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The interaction of native calf thymus DNA with the Zn(II) and Cu(II) complexes of 5-triethyl ammonium methyl salicylidene ortophenylendiimine (ZnL 2+ and CuL 2+ ), in 1 mM Tris-HCl aqueous solutions at neutral pH, has been monitored as a function of the metal complex-DNA molar ratio by UV absorption spectrophotometry, circular dichroism (CD) and fluorescence spectroscopy. The results support for an intercalative interaction of both ZnL 2+ and CuL 2+ with DNA, showing CuL 2+ an affinity of approximately 10 times higher than ZnL 2+ . In particular, the values of the binding constant, determined by UV spectrophotometric titration, equal to 7.3 · 10 4 and 1.3 · 10 6 M À1 , for ZnL 2+ and CuL 2+ , respectively, indicate the occurrence of a marked interaction with a binding size of about 0.7 in base pairs. The temperature dependence of the absorbance at 258 nm suggests that both complexes strongly increase the DNA melting temperature (Tm) already at metal complex-DNA molar ratios equal to 0.1. As evidenced by the quenching of the fluorescence of ethidium bromide-DNA solutions in the presence of increasing amounts of metal complex, ZnL 2+ and CuL 2+ are able to displace the ethidium cation intercalated into DNA. A tight ZnL 2+ -DNA and CuL 2+ -DNA binding has been also proven by the appearance, in both metal complex-DNA solutions, of a broad induced CD band in the range 350-450 nm. In the case of the CuL 2+ -DNA system, the shape of the CD spectrum, at high CuL 2+ content, is similar to that observed for w-DNA solutions. Such result allowed us to hypothesize that CuL 2+ induces the formation of supramolecular aggregates of DNA in aqueous solutions.

DFT calculations, using an all-electron basis set and with full geometry optimization, were performed on 34 Sn(II) and Sn(IV) compounds of known structure and (119)Sn Mössbauer parameters, to obtain the theoretical values of the electric field gradient components, V(xx), V(yy), and V(zz), at the tin nucleus. These were used to determine the quantity V = V(zz)[1+ 1/3((V(xx) - V(yy))/((V(zz))(2)](1/2), for each investigated compound, which is related to the quadrupole splitting (DeltaE) parameter according to DeltaE = 1/2eQV, where e is the electronic charge and Q is the quadrupole moment of the tin nucleus. The linear fitting of the correlation plot of the experimental DeltaE, versus the corresponding calculated V values, produced a slope that is equal to 0.93 +/- 0.03 and a correlation coefficient R = 0.982. The value of Q obtained, 15.2 +/- 4.4 fm(2), is in agreement with that previously experimentally determined or calculated by analogous procedures. The calculation method is able to establish the sign of the electric field gradient component V(zz), in agreement with the sign of DeltaE determined experimentally by Mössbauer-Zeeman spectroscopy. The calculated structural parameters are in good agreement with the corresponding experimental data, determined by X-ray crystallography in the solid state, with average structural deviations of about 3 % for bond lengths and angles in the tin environment. Calculated values of DeltaE were obtained from the calibration fitting constant and from the values of V. By comparing experimental and calculated DeltaE parameters, the structure assignment of configurational isomers was successful in two test cases, in agreement with the experimental X-ray crystallographic structures. These results indicate that the method can be used as a tool to support the routine structure interpretation of tin compounds by (119)Sn Mössbauer spectroscopy.

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The interaction between native calf thymus deoxyribonucleic acid (DNA) and

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