The photophysical and DNA-binding properties of the cationic zinc(II) complex of 5-triethyl ammonium methyl salicylidene ortho-phenylendiiminato, ZnL2+, have been investigated by a combined experimental-theoretical study. DFT calculations have been performed on both the ground and the first excited states of ZnL2+ and on its possible mono- and bi-oxidation products both in vacuo and in selected solvents mimicked by the polarizable continuum model (PCM). The calculated absorption and fluorescence transitions have been compared to the corresponding experimental data, allowing to conclude that the visible light induces a two-electron photo-oxidation process located on the phenylendiiminato ligand. Kinetic measurements, performed by monitoring absorbance changes in time in several solvents, are in agreement with a slow unimolecular photooxidation process, which is faster in water and slower in less polar solvents. Moreover, structural details of the ZnL-DNA binding have been obtained by DFT calculations performed on the intercalation complexes between ZnL and the two d(ApT)2 and d(GpC)2 dinucleoside-monophosphate duplexes. Two main complementary binding interactions are proposed: 1) the intercalation of the central phenyl ring of the ligand between the stacked DNA base pairs; 2) the external electrostatic attraction between the negatively charged phosphate groups and the two triethylammonium cationic groups of the Schiff base ligand. Such suggestions are supported by the fluorescence titrations performed on the ZnL/DNA system under different ionic strength and temperature conditions. In particular, the values of the DNA-binding constants obtained at different temperatures provided the enthalpic and entropic contributions in the binding and confirmed that two competitive mechanisms, intercalation and external interaction, are involved. The two mechanisms are coexistent at room temperature at physiological conditions.

A Theoretical and Experimental Investigation of the Spectroscopic Properties of a DNA-Intercalator Salphen-Type ZnIIComplex

BIANCARDI, ALESSANDRO;BIVER, TARITA;MENNUCCI, BENEDETTA
2014

Abstract

The photophysical and DNA-binding properties of the cationic zinc(II) complex of 5-triethyl ammonium methyl salicylidene ortho-phenylendiiminato, ZnL2+, have been investigated by a combined experimental-theoretical study. DFT calculations have been performed on both the ground and the first excited states of ZnL2+ and on its possible mono- and bi-oxidation products both in vacuo and in selected solvents mimicked by the polarizable continuum model (PCM). The calculated absorption and fluorescence transitions have been compared to the corresponding experimental data, allowing to conclude that the visible light induces a two-electron photo-oxidation process located on the phenylendiiminato ligand. Kinetic measurements, performed by monitoring absorbance changes in time in several solvents, are in agreement with a slow unimolecular photooxidation process, which is faster in water and slower in less polar solvents. Moreover, structural details of the ZnL-DNA binding have been obtained by DFT calculations performed on the intercalation complexes between ZnL and the two d(ApT)2 and d(GpC)2 dinucleoside-monophosphate duplexes. Two main complementary binding interactions are proposed: 1) the intercalation of the central phenyl ring of the ligand between the stacked DNA base pairs; 2) the external electrostatic attraction between the negatively charged phosphate groups and the two triethylammonium cationic groups of the Schiff base ligand. Such suggestions are supported by the fluorescence titrations performed on the ZnL/DNA system under different ionic strength and temperature conditions. In particular, the values of the DNA-binding constants obtained at different temperatures provided the enthalpic and entropic contributions in the binding and confirmed that two competitive mechanisms, intercalation and external interaction, are involved. The two mechanisms are coexistent at room temperature at physiological conditions.
Biancardi, Alessandro; Azzurra, Burgalassi; Alessio, Terenzi; Angelo, Spinello; Giampaolo, Barone; Biver, Tarita; Mennucci, Benedetta
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11568/493892
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