Different DNA structures bond differently, and their resistance to the denaturation process can be different than in the case of T4 bacteriophage DNA. Levine studied the chemical denaturation of six different types of DNA (including T4 bacteriophage) and showed that the stabilities of these different DNA molecules against denaturation in the presence of urea or pyridine increases with increasing guanine (G) plus cytosine (C) content of the DNA. This phenomenon can be explained by comparison of corresponding enthalpy changes.