ABSTRACT

The steady-state and time-resolved absorption and fluorescence help to identify the steady-state products and transient intermediates, respectively, generated through photoinduced electron transfer (PET), which may be one of the plausible phenomena in drug–protein/DNA interactions. However, the importance of application of low magnetic field of the order of 0.01–0.02 T lies in its ability to identify initial spin state, one of the deciding factors for ultimate product formation, as well as to assess the intermediate distance in geminating spin-correlated radical ion pairs/radical pairs produced as transients, an useful technique to study “distance-dependent” interactions in biomacromolecules. We have synthesized and studied five new copper(II) Schiff base complexes with differently substituted heterocyclic ligands, [CuL1]·2ClO4, [CuL2]·2ClO4, [CuL3]·2ClO4, [CuL4]·2ClO4, and [CuL5]·2ClO4, among which the first two metal complexes with N2O2 donor set of atoms and the other three metal complexes with N4 donor set of atoms with different aliphatic substitutions, to understand their effect on interaction with calf thymus DNA (CT-DNA). Laser flash photolysis coupled with an external magnetic field has helped to assess the efficiency of PET from CT-DNA to the complexes. The possibility of PET in triplet state between CT-DNA and the metal complexes having N2O2 donor set of atoms, CuL1 and CuL2, is insignificant due to the presence of oxygen as ligand atom. However, the other three complexes with N4 donor set atoms undergo PET with CT-DNA. The extent of PET is much more prominent with pyrrole containing complexes, CuL4 and CuL5, compared to pyridine-substituted complex, CuL3. The increase in the yield of radical ions in the presence of magnetic field depicts the initial spin correlation of the geminate radical ion pair as triplet. The difference between experimental and calculated B 1/2 values that determines the extent of hyperfine interactions present in the system is much higher for unsubstituted pyrrole copper complex, CuL4, compared to the substituted one, CuL5, since the former due to its smaller structure can approach DNA with greater proximity which leads to much more “through-space” hole hopping for intrastrand and interstrand DNA bases. However, the superexchange interaction, which reduces the hole-hopping rate on increasing the size of the nucleobases’ bridge, becomes much more prominent leading to a decrease in experimental B 1/2 value for methyl-substituted pyrrole–DNA system.