Potency of photoinduced electron transfer and antioxidant efficacy of pyrrole and pyridine based Cu(II)-Schiff complexes while binding with CT-DNA

• Charge-transfer occurs between copper-Schiff-base complexes (CSC) and CT-DNA.
• Magnetic Field Effect confirms photoinduced electron transfer, PET from DNA to CSCs.
• Estimation of HFI indicates intra- and inter-strand hole hopping among DNA bases.
• Structure and dimension of CSCs control PET, DNA-cleavage and antioxidant activity.
• The CSCs with pyrrole containing ligands are more effective than pyridine one.

Here we report a systematic and comparative study to define a correlation between the structure and function of a series of simple, biologically active small inorganic Schiff base copper complexes for the occurrence of charge transfer phenomenon in calf thymus DNA (CT-DNA) using transient absorption spectroscopy corroborated with magnetic field effect. Four copper(II) Schiff base complexes with differently substituted heterocyclic ligands with antioxidant activity have been used. The binding constants of the order of ∼104 support the moderate binding affinity of the complexes towards CT-DNA. The methyl-substituted pyrrole complex shows maximum binding affinity (Kb: 8.33 × 104) compared to others. The occurrence of photoinduced electron transfer (PET) from CT-DNA to pyrrole containing complexes has been confirmed by identifying the corresponding transient radical ions whereas the extent of PET with pyridine substituted complexes is too small to be observed. The increase of the yield of radical ions in presence of magnetic field depicts that the initial spin correlation in geminate radical ion pair is triplet. The difference between experimental and calculated B½ values, the measure of hyperfine interactions (HFI) present in the system, arises due to hole hopping through intrastrand and interstrand DNA bases. The unsubstituted pyrrole complexes cleave DNA much more than the methyl-substituted one. Therefore, the probability of intrastrand superexchange increases with methyl-substituted complexes, that reduces the rate of hole hopping and hence the B½ value.

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