Long distance PELDOR measurements on the histone core particle

The nucleosome core particle is the fundamental unit of chromatin structure and at its heart is the histone core octamer composed of histones H4, H3, H2A and H2B. To understand the structure dynamics and function of chromatin it is important to be able to probe the structures of its component parts in a variety of ways. Site directed spin-labeling technology has enabled the insertion of nitroxide spin labels at positions on the surface of the H3 histones and these have been assembled into histone octamers. Pulsed EPR, and in particular the PELDOR or DEER experiments have been performed and provided extremely well defined dipolar oscillations, over long time periods. From the PELDOR data we have been able to derive distance distributions of between 60 and 70 A. The distances measured, are among the longest well-defined PELDOR measurements on a biological system to date, spanning the width of the histone core particle and approaching what has been often defined as the limit of distance measurement by this technique. Relatively minor differences to the crystal structures have been observed.