Formation of Nickel(II)Porphyrin and Its Interaction with DNA in Aqueous Medium

In this work, kinetics of the reaction between 5,10,15,20-tetrakis(N-methylpyridium-4-yl)porphyrin and Ni2+ species were investigated in aqueous solution at 25 ±1 ºC in I = 0.10 M (NaNO3). Speciation of Ni2+ was carried out in I = 0.10 M (NaNO3) in order to provide the distribution of the Ni2+ species with different solution pH. The experimental data have been compared with the speciation diagram constructed from the values of hydrolysis constants of Ni2+ ion. Speciation data showed that the hexaaquanickel(II), [Ni(H2O)6]2+, ions take place in hydrolysis reactions through formation of [Ni(OH2)6-n(OH)n]2-n species with solution pH. Based on the speciation of Ni2+ and pH dependent rate constants, rate expression can be written as: d[Ni(TMPyP)4+]/dt = (k1[Ni2+(aq)] + k2[Ni(OH)+(aq)] + k3[Ni(OH)2o(aq)] + k4[Ni(OH)3-(aq)])[H2TMPyP4+], where k1, k2, k3 and k4 were found to be k1 = (0.62 ± 0.22) × 10-2; k2 = (3.60 ± 0.40) × 10-2; k3 = (2.09 ± 0.52) × 10-2, k4 = (0.53 ± 0.04) × 10-2 M-1s-1 at 25 ±1 °C, respectively. Kinetic results showed that monohydroxo, [Ni(H2O)5(OH)]+, is the most reactive among the [Ni(OH2)6-n(OH)n]2-n species. The enhanced reactivity has been ascribed to the formation of hydrogen bonding between oxygen atom of hydroxyl group of the [Ni(H2O)5(OH)]+ species and the pyrrolic hydrogen atom of the [H2TMPyP]4+. The rate of formation of [Ni(II)TMPyP]4+ complex was to be 3.99 × 10-2 M-1s-1 in I = 0.10 M, NaNO3 (25 ± 1 ºC). Ionic strength effect on the reaction rate is suggested that the net charge of the tetracationic porphyrin is to be +3.6 on the basis of Brønsted-Bjerrum equation. The UV-Vis and fluorescence data revealed that [Ni(II)TMPyP]4+ and H2(TMPyP)4+ interact with DNA, and UV-Vis results suggest that Ni(II)-porphyrin and free base porphyrin interact with DNA via outside binding with self-stacking and intercalation, respectively. Mechanism of kinetics of formation of the [Ni(II)TMPyP]4+ complex in aqueous medium is discussed. An investigation of application of the [Ni(II)TMPyP]4+ complex along with other metalloporphyrins such as Zn2+-, Ru2+-, Pt2+-, [Au(III)TMPyP]5+ as anti-COVID-19 agents is now in progress under international collaboration.