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Non-Switching 1,2-Dithienylethene-based Diplatinum(II) Complex Showing High Cytotoxicity Andreu Presa,† Leoní Barrios,† Jordi Cirera,*,†,Δ Luís Korrodi-Gregório,‡,□ Ricardo Pérez-Tomás,‡ Simon J. Teat,§ and Patrick Gamez*,†,∥ †

Departament de Química Inorgànica i Orgànica and ΔInstitut de Recerca de Química Teòrica i Computacional, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain ‡ Department of Pathology and Experimental Therapeutics, Faculty of Medicine, Universitat de Barcelona, Campus Bellvitge, Feixa Llarga s/n, 08907 L’Hospitalet de Llobregat, Spain □ Department of Medical Sciences, Institute for Research in Biomedicine, Health Sciences Program, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal § Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley California 94720, United States ∥ Catalan Institution for Research and Advanced Studies, Passeig Lluís Companys 23, 08010 Barcelona, Spain S Supporting Information *

ABSTRACT: A diplatinum(II) complex was prepared from a new 1,2dithienylethene-based ligand containing N-methylimidazole groups as metal-binding units. Reaction of the ligand 1,2-bis[2-methyl-5-(1-methyl1H-imidazol-2-yl)-3-thienyl]-cyclopentene (L2H) with cis-dichlorobis(dimethylsulfoxido)platinum(II) generated the bimetallic complex trans[Pt2Cl4(DMSO)2(L2H)] (DMSO = dimethyl sulfoxide), whose DNA-interacting properties were investigated using different techniques. Cytotoxicity assays with various cancer cell lines showed that this compound is active, with IC50 values in the micromolar range. Surprisingly, the diplatinum(II) complex does not exhibit the anticipated photoswitching properties; indeed, UV irradiation does not lead to the photocyclization of the ligand L2H or of the metal complex. Computational studies were performed and revealed significant differences in the electronic structure of L2H compared with L1H (i.e., 1,2-bis[2-methyl-5-(4pyridyl)-3-thienyl]-cyclopentene, which exhibits photoswitching properties), in terms of the relevant molecular orbitals involved in the UV−vis absorption features, which ultimately is responsible for the inertia of L2H toward photocyclization.

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INTRODUCTION The design and preparation of new potential therapeutic agents is of utmost importance in medicinal chemistry.1,2 For instance, the development of more efficient and selective drugs is essential for the pharmaceutical industry, and this is particularly true for chemotherapy, as it commonly induces adverse drug reactions and patient suffering.3 The many unpleasant side effects of most chemotherapeutic treatments are clearly due to their usual lack of selectivity; indeed, once the drug is distributed throughout the human body, the spatiotemporal regulation of its activity is normally uncontrolled, so that healthy cells are also affected.4−6 Light represents a versatile external stimulus that can be used to precisely control molecular processes;7 for instance, the application of light may allow spatial and temporal modification of the properties of chemical agents, for example, their therapeutic activity.8−10 Thus, the development of new molecules for photochemotherapeutic applications is of great interest, since reduced side effects and improved therapeutic efficacy can be obtained.11,12 Photoactivated chemotherapy (PACT) based on metal-containing compounds is fundamentally metal-centered,13 and may operate via three different mechanisms of action, specifically, photodissociation/photoreduction,14 photosensitization,15 or photothermal reaction.16 © XXXX American Chemical Society

Recently, we have described a hitherto unexplored approach to photoactivate coordination compounds, namely, through the photomodification of the ligand containing a 1,2-dithienylethene moiety.17,18 Hence, platinum(II) complexes from photoswitchable 1,2-dithienylethene-containing ligands, that is, 1,2-bis[2-methyl-5-(4-pyridyl)-3-thienyl]-cyclopentene (L1H) and 1,2-bis[2-methyl-5-(4-pyridyl)-3-thienyl]-perfluorocyclopentene (L1F) (Figure 1), have been prepared.19 Remarkably, the closed and open forms of the corresponding complexes trans-[Pt 2 Cl 4 (DMSO) 2 (L1 H )] and trans[Pt2Cl4(DMSO)2(L1F)] (DMSO = dimethyl sulfoxide) show clearly distinct cytotoxicity behaviors toward various cancer-cell lines,19 opening new avenues of research in PACT. The imidazole ring is a naturally occurring N-donor ligand, which is found in numerous important biomolecules (for instance, proteins and enzymes), mostly as part of the histidine residue. Compounds containing this heteroaromatic moiety have found many biological (medicinal) applications,20 including cancer.21−24 It should also be mentioned here that an important ruthenium anticancer drug, namely, imidazolium trans-imidazoledimethyl sulfoxidetetrachlororuthenate (ImHReceived: February 12, 2016

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DOI: 10.1021/acs.inorgchem.6b00362 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

insight as to why this particular molecule does not undergo photocyclization.

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RESULTS AND DISCUSSION Synthesis of 1,2-Bis[2-methyl-5-(1-methyl-1H-imidazol-2-yl)-3-thienyl]-cyclopentene (L2 H ) and trans[Pt2Cl4(DMSO)2(L2H)]. The ligand L2H was prepared by reaction of 2-bromo-1-methyl-1H-imidazole and the precursor 1,2-bis(5-chloro-2-methylthien-3-yl)cyclopentene, as reported earlier for the preparation of L1H (see Experimental Section for details).19 L2H was obtained with a yield of 37%, and its UV− vis spectrum is characterized by the presence of two clear bands at λ = 250 and 283 nm and a shoulder at λ = 306 nm (see Figure S1). Comparatively, L1H exhibits bands at λ = 230, 285, and 322 nm.19 Surprisingly, upon UV irradiation of L2H, no spectral changes are observed, even for exposure times of up to 5 min (it should be mentioned here that UV irradiation during 2 min is sufficient to convert L1H to its closed form). Thus, in contrast to L1H, it appears that L2H cannot be photocyclized. trans-[Pt2Cl4(DMSO)2(L2H)] was subsequently synthesized with a yield of 74%, by reaction of 2 equiv of cis[PtCl2(DMSO)2]29 with 1 equiv of L2H in methanol (see Experimental Section for details). Twinned crystals of trans[Pt2Cl4(DMSO)2(L2H)] could be obtained, which were used for X-ray diffraction studies with a synchrotron source. Despite the poor data collected (R value of 11.6%),30 a reasonable structure of the diplatinum(II) complex could be proposed (Figure 2), which actually is very similar to that of the related pyridine-based compounds trans-[Pt2Cl4(DMSO)2(L1H)] and trans-[Pt2Cl4(DMSO)2(L1F)], described earlier.19 The unit cell contains two slightly different diplatinum(II) moieties; only one of them is depicted in Figure 2. As for trans[Pt2Cl4(DMSO)2(L1F)],19 the metal centers exhibit a slightly distorted square-planar geometry (as reflected by the coordination bond angles listed in Table 1), with two trans chlorido ligands, a DMSO sulfur atom, and a imidazole nitrogen atom. The Pt−Cl, Pt−S, and Pt−N bond distances are comparable to those of trans-[Pt2Cl4(DMSO)2(L1F)]19 (Table 1). The distance between the two thiophene carbon atoms subjected to form a covalent C−C bond upon photocyclization (C12D and C13B in Figure 2) is ca. 3.38 Å (Table 1). For L1F,

Figure 1. Light-driven closing/opening process for 1,2-bis[2-methyl-5(4-pyridyl)-3-thienyl]-cyclopentene (L1H), 1,2-bis[2-methyl-5-(4-pyridyl)-3-thienyl]-perfluorocyclopentene (L1F), and 1,2-bis[2-methyl-5(1-methyl-1H-imidazol-2-yl)-3-thienyl]-cyclopentene (L2H). The carbon atoms involved in the cyclization are marked with a red asterisk.

[trans-RuCl4(DMSO)Im], NAMI-A),25 contains this heteroaromatic five-membered ring. Actually, NAMI-A is an efficient antimetastatic agent, which has entered phase II clinical trials,26,27 also in combination with other chemotherapeutic drugs.28 In the present study, we prepared the 1,2-dithienylethenebased ligand 1,2-bis[2-methyl-5-(1-methyl-1H-imidazol-2-yl)-3thienyl]-cyclopentene (L2H), for which the metal-binding unit is an N-methylimidazole group (Figure 1). It must be mentioned here that the N-methylated ring was chosen for synthetic reasons; indeed, the conditions used for the Suzuki cross-coupling reaction requires the protection of the NHimidazole (see Experimental Section). As for the previous pyridine-containing ligand,19 the diplatinum(II) complex trans[Pt2Cl4(DMSO)2(L2H)] was synthesized, and its DNAinteracting and cytotoxicity properties were investigated, which revealed a very efficient cytotoxic behavior (as reflected by IC50 values in the low μM range). Remarkably, the platinum(II) compound does not exhibit the expected photoswitching properties (i.e., the open form of the complex cannot be converted to its closed form upon application of light in the UV range). Computational studies have provided some

Figure 2. Representation of the molecular structure of trans-[Pt2Cl4(DMSO)2(L2H)] obtained from poor X-ray diffraction data, which nevertheless illustrates the atomic connectivity. B


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Inorganic Chemistry

required for the photocyclization to occur). This computed value is in good agreement with the experimental one, namely, λexp = 322 nm.19 In the case of L2H, however, two bands of significant intensity are found, which are centered at λcalcd = 320 nm ( f = 0.1897) and λcalcd = 296 nm ( f = 0.9487; see Table 2). The first absorption is due to the HOMO → LUMO transition, which moved to a higher energy, as experimentally observed. The second band, which is clearly more intense than the former one, is assigned to the HOMO−2 to LUMO transition; while this transition was partially forbidden for the L1H system, it becomes fully allowed for L2H. These results are also in agreement with the experimental absorption spectrum of L2H, in which the intense band at λexp = 283 nm and a shoulder peak at λexp = 306 nm are clearly noticeable (see Figure S1). Thus, it is likely that the presence of this strongly absorbing HOMO−2 to LUMO transition, which is unrelated with the formation of the new carbon−carbon bond, greatly contributes to hindering the photocyclization process. In addition, close inspection of the relevant molecular orbitals involved in the electronic transitions reveals significant differences between the electronic structure of L1H and that of L2H (see Figure 3). Both systems have (4n + 2) π-electrons, which according to the Woodward−Hoffmann rules, should lead to a conrotatory mechanism for the photoisomerization, via the allowed excited-state rotation.31,32 The composition of the HOMO orbital is comparable for both systems, whose LUMO is accessible through electronic excitation. The particular shape of this frontier molecular orbital in L2H (see Figures 3 and S3) further explains its inertia toward photocyclization. As evidenced in Figure 3, the LUMO of L1H exhibits a major delocalization over the pyridine groups, whereas the LUMO of L2H presents a major localization over the thiophene ring; this feature increases the antibonding character of the orbital, as evidenced by the larger p contribution of the C and S atoms next to the 2-position of the ring. It is this antibonding interaction that ultimately seems to hinder the conrotatory rearrangement that leads to the formation of the new carbon−carbon bond, thus impeding L2H to effectively undergo ring closing. So as to assess if these differences arise from the lower coplanarity of L2H (compared to L1H or L1F, as observed in the solid state; see Figure S4), an analogous structure in which the methyl groups from the imidazole rings were replaced with hydrogen atoms was also investigated. This structural modification indeed leads to a decrease of the dihedral angle

Table 1. Selected Bond Distances (Å) and Angles (deg) for trans-[Pt2Cl4(DMSO)2(L2H)] bond

crystallographic

Pt−Cl Pt−N Pt−S Cl−Pt−N Cl−Pt−S Cthiophene1···Cthiophene2 a

2.286(16)−2.320(14) 2.00(4)−2.01(4) 2.223(13)−2.236(13) 88.0(13)−89.6(13) 88.4(5)−94.1(5) 3.32(8)−3.37(8)

a

Carbon atoms involved in the photocyclization process, which generates a covalent C−C bond (see Figure 1).

the corresponding distance amounts to 3.44 Å (thus, the value is analogous). 1 9 The UV−vis spectrum of trans[Pt2Cl4(DMSO)2(L2H)] shows three bands centered at λ = 231, 250, and 274 nm (Figure S2), thus revealing a shift toward higher energies upon metal coordination. Similarly to the free ligand L2H, UV irradiation of the platinum complex does not modify the absorption spectrum, once again suggesting that the photocyclization process cannot take place. Computational Studies. The unexpected behavior of L2H (and thus of trans-[Pt2Cl4(DMSO)2(L2H)]), namely, its inability to undergo photocyclization, was examined theoretically, comparing the geometrical and electronic structure of L2H with that of L1H, whose open form can be converted to the closed one upon application of UV light.19 Calculations in the gas phase reveal no major structural differences between the two ligands; for instance, the distance separating the thiophene carbon atoms prone to forming a covalent C−C bond (see carbon atoms labeled with a red asterisk in Figure 1) through light activation is ∼3.69 Å in both cases. Furthermore, the inclusion of solvent effects (namely, dichloromethane, which is a solvent in which L1H and L2H are soluble) via a polarizable continuum model in the calculations does not give rise to significant variations; therefore, all data presented below were obtained in the presence of dichloromethane. A comparative analysis of the absorption spectrum of both molecules was performed using time-dependent density functional theory calculations (TD-DFT; see Computational Details). The calculated values for the two ligands are listed in Table 2. The absorption data for L1H show mostly the presence of one band centered at λcalcd = 351 nm (f = 0.2668), which is ascribed to a highest occupied molecular orbital (HOMO) → lowest unoccupied molecular orbital (LUMO) transition (that is

Table 2. Computed TD-DFT Absorption Spectrum for L1H and L2H in Dichloromethane L1H

a

band

f

energy (nm)

1a 2 3 4 5 6 7 8 9 10 11

0.2668 0.0015 0.3489 0.4694 0.2959 0.0556 0.1132 0.0017 0.0084 0.0024 0.0507

351.63 345.61 306.44 305.74 293.23 286.96 280.29 273.61 272.51 272.29 268.99

L2H transition

band

f

energy (nm)

→ → → → → → → → → → →

1a 2 3 4b 5 6 7 8 9 10 11

0.1897 0.0503 0.0266 0.9487 0.0195 0.1701 0.2178 0.0572 0.0307 0.0008 0.0892

320.37 312.71 300.41 295.79 285.47 273.78 273.53 259.89 245.89 242.42 238.48

109B 109A 108B 108B 107A 107B 109A 109B 105A 104B 109B

110B 111A 110B 111B 110A 111B 112A 113B 110A 111B 114B

transition 111B 111A 110A 109B 110B 109B 111A 110B 111B 111B 108B

→ → → → → → → → → → →

112B 113A 112A 112B 113B 113B 114A 114B 115B 116B 112B

The relevant HOMO → LUMO transitions are in this row. bThe relevant HOMO−2 → LUMO transitions are in this row. C


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Figure 3. Relevant molecular orbitals involved in the absorption spectrum of L1H and L2H.

between the thiophene and imidazole planes (from ca. 24° for L2H to ∼12° for the nonmethylated ligand; see Figure S5), which in the latter case is even lower than the angle of ca. 17° observed for L1H. This increase in the system’s coplanarity, however, does not substantially alter its electronic distribution or the antibonding character of the frontier orbital, thus indicating that this effect arises from the orbital distribution of this particular five-membered aromatic system. It can be mentioned here that comparable features have been reported by Staykov and co-workers for a closely related family of diarylperfluorocyclopentenes.33 To further verify this hypothesis, the change in free energy upon ring closing was calculated for both L1H and L2H. Actually, the energy difference between them is minor; the computed values in dichloromethane are 15.67 and 15.80 kcal mol−1 for L1H and L2H, respectively. Consequently, this would indicate that the photoreaction should be slightly more favorable for L1H than L2H, but within the same range of probabilities, which is obviously not the case and thus suggests that the nonoccurrence of ring-closing reaction in L2H implies kinetic issues rather than thermodynamic ones; the LUMO shape for L2H involves strong antibonding interactions impeding the photocyclization (see Figure S3). Biological Properties. Although trans[Pt2Cl4(DMSO)2(L2H)] does not show photoswitching properties, its interaction with DNA was investigated using various techniques. First, the interaction of trans[Pt2Cl4(DMSO)2(L2H)] with plasmid DNA was investigated by agarose gel electrophoresis. Electrophoretic measurements were performed after 24 h of incubation at 37 °C of increasing quantities (0.5 to 2 equiv with respect to DNA) of the diplatinum(II) compound with pBR322 DNA. The corresponding gel is shown in Figure 4. Lane 1 is a control showing the migration of free DNA, which reveals that the commercially available biomolecule is principally composed of form I (or normal supercoiled form) with some form II (or relaxed circular form). Interaction of pBR322 DNA with cisplatin, a reference compound, generates the typical pattern (lane 2), namely, an increase of the electrophoretic mobility of form II associated with a decrease of that of form I.34 Incubation of 0.5 equiv of diplatinum(II) complex (i.e., [complex] = 7.5 μM)

Figure 4. Agarose gel-electrophoresis images of pBR322 DNA incubated for 24 h at 37 °C with increasing amounts of trans[Pt2Cl4(DMSO)2(L2H)]. Lane 1: pure plasmid DNA; Lane 2: DNA + 0.5 equiv of cisplatin; Lane 3: DNA + 0.5 equiv of complex; Lane 4: DNA + 1.0 equiv complex; Lane 5: DNA + 1.5 equiv of complex; Lane 6: DNA + 2 equiv of complex. [DNA] = 15 μM (base pairs).

with DNA gives rise to an apparent decrease of the mobility of form I toward the band corresponding to form II (lane 3). In addition, the remaining band ascribed to form I is located at the position observed for form I interacting with cisplatin (see lanes 2 and 3). Clearly, trans-[Pt2Cl4(DMSO)2(L2H)] is strongly affecting the double helix, which is most likely unwinded through its action.35 The use of 1 equiv of complex (i.e., [complex] = 15 μM) leads to the alteration of the electrophoretic mobility of form I, which is found at the position of the DNA−cisplatin adduct (see lanes 2 and 4). Furthermore, a clear vanishing of the bands is observed, which may be due to the initial precipitation of DNA−complex species (lane 4). Actually, higher amounts of trans[Pt2Cl4(DMSO)2(L2H)], namely, 1.5 and 2 equiv (that correspond to complex concentrations of, respectively, 22.5 and 30 μM), confirm these features, with an almost complete disappearance of the DNA bands (see lanes 5 and 6). It can be mentioned here that comparison of gel-electrophoresis results achieved with the previous compounds trans[Pt2Cl4(DMSO)2(L1H)]19 and trans-[Pt2Cl4(DMSO)2(L1F)]19 (their open forms) with those of trans-[Pt2Cl4(DMSO)2(L2H)] reveals a significantly stronger effect of the latter on the biomolecule. In other words, it appears that trans[Pt2Cl4(DMSO)2(L2H)] is more strongly interacting with pBR322 DNA than both open trans-[Pt2Cl4(DMSO)2(L1H)] and open trans-[Pt2Cl4(DMSO)2(L1F)].19 To corroborate these observations, competitive binding studies with ethidium bromide (EtBr) bound to calf-thymus DNA (ct-DNA) were performed using fluorescence spectrosD


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[Pt2Cl4(DMSO)2(L1H)] and open trans[Pt2Cl4(DMSO)2(L1F)], specifically, (6.6 ± 0.8) × 103 and (4.8 ± 0.9) × 10 3 M −1 , respectively. 19 Thus, trans[Pt2Cl4(DMSO)2(L2H)] exhibits a better DNA affinity than the related compounds previously reported (as already suggested by the gel-electrophoresis results; see above). The possible binding mode of trans-[Pt2Cl4(DMSO)2(L2H)] to DNA was investigated further by circular dichroism (CD). The CD spectra of free DNA and with increasing amounts of the diplatinum(II) complex, that is, in the range of 5−25 μM, after 24 h of incubation at 37 °C, are shown in Figure 6. The spectrum of pure ct-DNA ([DNA] = 50 μM in base pairs) exhibits a positive band centered at 275 nm, a negative band at ca. 245 nm, and a crosspoint at ca. 257 nm. These two bands characterize a right-handed B-form of DNA,36,37 and they are very sensitive to their interaction mode with small molecules.38 The addition of increasing quantities of complex gives rise to a decrease of both the positive and negative bands (Figure 6). Moreover, a red shift is observed for the two bands, that of the positive band being more pronounced (3.5 nm) than for the negative one (1 nm). These data are indicative of an alteration of the B-DNA conformation induced by interaction with the complex, possibly through intercalation between neighboring base pairs.39 As observed by gel electrophoresis (see above), the platinum(II) complex appears to strongly interact with DNA, possibly through intercalation and subsequent binding to DNA bases (specifically to N7 atoms of purines), since the electrophoretic mobility of form I bonded to cisplatin is comparable to that noticed with [Pt2Cl4(DMSO)2(L2H)] (Figure 4, lanes 2 and 4). NMR studies are however required to determine the exact binding mode of the 1,2-dithienylethene-based platinum complex. Next, cell viability studies was carried out, first performing single-point assays using complex concentrations of 50 and 10 μM and applying an incubation time of 48 h. The corresponding results achieved with a panel of four different cancer-cell lines, namely, lung adenocarcinoma (A549), melanoma (A375), breast adenocarcinoma (MCF7), and colon adenocarcinoma (SW620), are depicted in Figure 7 and Table S1. At [complex] = 50 μM, trans-[Pt2Cl4(DMSO)2(L2H)] exhibits good cytotoxicity properties toward all cell lines, particularly for MCF7, A375, and SW620 with percentages of cell viability below 5%. Using a concentration of

copy. The release of EtBr from the emitting EtBr−DNA adduct (λexc = 514 nm; λem = 610 nm) through its interaction with a molecule will result in a quenching of the fluorescence. Thus, fluorescence spectra were recorded at constant concentrations of ct-DNA and EtBr, respectively, 15 and 75 μM, and adding increasing amounts of trans-[Pt2Cl4(DMSO)2(L2H)], from 1 to 25 μM. After 24 h of incubation at 37 °C, a clear decrease of the fluorescence is observed, which confirms that the diplatinum(II) compound interacts with the biomolecule, and the magnitude of this binding was evaluated determining the corresponding Stern−Volmer quenching constant KSV by applying Equation 1: I0 = 1 + KSV[complex] (1) I where I0 and I are the initial fluorescence intensity of EtBr− DNA and the fluorescence intensity in the presence of complex, respectively. A plot I0/I versus [complex] generates a straight line whose slope gives KSV. The results achieved for trans[Pt2Cl4(DMSO)2(L2H)] are depicted in Figure 5, and a KSV

Figure 5. Plot of I0/I vs [complex] for the titration of DNA−EB with trans-[Pt2Cl4(DMSO)2(L2H)] at λexc = 514 nm and λem = 610 nm: experimental data points and linear fitting of the data. [complex]: 1− 25 μM; [DNA]: 15 μM; [EB]: 75 μM.

value of (17.1 ± 0.9) × 103 M−1 was obtained. It can be noticed that this value is significantly higher than those of open trans-

Figure 6. CD spectra of ct-DNA (50 μM) in the absence and presence of trans-[Pt2Cl4(DMSO)2(L2H)], using concentrations in the range of 5−25 μM. E


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Figure 7. Cell-viability assays (single-point) of the of trans-[Pt2Cl4(DMSO)2(L2H)] in different cancer cell lines; i.e., A549, A375, MCF7, and SW620, using two different complex concentrations, namely, 10 and 50 μM (single-point screenings), and an incubation time of 48 h (the results are given in per cent of cell viability; see Table S1).

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10 μM, the complex is still very efficient against the cell lines A375 and SW620, as reflected by the per cent viability of 9.7 and 7.8%, respectively (Table S1). It must be pointed out that trans-[Pt2Cl4(DMSO)2(L2H)] is much more active than open trans-[Pt2Cl4(DMSO)2(L1H)] and open trans[Pt2Cl4(DMSO)2(L1F)]; actually, these two compounds are almost non-cytotoxic,19 which may be linked to their weaker interaction with DNA (compared to trans[Pt2Cl4(DMSO)2(L2H)]; see above). Subsequently, IC50 values were determined for the two cell lines more affected by the diplatinum(II) compound, specifically, A375 and SW620. The results achieved after an incubation time of 48 h are 3.4 ± 0.6 and 1.8 ± 0.3 μM, respectively. Under the same experimental conditions, the IC50 values obtained for the reference compound cisplatin are 6.6 ± 0.6 μM (A375) and 13.6 ± 1.9 μM (SW620). Thus, trans[Pt2Cl4(DMSO)2(L2H)] shows remarkable cytotoxic properties against the panel of cell lines tested, and it is significantly better than similar compounds previously described in both their open and closed forms, as well as the standard platinum-based chemotherapeutic drug cisplatin.

EXPERIMENTAL SECTION

Materials and Methods. Solvents and chemicals were purchased from Aldrich, Fisher Scientific, or TCI Europe and used as received. pBR322 plasmid DNA and ct-DNA were obtained from SigmaAldrich. All reactions were performed under an inert atmosphere of dinitrogen, and the purifications were commonly performed in the air. Proton magnetic resonance (1H NMR) spectra were recorded at room temperature on a Varian Unity 400 MHz spectrometer. Proton chemical shifts are expressed in parts per million (ppm, δ scale) and are referenced to the solvent peak. UV−vis spectra were recorded on a Varian Cary-100 spectrophotometer. Fluorescence measurements were performed with a KONTRON SFM 25 spectrofluorometer. The UV-irradiation experiments were performed with a MAX-303 light source from Asahi Spectra, using a XUS0425 Shortpass Filter/UV 425 nm (allowing the irradiation of the sample with λ = 325−425 nm). C, H, N, and S analyses were performed by the Servei de ́ (CSIC) Microanalisi, Consejo Superior de Investigaciones Cientificas of Barcelona. Electrospray ionization (ESI) mass spectroscopy was performed using a LC/MSD-TOF spectrometer (Agilent Technologies) equipped with an ESI source at the Serveis Cientificotècnics of the Universitat de Barcelona. Synthesis of 1,2-Bis[2-methyl-5-(1-methyl-1H-imidazol-2yl)-3-thienyl]-cyclopentene (L2H). The intermediate products of the three-step reaction to generate L2H were obtained as described earlier for the preparation of ligands L1H and L1F.19 L2H was obtained as follows: 1,2-bis(5-chloro-2-methylthien-3-yl)cyclopentene19 (1.0 g, 3.0 mmol) was dissolved in anhydrous tetrahydrofuran (THF; 10 mL) under a nitrogen atmosphere and subsequently treated with nbutyllithium (4 mL of a 1.6 M solution in hexane, 6.4 mmol) at room temperature. The reaction mixture was stirred in the dark for 45 min until a light brown precipitate appeared. Then, tributyl borate (2.4 mL, 9.0 mmol) was added, and the solution quickly turned to bright orange. This solution was further stirred in the dark for 1 h and subsequently used for the Suzuki cross-coupling reaction. In a separate flask, [Pd(PPh3)4] (350 mg, 5% mol) and 2-bromo-1-methylimidazole (1.0 g, 6.2 mmol) were dissolved in a mixture of anhydrous THF (15 mL) and 20% aqueous K2CO3 solution (20 mL) under a nitrogen atmosphere. This two-phase system was stirred at 50 °C for 15 min, and the freshly prepared solution of the boronic derivative was added dropwise with a syringe. The resulting reaction mixture was stirred in the dark overnight, after which it was cooled to room temperature before the addition of dichloromethane (25 mL) and water (15 mL). The organic layer was separated, washed with water (25 mL), dried over MgSO4, and filtered. After evaporation of the solvent, the crude product was purified by column chromatography on silica gel using ethyl acetate (containing 1% NH3) as eluent, to yield the final product L2H as a slightly pink solid (yield: 470 mg, 37%). 1 H NMR (400 MHz, CDCl3, 25 °C): δ = 2.08 (q, 2H, J = 7.6 Hz, cyclopent), 2.11 (s, 6H, Me), 2.83 (t, 4H, J = 7.6 Hz, cyclopent), 3.65 (s, 6H, N-Me), 6.85 (d, 2H, J = 1.2 Hz, imid), 6.96 (s, 2H, thioph),

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CONCLUSIONS In summary, a new 1,2-dithienylcyclopentene derivative containing two N-methylimidazole groups has been designed with the objective to prepare a diplatinum(II) complex with potential cytotoxic properties. Actually, the bimetallic compound trans-[Pt2Cl4(DMSO)2(L2H)] strongly interacts with duplex DNA and acts as a very efficient cytotoxic agent, as reflected by the low IC50 values obtained for various cell lines. Unexpectedly, the ligand 1,2-bis[2-methyl-5-(1-methyl-1Himidazol-2-yl)-3-thienyl]-cyclopentene (L2H), as well as its platinum complex trans-[Pt2Cl4(DMSO)2(L2H)] do not undergo photocyclization upon application of UV light. This striking phenomenon has been investigated theoretically. Computational calculations performed with the ligand L2H and 1,2-bis[2methyl-5-(4-pyridyl)-3-thienyl]-cyclopentene (L1H), that is, a ligand previously reported, which shows photoswitching properties, have revealed that the noncyclization of the former most likely arises from the electronic distribution of the LUMO of this particular system; this specific orbital leads to an increase of the antibonding interaction between the p orbitals of the S and the C atoms, which makes the conrotatory mechanism impossible to occur. F


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Inorganic Chemistry 7.01 (d, 2H, J = 1.2 Hz, imid) ppm (see Figure S5A). ESI-MS: m/z = 421 [M + H]+. Elemental analysis for C23H24N4S2: calcd.: C, 65.68, H, 5.75, N, 13.32, S, 15.25; found: C, 65.38, H, 6.07, N, 13.21, S, 15.43%. Preparation of Complex trans-[Pt2Cl4(DMSO)2(L2H)]. cis[PtCl2(DMSO)2] (211 mg, 0.50 mmol) was suspended in methanol (40 mL), and the mixture was refluxed until the platinum compound completely dissolved. After filtration of the resulting yellow solution (to remove any traces of platinum(0) impurities), ligand L2H (105 mg, 0.25 mmol) was added, and the resulting reaction mixture was stirred overnight in the dark at room temperature, producing an off-white precipitate, which was filtered, washed with methanol, and dried under reduced pressure (yield: 205 mg, 74%). 1H NMR (400 MHz, CDCl3, 25 °C): δ = 2.23 (s, 6H, Me), 2.08−2.16 (q, 2H, J = 7.6 Hz, cyclopent), 2.89 (t, 4H, J = 7.6 Hz, cyclopent), 3.33 (s, 12H, DMSO), 3.68 (s, 6H, N-Me), 6.95 (d, 2H, J = 1.2 Hz, imid), 7.15 (d, 2H, J = 1.2 Hz, imid), 7.25 (s, 2H, thioph) ppm (see Figure S5B). ESI-MS: m/z 1107 [M + H]+. Elemental analysis for C27H36Cl4N4O2Pt2S4: C, 29.25, H, 3.27, N, 5.05, S, 11.57; found: C, 29.21, H, 3.37, N, 5.04, S, 11.69%. X-ray Crystallography. Single-Crystal X-ray Diffraction. A colorless plate of dimension 0.25 × 0.15 × 0.04 mm3 was mounted on a MitGen loop in oil, and data were collected on Bruker PHTON100 CMOS diffractometer at beamline 11.3.1 of the Advanced Light Source (Synchrotron radiation, λ = 0.7749 Å) at 100 K. The frames were integrated, and the intensity data were corrected for Lorentz and polarization effects by the Bruker SAINT Software package. Absorption, twinning, and other effects were corrected for using the Multi-Scan method (TWINABS). The data consists of 43 733 reflections collected, of which 8076 were unique [R(int) = 0.1560] and 6142 were observed [I > 2σ(I)]. The structure was solved by SHELXT2014 and refined by SHELXL2014 using 442 parameters and 6 restraints. Only the heavy atoms were refined anisotropically, while all light atoms were refined isotropically. Hydrogen atoms were found in the difference map for the methyl groups; for the other carbon atoms, they were placed geometrically. All hydrogen atoms were then constrained and refined using a riding model. Bond distance restraints were used in modeling one of the ligands. The maximum and minimum peaks in the final difference Fourier map were 5.85 and −2.66 e.Å−3. Crystal data: C27H36Cl4N4O2Pt2S4, Mw = 1108.82, monoclinic, P1̅, a = 13.3390(17)Å, b = 16.241(3)Å, c = 17.767(3)Å, α = 109.263(10)°, β = 90.811(10)°, γ = 101.450(10)°, V = 3548.0(10)Å3, T = 100(2) K, Z = 4, R1 [I > 2σ(I)] = 0.1163, wR2 (all data) = 0.3369, GOF (on F2) = 1.040 (see Table S2). Computational Details. All calculations were performed using the Gaussian 09 electronic structure code (rev. D01).40 A survey of different density functional methods was done using as a reference value the position of the HOMO−LUMO band in L1H (λexp = 322 nm). From the different tested functionals (TPSSh, PBEPBE, BP86, BLYP, B3LYP, and ωB97xD), the one that better reproduces this value is the B3LYP functional (see Table S7 Supporting Information). Ground-state optimizations were done using the B3LYP functional.41,42 The fully optimized contracted triple-ζ all-electron Gaussian basis set with added polarization functions developed by Ahlrichs and co-workers was used for all the elements in the molecule.43 The studied systems were fully optimized in vacuum and modeling the dichloromethane solvent properties using a polarizable continuum model (self-consistent reaction field approximation), and subsequently vibrational analysis was performed (see Supporting Information for optimized structures and Cartesian coordinates). The UV−vis spectrum was calculated within the TDDFT methodology,44 using the same functional as for the optimizations. The number of excited states included in the TDDFT calculation is 30. Gel Electrophoresis. Plasmidic pBR322 DNA aliquots in 1 mM sodium cacodylate and 20 mM NaCl buffer (pH = 7.2) were treated with freshly prepared DMSO stock solutions of complex trans[Pt2Cl4(DMSO)2(L2H)]. The resulting samples, containing 15 μM DNA (base pair), 0.5−2.0 equiv of the complex, and 5% DMSO in a volume of 20 μL, were incubated for 24 h at 37 °C, treated with 4 μL of xylene cyanol 1× solution (30% glycerol), and subsequently electrophoretized on agarose gel (1% in tris-acetate-EDTA buffer) for 1 h at 6.25 V cm−1, using a BIO-RAD horizontal tank connected to a

PHARMACIA GPS 200/400 variable potential power supply. Analogous samples of free DNA and DNA bound to cisplatin (0.5 equiv, DMSO free) were also prepared and used as controls. Afterward, the DNA was stained with SYBR Safe, and the gel was photographed with a BIO-RAD Gel Doc EZ Imager. Fluorescence Spectroscopy. Fifteen micromolar ctDNA (base pair) and 75 μM ethidium bromide solutions in 1 mM sodium cacodylate and 20 mM NaCl buffer (pH = 7.2) were treated with increasing amounts of a freshly prepared DMSO stock solution of complex trans-[Pt2Cl4(DMSO)2(L2H)]. The final samples, containing 1−25 μM of the studied complex and up to 5% DMSO, were then incubated for 24 h at 37 °C. After incubation, the fluorescent emission spectra of all solutions were recorded at room temperature in the range of 530−800 nm, using a KONTRON SFM 25 spectrofluorometer and applying an excitation wavelength of λexc = 514 nm. Circular Dichroism. Fifty micromolar ct-DNA (base pair) solutions in 1 mM sodium cacodylate and 20 mM NaCl buffer (pH = 7.2) were treated with increasing amounts of a freshly prepared DMSO stock solution of complex trans-[Pt2Cl4(DMSO)2(L2H)]. The final samples, containing 5−25 μM of the studied complex and up to 5% DMSO, were then incubated for 24 h at 37 °C. After incubation, the CD spectra of all solutions were recorded at room temperature in the range of 235−315 nm to minimize DMSO interference, using a Jasco J-815 CD spectropolarimeter. Cell Lines and Culture. Human lung adenocarcinoma (A549), melanoma (A375), colorectal adenocarcinoma (SW620), and breast adenocarcinoma (MCF7) cell lines used in this study were purchased from the American Type Culture Collection (ATCC, Manassas, VA). A549, A375, and SW620 cell lines were cultured in Dulbecco’s Modified Eagle’s medium (DMEM) supplemented with 10% (v/v) fetal bovine serum, 100 unit/mL penicillin, 100 μg/mL streptomycin, and 2 mM L-glutamine. MCF7 cell line was cultured in DMEM-F12 (Ham) media (1:1) supplemented with 5% horse serum (v/v), 100 μM sodium pyruvate, 10 μg/mL insulin, 100 unit/mL penicillin, 100 μg/mL streptomycin, and 2 mM L-glutamine. All cells were grown at 37 °C under a 5% CO2 atmosphere. Cell Viability Assays. Cell proliferation was evaluated by the 3(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) assay. Cells were plated in 96-well sterile plates at a density of 1 × 105 cells per milliliter (100 μL) and allowed to grow for 24 h. After attachment to the surface, cells were then incubated with various concentrations of complex trans-[Pt2Cl4(DMSO)2(L2H)] (10 and 50 μM for single-point experiments and a 0.5−5 μM range for dose− response curves), freshly dissolved in DMSO and diluted in the corresponding culture medium (DMSO final concentration = 1%) for 48 h at 37 °C. Control cells were cultured in the corresponding culture medium plus the carrier (DMSO final concentration = 1%). After treatment, 10 μM MTT was added to each well for an additional 4 h. Afterward, the medium was aspirated, the blue formazan precipitate was dissolved in 100 μL of DMSO, and the absorbance at 570 nm was measured in a multiwell plate reader (Multiskan FC, Thermo Scientific). Cell viability was expressed as percentage values with respect to control cells, and data are shown as the mean value ± standard deviation of three independent experiments. Dose−response curves and the corresponding IC50 values were obtained by means of nonlinear fitting, calculated with GraphPad Prism 5.0 software. For comparison purposes, the cytotoxic effect of cisplatin was also evaluated under identical experimental conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00362. • Absorption spectra of free ligand L2H and trans[Pt2Cl4(DMSO)2(L2H)], schematic representation of the relevant molecular orbitals involved in the absorption spectrum of L2H and L1H, illustrations of the crystal G


Article

Inorganic Chemistry

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structures showing the angle between the thiophene ring and the N-containing aromatic ring for trans[Pt2Cl4(DMSO)2(L1H)] and trans[Pt2Cl4(DMSO)2(L2H)], cell-viability assays with different cancer-cell lines, NMR spectra of L2H and trans[Pt2Cl4(DMSO)2(L2H)], Cartesian coordinates for all calculated structures. (PDF) • Crystal data and structure refinement for trans[Pt2Cl4(DMSO)2(L2H)].(CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (J.C.) *E-mail: [email protected]. (P.G.) Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Ministerio de Economiá y Competitividad of Spain (Project Nos. CTQ2014-55293-P and CTQ2015-70371REDT) and COST Action No. CM1105 are kindly acknowledged. P.G. acknowledges the Institució Catalana de Recerca i Estudis Avançats (ICREA). J.C. gratefully acknowledges financial support from the Generalitat de Catalunya (7th program Beatriu de Pinos-Marie Curie cofund Fellowship, No. 2013 BP-B 00155). J.C. thanks the Consorci de Serveis Universitaris de Catalunya (CSUC) for computational resources. This work was partially supported by a grant from the Spanish government and the EU (FIS PI13/00089), a grant from La Marató de TV3 Foundation (20132730) and an individual grant from Fundaçaõ para a Ciência e Tecnologia of the Portuguese Ministry of Science and Higher Education to LKG (SFRH/BPD/91766/2012). The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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