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Syntheses, crystal structures and anticancer activities of three novel transition metal complexes with Schiff base derived from 2-acetylpyridine and l -tryptophan
Inorganic Chemistry Communications 22 (2012) 68–72
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Syntheses, crystal structures and anticancer activities of three novel transition metal complexes with Schiff base derived from 2-acetylpyridine and l-tryptophan Nan Zhang, Yu-hua Fan, Zhen Zhang, Jian Zuo, Peng-fei Zhang, Qiang Wang, Shan-bin Liu, Cai-feng Bi ⁎ Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong 266100, China
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Article history: Received 23 March 2012 Accepted 15 May 2012 Available online 24 May 2012 Keywords: L-tryptophan 2-acetylpyridine Schiff base Crystal structure Anticancer activity
a b s t r a c t Three novel transition metal coordination complexes, Cu(C18H16N3O2)2·2CH3OH (1), Zn(C18H16N3O2)2·2CH3CH2OH (2) and Cd(C18H16N3O2)2·2CH3OH (3) (C18H16N3O2: 2-acetylpyridine-L-tryptophan) have been synthesized and characterized by IR, UV, elemental analysis, 1H NMR and X-ray diffraction single crystal analysis. The three crystals crystallize in the tetragonal crystal system, space group P43212. They are all six-coordinated by two nitrogen atoms from C=N, two nitrogen atoms from pyridine rings and two carboxylic oxygen atoms in different ligands, forming a distorted octahedron geometry. Each ligand serves as a bridging ligand to link metal ions through carboxylic oxygen atoms, leading to a three-dimensional coordination polymer. The anticancer activities of these three complexes on MDA-MB-231 breast cancer cells were also investigated. The results indicate that all of the three complexes can inhibit the cellular proliferation. Furthermore, Cd(C18H16N3O2)2·2CH3OH (3) has the highest anti-proliferative activity among the three complexes. In addition, Cd(C18H16N3O2)2·2CH3OH (3) can inhibit proteasomal chymotrypsin-like activity and also can induce apoptosis on human breast cancer MDAMB-231 cells. © 2012 Elsevier B.V. All rights reserved.
Schiff base is an important organic ligand. Recent researches show that Schiff base complexes have potential applications as antibacterial, anticancer and antiviral agents [1–3]. Due to their important role in homogeneous or heterogeneous catalysis and magnetism [4,5], many complexes have been given enough attention by chemists all over the world. In addition, some of the Schiff bases and Schiff base complexes have special properties in luminescence and photochromism. Therefore, they have wide prospect of application [6]. On the other hand, because of high bio-activity, some Schiff base complexes derived from amino acids become the hot research topics for years [7–10]. What is more, the interest in metal-based anticancer drugs has increased since the discovery of cisplatin as one of the anticancer drugs [11]. Clinical studies suggest the use of proteasome inhibitors as potential novel anticancer agents [12–16]. Proteasome inhibitors are known to induce cell death rapidly and selectively in oncogene-transformed but not untransformed and normal cells. It has been reported that copper, zinc and cadmium complexes can induce apoptosis by inhibiting the tumor cellular proteasome [17–19]. In the present study, we describe the preparations and crystal structures of copper, zinc and cadmium complexes Cu(C18H16N3O2)2·2CH3OH (1), Zn(C18H16N3O2)2·2CH3CH2OH (2) and Cd(C18H16N3O2)2·2CH3OH (3) with Schiff base derived from 2-acetylpyridine and L-tryptophan [20–22] and also investigate their potential apoptosis induction and proteasomal inhibition activities.
⁎ Corresponding author. E-mail address: [email protected] (C. Bi). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2012.05.022
The general synthesis reaction of the three complexes is shown in Scheme 1. We found that Cd(C18H16N3O2)2·2CH3OH (3) has potential to be used as a proteasome inhibitor and anticancer agent. Crystallographic data [23,24] and processing parameters for the three complexes were obtained. The molecular structures and the crystal packing diagrams of the three complexes are shown in Figs. 1, 2 and 3, respectively. Since the three complexes were derived from the same ligand of 2-acetylpyridine-L-tryptophan, they have the similar structures. The crystallographic structural analysis reveals that all of the three crystals crystallize in the tetragonal crystal system, space group P43212. Each metal atom coordinates with two Schiff base ligands. It is coordinated by six atoms, namely, two nitrogen atoms from C=N, two nitrogen atoms from pyridine rings and two carboxylic oxygen atoms in different ligands, forming a type of the 4 N + 2O neutrality complex. In addition, there are two solvent methanol or ethanol molecules not coordinating with metal ions in these complexes. Take the copper complex Cu(C18H16N3O2)2·2CH3OH (1) for example. The crystal structure of 1 is shown in Fig. 1a. The corresponding bond angles of O(1A)Cu(1)N(3) (155.4°) and O(1)Cu(1)N(3A) (155.4°) are both less than 180°, and the bond angles N(1A)Cu(1)O(1) (93.3°), N(1A) Cu(1)N(3) (77.4°), N(1A)Cu(1)O(1A) (78.3°) and N(1A)Cu(1)N(3A) (111.3°) are less or more than 90°, indicating that the central Cu(II) adopts a distorted octahedron geometry. Four atoms N(3), O(1), O(1A) and N(3A) occupy each vertex of the basal site, while the N(1) and N(1A) atoms locate in the apical position of the octahedral structure. The bond length of C(13)–N(1) is 1.293 Å, which is close to normal
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Scheme 1. The general synthesis reaction of the three complexes. (M=Cu, Zn, Cd; R–OH=CH3OH, CH3CH2OH.).
C= N 1.306 Å [25], indicating that the double bond between C(13) and N(1) is formed in the complex. The bond lengths of Cu(1)–O(1) and Cu(1)–O(1A) are both 2.125 Å, which is in good agreement with the relative values of Cu–O in literature [26]. As for the bond lengths of Cu–N, Cu(1)–N(1)(2.002 Å), Cu(1)–N(1A) (2.002 Å) are shorter than that of Cu(1)–N(3) (2.148 Å ) and Cu(1)–N(3A) (2.148 Å), suggesting that the coordination ability of N(1) and N(1A) in imines is stronger than that of the N(3) and N(3A) atoms in pyridine rings. What's more, each ligand serves as a bridging ligand to link Cu(II) ions through carboxylic oxygen atoms, leading to a three-dimensional coordination polymer. On the whole, we notice that the bond length of M(1)–O(1A) (2.235 Å), M(1)–N(1A) (2.299 Å) and M(1)–N(3) (2.305 Å) in Cd(C18H16N3O2)2·2CH3OH (3) are much longer than that of Cu(C18H16N3O2)2·2CH3OH (1) and Zn(C18H16N3O2)2·2CH3CH2OH (2), showing that the coordination ability of M-O and M-N in Cd(C18H16N3O2)2·
Fig. 2. The molecular structure (a) and the crystal packing diagram analog the a axis (b) of Zn(C18H16N3O2)2·2CH3CH2OH (2).
Fig. 1. The molecular structure (a) and the crystal packing diagram analog the a axis (b) of Cu(C18H16N3O2)2·2CH3OH (1).
Fig. 3. The molecular structure (a) and the crystal packing diagram analog the a axis of Cd(C18H16N3O2)2·2CH3OH (3).
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0.55
1
0.50
Absorption
0.45
3
0.40
2
0.35
L
0.30 0.25 0.20 0.15 0.10 0.05 0.00 220
240
260
280
300
Wavelength, nm Fig. 4. UV spectra of the ligand (L) and its complex: 1, complex (1); 2, complex (2); 3, complex (3).
2CH3OH (3) are much weaker than that of Cu(C18H16N3O2)2·2CH3OH (1) and Zn(C18H16N3O2)2·2CH3CH2OH (2). The IR spectra of the complexes 1, 2 and 3 display strong absorptions at 1622, 1654 and 1642 cm− 1, respectively, which are assigned to the C=N stretching mode in the Schiff bases [27]. The absorptions at 1601, 1593 and 1585 cm− 1 are attributed to the stretching vibration of aromatic rings. Furthermore, all of the three complexes have strong absorptions at 747 cm− 1, and they all have weak absorptions at 870, 874 and 870 cm− 1, moderate intensity absorptions at 780, 784 and 788 cm− 1, and weak absorptions at 698, 698 and 694 cm− 1, respectively, which are almost the same, showing the C–H out-of-plane bend vibrations of aromatic rings. The appearance of the bands at 580, 584 and 575 cm− 1 are assigned to ν(M–N), and 465, 461 and 461 cm− 1 are assigned to ν(M–O), respectively. Fig. 4 shows the UV spectra of the complexes and the ligand. There are two absorption peaks at 222 and 273 nm in the range of 215–300 nm in the spectrum of the ligand. The counterparts of the
complexes 1, 2 and 3 appear at 221, 222, 221 and 281, 279, 279 nm, respectively. The first peaks are assigned to π–π* transition of the pyridyl or indol. The second peaks are assigned to n–π* transition of conjugation between a lone pair electron of the N atom in the C = N group and delocalized π bond of the aromatic ring. The shifts of 8 or 6 nm are caused by the coordination of the N atom to the metal atoms, which can also provide the evidence for the coordination [28]. To investigate the proliferation-inhibitory effect of the three complexes, human breast cancer MDA-MB-231 cells [29] were treated with both of them dissolved in DMSO in different concentrations followed by MTT assay [30,31]. Cells treated with DMSO were used as solvent control. We found that both of them potentially inhibited cellular proliferation (Fig. 5). Moreover, Cd(C18H16N3O2)2·2CH3OH (3) had the highest anti-proliferative activity compared with the other two complexes under our experimental condition. The IC50 value of 3 is 27 μmol/L. The proliferation-inhibitory activity of 3 on human breast cancer MDA-MB-231 is higher than that of cispatin (IC50: 82 μmol/L) [32]. It was reported that some cadmium complexes can inhibit proteasome activity [19]. To examine whether Cd(C18H16N3O2)2·2CH3OH (3) is capable of inhibiting the proteasome activity, we incubated it at 5, 10, 20 and 50 μmol/L with a purified rabbit 20 S proteasome for 1, 2, 3 and 4 hours [33]. The results showed that 3 inhibited the proteasomal CT-like activity in a concentration-dependent manner: take the point of 4 h for example, complex 3 at 20 and 50 μmol/L caused 92.5% and 94.6% inhibition, respectively (Fig. 6). Furthermore, the IC50 value of 3 is 4 μmol/L. It has been shown that inhibition of tumor cellular proteasome activity is associated with apoptosis induction [34-37]. To explore whether Cd(C18H16N3O2)2·2CH3OH (3) has apoptosis-inducing activity, cellular morphologic changes were studied. Significant changes in cellular apoptotic morphology (fragmentation and condensation cells and characteristic apoptotic blebbing) were observed only in the cells treated with 3 at 50 μM (Fig. 7), indicating that 3 has a high apoptosis induction potency. Morphologically, apoptotic changes were observed after 8 to 24 h treatment (Fig. 8). These results support the notion that the apoptosis induced by 3 occurred after the proteasome inhibition. In conclusion, we synthesized and characterized three novel transition metal coordination complexes, Cu(C18H16N3O2)2·2CH3OH (1),
Fig. 5. Anti-proliferation activities of the three complexes on MDA-MB-231 cells.
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Fig. 6. Inhibition of proteasomal CT-like activity of purified 20S proteasome by Cd(C18H16N3O2)2·2CH3OH (3).
Acknowledgement
Zn(C18H16N3O2)2·2CH3CH2OH (2) and Cd(C18H16N3O2)2·2CH3OH (3). They all crystallize in the tetragonal crystal system, space group P43212. What is more, all of them are six-coordinated by two nitrogen atoms from C=N, two nitrogen atoms from pyridine rings and two carboxylic oxygen atoms in different ligands, forming a distorted octahedron geometry. The complexes display a three-dimensional coordination polymer through carboxylic oxygen atoms. We also investigated the anticancer activities of these three complexes on MDA-MB-231 breast cancer cells. The results indicate that all of the three complexes can inhibit the cellular proliferation. Furthermore, Cd(C18H16N3O2)2·2CH3OH (3) has the highest anti-proliferative activity among the three complexes. In addition, complex 3 can inhibit proteasomal chymotrypsinlike activity and also can induce apoptosis on human breast cancer MDA-MB-231 cells. Cd(C18H16N3O2)2·2CH3OH (3) has potential to be used as a proteasome inhibitor and anticancer agent.
Crystallographic information of the Schiff base coordination complexes have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number s CCDC (818893, 827172 and 821439). The copies of the data may be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EB, UK (Fax: +44-1223-336-033; E-mail: [email protected] or http://www.ccdc.cam.ac.uk). Supplementary related to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2012.05.022.
Fig. 7. Cellular morphological changes of MDA-MB-231 cells treated with Cd(C18H16N3O2)2·2CH3OH (3) for 24 h.
Fig. 8. Cellular morphological changes of MDA-MB-231 cells treated with Cd(C18H16N3O2)2·2CH3OH (3) at 50 μmol/L.
This work was supported by the National Science Foundation of China (No. 21071134 and No. 20971115). Appendix A. Supplementary data
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The reaction mixture was heated to 50 °C with stirring and refluxed for 6 h to give a bright yellow transparent solution. After that, 10 mL of methanol solution of CuCl2·2H2O (0.170 g, 1.0 mmol) was added to 50 mL of methanol solution of the ligand (0.691 g, 2.0 mmol), and the mixture was stirred and refluxed at 50 °C for 6 h. The resulting solution was cooled at room temperature and then filtered. The filtrate was left for slow evaporation at room temperature. The green block-shaped crystals were formed two days later (yield 38% based on crystals). Elemental Anal Calc (%) for 1: C, 61.65; H, 5.45; N, 11.35. Found: C, 61.24; H, 5.51; N, 11.47. IR (KBr, cm- 1): 1622 (vs), 1601 (vs), 1434 (s), 1373 (s), 1348 (s), 1311 (s), 1254 (m), 1234 (w), 1164 (vw), 1103 (w), 1042 (vw), 1017 (w), 870 (w), 780 (m), 747 (s), 698 (w), 580 (w), 531 (vw), 506 (vw), 465 (w), 420 (m). Decomposition point: 233 °C. 1 H NMR (DMSO-d6 600 MHz) δ (ppm): 1.43 (s, 3 H), 3.18 (t, 1 H, J = 12.6 Hz), 3.38 (d, 1 H, J = 12.0 Hz), 4.42 (s, 1 H), 6.73 (t, 1 H, J = 7.2 Hz), 6.93 (t, 1 H, J = 7.8 Hz), 7.11 (s, 1 H), 7.33 (d, 1 H, J = 8.4 Hz), 7.45 (t, 1 H, J = 7.2 Hz), 7.63 (t, 2 H, J = 10.2 Hz), 7.96 (t, 1 H, J = 8.4 Hz), 8.03 (s, 1 H), 10.89 (s, 1 H). [21] The complex Zn(C18H16N3O2)2·2CH3CH2OH (2) was synthesized in a procedure similar to that of complex 1, but the Zn(CH3COO)2 (0.220 g, 1.0 mmol) which was added to ligand solution was used and dissolved in 20 mL of ethanol. The yellow block crystals of 2 were obtained one week later with 61% yield on crystals base. Elemental Anal Calc (%) for 2: C, 62.38; H, 5.76; N, 10.91. Found: C, 61.87; H, 5.94; N, 11.03. IR (KBr, cm- 1): 1654 (vs), 1593 (vs), 1475 (w), 1438 (s),
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1397 (m), 1377 (s), 1311 (s), 1254 (s), 1160 (vw), 1140 (vw), 1099 (w), 1046 (vw), 1009 (w), 976 (vw), 919 (vw), 874 (w), 809 (w), 784 (m), 747 (s), 698 (w), 637 (w), 584 (w), 506 (vw), 461 (w), 425 (m). Decomposition point: 196 °C. 1 H NMR (DMSO-d6 600 MHz) δ (ppm): 1.43 (s, 3 H), 3.22 (t, 1 H, J = 12.6 Hz), 3.50 (d, 1 H, J = 12.0 Hz), 4.46 (dd, 1 H, J = 2.4, 10.2 Hz), 6.80 (t, 1 H, J = 7.2 Hz), 7.02 (t, 1 H, J = 7.2 Hz), 7.07 (s, 1 H), 7.34 (d, 1 H, J = 8.4 Hz), 7.40 (t, 1 H, J = 6.0 Hz), 7.69 (t, 2 H, J = 9.6 Hz), 7.92 (t, 1 H, J = 8.4 Hz), 7.98 (d, 1 H, J = 4.2 Hz), 10.94 (s, 1 H). Cd(C18H16N3O2)2·2CH3OH (3) was prepared in a similar manner as complex 1 by Cd(CH3COO)2·2H2O (0.267 g,1.0 mmol) in place of CuCl2·2H2O. The yellow block crystals of 3 were obtained two days later. The yield was 65% (based on crystals). Elemental Anal Calc (%) for 3: C, 57.83; H, 5.11; N, 10.65. Found: C, 57.35; H, 5.34; N, 10.80. IR (KBr, cm-1): 1642 (vs), 1585 (vs), 1458 (m), 1438 (s), 1368 (s), 1332 (w), 1311 (s), 1250 (s), 1197 (vw), 1169 (w), 1136 (w), 1099 (w), 1042 (vw), 1013 (m), 972 (vw), 935 (vw), 911 (vw), 886 (vw), 870 (w), 809 (w), 788 (m), 747 (s), 694 (w), 637 (w), 617 (vw), 575 (w), 559 (vw), 502 (vw), 461 (vw), 433 (w), 408 (vw). Decomposition point: 191 °C. 1 H NMR (DMSO-d6 600 MHz) δ (ppm): 1.45 (s, 3 H), 3.17 (t, 1 H, J = 11.4 Hz), 3.47 (d, 1 H, J = 13.2 Hz), 4.41 (s, 1 H), 6.83 (t, 1 H, J = 7.2 Hz), 7.02 (t, 1 H, J = 7.2 Hz), 7.10 (s, 1 H), 7.33 (d, 1 H, J = 8.4 Hz), 7.55 (t, 1 H, J = 6.0 Hz), 7.65 (d, 1 H, J = 7.8 Hz), 7.83 (d, 1 H, J = 7.2 Hz), 8.05 (t, 1 H, J = 7.2 Hz), 8.13 (s, 1 H), 10.91 (s, 1 H). The single crystals were mounted on an Enraf-Nonius CAD-4 X-ray single-crystal diffractometer. All data were collected at 293(2) K with a graphite monochromatized MoKα radiation (λ = 0.71073 Å) by using an ω-2θ scan mode. The structures were solved by direct methods using SHELXS-97. 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Syntheses, crystal structures and anticancer activities of three novel transition metal complexes with Schiff base derived from 2-acetylpyridine and l-tryptophan Nan Zhang, Yu-hua Fan, Zhen Zhang, Jian Zuo, Peng-fei Zhang, Qiang Wang, Shan-bin Liu, Cai-feng Bi ⁎ Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao, Shandong 266100, China
a r t i c l e
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Article history: Received 23 March 2012 Accepted 15 May 2012 Available online 24 May 2012 Keywords: L-tryptophan 2-acetylpyridine Schiff base Crystal structure Anticancer activity
a b s t r a c t Three novel transition metal coordination complexes, Cu(C18H16N3O2)2·2CH3OH (1), Zn(C18H16N3O2)2·2CH3CH2OH (2) and Cd(C18H16N3O2)2·2CH3OH (3) (C18H16N3O2: 2-acetylpyridine-L-tryptophan) have been synthesized and characterized by IR, UV, elemental analysis, 1H NMR and X-ray diffraction single crystal analysis. The three crystals crystallize in the tetragonal crystal system, space group P43212. They are all six-coordinated by two nitrogen atoms from C=N, two nitrogen atoms from pyridine rings and two carboxylic oxygen atoms in different ligands, forming a distorted octahedron geometry. Each ligand serves as a bridging ligand to link metal ions through carboxylic oxygen atoms, leading to a three-dimensional coordination polymer. The anticancer activities of these three complexes on MDA-MB-231 breast cancer cells were also investigated. The results indicate that all of the three complexes can inhibit the cellular proliferation. Furthermore, Cd(C18H16N3O2)2·2CH3OH (3) has the highest anti-proliferative activity among the three complexes. In addition, Cd(C18H16N3O2)2·2CH3OH (3) can inhibit proteasomal chymotrypsin-like activity and also can induce apoptosis on human breast cancer MDAMB-231 cells. © 2012 Elsevier B.V. All rights reserved.
Schiff base is an important organic ligand. Recent researches show that Schiff base complexes have potential applications as antibacterial, anticancer and antiviral agents [1–3]. Due to their important role in homogeneous or heterogeneous catalysis and magnetism [4,5], many complexes have been given enough attention by chemists all over the world. In addition, some of the Schiff bases and Schiff base complexes have special properties in luminescence and photochromism. Therefore, they have wide prospect of application [6]. On the other hand, because of high bio-activity, some Schiff base complexes derived from amino acids become the hot research topics for years [7–10]. What is more, the interest in metal-based anticancer drugs has increased since the discovery of cisplatin as one of the anticancer drugs [11]. Clinical studies suggest the use of proteasome inhibitors as potential novel anticancer agents [12–16]. Proteasome inhibitors are known to induce cell death rapidly and selectively in oncogene-transformed but not untransformed and normal cells. It has been reported that copper, zinc and cadmium complexes can induce apoptosis by inhibiting the tumor cellular proteasome [17–19]. In the present study, we describe the preparations and crystal structures of copper, zinc and cadmium complexes Cu(C18H16N3O2)2·2CH3OH (1), Zn(C18H16N3O2)2·2CH3CH2OH (2) and Cd(C18H16N3O2)2·2CH3OH (3) with Schiff base derived from 2-acetylpyridine and L-tryptophan [20–22] and also investigate their potential apoptosis induction and proteasomal inhibition activities.
⁎ Corresponding author. E-mail address: [email protected] (C. Bi). 1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2012.05.022
The general synthesis reaction of the three complexes is shown in Scheme 1. We found that Cd(C18H16N3O2)2·2CH3OH (3) has potential to be used as a proteasome inhibitor and anticancer agent. Crystallographic data [23,24] and processing parameters for the three complexes were obtained. The molecular structures and the crystal packing diagrams of the three complexes are shown in Figs. 1, 2 and 3, respectively. Since the three complexes were derived from the same ligand of 2-acetylpyridine-L-tryptophan, they have the similar structures. The crystallographic structural analysis reveals that all of the three crystals crystallize in the tetragonal crystal system, space group P43212. Each metal atom coordinates with two Schiff base ligands. It is coordinated by six atoms, namely, two nitrogen atoms from C=N, two nitrogen atoms from pyridine rings and two carboxylic oxygen atoms in different ligands, forming a type of the 4 N + 2O neutrality complex. In addition, there are two solvent methanol or ethanol molecules not coordinating with metal ions in these complexes. Take the copper complex Cu(C18H16N3O2)2·2CH3OH (1) for example. The crystal structure of 1 is shown in Fig. 1a. The corresponding bond angles of O(1A)Cu(1)N(3) (155.4°) and O(1)Cu(1)N(3A) (155.4°) are both less than 180°, and the bond angles N(1A)Cu(1)O(1) (93.3°), N(1A) Cu(1)N(3) (77.4°), N(1A)Cu(1)O(1A) (78.3°) and N(1A)Cu(1)N(3A) (111.3°) are less or more than 90°, indicating that the central Cu(II) adopts a distorted octahedron geometry. Four atoms N(3), O(1), O(1A) and N(3A) occupy each vertex of the basal site, while the N(1) and N(1A) atoms locate in the apical position of the octahedral structure. The bond length of C(13)–N(1) is 1.293 Å, which is close to normal
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Scheme 1. The general synthesis reaction of the three complexes. (M=Cu, Zn, Cd; R–OH=CH3OH, CH3CH2OH.).
C= N 1.306 Å [25], indicating that the double bond between C(13) and N(1) is formed in the complex. The bond lengths of Cu(1)–O(1) and Cu(1)–O(1A) are both 2.125 Å, which is in good agreement with the relative values of Cu–O in literature [26]. As for the bond lengths of Cu–N, Cu(1)–N(1)(2.002 Å), Cu(1)–N(1A) (2.002 Å) are shorter than that of Cu(1)–N(3) (2.148 Å ) and Cu(1)–N(3A) (2.148 Å), suggesting that the coordination ability of N(1) and N(1A) in imines is stronger than that of the N(3) and N(3A) atoms in pyridine rings. What's more, each ligand serves as a bridging ligand to link Cu(II) ions through carboxylic oxygen atoms, leading to a three-dimensional coordination polymer. On the whole, we notice that the bond length of M(1)–O(1A) (2.235 Å), M(1)–N(1A) (2.299 Å) and M(1)–N(3) (2.305 Å) in Cd(C18H16N3O2)2·2CH3OH (3) are much longer than that of Cu(C18H16N3O2)2·2CH3OH (1) and Zn(C18H16N3O2)2·2CH3CH2OH (2), showing that the coordination ability of M-O and M-N in Cd(C18H16N3O2)2·
Fig. 2. The molecular structure (a) and the crystal packing diagram analog the a axis (b) of Zn(C18H16N3O2)2·2CH3CH2OH (2).
Fig. 1. The molecular structure (a) and the crystal packing diagram analog the a axis (b) of Cu(C18H16N3O2)2·2CH3OH (1).
Fig. 3. The molecular structure (a) and the crystal packing diagram analog the a axis of Cd(C18H16N3O2)2·2CH3OH (3).
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0.55
1
0.50
Absorption
0.45
3
0.40
2
0.35
L
0.30 0.25 0.20 0.15 0.10 0.05 0.00 220
240
260
280
300
Wavelength, nm Fig. 4. UV spectra of the ligand (L) and its complex: 1, complex (1); 2, complex (2); 3, complex (3).
2CH3OH (3) are much weaker than that of Cu(C18H16N3O2)2·2CH3OH (1) and Zn(C18H16N3O2)2·2CH3CH2OH (2). The IR spectra of the complexes 1, 2 and 3 display strong absorptions at 1622, 1654 and 1642 cm− 1, respectively, which are assigned to the C=N stretching mode in the Schiff bases [27]. The absorptions at 1601, 1593 and 1585 cm− 1 are attributed to the stretching vibration of aromatic rings. Furthermore, all of the three complexes have strong absorptions at 747 cm− 1, and they all have weak absorptions at 870, 874 and 870 cm− 1, moderate intensity absorptions at 780, 784 and 788 cm− 1, and weak absorptions at 698, 698 and 694 cm− 1, respectively, which are almost the same, showing the C–H out-of-plane bend vibrations of aromatic rings. The appearance of the bands at 580, 584 and 575 cm− 1 are assigned to ν(M–N), and 465, 461 and 461 cm− 1 are assigned to ν(M–O), respectively. Fig. 4 shows the UV spectra of the complexes and the ligand. There are two absorption peaks at 222 and 273 nm in the range of 215–300 nm in the spectrum of the ligand. The counterparts of the
complexes 1, 2 and 3 appear at 221, 222, 221 and 281, 279, 279 nm, respectively. The first peaks are assigned to π–π* transition of the pyridyl or indol. The second peaks are assigned to n–π* transition of conjugation between a lone pair electron of the N atom in the C = N group and delocalized π bond of the aromatic ring. The shifts of 8 or 6 nm are caused by the coordination of the N atom to the metal atoms, which can also provide the evidence for the coordination [28]. To investigate the proliferation-inhibitory effect of the three complexes, human breast cancer MDA-MB-231 cells [29] were treated with both of them dissolved in DMSO in different concentrations followed by MTT assay [30,31]. Cells treated with DMSO were used as solvent control. We found that both of them potentially inhibited cellular proliferation (Fig. 5). Moreover, Cd(C18H16N3O2)2·2CH3OH (3) had the highest anti-proliferative activity compared with the other two complexes under our experimental condition. The IC50 value of 3 is 27 μmol/L. The proliferation-inhibitory activity of 3 on human breast cancer MDA-MB-231 is higher than that of cispatin (IC50: 82 μmol/L) [32]. It was reported that some cadmium complexes can inhibit proteasome activity [19]. To examine whether Cd(C18H16N3O2)2·2CH3OH (3) is capable of inhibiting the proteasome activity, we incubated it at 5, 10, 20 and 50 μmol/L with a purified rabbit 20 S proteasome for 1, 2, 3 and 4 hours [33]. The results showed that 3 inhibited the proteasomal CT-like activity in a concentration-dependent manner: take the point of 4 h for example, complex 3 at 20 and 50 μmol/L caused 92.5% and 94.6% inhibition, respectively (Fig. 6). Furthermore, the IC50 value of 3 is 4 μmol/L. It has been shown that inhibition of tumor cellular proteasome activity is associated with apoptosis induction [34-37]. To explore whether Cd(C18H16N3O2)2·2CH3OH (3) has apoptosis-inducing activity, cellular morphologic changes were studied. Significant changes in cellular apoptotic morphology (fragmentation and condensation cells and characteristic apoptotic blebbing) were observed only in the cells treated with 3 at 50 μM (Fig. 7), indicating that 3 has a high apoptosis induction potency. Morphologically, apoptotic changes were observed after 8 to 24 h treatment (Fig. 8). These results support the notion that the apoptosis induced by 3 occurred after the proteasome inhibition. In conclusion, we synthesized and characterized three novel transition metal coordination complexes, Cu(C18H16N3O2)2·2CH3OH (1),
Fig. 5. Anti-proliferation activities of the three complexes on MDA-MB-231 cells.
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Fig. 6. Inhibition of proteasomal CT-like activity of purified 20S proteasome by Cd(C18H16N3O2)2·2CH3OH (3).
Acknowledgement
Zn(C18H16N3O2)2·2CH3CH2OH (2) and Cd(C18H16N3O2)2·2CH3OH (3). They all crystallize in the tetragonal crystal system, space group P43212. What is more, all of them are six-coordinated by two nitrogen atoms from C=N, two nitrogen atoms from pyridine rings and two carboxylic oxygen atoms in different ligands, forming a distorted octahedron geometry. The complexes display a three-dimensional coordination polymer through carboxylic oxygen atoms. We also investigated the anticancer activities of these three complexes on MDA-MB-231 breast cancer cells. The results indicate that all of the three complexes can inhibit the cellular proliferation. Furthermore, Cd(C18H16N3O2)2·2CH3OH (3) has the highest anti-proliferative activity among the three complexes. In addition, complex 3 can inhibit proteasomal chymotrypsinlike activity and also can induce apoptosis on human breast cancer MDA-MB-231 cells. Cd(C18H16N3O2)2·2CH3OH (3) has potential to be used as a proteasome inhibitor and anticancer agent.
Crystallographic information of the Schiff base coordination complexes have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication number s CCDC (818893, 827172 and 821439). The copies of the data may be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EB, UK (Fax: +44-1223-336-033; E-mail: [email protected] or http://www.ccdc.cam.ac.uk). Supplementary related to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2012.05.022.
Fig. 7. Cellular morphological changes of MDA-MB-231 cells treated with Cd(C18H16N3O2)2·2CH3OH (3) for 24 h.
Fig. 8. Cellular morphological changes of MDA-MB-231 cells treated with Cd(C18H16N3O2)2·2CH3OH (3) at 50 μmol/L.
This work was supported by the National Science Foundation of China (No. 21071134 and No. 20971115). Appendix A. Supplementary data
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Abrams, B.A. Murrer, Metal compounds in cancer therapy and diagnosis, Science 261 (1993) 725–730. [12] J. Adams, V.J. Palombella, E.A. Sausville, Proteasome inhibitors: a novel class of potent and effective antitumor agents, Cancer Res. 59 (1999) 2615–2622. [13] R.Z. Orlowski, T.E. Stinchcombe, B.S. Mitchell, Phase I trial of the proteasome inhibitor PS-341 in patients with refractory hematologic malignancies, J. Clin. Oncol. 20 (2002) 4420–4427. [14] J.A. Dams, Potential for proteasome inhibition in the treatment of cancer, Drug Discov. Today 8 (2003) 307–315. [15] J.B. Almond, G.M. Cohen, The proteasome: a novel target for cancer chemotherapy, Leukemia 16 (2002) 433–443. [16] Q.P. Dou, B. Li, Pharmacological proteasome inhibitors and their therapeutic potential, Expert. Opin. Ther. Pat. 10 (2000) 1263–1272. [17] B. Cvek, V. Milacic, J. Taraba, Q.P. 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The reaction mixture was heated to 50 °C with stirring and refluxed for 6 h to give a bright yellow transparent solution. After that, 10 mL of methanol solution of CuCl2·2H2O (0.170 g, 1.0 mmol) was added to 50 mL of methanol solution of the ligand (0.691 g, 2.0 mmol), and the mixture was stirred and refluxed at 50 °C for 6 h. The resulting solution was cooled at room temperature and then filtered. The filtrate was left for slow evaporation at room temperature. The green block-shaped crystals were formed two days later (yield 38% based on crystals). Elemental Anal Calc (%) for 1: C, 61.65; H, 5.45; N, 11.35. Found: C, 61.24; H, 5.51; N, 11.47. IR (KBr, cm- 1): 1622 (vs), 1601 (vs), 1434 (s), 1373 (s), 1348 (s), 1311 (s), 1254 (m), 1234 (w), 1164 (vw), 1103 (w), 1042 (vw), 1017 (w), 870 (w), 780 (m), 747 (s), 698 (w), 580 (w), 531 (vw), 506 (vw), 465 (w), 420 (m). Decomposition point: 233 °C. 1 H NMR (DMSO-d6 600 MHz) δ (ppm): 1.43 (s, 3 H), 3.18 (t, 1 H, J = 12.6 Hz), 3.38 (d, 1 H, J = 12.0 Hz), 4.42 (s, 1 H), 6.73 (t, 1 H, J = 7.2 Hz), 6.93 (t, 1 H, J = 7.8 Hz), 7.11 (s, 1 H), 7.33 (d, 1 H, J = 8.4 Hz), 7.45 (t, 1 H, J = 7.2 Hz), 7.63 (t, 2 H, J = 10.2 Hz), 7.96 (t, 1 H, J = 8.4 Hz), 8.03 (s, 1 H), 10.89 (s, 1 H). [21] The complex Zn(C18H16N3O2)2·2CH3CH2OH (2) was synthesized in a procedure similar to that of complex 1, but the Zn(CH3COO)2 (0.220 g, 1.0 mmol) which was added to ligand solution was used and dissolved in 20 mL of ethanol. The yellow block crystals of 2 were obtained one week later with 61% yield on crystals base. Elemental Anal Calc (%) for 2: C, 62.38; H, 5.76; N, 10.91. Found: C, 61.87; H, 5.94; N, 11.03. IR (KBr, cm- 1): 1654 (vs), 1593 (vs), 1475 (w), 1438 (s),
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1397 (m), 1377 (s), 1311 (s), 1254 (s), 1160 (vw), 1140 (vw), 1099 (w), 1046 (vw), 1009 (w), 976 (vw), 919 (vw), 874 (w), 809 (w), 784 (m), 747 (s), 698 (w), 637 (w), 584 (w), 506 (vw), 461 (w), 425 (m). Decomposition point: 196 °C. 1 H NMR (DMSO-d6 600 MHz) δ (ppm): 1.43 (s, 3 H), 3.22 (t, 1 H, J = 12.6 Hz), 3.50 (d, 1 H, J = 12.0 Hz), 4.46 (dd, 1 H, J = 2.4, 10.2 Hz), 6.80 (t, 1 H, J = 7.2 Hz), 7.02 (t, 1 H, J = 7.2 Hz), 7.07 (s, 1 H), 7.34 (d, 1 H, J = 8.4 Hz), 7.40 (t, 1 H, J = 6.0 Hz), 7.69 (t, 2 H, J = 9.6 Hz), 7.92 (t, 1 H, J = 8.4 Hz), 7.98 (d, 1 H, J = 4.2 Hz), 10.94 (s, 1 H). Cd(C18H16N3O2)2·2CH3OH (3) was prepared in a similar manner as complex 1 by Cd(CH3COO)2·2H2O (0.267 g,1.0 mmol) in place of CuCl2·2H2O. The yellow block crystals of 3 were obtained two days later. The yield was 65% (based on crystals). Elemental Anal Calc (%) for 3: C, 57.83; H, 5.11; N, 10.65. Found: C, 57.35; H, 5.34; N, 10.80. IR (KBr, cm-1): 1642 (vs), 1585 (vs), 1458 (m), 1438 (s), 1368 (s), 1332 (w), 1311 (s), 1250 (s), 1197 (vw), 1169 (w), 1136 (w), 1099 (w), 1042 (vw), 1013 (m), 972 (vw), 935 (vw), 911 (vw), 886 (vw), 870 (w), 809 (w), 788 (m), 747 (s), 694 (w), 637 (w), 617 (vw), 575 (w), 559 (vw), 502 (vw), 461 (vw), 433 (w), 408 (vw). Decomposition point: 191 °C. 1 H NMR (DMSO-d6 600 MHz) δ (ppm): 1.45 (s, 3 H), 3.17 (t, 1 H, J = 11.4 Hz), 3.47 (d, 1 H, J = 13.2 Hz), 4.41 (s, 1 H), 6.83 (t, 1 H, J = 7.2 Hz), 7.02 (t, 1 H, J = 7.2 Hz), 7.10 (s, 1 H), 7.33 (d, 1 H, J = 8.4 Hz), 7.55 (t, 1 H, J = 6.0 Hz), 7.65 (d, 1 H, J = 7.8 Hz), 7.83 (d, 1 H, J = 7.2 Hz), 8.05 (t, 1 H, J = 7.2 Hz), 8.13 (s, 1 H), 10.91 (s, 1 H). The single crystals were mounted on an Enraf-Nonius CAD-4 X-ray single-crystal diffractometer. All data were collected at 293(2) K with a graphite monochromatized MoKα radiation (λ = 0.71073 Å) by using an ω-2θ scan mode. The structures were solved by direct methods using SHELXS-97. 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