Microcalorimetry was used to measure the change in enthalpy for the scission of calf thymus DNA (ct-DNA) induced by adriamycin (ADM) in the presence of ferric ions, Vitamin C, and oxygen. At 298.15K and pH 7.4, the overall molar reaction enthalpy for this cleavage was-147.1kJ/mol, noticeably higher than that by the mixture of Fe3á, Vitamin C, and O2. Under the same conditions, the enthalpy change for the damage of ct-DNA by the mixture of adriamycin, ferrous ions, and oxygen, however, was nearly zero, indicating that this mixture can not induce any detectable degradation of DNA. These esults suggest that both the activated adriamycin and hydroxyl radical attack DNA strands during the cleavage. A possible mechanism for the cleavage of DNA induced by adriamycin is proposed based on the calorimetric measurements. A novel thermodynamic model for the interactions of DNA with small molecules is also suggested. This is a convenient method tocalculate both the binding constant (Kb) and the standard thermodynamic parameters (ΔbH0m, ΔbG0m, and ΔbS0m) for the binding of adriamycin-Fe3á complex to ct-DNA by the calorimetric data. This nucleotide binding reaction is driven by a favorable enthalpy change, with a large unfavorable entropy change. This result indicates that the binding results in structural changes accompanied by an increase in the order of the whole system, implying that an intercalation mode is involved in adriamycinmediated breakage of DNA.

Aluminum is a known neurotoxic agent and its neurotoxic effects may be due to its binding to DNA. However, the mechanism for the interaction of aluminum ions with DNA is not well understood. Here, we report the application of isothermal titration calorimetry (ITC), fluorescence spectroscopy, and UV spectroscopy to investigate the thermodynamics of the binding of aluminum ions to calf thymus DNA (CT DNA) under various pH and temperature conditions. The binding reaction is driven entirely by a large favorable entropy increase but with an unfavorable enthalpy increase in the pH range of 3.5-5.5 and at all temperatures examined. Aluminum ions show a strong and pH-dependent binding affinity to CT DNA, and a large positive molar heat capacity change for the binding, 1.57 kcal mol 1K 1, demonstrates the burial of the polar surface of CT DNA upon groove binding. The fluorescence of ethidium bromide bound to CT DNA is quenched by aluminum ions in a dynamic way. Both Stern-Volmer quenching constant and the binding constant increase with the increase of the pH values, reaching a maximum at pH 4.5, and decline with further increasing the pH to 5.5. At pH 6.0 and 7.0, aluminum ions precipitate CT DNA completely and no binding of aluminum ions to CT DNA is observed by ITC. Combining the results from these three methods, we conclude that aluminum ions bind to CT DNA with high affinity through groove binding under aluminum toxicity pH conditions and precipitate CT DNA under physiological conditions.

Electrospray ionization mass spectrometry, isothermal titration calorimetry (ITC), fluorescence spectroscopy, and glutaraldehyde cross-linking SDS-PAGE have been used to study the unfolding of rabbit muscle creatine kinase (MM-CK) induced by acid. The mass spectrometric experiments show that MM-CK is unfolded gradually when titrated with acid. MM-CK is a dimer (the native state) at pH 7.0 and becomes an equilibrium mixture of the dimer and a partially folded monomer (the intermediate) between pH 6.7 and 5.0. The dimeric protein becomes an equilibrium mixture of the intermediate and an unfolded monomer (the unfolded state) between pH 5.0 and 3.0 and is almost fully unfolded at pH 3.0 reached. The results from a "phase diagram" method of fluorescence show that the onformational transition between the native state and the intermediate of MM-CK occurs in the pH range of 7.0-5.2, and the transition between the intermediate and the unfolded state of the protein occurs between pH 5.2 and 3.0. The intrinsic molar enthalpy changes for formation of the unfolded state of MM-CK induced by acid at 15.0, 25.0, 30.0, and 37.0 ℃ have been determined by ITC. A large positive molar heat capacity change of the unfolding, 8.78 kcal mol 1K 1, at all temperatures examined indicates that hydrophobic interaction is the dominant driving force stabilizing the native structure of MM-CK. Combining the results from these four methods, we conclude that the acid-induced unfolding of MM-CK follows a "threestate" model and that the intermediate state of the protein is a partially folded monomer.

Microcalorimetry and UV-vis spectroscopy were used to conduct thermodynamic and kinetic investigations of the scission of calf thymus DNA catalyzed by bleomycin A5 (BLM-A5) in the presence of ferrousion and oxygen. The molar reaction enthalpy for the cleavage, the Michaelis-Menten constant for calf thymus DNA and the turnover number of BLM-A5 were calculated by a novel thermokinetic method for an nzyme-catalyzed reaction to be -577