Authors:M. Akaboshi, Y. Tanaka, T. Sumino, J. Takada, and K. Kawai
The literature indicates that the interaction of Tb3+ with DNA modified by the antitumour drug cis-diaminedichloroplatinum(II) (CDDP) results in substantial enhancement of the
fluorescence of this cation, while no enhancement is observed in the case of DNA modified by irradiation with ionizing radiation.
This study investigates the effect of Tb3+ on the survival of cultured mammalian cells treated with CDDP. HeLa cells were treated with a combination of195mPt-CDDP and TbCl3, and the relationship between lethal effect and the numbers of Tb and/or Pt atoms binding to DNA, RNA and proteins was examined.
The Tb content in each fraction was determined using instrumental neutron activation analysis. It was found that the cytotoxic
effect of CDDP was greatly enhanced by the presence of Tb ions (D0 of CDDP fell from 8.3 μM without Tb to 3.2 μM with 0.75 mM Tb), while no such effect was found in radiation-induced cell-killing.
The number of Tb atoms bound to DNA molecules in a cell was calculated to be about 4.5·107, namely 1 per 1.400 nucleotides, under that situation.
The molar enthalpies of the solid–solid and solid–liquid phase transitions were determined by differential scanning calorimetry
for pure TbCl3 and KTb2Cl7, RbTb2Cl7, CsTb2Cl7, K3TbCl6, Rb3TbCl6 and Cs3TbCl6 compounds. Both types of compounds, i.e. M3TbCl6 and MTb2Cl7 (M=K, Rb, Cs) melt congruently and show additionally a solid–solid phase transition with a corresponding enthalpy ΔtrsH0 of 6.1, 7.6 and 7.0 kJ mol–1 for potassium, rubidium and caesium M3TbCl6 compounds andΔtrsH0 of 17.1 (rubidium) and of 12.1 and 10.9 kJ mol–1 (caesium) for MTb2Cl7 compounds, respectively. The enthalpies of fusion were measured for all the above compounds with the exception of Rb3TbCl6 and Cs3TbCl6. The heat capacities of the solid and liquid compounds have been determined by differential scanning calorimetry (DSC) in
the temperature range 300–1100 K. The experimental heat capacity strongly increases in the vicinity of a phase transition,
but varies smoothly in the temperature ranges excluding these transformations. Cp data were fitted by an equation, which provided a satisfactory representation up to the temperatures of Cp discontinuity. The measured heat capacities were checked for consistency by calculating the enthalpy of formation of the
liquid phase, which had been previously measured. The results obtained agreed satisfactorily with these experimental data.