The number of charge-charge interactions stabilizing the ends of nucleosome DNA

It has been shown by others that the melting of DNA in the nucleosome core particle is biphasic (ref. 1) and that the initial denaturation phase is due to melting of the DNA termini (refs. 1 & 2). We analyze the salt dependence of the melting temperature of this first transition and estimate that only 15% of the phosphates of the DNA termini are involved in intimate charge-charge interactions with histones. (The simplest model yields approximately 9%, whereas a calculated overestimate yields approximately 21% neutralization.) This is a surprisingly small number of interactions but we suggest that it may nonetheless be representative of all the core particle DNA.