The structure of the histone dimer H2A-H2B studied by spectroscopy.

The spatial organization of the histone dimer (H2A-H2B) in 0.1-1.0 M NaCl is characterized by the inclusion of 38% of the residues in alpha-helical segments, an average fluorescence quantum yield of 0.085 +/- 0.003, a red shift of absorption (lambda max = 278 +/- 0.5 nm) and fluorescent spectra (lambda max = 304.4 +/- 0.3 nm) as compared to the respective spectra of free tyrosine. The changing of position lambda max of tyrosine fluorescence of histones during denaturation has been shown. The dimer (H2A-H2B) exhibited a conformational change in a transition centred at about 0.5 M NaCl. The dimer denaturation takes place at higher urea concentrations as the ionic strength of the medium increases. The quenching of tyrosine fluorescence of the histone dimer (H2A-H2B) was performed using the ions I-, Cs+ and acrylamide. It has been shown that, at a concentration of NaCl over 0.5 M, dimer compactization takes place, as well as the screening of some part of tyrosyls for te against the quenching effect of Cs+. Our experiments made it possible to identify three zones in the composition of the histone dimer (H2A-H2B) and determine the number (ni) and fluorescence quantum yields (qi) of tyrosyls included in the following specific zones: zone I, n1 = 2, q1 = 0.136; zone II, n2 = 3; q2 = 0.08; zone III, n3 = 3; q3 = 0.055.