Conformation and folding in histones H1 and H5

Denatured histones H1 and H5 can be readily refolded on salt addition. Their digestion by trypsin leads to limit peptides of about 80 residues having the same nmr and CD spectra as those of the intact parent histones. Scanning microcalorimetry shows that (1) the folded structures of H1 and H5 are located entirely in their limit peptides; (2) both have values of the specific denaturation enthalpy typical for small globular proteins; and that (3) both exhibit a classic “2-state” transition (ΔH = ΔH). The heat-denaturation profiles of H5 measured using intrinsic and extrinsic Cotton effect and side-chain nmr peaks do not coincide at all. Only the intrinsic Cotton effects give a T_m and ΔH close to that from microcalorimetry. We conclude that these proteins exhibit large-scale side-chain motions that precede the macroscopic cooperative transition.