Reactivity of individual functional groups of histones in calf thymus chromatin

Using the technique of competitive labelling (Kaplan, H., Stevenson, K.J. and Hartley, B.S. (1971) Biochem. J. 124, 289-299) with 1-fluoro-2,4-dinitro-[3,5-3H]benzene ([3H]Dnp-F), the chemical reactivity of the alanyl amino-terminus of H3, the prolyl amino-terminus of H2B and the sulphydryl groups of H3 within calf thymus chromatin were determined. Over pH values from 6 to 10, none of the groups had pH-reactivity profiles which fit titration curves, indicating that they are involved in specific interactions with other structural elements of chromatin. The alanyl amino-terminus is completely nonreactive over the entire pH range studied. The prolyl amino-terminus is reactive but its reactivity is considerably lower than expected for a secondary amino group. An unusual feature of this group is that its pH-reactivity profile has sharp discontinuities at pH values of 7.8 and 9. The average reactivity of the two sulphydryl groups of H3, unlike the two amino-termini, appears to approximate that expected of a free sulphydryl group.