English

• 0.   Introduction

  Metal-organic frameworks (MOFs) are coordination polymers self-assembled from polydentate organic ligands (mostly aromatic polyacids and polybases) and metal ions. Compared with other porous materials, MOFs have broader application prospects in various fields^[1-2], including bacteriostasis^[3], gas storage^[4], catalysis^[5], optical materials^[6-7], and biomedicine^[8-9]. In recent years, the interaction between MOFs and DNA has become an important issue in life sciences^[10]. The combination of MOFs and DNA can provide a basis for the early diagnosis and treatment of diseases by detecting gene mutations, expression levels, and so on^[11-12]. In addition, the low toxicity and easy functionalization of MOFs have expanded their application scope to the antibacterial field^[13-14]. MOFs constructed by rare-earth metal ions and Schiff bases can inhibit the proliferation of bacteria. Therefore, research on polydentate Schiff-base ligands and their complexes is expected to promote the development of the life sciences field^[15-17].

  Bacteria, as the most abundant organisms, threatened human lives until the advent of antibiotics. However, with the widespread use of antibiotics^[18] over a long period of time, bacteria′s tolerance to antibiotics has gradually increased, further intensifying the threats to human and environmental safety. Therefore, various types of new antibacterial agents have been developed^[19-20]. Among them, MOFs seem to be playing an increasingly important role^[21]. The unique properties of MOFs make them a suitable multifunctional platform. By selecting appropriate metastable coordination bonds, MOFs can act as reservoirs to release antibacterial rare-earth metal ions or organic linkers^[22]. Importantly, when combined with other materials, MOFs can serve as a platform to kill bacteria effectively through the synergistic effect of multiple mechanisms. Therefore, MOFs have significant implications for research on antibacterial agents ^[23-24].

  Research has shown that Schiff-base ligands containing heteroatoms such as N, O, P, and S exhibit strong coordination abilities, enabling them to form stable metal complexes with rare-earth metal ions and bind to DNA with ease^[25]. Based on previous efforts to synthesize lanthanide rare-earth metal ions complexes, we used two Schiff-base ligands [Scheme 1 and S1 (Supporting information)] to construct two new binuclear gadolinium (Gd₂) complexes. Their general formula are [Gd₂(dbm)₂(HL₁)₂(CH₃OH)₂]·4CH₃OH (1) and [Gd₂(dbm)₂(L₂)₂(CH₃OH)₂]·2CH₃OH (2), where Hdbm=dibenzoylmethane. The results of subsequent investigations demonstrated that the two Gd₂ complexes interact with calf thymus DNA (CT-DNA) via an intercalation mechanism. Meanwhile, the complexes exhibited good inhibitory activity against four types of bacteria [Bacillus subtilis (B. subtilis), Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), Candida albicans (C. albicans)].

  Scheme 1

  

  Scheme 1.  Structures of two Schiff-base ligands

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  1.   Experimental

  1.1   Materials

  Gd(dbm)₃·6H₂O (AR, Energy Chemical), Hdbm (AR, Aladdin), hydroxyacetohydrazide (AR, Energy Chemical), 4-(diethylamino)salicylaldehyde (AR, Energy Chemical), nicotinohydrazide (AR, Energy Chemical), 5-bromo-2-hydroxy-3-methoxybenzaldehyde (AR, Energy Chemical), CH₃OH (AR, Kermel), CH₃CN (AR, Kermel), DMF (AR, Kermel), and CHCl₃ (AR, Kermel) were used. Deionized water (DI water) was produced by the Milli-Q water purification system. All chemicals and solvents were utilized as received without further purification. CT-DNA was obtained from Shanghai Institute of Materia Medica. The details of the synthesis of the ligands H₃L₁ and H₂L₂ and physical measurements are in the Supporting information.

  1.2   Synthesis of complexes 1 and 2

  Gd(dbm)₃·6H₂O (0.025 mmol), H₃L₁ (0.025 mmol) [or H₂L₂ (0.025 mmol)], CH₃OH (8 mL), CH₃CN (2 mL), DMF (0.5 mL), and CHCl₃ (2 mL) were enclosed in a 15 mL vial by stirring for 5-10 min. The vial was sealed and heated to 70 ℃ for 10 h, then cooled to room temperature at a rate of about 5 ℃·h^-1. Yellow block crystals of the products were collected. The synthetic routes of complexes 1 and 2 are shown in Scheme S2.

  Complex 1. Yield based on Gd: 48%. Anal. Calcd. for C₆₂H₈₀Gd₂N₆O₁₆(%): C, 50.32; H, 5.41; N, 5.68. Found(%): C, 50.41; H, 5.36; N, 5.63.

  [Gd₂(dbm)₂(L₂)₂(CH₃OH)₂]·2CH₃OH (2). Yield based on Gd: 52%. Anal. Calcd. for C₆₂H₅₈Br₂Gd₂N₆O₁₄(%): C, 46.96; H, 3.66; N, 5.30. Found(%): C, 49.95; H, 5.39; N, 5.61.

  1.3   X-ray crystallography

  A computer-controlled Rigaku Saturn CCD area detector diffractometer equipped with confocal monochromatized Mo Kα radiation (λ=0.071 073 nm) was utilized to get single-crystal X-ray diffraction data of complexes 1 and 2 using the ω-φ scan technique. Through SHELXS-2016 and SHELXL-2016 programs with a full-matrix least-squares technique based on F ², the structures were solved and refined. All non-hydrogen atoms obtained anisotropic thermal parameters. The crystals were measured at the moment when they were isolated from the solution, so certain disordered solvent molecules were present. Table 1 covers crystal parameters, data collection, and refinement details for complexes 1 and 2.

  Table 1

  

  Table 1.  Crystallographic data and structure refinements for complexes 1 and 2

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  Parameter	1	2
  Formula	C62H80Gd2N6O16	C62H58Br2Gd2N6O14
  Formula weight	1 479.82	1 585.46
  T / K	150.0	150.0
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/n
  a / nm	1.515 72(5)	1.348 16(4)
  b / nm	1.212 11(5)	1.329 75(3)
  c / nm	1.887 29(7)	1.763 92(5)
  β / (°)	112.730 7(12)	105.093(1)
  V / nm3	3.198 1(2)	3.053 12(14)
  Z	2	2
  Dc / (g·cm-3)	1.537	1.725
  μ / mm-1	2.126	3.532
  θ range / (°)	2.047-26.394	1.943-26.400
  F(000)	1 500	1 564
  Reflection collected	36 429	57 467
  Unique reflection	6 525	6 232
  Rint	0.056 5	0.052 5
  GOF (F 2)	1.004	1.025
  R1, wR2 [I > 2σ(I)]	0.036 1, 0.096 3	0.039 9, 0.099 1
  R1, wR2 (all data)	0.049 6, 0.111 4	0.047 1, 0.109 7

  1.4   Interaction between complexes 1 and 2 and DNA

  The CT-DNA mother liquor (3 mL, 2 μg·mL^-1) was used to determine the absorbance at 260 and 280 nm by a UV spectrophotometer. Tris-HCl/NaCl buffer solution (500 mL, 5 mmol·L^-1, pH=7.25) was obtained by mixing Tris (0.302 5 g), HCl (6 mol·L^-1, 0.55 mL), NaCl (0.146 3 g), and distilled water. Complex 1 (complex 2, H₃L₁, and H₂L₂) was dissolved in Tris-HCl/NaCl buffer solution to give a final concentration of 1.7×10^-2 mmol·L^-1.

  1.4.1   Ultraviolet spectrophotometry

  At room temperature, the concentration of complex 1 (complex 2, H₃L₁, and H₂L₂) was maintained at a constant level. Meanwhile, the absorbance of CT-DNA was measured within the wavelength range of 190-450 nm and determined via ultraviolet spectrophotometric analysis, taking Tris-HCl/NaCl buffer solution as the reference solution and CT-DNA solution as the control liquid. Subsequently, a consistent volume of CT-DNA solution (10 μL) was incrementally added, and the mixture was tested after 30 min of reaction.

  1.4.2   Fluorescence spectrometry

  In fluorescence quenching research, a solution consisting of 0.06 mg·mL^-1 of ethidium bromide (EB) and 2 μg·mL^-1 of CT-DNA was prepared in advance and stored in the dark for 12 h before utilization. The specific test process was as follows: 3 mL of the mixed solution of EB-DNA was added to the sample pool, and the fluorescence emission spectrum in the range of 550-700 nm was measured at the excitation wavelength of 270 nm. Subsequently, the same volume of complex 1 (complex 2, H₃L₁, and H₂L₂) solution (1.7×10^-2 mmol·L^-1, 10 μL) was added to the EB-DNA system in turn. After each addition of the mixed solution, the reaction mixture was shaken well at room temperature for 5 min, and then the emission spectrum was determined.

  1.4.3   Cyclic voltammetry

  In the cyclic voltammetric measurement, the three-electrode system was used. The glassy carbon electrode was used as the working electrode, the saturated calomel electrode was used as the reference electrode, and the platinum wire electrode was used as the counter electrode. The scanning potential range was set between -0.15 and 0.8 V. The scanning rate was 0.1 V·s^-1, the sample interval was 0.001 V, and the settling time was 2 s. The experimental procedures were carried out using 1.7×10^-2 mmol·L^-1 of complex 1 (complex 2, H₃L₁, and H₂L₂) together with 2 μg·mL^-1 of DNA.

  1.5   Antibacterial activity of the two complexes

  An appropriate amount of complex 1 (complex 2, H₃L₁, and H₂L₂) was taken, and solutions were prepared with DMSO as the solvent. Then, the punched inhibition zone method was used to determine the antibacterial activities of the complexes against four kinds of bacteria (B. subtilis, C. albicans, E. coli, and S. aureus). The activated bacterial liquid was diluted to different concentrations. 100 μL of the diluted bacterial liquid was taken and evenly spread on the LB solid medium. Then it was placed in a biochemical incubator and incubated at 37 ℃ for about 10 h. Finally, the appropriate concentration of the bacterial liquid was determined as 8×10⁴ CFU·mL^-1. The inhibition zone test was conducted at this concentration. 4 mL stainless steel tubes were used for disinfection and sterilization to punch holes in the plate. 60 μL of antibacterial agent was injected into the holes, pre-diffusion was carried out at 4 ℃ for 2 h, and then it was incubated in a biochemical incubator at 37 ℃ for about 10 h. Vernier calipers were used to measure the diameter of the inhibition zone.

  2.   Results and discussion

  2.1   Crystal structures of complexes 1 and 2

  Complex 1 crystallizes in the monoclinic system and P2₁/c space group with Z=2. As shown in Fig.1, the molecular structure of complex 1 consists of two Gd³⁺ ions, two Schiff-base ligands (HL₁^2-), two dbm^- ions, two coordinated CH₃OH molecules, and four free CH₃OH molecules. Moving on to the specific structural features, as shown in Fig.S1a, two Gd³⁺ ions are bridged by two μ₂-O atoms (O2 and O2a) from two HL₁^2- ligands, respectively, to form a dinuclear structure. As shown in Fig.S1b, further examination of the coordination environment around each Gd³⁺ ion reveals that it is coordinated by one nitrogen atom (N2) from one HL₁^2- ligand, and the Gd1—N2 distance is 0.246 8(4) nm. It is also coordinated by seven oxygen atoms (O1a, O2, O2a, O3, O4, O5, O6), with the Gd—O distances ranging from 0.223 5(3) to 0.245 1(3) nm. Therefore, the Gd³⁺ ion is eight-coordinated. The coordination mode of HL₁^2- ligand is shown in Fig.S2a, one HL₁^2- ligand connects two Gd³⁺ ions [Gd1—O1 0.245 1(3) nm, Gd1—O3 0.223 5(3) nm, Gd1—O2—Gd1 114.64(11)°]. It clearly defines how the HL₁^2- ligand coordinates with the Gd³⁺ ions. The coordination mode of dbm^- ion is shown in Fig.S2b, one dbm^- ion is connected to one Gd³⁺ ion [Gd1—O4 0.237 3(3) nm, Gd1—O5 0.237 5(3) nm, O4—Gd1—O5 71.14(10)°]. The data of bond lengths and bond angles for complex 1 are presented in Table S1.

  Figure 1

  

  Figure 1.  Molecular structure for complex 1

  Hydrogen atoms have been omitted; Symmetry code: a: -x+1, -y+2, -z+1; Ellipsoids are drawn at a 30% probability level.

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  Complex 2 crystallizes in the P2₁/n space group with Z=2 of the monoclinic system. As shown in Fig.2, complex 2 consists of two Gd³⁺ ions, two L₂^2- ligands, two dbm^- ions, two coordinated CH₃OH molecules, and two free CH₃OH molecules. Turning to the structural particulars, as shown in Fig.S3a, the structure of complex 2 is similar to complex 1. In complex 2, two Gd³⁺ ions are respectively bridged by two oxygen atoms (O2 and O2a) from two L₂^2- ligands, thereby forming a dinuclear structure. Fig.S3b further reveals the coordination environment around each central Gd³⁺ ion. Each central Gd³⁺ ion adopts an eight-coordinated mode by one nitrogen atom (N1) from one L₂^2- ligand, and seven oxygen atoms (O1a, O2a, O2, O3, O4, O5, O6). The coordination mode of the L₂^2- ligand is shown in Fig.S4a. One L₂^2- ligand connects two Gd³⁺ ions [Gd1—O3 0.225 3(3) nm, Gd1—O1 0.252 7(3) nm, Gd1—O2—Gd1 105.02(11)°]. Similarly, the coordination mode of dbm^- ion is shown in Fig.S4b. One dbm^- ion is connected to one Gd³⁺ ion [Gd1—O5 0.229 2(3) nm, Gd1—O4 0.230 2(3) nm, O4—Gd—O5 72.56(10)°]. These bond lengths and bond angles of complexes 1 and 2 are comparable to those of the reported Ln₂ complexes^[26-29]. The data of bond lengths and bond angles for complex 2 are presented in Table S2.

  Figure 2

  

  Figure 2.  Molecular structure for complex 2

  Hydrogen atoms have been omitted; Symmetry code: a: -x+1, -y+1, -z+1; Ellipsoids are drawn at a 30% probability level.

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  2.2   Powder X-ray diffraction and thermogravimetric analyses

  To ascertain the phase purity of complexes 1 and 2, the single crystal specimens of both complexes underwent powder X-ray diffraction (PXRD) analysis at room temperature. As shown in Fig.S5, the PXRD data from the experimental trials were carefully compared with the theoretical values calculated from the simulation of the single-crystal structure. The analysis revealed that the position and morphology of the primary peaks within the experimental dataset were essentially coincident with those of the simulated PXRD pattern, signifying that the crystalline samples of the assayed complexes 1 and 2 exhibited a high degree of phase purity.

  The thermal stability analysis of complexes 1 and 2 within the temperature range from 30 to 800 ℃ is shown in Fig.S6. When the temperature of complex 1 ranged from 30 to 150 ℃, a weight loss of 2.06% (compared with the calculated value of 2.17%) was observed, indicating the loss of one CH₃OH molecule. As the temperature increased, its molecular structure was gradually broken down, and the remainder was Gd₂O₃ (Found 32.06%, Calcd. 24.51%). For complex 2, within the temperature range from 30 to 80 ℃, a weight loss of 2.83% was observed (compared with the calculated value of 2.02%), suggesting the loss of one CH₃OH molecule as well. Subsequently, its molecular structure was also gradually broken down with the rise in temperature.

  2.3   Interaction between complexes 1-2 and DNA

  2.3.1   Ultraviolet spectrometric analysis

  As a direct method for exploring the interaction between complexes and DNA^[30-31], UV-Vis spectroscopy is used to exploit the binding of complexes with DNA, which causes absorption band broadening, absorption peak red shift, and hypochromic effect at ambient temperature. The absorption spectra were recorded within the wavelength range of 190-450 nm. The concentrations of H₃L₁, H₂L₂, complexes 1 and 2 were kept constant while the CT-DNA concentration was systematically varied. As shown in Fig.3, complexes 1 and 2 showed different levels of hypochromic effect at the absorption peak around 209 nm. The absorption peak of complex 1 showed a red shift of 5 nm, and complex 2 had a red shift of 7 nm. In contrast, for H₃L₁ and H₂L₂, as shown in Fig.S7, the absorption peaks at around 205 nm exhibited a relatively smaller hypochromic effect compared to the complexes, along with a relatively small red shift. The red shift and hypochromic effect might result from the accumulation of π electrons between the complex and the DNA base pair after the complex inserts into DNA. The coupling between the ligand′s π* empty orbital and the DNA base pair′s π orbital reduces energy levels. Subsequently, the π* orbital gets partially filled with electrons, decreasing the probability of π-π* transitions and leading to the hypochromic effect. The results indicate that complexes 1, 2, H₃L₁, and H₂L₂ can bind to CT-DNA by intercalation. As the intercalation strength increases, the discoloration effect becomes progressively more pronounced.

  Figure 3

  

  Figure 3.  UV-Vis spectra for the interaction between complex 1 (a)/2 (b) (1.7×10^-2 mmol·L^-1) and CT-DNA

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  2.3.2   Fluorescence spectrometry analysis

  The fluorescence of the EB molecule is inherently weak due to its conjugated planar structure. However, when it inserts specifically parallel between the base pairs within the DNA double helix or triple helix, forming an EB-DNA complex, its fluorescence intensity is significantly enhanced^[32]. The Stern-Volmer equation, expressed as I₀/I=1+K_svr, describes the relationship between the fluorescence intensities of the EB-DNA complex. Here, I₀ represents the initial fluorescence intensity of the EB-DNA complex without any additional compound, and I is the fluorescence intensity measured after introducing various concentrations of the compound. The variable r denotes the concentration ratio of the compound to DNA. The constant K_sv, known as the linear Stern-Volmer quenching constant, can be determined from the slope of the linear plot of I₀/I against r. This data provides a measure of the compound′s ability to bind to DNA, with a higher K_sv indicating stronger binding affinity.

  Based on the above theoretical analysis, we conducted EB-DNA fluorescence experiments to explore the binding ability of complex 1 (complex 2, H₃L₁, and H₂L₂) with DNA. The experimental results are shown in Fig.4 as well as Fig.S8. As shown in Fig.4a and 4b, the top line indicates the fluorescence generated by the EB-DNA complex when complexes 1 and 2 were absent. As the concentrations of complexes 1 and 2 gradually increased, the fluorescence intensities decreased. Similarly, as shown in Fig.S8, the top line indicates the fluorescence generated by the EB-DNA complex when H₃L₁ and H₂L₂ were absent. With the gradual addition of H₃L₁ and H₂L₂, the fluorescence intensities also decreased. The above results indicate that complex 1 (complex 2, H₃L₁, and H₂L₂) replaces EB, which intercalates between DNA base pairs, generating a new system composed of CT-DNA and the compound. In other words, these compounds inserted themselves into CT-DNA. Furthermore, by analyzing the data in Fig.4c and 4d, we can obtain the respective K_sv values for complexes 1 and 2, which were 1.847 9 and 1.216 6. From Fig.S8c and S8d, the K_sv values for H₃L₁ and H₂L₂ were determined to be 1.178 0 and 0.656 8. These numerical values demonstrated that the bond strength follows the order of 1 > 2 > H₃L₁ > H₂L₂. This order implies that complex 1 has a relatively stronger binding affinity with CT-DNA compared to complex 2, H₃L₁, and H₂L₂. Moreover, the interaction strength between the complex and DNA is higher than the corresponding ligand. This result was consistent with the ultraviolet titration result, indicating that after the rare-earth metal salt forms a complex with the ligand, a synergistic effect occurs between them. Compared with some Ln₂ complexes reported in the literature^[33-37], the K_sv values of 1 and 2 are larger than those of some Ln₂ complexes (Table S3).

  Figure 4

  

  Figure 4.  Fluorescence spectra of EB-DNA (c_DNA=4.2 μmol·L^-1) complex in the presence of complexes 1 (a) and 2 (b) with different concentrations and corresponding Stern-Volmer plots [c (1), d (2)]

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  2.3.3   Cyclic voltammetry analysis

  The mode of interaction between the complex and DNA can be judged according to the changes in the electrochemical properties of the complex before and after adding DNA^[38]. As shown in Fig.5, the anodic peak potential (E_pa) and the cathodic peak potential (E_pc) of complex 1 (2) were 0.249 0 V (0.271 0 V) and 0.138 0 V (0.071 0 V), respectively. Correspondingly, the equation potential of complex 1 (2) was 0.193 5 V (0.171 0 V). After adding DNA, the E_pa and the E_pc of complex 1 (2) were 0.260 0 V (0.271 0 V) and 0.133 0 V (0.073 0 V), respectively. Equation potential of complex 1 (2) was 0.196 5 V (0.172 0 V). The equation potential of complexes 1 and 2 demonstrated a positive movement, suggesting that complexes 1 and 2 interact with the DNA molecule through an intercalative binding mode. As shown in Fig.S9, the E_pa and the E_pc of H₃L₁ (H₂L₂) were 0.149 8 V (0.268 0 V) and 0.256 1 V (0.076 0 V), respectively. Equation potential of H₃L₁ (H₂L₂) was 0.203 0 V (0.172 0 V). After adding DNA, the E_pa and the E_pc of H₃L₁ (H₂L₂) were 0.276 3 V (0.292 0 V) and 0.130 1 V (0.052 0 V), respectively. Equation potential of H₃L₁ (H₂L₂) was 0.203 2 V (0.172 5 V). The equation potential of H₃L₁ and H₂L₂ also showed a relatively small positive movement. In summary, the positive movement value of complex 1 was greater than that of H₃L₁, and the value of complex 2 was also greater than that of H₂L₂. It demonstrates that the interaction strength between complex 1 (2) and DNA is greater than that between its corresponding ligand H₃L₁ (H₂L₂) and DNA. The above results indicate that a synergistic effect has occurred between the rare-earth metal salts and the ligands. Thus, the results of the cyclic voltammetry analysis were consistent with those of the ultraviolet spectrometric analysis and fluorescence spectrometry analysis, and they confirmed each other.

  Figure 5

  

  Figure 5.  Cyclic voltammetry curves for the interaction between complexes 1 (a)/2 (b) (1.7×10^-2 mmol·L^-1) and CT-DNA

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  2.4   Antibacterial activity of the two complexes

  The antimicrobial activities of DMF, Gd(dbm)₃·6H₂O, ligands H₃L₁ and H₂L₂, and complexes 1 and 2 were determined by employing the bacteriostatic zone test and the minimum inhibitory concentration determination method^[39]. According to the results shown in Fig.S10, the inhibition zone of DMF against the four kinds of bacteria (B. subtilis, E. coli, S. aureus, and C. albicans) was less than 7 mm, and there was no antibacterial effect. Gd(dbm)₃·6H₂O, H₃L₁, and H₂L₂ showed relatively weak antibacterial activity against these four bacteria, with the inhibition zones ranging between 10 and 20 mm. Complexes 1 and 2 had good antimicrobial activity against these four bacteria. In addition, complex 1 showed strong antibacterial activity against B. subtilis and S. aureus, with an inhibition zone of up to 26 mm. Complex 2 demonstrated strong antibacterial activity against S. aureus, with an inhibition zone reaching 23 mm. By comparison, the antimicrobial activity of the two complexes was stronger than Gd(dbm)₃·6H₂O, ligands H₃L₁ and H₂L₂. This is attributed to the synergistic effect between the metal and ligands within the complex in inhibiting bacterial proliferation.

  3.   Conclusions

  In short, we synthesized two kinds of rare earth com-plexes, namely [Gd₂(dbm)₂(HL₁)₂(CH₃OH)₂]·4CH₃OH (1) and [Gd₂(dbm)₂(L₂)₂(CH₃OH)₂]·2CH₃OH (2), where H₃L₁=(Z)-N′-(4-(diethylamino)-2-hydroxybenzylidene)propionohydrazide, H₂L₂=(E)-N′-(5-bromo-2-hydroxy-3-methoxybenzylidene)nicotinohydrazide), Hdbm=dibenzoylmethane. X-ray structural analysis indicates that these two complexes are dinuclear, with each Gd³⁺ ion being eight-coordinated. A systematic study on the structures of the two complexes and their interactions with DNA has been conducted. Subsequent studies strongly indicate that the two Gd₂ complexes interact with CT-DNA through an intercalation mechanism. This is a certain reference value in the study of the interaction between the complexes and DNA, and also provides a reference basis for antibacterial activity. The understanding of the interaction between the Gd₂ complexes and DNA may be related to their antibacterial mechanism. Additionally, our investigation revealed that two complexes exhibited remarkable antibacterial activity against four types of bacteria, which has certain reference value for the development of new MOFs with antibacterial properties.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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• 

  Scheme 1  Structures of two Schiff-base ligands

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  Figure 1  Molecular structure for complex 1

  Hydrogen atoms have been omitted; Symmetry code: a: -x+1, -y+2, -z+1; Ellipsoids are drawn at a 30% probability level.

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  Figure 2  Molecular structure for complex 2

  Hydrogen atoms have been omitted; Symmetry code: a: -x+1, -y+1, -z+1; Ellipsoids are drawn at a 30% probability level.

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  Figure 3  UV-Vis spectra for the interaction between complex 1 (a)/2 (b) (1.7×10^-2 mmol·L^-1) and CT-DNA

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  Figure 4  Fluorescence spectra of EB-DNA (c_DNA=4.2 μmol·L^-1) complex in the presence of complexes 1 (a) and 2 (b) with different concentrations and corresponding Stern-Volmer plots [c (1), d (2)]

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  Figure 5  Cyclic voltammetry curves for the interaction between complexes 1 (a)/2 (b) (1.7×10^-2 mmol·L^-1) and CT-DNA

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  Table 1.  Crystallographic data and structure refinements for complexes 1 and 2

  Parameter	1	2
  Formula	C62H80Gd2N6O16	C62H58Br2Gd2N6O14
  Formula weight	1 479.82	1 585.46
  T / K	150.0	150.0
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	P21/n
  a / nm	1.515 72(5)	1.348 16(4)
  b / nm	1.212 11(5)	1.329 75(3)
  c / nm	1.887 29(7)	1.763 92(5)
  β / (°)	112.730 7(12)	105.093(1)
  V / nm3	3.198 1(2)	3.053 12(14)
  Z	2	2
  Dc / (g·cm-3)	1.537	1.725
  μ / mm-1	2.126	3.532
  θ range / (°)	2.047-26.394	1.943-26.400
  F(000)	1 500	1 564
  Reflection collected	36 429	57 467
  Unique reflection	6 525	6 232
  Rint	0.056 5	0.052 5
  GOF (F 2)	1.004	1.025
  R1, wR2 [I > 2σ(I)]	0.036 1, 0.096 3	0.039 9, 0.099 1
  R1, wR2 (all data)	0.049 6, 0.111 4	0.047 1, 0.109 7

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