Just like the electronic energy levels in the hydrogen atom, structural transitions in the nucleosome are characterized by the appropriate (free) energy scales, which sets a useful framework for understanding the nucleosome function. Further, these energy scales and the corresponding transitions can be modulated by external ``fields", which, in the case of the nucleosome, includes intra-cellular pH. Surprisingly, biologically relevant small variations of intra-cellular pH can have significant effect on stability of the nucleosome, which may lead to multiple biological consequences.