31
intercalation or/and groove binding of the drug with DNA. The binding constant,
the sizes of the drug binding sites with DNA and the cooperativity parameters for
both types of complexes were calculated [36, 37, 89].
The thermodynamic parameters of the DNA-drug complexes were obtained by
using UV-VIS optical melting [18] and DSC methods [89]. From the UV melting
curves of DNA alone and its mixture with ligands (ActII-ActV) the melting (melting temperature, melting interval) and the binding parameters (changes of binding
free energy, binding enthalpy and entropy) for all the samples studied were determined. From these data it was found that the largest value of the binding free energy
change is associated with DNA-ActII complex. This result indicates that within the
set of the actinocin derivatives having different number of methylene groups in
dimethylaminoalkyl side chains studied in this work, specifically ActII containing
two CH 2 groups features the strongest interaction with DNA.
Quantitative estimation of the binding parameters accomplished using the DSC
data showed that the stability of the DNA-ligand complexes is higher than that
in the case of free DNA. On decrease in the number of methylene groups in the
ligands’ side chains both the binding enthalpy and the free energy changes increase
non-linearly reaching the maximal value for the number of CH 2 groups equal to 2
(i.e. DNA-ActII). Hence, there is a satisfactory agreement between the values of
thermodynamic parameters and their dependence on the number of CH 2 groups in
the ligands’ side chains, obtained from DSC and UV-VIS optical melting. The magnitude of the binding enthalpy can be explained by the intercalative type of interaction, which may additionally be stabilized by hydrogen bonds and water bridges.
The melting entropy of the complexes is higher by absolute value than that of free
DNA. It is due to more ordered structure of the hydration environment around the
complexes in comparison with free DNA.
Some peculiarities of the heat absorption curves caused by melting of DNAs
having different nucleotide compositions were observed for the solutions of free
DNA and its complexes with actinocin derivatives ActII-ActV. Notably, for the
DNA-ActII complex the heat absorption curve is significantly distorted in hightemperature area when the GC-rich blocks of DNA are melted.
The role of water in the DNA-drug complexation was investigated by piezogravimetry and IR spectroscopy. Hydration isotherms and IR-spectra of the free DNA
and the DNA-drug complexes were obtained in films. Analysis of the spectra was
carried out using reliably-assigned DNA absorption bands sensitive to hydration
and conformational states of nucleic acids, as well as by the absorption bands of
the drugs. Investigation of the properties of water absorption to DNA and DNAdrug complexes had led to the conclusion that the energy of interaction between
the water molecules and the complexes depends on the length of side chains of
synthetic phenoxazone drugs ActII-ActV. The hydrated environment makes significant contribution to stabilization of double-helical structure of either free DNA and
of its complexes with the drugs. Increase of relative humidity of the films in the
range of 0 to 90 % leads to increase in the intensity of the IR-absorption bands for
the sugar-phosphate backbone vibration, in- and out-ring groups of DNA base pairs
in the drug-DNA complexes, and also of the absorption bands of C = O, C = N
2 Structure, Thermodynamics and Energetics of Drug-DNA Interactions
intercalation or/and groove binding of the drug with DNA. The binding constant,
the sizes of the drug binding sites with DNA and the cooperativity parameters for
both types of complexes were calculated [36, 37, 89].
The thermodynamic parameters of the DNA-drug complexes were obtained by
using UV-VIS optical melting [18] and DSC methods [89]. From the UV melting
curves of DNA alone and its mixture with ligands (ActII-ActV) the melting (melting temperature, melting interval) and the binding parameters (changes of binding
free energy, binding enthalpy and entropy) for all the samples studied were determined. From these data it was found that the largest value of the binding free energy
change is associated with DNA-ActII complex. This result indicates that within the
set of the actinocin derivatives having different number of methylene groups in
dimethylaminoalkyl side chains studied in this work, specifically ActII containing
two CH 2 groups features the strongest interaction with DNA.
Quantitative estimation of the binding parameters accomplished using the DSC
data showed that the stability of the DNA-ligand complexes is higher than that
in the case of free DNA. On decrease in the number of methylene groups in the
ligands’ side chains both the binding enthalpy and the free energy changes increase
non-linearly reaching the maximal value for the number of CH 2 groups equal to 2
(i.e. DNA-ActII). Hence, there is a satisfactory agreement between the values of
thermodynamic parameters and their dependence on the number of CH 2 groups in
the ligands’ side chains, obtained from DSC and UV-VIS optical melting. The magnitude of the binding enthalpy can be explained by the intercalative type of interaction, which may additionally be stabilized by hydrogen bonds and water bridges.
The melting entropy of the complexes is higher by absolute value than that of free
DNA. It is due to more ordered structure of the hydration environment around the
complexes in comparison with free DNA.
Some peculiarities of the heat absorption curves caused by melting of DNAs
having different nucleotide compositions were observed for the solutions of free
DNA and its complexes with actinocin derivatives ActII-ActV. Notably, for the
DNA-ActII complex the heat absorption curve is significantly distorted in hightemperature area when the GC-rich blocks of DNA are melted.
The role of water in the DNA-drug complexation was investigated by piezogravimetry and IR spectroscopy. Hydration isotherms and IR-spectra of the free DNA
and the DNA-drug complexes were obtained in films. Analysis of the spectra was
carried out using reliably-assigned DNA absorption bands sensitive to hydration
and conformational states of nucleic acids, as well as by the absorption bands of
the drugs. Investigation of the properties of water absorption to DNA and DNAdrug complexes had led to the conclusion that the energy of interaction between
the water molecules and the complexes depends on the length of side chains of
synthetic phenoxazone drugs ActII-ActV. The hydrated environment makes significant contribution to stabilization of double-helical structure of either free DNA and
of its complexes with the drugs. Increase of relative humidity of the films in the
range of 0 to 90 % leads to increase in the intensity of the IR-absorption bands for
the sugar-phosphate backbone vibration, in- and out-ring groups of DNA base pairs
in the drug-DNA complexes, and also of the absorption bands of C = O, C = N
2 Structure, Thermodynamics and Energetics of Drug-DNA Interactions