Differential dissociation of histone tails from core chromatin.

The dissociation of the trypsin-sensitive basic tails of the core histones in core chromatin has been followed as a function of [NaCl] using proton NMR spectroscopy. The tails dissociate in a highly cooperative all or none manner over the salt concentration range 0.2-0.6 M, that is, below the salt concentration required to dissociate the complete molecule. Assuming that each basic tail dissociates independently, the total number of salt linkages involved in binding the tails to DNA is 103. This equals the number of basic side chains in the tails of an octamer. The standard free energy of dissociation, delta G degree, in 1 M NaCl at 297 K is 3.6 kcal/mol. Temperature had no effect on the extent of dissociation up to 45 degrees C. However, between 45 and 65 degrees C, where the premelting transition in the core chromatin occurs, the tails dissociated completely. Dissociation of the tails was associated with a conformational transition in the DNA consistent with loss of supercoiling. From this, and the results of a previous study, it can be shown that the structured, trypsin-resistant domain of each core histone octamer makes 100 salt linkages to DNA. Thus, in 10 mM salt, each core octamer makes a total of 203 salt linkages to DNA.