Synthesis of 10- O-aryl-substituted berberine derivatives by Chan-Evans-Lam coupling and investigation of their DNA-binding properties

Abstract

Eleven novel 10-O-aryl-substituted berberrubine and berberine derivatives were synthesized by the Cu²⁺-catalyzed Chan-Evans-Lam coupling of berberrubine with arylboronic acids and subsequent 9-O-methylation. The reaction is likely introduced by the Cu²⁺-induced demethylation of berberrubine and subsequent arylation of the resulting 10-oxyanion functionality. Thus, this synthetic route represents the first successful Cu-mediated coupling reaction of berberine substrates. The DNA-binding properties of the 10-O-arylberberine derivatives with duplex and quadruplex DNA were studied by thermal DNA denaturation experiments, spectrometric titrations as well as CD and LD spectroscopy. Fluorimetric DNA melting analysis with different types of quadruplex DNA revealed a moderate stabilization of the telomeric quadruplex-forming oligonucleotide sequence G₃(TTAG₃)₃. The derivatives showed a moderate affinity towards quadruplex DNA (K _b = 5-9 × 10⁵ M^-1) and ct DNA (K _b = 3-5 × 10⁴ M^-1) and exhibited a fluorescence light-up effect upon complexation to both DNA forms, with slightly higher intensity in the presence of the quadruplex DNA. Furthermore, the CD- and LD-spectroscopic studies revealed that the title compounds intercalate into ct DNA and bind to G4-DNA by terminal stacking.

Keywords: Cu-mediated coupling reactions; DNA recognition; berberine alkaloids; nucleic acids; quadruplex DNA.

Figure 1

Structures and numbering of berberine (1a), berberrubine (1b) and 9-O-aryl-substituted berberine derivatives 2a–i.

Scheme 1

Synthesis of 10-O-arylated berberine derivatives 5a–e.

Scheme 2

Cu²⁺-catalyzed demethylation of berberrubine (1b).

Figure 2

Temperature dependent emission spectra of derivatives 5a and 5d (c = 10 µM, with 0.25% v/v DMSO) in glycerol; λ_ex = 430 nm. The arrows indicate the changes of the emission intensity with increasing temperature. Insets: Plots of the relative fluorescence intensity versus the temperature of the solution.

Figure 3

Photometric titration of 5a (A) and 5d (B) (c_Ligand = 20 μM) with ct DNA (1) in BPE buffer (c_Na+ = 16 mM, pH 7.0, with 5% v/v DMSO) and with 22AG (2) in K-phosphate buffer (c_K+ = 110 mM, pH 7.0, with 5% v/v DMSO). The arrows indicate the changes of the absorption bands upon addition of DNA. Inset: Plot of the ligand absorption versus c_DNA (in base pairs).

Figure 4

Fluorimetric titration of 5a (A) and 5d (B, c_Ligand = 20 μM) with ct DNA (1) in BPE buffer (c_Na+ = 16 mM, pH 7.0, with 5% v/v DMSO) and with 22AG (2) in K-phosphate buffer (c_K+ = 110 mM, pH 7.0, with 5% v/v DMSO); λ_ex = 430 nm. The arrows indicate the changes of the emission bands upon addition of DNA. Insets: Plots of the relative fluorescence intensity versus c_DNA (in base pairs).

Figure 5

CD and LD spectra of ct DNA (1 and 2, c_DNA = 20 μM; in BPE buffer: 10 mM, pH 7.0; with 5% v/v DMSO) in the absence and presence of 5a (A), and 5d (B) at LDR = 0 (black), 0.05 (orange), 0.2 (blue), 0.5 (green), 1.0 (red). The arrows indicate the changes of CD and LD bands with increasing LDR.