Zinc(II)-5,10,15,20-tetrakis(α-pyridino-m-tolyl)porphyrin Tetrabromide

Abstract

Cationic porphyrins interact strongly with guanine quadruplex (G-quadruplex) DNA. We report the preparation of the zinc(II) complex of a porphyrin bearing cationic side arms, zinc(II)-5,10,15,20-tetrakis(α-pyridino-m-tolyl)porphyrin tetrabromide (ZnmPy), as a novel probe for the analysis of G-quadruplex/porphyrin interaction.

Guanine quadruplex (G-quadruplex) of a single-stranded overhang at the end of chromosomes is an attractive drug target for cancer therapy, because macrocyclic compounds like cationic tetra-(N-methyl-4-pyridyl)porphyrin (TMPyP4) stabilize G-quadruplex structures, and thus show anti-telomerase and anti-cancer activity [1,2,3,4]. We previously synthesized G-quadruplex-interacting porphyrins with cationic side arms at para- or meta-position of all phenyl groups of tetratolyl porphyrin [5]. These porphyrins were found to stabilize an anti-parallel G-quadruplex DNA more effectively than TMPyP4. On the basis of spectrophotometric and molecular modeling results, it is assumed that the chromophore of the cationic porphyrins should interact with unique sites of the anti-parallel G-quadruplex [6]. Insertion of diamagnetic zinc(II) to metal-free, cationic bis-porphyrins alters their characteristics in aqueous solution and improves the DNA-interacting and photocleaving abilities [7,8,9,10]. Thus, we herein report the preparation of the zinc(II) complex of a porphyrin bearing cationic side arms, zinc(II)-5,10,15,20-tetrakis(α-pyridino-m-tolyl)porphyrin tetrabromide(ZnmPy), as a novel probe for the analysis of G-quadruplex/porphyrin interaction.

The preparation of ZnmPy from mPy, 5,10,15,20-tetrakis(α-pyridino-m-tolyl)porphyrin, and zinc bromide failed to give the desired compound, because the purification cannot be achieved due to the highly polar character of the materials. Alternatively, the preparation of ZnmBr, zinc(II)-5,10,15,20-tetrakis(α-bromo-m-tolyl)porphyrin, followed by the introduction of pyridine was successful. A mixture of mBr and zinc acetate in chloroform was refluxed for 2 h, and was then passed through silica gel. After addition of heptane to the eluate, crystallization of ZnmBr was achieved by slow evaporation. ZnmPy was prepared successfully by the reaction of ZnmBr with an excess of pyridine. The ¹H NMR, MS and elemental analyses for ZnmBr and ZnmPy gave satisfactory results.

Experimental

¹H NMR spectra were recorded on a JEOL GX-400 spectrometer. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrum was recorded on a Bruker REFLEX^TM. Electrospray-ionization time-of-flight (ESI-TOF) mass spectrum was recorded on a Micromass LCT Premier™. Elemental analysis was performed at the Analytical Center, Kumamoto University. The starting material mBr was synthesized according to the previous method [11].

Zinc(II)-5,10,15,20-tetrakis(α-bromo-m-tolyl)porphyrin (ZnmBr)

To a solution of mBr (100 mg, 0.101 mmol) in CHCl₃ (20 mL) was added a solution of zinc acetate (22.3 mg, 0.127 mmol) in MeOH (2 mL), and it was refluxed for 2 h with stirring. After cooling to room temperature, the mixture was passed through silica gel (CHCl₃). After the addition of heptane to the eluate, the solution was slowly evaporated. The purple solids were collected, dried in vacuo (yield: 96.5%). ¹H NMR (CDCl₃): 4.77 (s, 8H, -CH₂-), 7.72 (t, J = 7.6 Hz, 4H, H-5), 7.80 (d, J = 7.6 Hz, 4H, H-6), 8.16 (d, J = 7.6 Hz, 4H, H-4), 8.25 (s, 4H, H-2), 8.96 (s, 8H, β-pyrrolic H). MALDI-TOF MS (m/z): Calcd for C₄₈H₃₂N₄Br₄Zn, 1049.9 [M]⁺. Found: 1049.7. Elemental analysis: Calcd. for C₄₈H₃₂N₄Br₄Zn: C, 54.92; H, 3.07; N, 5.34. Found: C, 55.10; H, 3.10; N, 5.40.

Zinc(II)-5,10,15,20-tetrakis(α-pyridino-m-tolyl)porphyrin tetrabromide (ZnmPy)

A solution of ZnmBr (50.0 mg, 0.0476 mmol) in pyridine (5 mL) was refluxed for 1.5 h with stirring. After cooling to room temperature, the purple solid was collected and dried in vacuo (yield: 76.1%). ¹H NMR (DMSO-d₆): 6.25 (dd, J = 8.0 Hz, 8H, -CH₂-), 7.90 (m, 4H, phenyl H-5), 8.03 (br d, J = 7.2 Hz, 4H, phenyl H-6), 8.23 (t, J = 6.4 Hz, 4H, phenyl H-4) 8.29 (m, 8H, pyridyl H-3 and H-5), 8.48 (m, 4H, phenyl H-2), 8.70-8.75 (m, 12H, pyridyl H-4 and β-pyrrolic H), 9.49 (m, 8H, pyridyl H-2 and H-6). ESI-TOF MS (m/z): Calcd for C₄₈H₃₂N₄Zn, 261.09 [M]⁺⁴. Found: 261.07. Elemental analysis: Calcd. for C₆₈H₅₂N₈Br₄Zn·4H₂O: C, 56.79; H, 4.20; N, 7.79. Found: C, 56.90; H, 3.97; N, 7.96.