DNA Complexes with Cobalt(II) Phthalocyanine Disodium Disulfonate

Abstract

The interaction of cobalt phthalocyanine disodium disulfonate (CoPc) with calf thymus DNA in solutions was investigated by UV/vis spectrophotometry, circular dichroism (CD), and hydrodynamic methods (viscosity and flow birefringence). Two types of CoPc binding to DNA were observed. Fast CoPc interactions with DNA via external binding to phosphates were accompanied by the formation of stack-type phthalocyanine structures on the periphery of the DNA helix. The optical absorption spectra of such CoPc complexes with DNA were analyzed in order to obtain a binding constant K = (4.8 ± 0.4) × 10⁴ M^-1. CD spectra show the increasing optical activity of phthalocyanines bonded to DNA. DNA plays the role of a matrix, contributing to an increase in their stacking interactions. The CD spectrum of DNA varies slightly. The second type of cobalt-to-DNA binding manifests itself over a certain time. It can be associated with the reorganization of ligands in the cobalt coordination sphere by introducing DNA atoms. In our experiments, such binding was observed after storage of solutions for approximately 20 h at a temperature of 4 °C. It was shown that the minor groove of DNA remains free in CoPc-DNA complexes. CoPc does not bind with the most important group for metal coordinating to DNA in the major groove (N7 guanine). We completely excluded the intercalation binding model. The planes of phthalocyanines in CoPc-DNA complexes are oriented predominantly normal to the axis of the DNA helix. DNA rigidity (persistent length) does not change. This follows from the data on the measurement of the optical anisotropy and intrinsic viscosity of DNA in complexes.

Figure 1

Structure of [CoPc(NaSO₃)₂].

Figure 2

Absorption spectra (a) of CoPc (1) and DNA (2) in 0.005 M NaCl with Tris-HCl and the dependence of CoPc absorption on C(CoPc) (b) at 663 nm (1) and 345 nm (2).

Figure 3

Results of spectrophotometric titration in 0.005 M NaCl: the Soret band (a) and the Q band (b), [CoPc] = 2.5 × 10^–5 M, [DNA] is shown in the figure; the binding curves for two CoPc absorption bands at 330 and 663 nm (c) and Wolf–Shimmer plot (d) with the inset (the inset shows the Scatchard isotherm).

Figure 4

Melting curves of DNA (dependences of DNA absorption at 260 nm on temperature) (a) and the first derivatives of melting curves (b) for DNA in complexes with CoPc (1) and for free DNA (2) in 5 mM NaCl with Tris-HCl. [DNA] = 7.6 × 10^–5 M, [CoPc] = 5 × 10^–5 M.

Figure 5

Influence of CoPc on DAPI luminescence in complexes with DNA immediately after the preparation of solutions (a) and after 2 days (b). (1–4) z = 0.02, λ_ex = 380 nm; (5–8)—z = 0.3, λ_ex = 420 nm; (1, 5)—(DAPI + DNA) without CoPc; (2, 6)—(DNA + CoPc) + DAPI for the addition of DAPI to CoPc–DNA complexes; (4, 8)—(DNA + DAPI) + CoPc for the addition of CoPc to DNA–DAPI complexes; (3, 7)—(DAPI + CoPc) + DNA for the addition of DNA to DAPI solution with CoPc. [DNA] = 1.5 × 10^–5 M, [CoPc] = 5 × 10^–6 M, [DAPI] = 3 × 10⁻⁷ M (for z = 0.02), [DAPI] = 5 × 10⁻⁶ M (for z = 0.3), pH 7. Note that the real intensity for the spectra (5–8) is 6 times greater than that shown in the figure.

Figure 6

Adsorption spectra: Soret (a,c), Q (b,d) bands and DNA absorption region (e) for CoPc–DNA complexes formed after and before the binding of compounds located in the DNA major groove: Mn²⁺ (a,b) and cis-DDP (c–e). Spectrum 8 in e demonstrates the sum of spectrum 1 and spectrum 6. The order of the addition of components is shown near the lines.

Figure 7

CD spectra of DNA in solutions with different CoPc concentrations (a) and CD spectra of CoPc without and with DNA in 0.005 M NaCl (b). Concentrations of components are shown near the curves.

Figure 8

Results of viscometric experiments: dependence of the reduced viscosity of DNA solutions in 0.005 M NaCl on DNA concentration at r = 0 (1) and r = 0.36 (2) for determining the DNA intrinsic viscosity by extrapolation of dependences to C([DNA)] = 0.

Figure 9

Dependence of relative changes in optical anisotropy of the DNA statistical segment on [CoPc] in 0.005 M NaCl. C (DNA) = 0.007%.