Fluorescence Analysis of 7-Aminoactinomycin–Telomeric Oligonucleotide d[AGGG(TTAGGG)3] Complexes

Abstract

Interactions of 7-aminoactinomycin D (a fluorescent analogue of actinomycin D, an anticancer antibiotic) and two structural forms of the model guanine-rich telomeric oligonucleotide d[AGGG(TTAGGG)₃] have been studied. We have shown that 7-aminoactinomycin D induces fluorescence in two G-quadruplex structures formed with the presence of potassium or sodium ions. The enthalpy of the interaction between the phenoxasone chromophore of the antibiotic and the telomeric oligonucleotide as determined by analysis of excitation spectra is 5.5 kcal/mol. This value differs little from those obtained for complexes with guanine, adenine, or thymine aggregates (6–7 kcal/mol). In the oligonucleotide, the antibiotic is located within dynamic cavities. Therefore, 7-aminoactinomycin D is not released from telomeric structures to the aqueous phase spontaneously or even by photoexcitation, but it is easily released from the surface of aggregates of the respective nucleobases. The entropy term of the interaction energy calculated as a difference between the total energy determined from the binding constant and the enthalpy determined from excitation spectra constitutes approximately 30% for the telomeric oligonucleotide and is virtually null in interactions with nucleobase aggregates.