 10 mM PtGR, 100 mM HA, 100 mM amino acids, 100 mM nucleosides, 10 mM small molecular neurotransmitters, and 10 mM metabolites were prepared in DMSO (Sigma-Aldrich, biotech grade). A 3 μL portion of PtGR was diluted in 3 mL of an air-equilibrated aqueous solution buffered to pH 7.4 (25 mM PIPES, 100 mM KCl) to a 10 μM concentration. Phosphorescence spectra of the PtGR solution were recorded before and after the addition of 1.0 mM amino acids, 1.0 mM nucleosides, 100 μM small molecular neurotransmitters, or 100 μM metabolites. HA (1.0 mM) was subsequently added to the mixture, and the phosphorescence spectrum was acquired. The competitive HA assay experiment was performed in triplicate. Quantum Chemical Calculations. Geometry optimization was performed using Becke’s three-parameter B3LYP exchange-correlation functional, the “double-ξ” quality LANL2DZ basis set for the Pt atom, and the 6-311+G(d,p) basis set for all the other atoms. A pseudopotential (LANL2DZ) was applied to replace the inner core electrons of the Pt atom, leaving the outer core [(5s)2(5p)6] electrons and the (5d)8 valence electrons. On the basis of the crystal structure of an antitumor drug, cis-[Pt(HA)Cl2],49 two possible coordination structures were used for the geometry optimization of PtHA: one structure had a trans configuration of the quinolinyl and imidazolyl I


Article

Inorganic Chemistry

Colocalization Experiments. Photoluminescence microscopic experiments were performed at the Korea Basic Science Institute (KBSI), Chuncheon Center, Korea. RAW 264.7 cells were pretreated with organelle-specific fluorescence stains, including 4′,6-diamidino-2phenylindole (DAPI, 300 nM, 5 min), CellMask green (Molecular Probes, ×1000, 10 min), LysoTracker Green (Molecular Probes, 50 nM, 40 min), ER-Tracker Green (Molecular Probes, 1 μM, 20 min), and MitoTracker Green (Molecular Probes, 100 nM, 30 min), to stain the nuclei, plasma membranes, lysosomes, endoplasmic reticulum, and mitochondria, respectively. After the treated cells were washed with Dulbecco’s phosphate-buffered saline (DPBS, Welgene) twice, the cells were incubated with 100 μM PtGR for 20 min at 37 °C. Prior to microscopic visualization, the cells were washed with DPBS and supplemented with fresh DPBS. A Carl Zeiss LSM 510 META confocal laser scanning microscope was used to obtain photoluminescence images. Photoluminescence microscopic visualization was performed through the following channels: DAPI, λex 405 nm, λobs 415−457 nm; CellMask Green, λex 488 nm, λobs 499−531 nm; LysoTracker Green, λex 488 nm, λobs 499−531 nm; ER-Tracker Green, λex 488 nm, λobs 497− 546 nm; MitoTracker Green, λex 488 nm, λobs 499−531 nm; PtGR, λex 405 nm, λobs 593−635 nm. Photoluminescence micrographs and colocalization coefficients were analyzed using Leica LSM 510 version 4.0 software. Fluorescence Visualization of Intracellular Histamine Using OPA. One day prior to imaging, RAW 264.7 cells were plated onto glass-bottom culture dishes (SPL Life Sciences). Stock solutions of 10 mM o-phthaldialdehyde (OPA), 1.0 M HA, and 1.0 mM thapsigargin were prepared in DMSO (Sigma-Aldrich, biotech grade). A 10 μM OPA solution was prepared by diluting the stock solution in DMEM. The cells were washed twice with fresh DPBS and incubated with the 10 μM OPA for 20 min at 37 °C under 5% CO2 in a humidified incubator. The cells were washed with DPBS and supplemented with fresh DPBS for microscopic visualization. An excitation beam (405 nm) was focused onto the dish, and the fluorescence signals were acquired through 32 emission channels covering the range 418−551 nm. A 1.0 mM HA solution was prepared by diluting the stock solution in DMEM. In order to elevate intracellular HA levels, RAW 264.7 cells were washed twice with fresh DPBS and incubated with 1.0 mM HA (DMEM) for 2 h at 37 °C. The cells were subsequently treated with 10 μM OPA (20 min), washed with DPBS, and supplemented with fresh DPBS. Fluorescence images of the treated cells were acquired, employing the conditions described above. Finally, DMEM containing 300 nM thapsigargin was prepared. RAW 264.7 cells were washed twice with fresh DPBS and incubated with the 300 nM thapsigargin solution (DMEM) for 20 h at 37 °C under 5% CO2 in a humidified incubator. OPA (10 μM) was placed in the culture dish, and the cells were incubated for 20 min at 37 °C under 5% CO2 in a humidified incubator. After the cells were washed with DPBS and supplemented with fresh DPBS, fluorescence images were taken. Analyses of the micrographs were performed using Leica LSM 510 version 4.0 software. Phosphorescence Microscopic Visualization of Intracellular Histamine Using PtGR. One day prior to imaging, RAW 264.7 cells were plated onto glass-bottom culture dishes (SPL Life Sciences). Stock solutions of 10 mM PtGR, 1.0 M HA, and 1.0 mM thapsigargin were prepared in DMSO (Sigma-Aldrich, biotech grade). A 100 μM PtGR solution was prepared by diluting the stock solution in DMEM. The cells were washed twice with fresh DPBS and incubated with 100 μM PtGR for 20 min at 37 °C under 5% CO2 in a humidified incubator. The cells were washed with DPBS and supplemented with fresh DPBS. An excitation beam (405 nm) was focused onto the dish, and the phosphorescence signals were acquired through 32 emission channels covering the range 411−691 nm. A 1.0 mM HA solution was prepared by diluting the stock solution in DMEM. In order to elevate intracellular HA levels, RAW 264.7 cells were washed twice with fresh DPBS and incubated with 1.0 mM HA (DMEM) for 2 h at 37 °C. The cells were subsequently treated with 100 μM PtGR (20 min), washed with DPBS, and supplemented with fresh DPBS. Phosphorescence images of the treated cells were acquired employing the conditions described above. Finally, DMEM containing 300 nM thapsigargin was prepared. RAW 264.7 cells were washed twice with fresh DPBS and incubated with the

nitrogens, and the other had a cis configuration of the two nitrogens. Frequency calculations were performed subsequently to assess the stability of the convergence. Time-dependent density functional theory (TD-DFT) calculations were carried out for the optimized geometries using the same functional and basis sets. Geometry optimization and single-point calculations were performed using the Gaussian 09 program.61 A GaussSum software was employed for the simulation of the predicted electronic absorption spectra.62 Phosphorescence Job Plot. Stock solutions of 10 mM PtGR and 10 mM HA were prepared in DMSO (Sigma-Aldrich, biotech grade). A 3 μL portion of the PtGR stock solution was diluted in 3.0 mL of an airequilibrated aqueous solution (pH 7.4; 25 mM PIPES, 100 mM KCl) to a 10 μM concentration, which was delivered into a quartz cell (Hellma, beam path length 1.0 cm). A 10 μM HA solution was prepared similarly. A phosphorescence spectrum of the PtGR solution was acquired. A 300 μL portion was removed from the solution, followed by the addition of 300 μL of the 10 μM HA solution. A phosphorescence spectrum of the resulting solution was acquired. The method of continuous variations was repeated in order to vary the mole fraction of PtGR. The exchange volumes were 300, 330, 390, 420, 510, 600, 750, 990, and 1500 μL, which corresponded to mole fractions of PtGR of 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, and 0.1, respectively. Thus, the total volume of the solution was maintained at 3.0 mL throughout the experiment, and the total concentration was kept at 10 μM. Cell Culture. RAW 264.7 cells purchased from the Korean Cell Line Bank were cultured in RPMI (Gibco) supplemented with 10% fetal bovine serum and penicillin (100 units mL−1) at 37 °C in a humidified incubator under 5% CO2. MTT Cell Proliferation Assays. RAW 264.7 cells were seeded into 6 × 10 wells in a 96-well plate, and the remaining wells were filled with the RPMI medium. The cells were incubated for 24 h at 37 °C in a humidified incubator at 5% CO2. After removal of the medium, the cells were treated with varied concentrations (2−500 μM) of PtGR which was diluted in Dulbecco’s modified Eagle’s medium (DMEM containing 4.5 g L−1 glucose, 584.0 mg L−1 L-glutamine, 110.0 mg L−1 sodium pyruvate; Gibco) (100 μL). The cells were incubated for 40 min in a humidified incubator at 37 °C in the dark. A 15 μL portion of 3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, 2 mg mL−1, Sigma) was delivered into each well, and the cells were incubated for an additional 3 h at 37 °C. Finally, 100 μL of stop solution (Sigma) was added. After 30 min, the absorbance at 570 nm of purple formazan was recorded by using a Molecular Devices SPECTRAMAX i3 microplate reader. Inductively Coupled Plasma Mass Spectrometry. RAW 264.7 cells were treated with 100 μM PtGR at 37 °C for 20 min in a humidified incubator under 5% CO2. Cells incubated without PtGR served as control samples. The cells were thoroughly washed with PBS twice and collected with employing a cell scraper. The collected cells were transferred to a 15 mL conical tube. The total volume of the cell suspension was kept to 5.0 mL by supplementing PBS. Centrifugation at 1000 rpm and 4 °C for 3 min resulted in pelletization. The supernatant was removed, and the cell pellet was resuspended in 5.0 mL of Milli-Q water. The centrifugation and resuspension cycle was repeated three times. The cells were finally pelletized, and 50 μL of the cell pellet was delivered to a 5.0 mL conical tube. A 337.5 μL portion of concentrated HCl and 112.5 μL of concentrated HNO3 were placed in the conical tube, which was heated at 80 °C for 1 h. After it was cooled to room temperature, the solution was transferred into a 10 mL volumetric flask. HNO3(aq) (2 wt %) was subsequently placed in the flask until the total volume of the solution became 10 mL. Inductively coupled plasma mass spectrometry (ICP-MS) analyses were performed for the sample solution with an Agilent 7900 mass spectrometer equipped with a Scott spray chamber and a MicroMist nebulizer at the Environmental Analysis Center in the Gwangju Instutite of Science and Technololgy (GIST), Gwangju, Korea. A Pt standard solution (1000 mg L−1) was purchased from Sigma-Aldrich and diluted in 2 wt % HNO3(aq) to concentrations of 1, 5, 10, 20, 50, and 100 ppb. ICP MS conditions: rf power, 1550 W; auxiliary gas flow rate, 1 L min−1; plasma gas flow rate, 15 L min−1; nebulizer gas flow rate, 1.07 L min−1; He gas flow rate, 5 L min−1. J


Article

Inorganic Chemistry 300 nM thapsigargin solution (DMEM) for 0.5, 1, 2, 4, and 20 h at 37 °C under 5% CO2 in a humidified incubator. PtGR (100 μM) was placed in the culture dishes, and the cells were incubated for 20 min at 37 °C under 5% CO2 in a humidified incubator. After the cells were washed with DPBS and supplemented with fresh DPBS, phosphorescence images were taken. Analyses of the micrographs were performed using Leica LSM 510 version 4.0 software. Photoluminescence Lifetime Imaging Microscopy. Photoluminescence lifetime imaging microscopic experiments were performed at the Korea Basic Science Institute (KBSI), Daegu Center, Korea. One day prior to imaging, RAW 264.7 cells were plated onto glass-bottom culture dishes (SPL Life Sciences). Stock solutions of 10 mM PtGR, 1.0 M HA, and 1.0 mM thapsigargin were prepared in DMSO (Sigma-Aldrich, biotech grade). A 100 μM PtGR solution was prepared by diluting the stock solution in DMEM. The cells were washed twice with fresh DPBS and incubated with 100 μM PtGR for 20 min at 37 °C under 5% CO2 in a humidified incubator. The cells were washed with DPBS and supplemented with fresh DPBS. Photoluminescence lifetime imaging microscopy studies were performed using an inverted-type scanning confocal microscope (PicoQuant MicroTime-200) with a 40× (air) objective. A 375 nm single-mode pulsed diode laser (PicoQuant LDH-P-C-375) with an ∼30 ps pulse width and ∼2 μW power was used as an excitation source. A dichroic mirror (AHF, Z375RDC), a long-pass filter (AHF, HQ405lp), a 75 μm pinhole, band pass filters (Thorlabs, 500−600 nm), and an avalanche photodiode detector (MPD PDM series) were used to collect emissions from the samples. Time-correlated single-photon counting (TCSPC) techniques were used to collect photons. Time-resolved photoluminescence lifetime (TRPL) images consisting of 200 × 200 pixels were recorded using the time-tagged time-resolved (TTTR) data acquisition method. The acquisition time of each pixel was 1.0 ms. A 1.0 mM HA solution was prepared by diluting the stock solution in DMEM. In order to elevate intracellular HA levels, RAW 264.7 cells were washed twice with fresh DPBS and incubated with 1.0 mM HA (DMEM) for 2 h at 37 °C. The cells were subsequently treated with 100 μM PtGR (20 min), washed with DPBS, and supplemented with fresh DPBS. TRPL images of the treated cells were acquired employing the conditions described above. Finally, DMEM containing 300 nM thapsigargin was prepared. RAW 264.7 cells were washed twice with fresh DPBS and incubated with the thapsigargin solution (DMEM) for 0.5, 1, 2, 4, and 20 h at 37 °C under 5% CO2 in a humidified incubator. PtGR (100 μM) was placed in the culture dish, and the cells were incubated for 20 min at 37 °C under 5% CO2 in a humidified incubator. After the cells were washed with DPBS and supplemented with fresh DPBS, TRPL images were taken. Exponential fittings for the obtained photoluminescence lifetime images were performed using Symphotime-64 software (version 2.2) using multiexponential decay models, I(t) = ∑Ai exp(−t/τi), where I(t) is the time-dependent photoluminescence intensity, Ai is the amplitude, τi is the photoluminescence lifetime, and ordinal i is 1−3 in this study.

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phosphorescence responses to histidine, cysteine, or homocysteine, MTT cell proliferation assay, ICP MS results, intracellular colocalization experiments, fluorescent visualization of intracellular HA using o-phthaldialdehyde, photoluminescence decay traces of PtGR after pulsed laser photoexcitation, photoluminescence decay traces of the photoluminescence lifetime micrographs, 1H and 13C NMR spectra, and fit results for the photoluminescence decay traces shown in Figure S17 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.Y.: [email protected]. ORCID

Youngmin You: 0000-0001-5633-6599 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MSIP of Korea through the GFP (Grant CISS-2012M3A6A6054204). The authors acknowledge Dr. Seung-Hae Kwon and Taemin Wi at the Korea Basic Science Institute, Chuncheon Center, for microscopic experiments.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site: Figures S1−S29 displaying The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02612. ATR FT-IR spectra for PtGR, HA, and PtHA, a comparison of the experimental and simulated absorption spectra of PtHA, phosphorescence rise traces of 10 μM PtGR after the addition of HA, concentration-dependent phosphorescence spectra, phosphorescence spectra of PtHA recorded in various solvents, phosphorescence responses to HA in the presence of various biometals, phosphorescence responses to HA obtained at various pHs, phosphorescence HA selectivity over biological competing Lewis bases, phosphorescence Job plot, K


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