Selective Dendritic Fluorescent Sensors for Zn(II)

Abstract

A series of dendritic 8-hydroxyquinoline (8-HQ) and 5-dialkyl(aryl)amino-methyl -8-HQ derivatives were synthesized and their fluoroionophoric properties toward representative alkali, alkaline earth, group IIIA and transition metal ions were investigated. Among the selected ions, Zn(II) enhanced the fluorescence of N-di- (methoxycarbonylethyl)aminoethyl-3-[4-(8-hydroxyquinolin-5-ylmethyl)piperazin-1-yl]-propanoic amide] (7) by 31-fold, while Al(III) caused enhancement to some extent. The absence of any significant fluorescence enhancement by the other ions examined renders 7 a highly useful Zn(II)-selective fluorescent sensor.

Introduction

Zinc, which always occurs as a divalent cation [Zn(II)] in biological systems, is the second most abundant transition metal after iron. Its amounts in brain tissues can reach 0.1-0.5 mM [1]. The Zn²⁺ ion is well known to play diverse roles in biological processes [2], such as its presence in hundreds of structural metalloproteins [3] and at the catalytic centre of human carbonic anhydrase [4] and its involvement in the regulation of gene expression [5] and cellular apoptosis [6]. Disorders of zinc metabolism are closely associated with many severe neurological diseases, such as Alzheimer’s disease, amyotrophic lateral sclerosis (ALS), Guam ALS-Parkinsonism dementia, Parkinson’s disease, hypoxia-ischemia and epilepsy [7]. There is huge scope for exploring the diverse physiological roles of biological zinc. Therefore, a sensitive and particularly selective chemosensor for Zn(II) is desired, since there are also many other biologically important transition metal ions.

A fluorescent chemosensor is the first choice among various types of sensors. 8-Aminoquinoline (8-AQ,) and 8-hydroxyquinoline (8-HQ, 1) and their derivatives have traditionally been used as fluorogenic agents for quantitative chemical assays of Zn(II) and other metal ions. 8-HQ, 8-AQ and their derivatives are non-fluorescent and are not influenced by changes in pH [8] due to the existence of an intramolecular hydrogen bond between the heterocyclic nitrogen atom and the 8-substituted group (i.e. –OH or –NH₂) [9,10]. However, when these quinolines and their derivatives chelate Zn(II) and other metal ions, they exhibit intense yellow-greenish fluorescence. Interestingly, only Zn(II) and Ca²⁺ ions can interrupt hydrogen bonding in 8-AQ and its derivatives, and subsequently stimulate fluorescence, thereby forming the basis for selective detection of these two cations. It was not until 1987 that one of the sulphonamido compounds, namely 6-methoxy-8-p-toluenesulphonamido-quinoline (TSQ), was first applied to in vitro imaging of divalent zinc ions [11]. However, TSQ has several limitations. The most notable among these is its poor water solubility, since this means that TSQ stock solutions have to be prepared in dimethylsulfoxide (DMSO) or ethanol. Although some efforts toward the development of new sensors have been made [12], TSQ remains the most widely used compound for biological zinc staining.

Although 8-HQ is regarded as the second most important chelating agent for cation analysis after EDTA, most indicators derived from it are not sufficiently specific for the selective detection of particular metal ions. However, while working on the synthesis of 8-HQ metallodendrimers for Organic Light Emitting Display (OLED), we found that, among the common transitional metal ions, only Zn²⁺ could form stable complexes with the dendritic 8-HQ ligands [13]. In particular, these ligands are easily soluble in common solvents and water. This experience inspired us to investigate the Zn²⁺-selective sensing properties of dendritic 8-HQ ligands.

Results and Discussion

The synthesis of new dendritic 8-HQ ligands is shown in Scheme 1 and Scheme 2. First, 8-HQ (1) was chloromethylated to give crude 5-chloromethyl-8-HQ (2). High purity products were obtained by washing with concentrated hydrogen chloride instead of acetone [8]. The residual 8-HQ could not be removed completely by washing with acetone and substitution reactions occurred during the crystallization from ethanol.

Scheme 1.

Scheme 1.

Next, 2 was reacted for about 10 minutes with different secondary amines in dichloromethane at room temperature to give 3a-3e in high yields. Triethylamine was used as the base. The use of inorganic bases, such as sodium carbonate, sodium hydrogen carbonate and sodium hydroxide, led to either very slow reaction rates or many unwanted byproducts.

Scheme 2.

Scheme 2.

Compound 4 was obtained in 93% yield by treatment of 3e with 30% hydrochloric acid for about 8 hours, followed by neutralization with potassium hydroxide. Michael addition of 4 with methyl acrylate in methanol gave 5 in 88% yield. Compound 6 was synthesized in 62% yield by amidation of 5 with ethylene diamine. Further Michael addition of 6 with methyl acrylate gave 7 in 75% yield. The structures were identified by ¹H-NMR, ¹³C-NMR, ESI-MS and elemental analysis.

The UV spectrum of 3a, selected as a representative compound, showed two absorption peaks at 255 and 327 nm. When equivalent Zn(II) was added, the absorption peaks shifted to 257 and 405 nm, respectively (Figure 1). When excited at 370 nm, 3a-3e, 5 and 7 showed similar emission patterns with maximum emission wavelengths around 406 and 432 nm.

The fluoroionophoric properties of the 8-HQ derivatives toward representative alkali ions (K⁺), alkaline earth ions (Mg²⁺, Ca²⁺, Ba²⁺), group IIIA ions (Al³⁺, Ga³⁺) and transition metal ions (Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺ Pb²⁺, Mn²⁺, Cr³⁺) were investigated in chloroform solutions. The results revealed that only Zn²⁺ and Al³⁺ caused new emission peaks, while the other ions hardly had any effects. Following addition of Zn²⁺ or Al³⁺, the maximum emission wavelengths changed to around 500 or 530 nm, respectively (Figure 2). Furthermore, Zn²⁺ enhanced the fluorescence intensity to a greater extent than Al³⁺ for all the sensors examined. However, there were differences in the sensitivities and selectivities among 3a-3e, 5 and 7. The corresponding data are summarized in Table 1. The best sensor was 7, which showed 31-fold fluorescence enhancement for Zn²⁺ and 2.4-fold enhancement for Al³⁺ (Figure 3).

Figure 1. UV spectra of 3a (10 μM) in chloroform, before (A) and after (B) the addition of 1.0 equiv of Zn²⁺.

Figure 1. UV spectra of 3a (10 μM) in chloroform, before (A) and after (B) the addition of 1.0 equiv of Zn²⁺.

Figure 2. Fluorescence spectra of 3a (10 μM) in chloroform, before (A) and after the addition of 1.0 equiv of Zn²⁺ (B) and Al³⁺ (C).

Figure 2. Fluorescence spectra of 3a (10 μM) in chloroform, before (A) and after the addition of 1.0 equiv of Zn²⁺ (B) and Al³⁺ (C).

Table 1. Fluorescence enhancement of 3a-3d, 4, 5 and7 after addition of 1.0 equiv of Zn²⁺ or Al³⁺

Compd.	λ0max(nm)	λ( Al3+)max(nm)	λ( Zn2+)max(nm)	Enhancing percent. of Zn2+	Enhancing percent. of Al3+
3a	406, 432	531	507	3	2
3b	407, 431	530	506	4	2
3c	406, 432	529	505	4	3
3d	407, 431	530	506	4	2
4 5 7	407, 431 406, 432 405, 433	530 529 529	506 505 505	5 8 31	2 3 3

Table 1. Fluorescence enhancement of 3a-3d, 4, 5 and7 after addition of 1.0 equiv of Zn²⁺ or Al³⁺

Compd.	λ0max(nm)	λ( Al3+)max(nm)	λ( Zn2+)max(nm)	Enhancing percent. of Zn2+	Enhancing percent. of Al3+
3a	406, 432	531	507	3	2
3b	407, 431	530	506	4	2
3c	406, 432	529	505	4	3
3d	407, 431	530	506	4	2
4 5 7	407, 431 406, 432 405, 433	530 529 529	506 505 505	5 8 31	2 3 3

The fluorescence enhancement data and selectivity of sensor 7 are shown in Figure 3. Zn²⁺ caused the highest level of fluorescence enhancement, while Al³⁺ enhanced the fluorescence to some extent. The other ions examined showed little fluorescence enhancement.

Figure 3. Relative fluorescence intensity (I_f/I₀) of 7 (10 μM) in the presence of 1.0 equiv. of various metal ions (10 μM) in chloroform.

Figure 3. Relative fluorescence intensity (I_f/I₀) of 7 (10 μM) in the presence of 1.0 equiv. of various metal ions (10 μM) in chloroform.

In order to gain further insight into the analytical possibilities, fluorescence titration of 7 with Zn²⁺ was performed and the obtained curves are shown in Figure 4. The fluorescence enhancement was very effective and 31-fold enhancement was obtained at the ratio of 1:1. The break around 0.5 equiv of Zn²⁺ suggested a 1:1 stoichiometry for the 7-Zn²⁺ complex system.

Figure 4. Fluorescence titration curves of 7 (10 μM) by addition of 1μM Zn²⁺ each time in chloroform.The inset shows a plot of the fluorescence intensities against [Zn²⁺].

Figure 4. Fluorescence titration curves of 7 (10 μM) by addition of 1μM Zn²⁺ each time in chloroform.The inset shows a plot of the fluorescence intensities against [Zn²⁺].

Conclusions

A series of novel Zn²⁺ fluorescent sensors based on 8-HQ were synthesized and their fluoroionophoric properties toward K⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺, Ga³⁺ Fe³⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺, Pb²⁺, Mn²⁺ and Cr³⁺ were investigated in chloroform solutions. Only Zn²⁺ caused considerable enhancement of the fluorescence emission, and the other metal ions examined showed little interference with the detection of Zn²⁺. Among these potential metal ion fluorescent sensors, dendritic ligand 7 showed the best efficiency and selectivity toward Zn²⁺. These results are consistent with our earlier discovery.

Experimental

General

All reagents were purchased from Aldrich and used without further purification. ¹H-NMR and ¹³C-NMR spectra were recorded in CDCl₃ on a JEOL JNM-ECA300 spectrometer operating at 300 and 75 MHz, respectively. Chemical shifts (δ) are reported in parts per million (ppm) relative to tetramethylsilane (TMS) as an internal standard, and coupling constants (J) are given in hertz (Hz). All ESI-MS experiments were undertaken on a Bruker ESQUIRE-LC. Fluorescence intensity was measured using a HORABA Fluoromax 3.

General procedure for the preparation of 5-chloromethyl-8-HQ hydrochloride (2)

Into a solution of 8-HQ (1, 0.1 mol) and formaldehyde (20 mL, 37%) in 37% hydrochloric acid (50 mL) was blown a stream of hydrogen chloride gas for 8 hours at 25°C. After filtration, the product was washed with 37% hydrochloric acid and dried to afford 2 in 85% yield.

General procedure for the preparation of 5-dialkyl(aryl)aminomethyl-8-HQ compounds 3a-3e

To a solution of dialkyl(aryl)amine (10 mmol) and triethylamine (30 mmol for dimethylamine hydrochloride and 20 mmol for other secondary amines) in dichloromethane (50 mL) was added dropwise a solution of 2 (10 mmol) in dichloromethane at room temperature over 10 minutes. After a further 15 minutes of reaction, the solution was washed thoroughly with brine, dried over sodium sulfate and concentrated to afford 3a-3e without further purification (yields are listed in Scheme 1 and Scheme 2).

5-Dimethylaminomethyl-quinolin-8-ol (3a). ¹H-NMR δ 8.77 (dd, 1H, J₁=4.4, J₂=1.4), 8.61 (dd, 1H, J₁=8.6, J₂=1.4), 7.48 (dd, 1H, J₁=8.6, J₂=4.4), 7.30 (d, 1H, J=8.3), 7.07 (d, 1H, J=8.3), 3.72 (s, 2H), 2.25 (s, 6H); ¹³C-NMR δ 153.4, 148.2, 137.4, 132.6, 132.2, 127.4, 122.6, 115.8, 111.1, 56.6, 42.2; ESI-MS: calcd. for (M+H)/z: 203.1. Found: (M+H)/z: 203.1; Elemental analysis: C₁₁H₁₁N₂O calcd. (%) C, 70.57; H, 5.92; N, 14.96. Found (%) C, 70.49; H, 6.04; N, 14.64.

5-Dipropylaminomethyl-quinolin-8-ol (3b). ¹H-NMR δ 8.75 (m, 2H), 7.44 (dd, 1H, J₁=6.5, J₂=4.4), 7.34 (d, 1H, J=7.56), 7.06 (d, 1H, J=7.56), 3.85 (s, 2H), 2.38 (t, 4H, J=7.2), 1.49 (m, 4H), 0.79 (t, 6H, J=7.2); ¹³C-NMR δ 151.6, 147.5, 138.8, 134.4, 128.7, 128.0, 126.4, 121.2, 108.8, 57.3, 55.8, 19.9, 12.1; ESI-MS: calcd. for (M+H)/z: 259.2. Found: (M+H)/z: 259.2; Elemental analysis: C₁₆H₂₂N₂O calcd. (%) C, 74.38; H, 8.58; N, 10.84. Found (%) C, 74.49; H, 8.44; N, 11.01.

5-(Diphenylaminomethyl)quinoline-8-ol (3c). ¹H-NMR δ 8.80 (dd, 1H, J₁=4.1, J₂=1.0), 8.36 (dd, 1H, J₁=8.5, J₂=1.0), 7.46 (m, 2H), 7.23 (m, 4H), 7.05(m, 5H), 6.95 (t, 2H, J=8.5), 5.30 (s, 2H); ¹³C-NMR δ 151.4, 148.1, 147.5, 138.7, 131.9, 129.4, 126.5, 126.4, 123.7, 121.8, 121.7, 121.0, 109.5, 53.3; ESI-MS: calcd. for (M+H)/z: 327.2. Found: (M+H)/z: 327.2; Elemental analysis: C₂₂H₁₈N₂O calcd. (%) C, 80.96; H, 5.56; N, 8.58. Found (%) C, 80.61; H, 5.94; N, 8.39

5-(Morpholin-4-ylmethyl)-quinolin-8-ol (3d). ¹H-NMR δ 8.78 (dd, 1H, J₁=4.2, J₂=1.3), 8.67 (dd, 1H, J₁=8.6, J₂=1.3), 7.45 (dd, 1H, J₁=8.6, J₂=4.2), 7.32 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9), 3.79 (s, 2H), 3.66 (m, 4H), 2.45 (m, 4H); ¹³C-NMR δ 152.1, 147.7, 138.8, 134.2, 129.2, 127.9, 124.0, 121.6, 108.7, 67.1, 61.1, 53.6; ESI-MS: calcd for (M+H)/z: 245.1. Found: (M+H)/z: 245.1; Elemental analysis: C₁₄H₁₆N₂O₂ calcd. (%) C, 68.83; H, 6.60; N, 11.47. Found (%) C, 68.91; H, 6.35; N, 11.65.

5-(4'-Benzoylpiperazin-1-ylmethy)l-quinolin-8-ol (3e). ¹H-NMR δ 8.78 (dd, 1H, J₁=4.2, J₂=1.3), 8.66 (dd, 1H, J₁=8.6, J₂=1.3), 7.48 (dd, 1H, J₁=8.6, J₂=4.2), 7.39 (m, 5H), 7.32 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9), 3. 83 (s, 2H), 3.76 (m, 2H), 3.37 (m, 2H), 2.57 (m, 2H), 2.39 (m, 2H);¹³C- NMR δ 170.2, 152.0, 147.6, 138.7, 135.8, 133.9, 129.6, 129.1, 128.4, 128.0, 127.7, 127.0, 121.5, 108.6, 67.9, 60.5, 52.2; ESI-MS: calcd for (M+H)/z: 347.2. Found: (M+H)/z: 347.2.

5-(Piperazin-1-ylmethyl)-quinolin-8-ol (4). ¹H-NMR δ 8.77 (dd, 1H, J₁=4.2, J₂=1.3), 8.67 (dd, 1H, J₁=8.6, J₂=1.3), 7.46 (dd, 1H, J₁=8.6, J₂=4.2), 7.32 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9), 3.77 (s, 2H), 2.83 (m, 4H), 2.43 (m, 4H); ¹³C-NMR δ 151.9, 147.6, 138.8, 134.2, 129.0, 128.0, 124.6, 121.4, 108.8, 61.4, 54.4, 46.2; ESI-MS: calcd. for (M+H)/z: 244.1. Found: (M+H)/z: 244.1; Elemental analysis: C₁₄H₁₇N₃O calcd. (%) C, 69.11; H, 7.04; N, 17.27. Found (%) C, 69.43; H, 7.11; N, 17.04.

The compounds 5,6 and 7 were obtained by a reported method [13].

3-[4'-(8-Hydroxyquinolin-5-ylmethyl)-piperazin-1'-yl]-propionic acid methyl ester (5). ¹H-NMR δ 8.77 (d, 1H, J=4.1), 8.65 (d, 1H, 8.2), 7.46 (dd, 1H, J1=8.2, J2=4.1), 7.33 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9), 3.79 (s, 2H), 3.67 (s, 3H), 2.67 (m, 2H), 2.48 (m, 10H); ¹³C-NMR δ 171.6, 153.4, 148.2, 139.8, 132.2, 129.3, 128.4, 124.6, 121.8, 110.4, 60.2, 54.4, 51.7, 48.7, 46.2, 34.2; ESI-MS: calcd. for (M+H)/z: 330.2. Found: (M+H)/z: 330.2; Elemental analysis: C₁₈H₂₃N₃O₃ calcd. (%) C, 65.63; H, 7.04; N, 12.76. Found (%) C, 65.85; H, 6.98; N, 12.73.

N-Aminoethyl-3-[4-(8-hydroxyquinolin-5-ylmethyl)piperazin-1-yl]propanamide (6). ¹H-NMR δ 8.77 (d, 1H, J=4.1), 8.65 (d, 1H, 8.2), 7.46 (dd, 1H, J₁=8.2, J₂=4.1), 7.33 (d, 1H, J=7.9), 7.06 (d, 1H, J=7.9), 3.77 (s, 2H), 3.66 (t, 2H), 2.75 (t, 2H), 2.69 (m, 2H), 2.46 (m, 10H); ¹³C-NMR δ 171.6, 153.4, 148.2, 139.8, 132.2, 129.3, 128.4, 124.6, 121.8, 110.4, 60.2, 54.4, 48.7, 46.2, 40.6, 39.8, 34.2; ESI-MS: calcd. for (M+H)/z: 357.2. Found: (M+H)/z: 357.2; Elemental Analysis: C₁₉H₂₇N₅O₂ calcd. (%) C, 63.84; H, 7.61; N, 19.59; O, 8.95. Found (%) C, 65.85; H, 7.64; N, 19.57; O, 8.93.

N-di(Methoxycarbonylethyl)aminoethyl-3-[4-(8-hydroxyquinolin-5-ylmethyl)piperazin-1-yl]propanoic amide] (7). ¹H-NMR δ 8.85 (d, 1H, J=3.0), 8.62 (d, 1H, J=7.9), 7.57 (dd, 1H, J₁=7.9, J₂=3.0), 7.32 (d, 1H, J=7.9), 7.00 (d, 1H, J=7.9), 3.74 (s, 2H), 3.61 (s, 6H), 3.05 (t, 2H, J=6.5), 2.46-2.55 (m, 10H), 2.37 (m, 10H), 2.22 (t, 2H, J=6.9); ¹³C-NMR δ 172.4, 172,0, 171.3, 153.4, 147.9, 139.4, 132.1, 129.4, 128.0, 124.7, 121.9, 111.2, 60.8, 54.4, 53.2, 51.8, 51.7, 48.7, 46.2, 37.1, 34.2, 33.5; ESI-MS: calcd. for (M+H)/z: 530.3. Found: (M+H)/z: 530.3; Elemental analysis: C₂₇H₃₉N₅O₆ calcd. (%) C, 61.23; H, 7.42; N, 13.22. Found (%) C, 61.55; H, 7.38; N, 13.04.