Aggregation and DNA binding characteristics of tetrakis(N,N',N″,N‴-tetramethyltetra-3,4-pridino)porphyrazine cobalt(II)

Abstract

The association behavior of tetrakis( N , N ', N ″, N ‴-tetramethyltetra-3,4-pridino)porphyrazine cobalt(II) ([ Co (II)3,4- TMTPPA ]4+) was investigated in aqueous solution at 25°C and various ionic strengths using optical absorption and resonance light scattering spectroscopies. The results show that [ Co (II)3,4- TMTPPA ]4+ does not have any affinity for aggregation due to increasing salt concentration and exists as the monomer form even in homogeneous aqueous solutions at high ionic strengths (more than 1 M concentration of NaCl ). The interaction of [ Co (II)3,4- TMTPPA ]4+ with calf thymus DNA in aqueous solution has also been studied in 5 mM phosphate buffer pH 7.2, by optical absorption and resonance light scattering spectroscopies, and thermal denaturation experiments. The appearance of hypochromicity of less than 10% and a bathochromic shift of Δλ ≤ 8 nm in the UV-vis spectra of [ Co (II)3,4- TMTPPA ]4+, an increase in the thermal melting point of DNA, and no enhancement in resonance light scattering spectra of porphyrazine due to interaction with DNA, indicates the minor outside-groove binding mode without any stack aggregate formation. The thermodynamics of the binding of [ Co (II)3,4- TMTPPA ]4+-DNA has also been studied. The binding constant (K) was obtained by analysis of optical absorption spectra of the above-mentioned complex at various DNA concentrations using SQUAD software. The value of K was estimated as 1.31 × 106 ± 1.174 M-1 at 25°C. The thermodynamic parameters were calculated by van't Hoff equation. The enthalpy and entropy changes were 48.77 ± 2.50 kJ . mol -1 and 280.77 ± 9.62 J . mol -1. K -1 at 25°C, respectively. The results indicate that the process is entropy driven suggesting that hydrophobic interactions are the main driving forces for complex formation. The increase of ionic strength due to the addition of NaCl , destabilized porphyrazine-DNA complexes, which indicates the competition of Na + ions with porphyrazine complexes for occupation of minor groove binding sites.