Synthesis of phenylazonaphtol-β-D-O-glycosides, evaluation as substrates for beta-glycosidase activity and molecular studies
© Brito-Arias et al.; licensee Springer. 2014
Received: 18 October 2013
Accepted: 2 May 2014
Published: 17 May 2014
Phenylazonaphtol-β-D-O-glycosides are alternative substrates for the detection of enzymatic activity of β-glycosidases which are involved in various important processes. These azoic compounds are currently exploited as prodrugs for colonic disease due the presence of β-glycosidase activity in the gut flora and therefore allowing the release of the drug at the specific site.
Phenylazonaphtol-β-D-O-glucoside 3a and galactoside 3b were prepared via diazonium salt conditions under weak acidic conditions which do not compromise the O-glycosidic bond stability, by coupling reaction between 2-naphtol sodium salt with aminoglycosides 1a and 1b. The resulting phenylazonaphtol glycosides 2a and 2b were deprotected affording the phenylazonaphtol glycosides 3a and 3b in quantitative yield. The galactoside glycoside 3b was assayed as substrate for in vitro β-galactosidase enzymatic activity showing strong absorbance after releasing of the azoic chromophore. Also, docking studies were performed to determine the best pose as well as the interactions between the ligand and the residues located at the active site.
The methodology developed for synthesizing the phenylazonaphtol glycosides described proved to be convenient for generating azoic functionalities in the presence of glycosidic bonds and the glycosides suitable as alternative substrates and potentially useful prodrugs in the treatment of colonic diseases.
Keywordsβ-galactosidase Synthesis Phenylazonaphtol glycoside Substrate Docking
β-glycosidases are hydrolytic enzymes involved in a number of important processes such as defense mechanism, growth regulation , gene markers in transgenic plants , and prodrugs . The most commonly used substrates for the histochemical localization of β-glycosidases contain the 5-bromo-4-chloro-3-indolyl chromophore attached to most of the known monosaccharides through an O-glycosidic bond . After enzymatic hydrolysis, the water soluble indoxyl intermediate must undergo an oxidative dimerization to produce a blue precipitate. However, in some cases before dimerization occurs to produce the indigo dye, some diffusion takes place, producing false positives at some regions in cells lacking enzymatic activity . In addition, phenylazonaphtol glucosides have been synthetically prepared and tested in transgenic plants containing the β-glucuronidase activity which is a widely used gene marker. A comparative test between the commercially available glycosidic indoxyl substrates with the alternative phenylazonaphtol glucuronides resulted in partial diffusion for the indigo dye and no detectable diffusion for the phenylazonaphtol chromophore [6, 7].
On the other hand, glycosides when attached to pharmacologically active substances can be used as prodrugs for improving availability, stability, and particularly for specific delivery of the active compound at the target organ. For instance, steroids, antitumor, and anti-inflammatory compounds have been attached to glycosides and evaluated as specific delivery prodrugs . Also, azoic β-glycosides provided to be useful as prodrugs particularly against colitis, Crohn's disease, and colorectal cancer since the delivery at colon level might be achieved by the action of azo-reductase or glycosidase present at the colonic microflora .
Due the potential usefulness of phenylazonaphtol glycosides in processes mentioned above, we developed a methodology for preparing phenylazonaphtol glycosides under weak acidic conditions and the resulting azoic glycosides evaluated as β-galactosidase substrates with potential application as prodrugs for colon diseases.
The beta galactosidase assay. β-galactosidase (2 mg; 2.1 U/mg) was suspended in 0.8 mL of buffer containing 0.06 M Na2HPO4, 0.04 M NaH2PO4, 0.01 M KCl, 0.001 M MgSO4, and 1 mM DTT, pH 7.0, and then incubated with glycoside 3b (4 mg; 9.3 × 10-3 mM) in a mixture of 50 mM acetate buffer with pH 5.0 (0.5 mL) and methanol (0.5 mL) at 37°C for 3 h. The reaction was stopped by adding 0.5 mL 1 M Na2CO3. The amount of the released phenylazonaphtol chromophore was measured spectrophotometrically at 410 and 455 nm .
Results and discussion
All the reagents were purchased from Aldrich Chemical Co. (St. Louis, MO, USA) except HBr/acetic acid which was purchased from Fluka Chemical Co. (Buchs, Switzerland). Column chromatography was performed on silica gel with a 70-230 mesh, and thin-layer chromatography on Kieselgel, both from Merck Co. (Darmstadt, Germany), which was used as a detection system. A cerium sulfate solution followed by heating on hot plate. Infrared (IR) spectra were obtained on Perkin-Elmer spectrometer (Waltham, MA, USA); nuclear magnetic resonance (NMR) spectra were recorded on Varian 300-MHz spectrometer (Palo Alto, CA, USA) and Bruker 500 MHz (Karlsruhe, Germany). Mass spectrum (MS) spectra were obtained using a Hewlett-Packard 5989A (Palo Alto, CA, USA). Optical rotations were measured with a Perkin-Elmer 341 polarimeter at 23°C. Enzyme galactosidase was purchased from Sigma Chemical Company (St. Louis, MO, USA).
1-[(4-tetra-O-acetyl-β-D-glucopyranosyloxy phenyl)azo]-2-naphtol (2a)
1-[(4-tetra-O-acetyl-β-D-galactopyranosyloxy phenyl azo]-2-naphtol (2b)
Same reaction conditions as for 2a except that compound 1b is used instead of 1a.
1H NMR (CDCl3) δ 2.09 to 2.12 (4 s, 12 H), 4.12 (1H, m, H-5), 4.22 (1H, dd, H-6), 4.26 (1-H, dd, H6′), 5.11 (1H, d, H-1), 5.13 (1H, dd, H-4), 5.48 (1H, t, H-2), 5.54 (1H, dd, H-3), 6.97 (d, H-13, J = 9.3), 7.15 (d, H-8, 8′, J = 5.1 Hz), 7.41 (t, H-16, J = 7.5 Hz, 7.2 Hz), 7.58 (t, H-17, J = 7.5 Hz, 7.2 Hz), 7.66 (d, H-15, J = 8.1 Hz), 7.75 (d, H-14, J = 9.6 Hz), 7.76 (d, H-9,9′, J = 9.0 Hz), 8.62 (d, H-18, J = 7.8 Hz). 13C NMR (CDCl3) δ 20.8, 20.9 (4 CH3-), 61.5 (C-6), 67.0 (C-4), 68.7 (C-2), 71.2 (C-3), 71.4 (C-5), 99.7 (C-1), 117.8 (C-8), 121.0 (C-9), 121.5 (C-18), 123.0 (C-13), 125.2 (C-16), 127.7 (C-14a), 128.1 (C-11), 128.4 (C-17), 129.7 (C-15), 133.3 (C-18a), 138.2 (C-10), 142.6 (C-14), 156.8 (C-7), 165.1 (C-12), 169.2, 169.3, 170.1, 170.4 (4 C = O). MS (EI) m/z 594.18 [M+], 169 (100), 264 (62).
1-[(4-β-D-glucopyranosyloxy phenyl)azo]-2-naphtol (3a)
m.p. 177°C to 178°C, [α]D -23.4 (c 0.9, MeOH), 1H NMR (DMSO d-6) δ 3.13 (H-2), 3.30 (H-4), 3.35 (H-5), 3.37 (H-3), 3.60 (H-6), 3.75 (H-6′), 4.99 (H-1, J = 7.8), 6.97 (d, H-13, J = 9.3), 7.15 (d, H-8, 8′, J = 5.1 Hz), 7.41 (t, H-16, J = 7.5 Hz, 7.2 Hz), 7.58 (t, H-17, J = 7.5 Hz, 7.2 Hz), 7.66 (d, H-15, J = 8.1 Hz), 7.75 (d, H-14, J = 9.6 Hz), 7.76 (d, H-9,9′, J = 9.0 Hz), 8.62 (d, H-18, J = 7.8 Hz). 13C NMR (DMSO d-6) δ 61.7 (C-6), 76.8 (C-5), 71.6 (C-4), 74.1 (C-2), 76.7 (C-3), 96.7 (C-1), 117.8 (C-8), 121.0 (C-9), 121.5 (C-18), 123.0 (C-13), 125.2 (C-16), 127.7 (C-14a), 128.1 (C-11), 128.4 (C-17), 129.7 (C-15), 133.3 (C-18a), 138.2 (C-10), 142.6 (C-14), 156.8 (C-7), 165.1 (C-12), 169.2, 169.3, 170.1, 170.4 (4 C = O). HRMS calculated for C22H22N2O7 (M + H) 427.1505, found 427.1509.
1-[(4-β-D-galactopyranosyloxy phenyl)azo]-2-naphtol (3b)
Same reaction conditions as for 3a except that compound 2b is used instead of 2a. 1H NMR (DMSO d-6) δ 3.41 (t, H-2), 3.56 (dd, H-3), 3.61 (m, H-5), 3.62 (H-6), 3.70 (H-6′), 3.84 (dd, H-4), 4.98 (d, H-1, J = 8.0), 6.97 (d, H-13, J = 9.3), 7.15 (d, H-8, 8′, J = 5.1 Hz), 7.41 (t, H-16, J = 7.5 Hz, 7.2 Hz), 7.58 (t, H-17, J = 7.5 Hz, 7.2 Hz), 7.66 (d, H-15, J = 8.1 Hz), 7.75 (d, H-14, J = 9.6 Hz), 7.76 (d, H-9,9′, J = 9.0 Hz), 8.62 (d, H-18, J = 7.8 Hz). 13C NMR (DMSO d-6) δ 62.0 (C-6), 69.7 (C-4), 72.9 (C-2), 73.8 (C-3), 76.0 (C-5), 97.3 (C-1), 117.8 (C-8), 121.0 (C-9), 121.5 (C-18), 123.0 (C-13), 125.2 (C-16), 127.7 (C-14a), 128.1 (C-11), 128.4 (C-17), 129.7 (C-15), 133.3 (C-18a), 138.2 (C-10), 142.6 (C-14), 156.8 (C-7), 165.1 (C-12), 169.2, 169.3, 170.1, 170.4 (4 C = O). FABMS (M + 1) 427.1.
The Additional file 1 (as word document) shows the 1H, 13C NMR spectra for the synthesized compounds 2a-b and 3a-b, and high-resolution mass spectrometry and elemental analysis are also included.
The methodology developed for synthesizing the phenylazonaphtol glycosides described proved to be convenient for generating azoic functionalities in the presence of glycosidic bonds, and the glycosides are suitable as alternative substrates and potentially useful prodrugs in the treatment of colonic diseases.
We wish to thank COFAA-IPN for the scholarship and SIP-IPN for the financial support. We also want to thank Dr. Carlos Cerda Garcia-Rojas for MS and optical rotation, Mabel Fragoso for mass spectroscopy, and Gerardo Zepeda for NMR bidimensional determinations.
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