| Abstract: | The interaction between total histone and deoxyribonucleoprotein (DNP) preparations from calf thymus with mercury-containing nitroxyl radicals in low ionic strength solutions, 2 M NaCl and urea was investigated. It was found that the label is rapidly incorporated into the SH-groups of histone H3 to produce characteristic EPR signals. Titration of SH-groups within DNP demonstrated that in low ionic strength solutions only one SH-group (presumably, the SH-group of the cysteine residue in position 110) is accessible to the reagents. After dissociation by 2 M NaCl, two SH-groups become titrable; however, the EPR spectra point to differences in the conformational state of these two groups. In 4 M urea, these differences are compensated for by structural disintegration. The spin labels may be used for the analysis of SH-groups under different conditions and at different functional states of nucleoproteins. |
