Nuclear magnetic resonance studies on the solution conformation of histone IV fragments obtained by cyanogen bromide cleavage.

Two histone IV fragments obtained by cleavage at Met-84 by cyanogen bromide have been examined by proton magnetic resonance (PMR) spectroscopy as a function of temperature, peptide concentration, ionic strength, and pD. Sedimentation and gel electrophoresis studies on these peptides have also been carried out. The 220-MHz PMR spectrum of the N-peptide in both the high- and low-field regions was shown to be almost identical with that calculated for an extended coil N-peptide monomer. The calculated random coil and experimental C-peptide spectra, on the other hand, differ in many respects. Evidence was obtained for the presence of rigid secondary structure in the C-peptide. In addition, the Val, Leu, Ile CH3 resonance displays a prominent high-field satellite band which shifts downfield with increasing temperature. Sedimentation studies on the N-peptide reveal the formation of extremely large, remarkably homogeneous aggregates at ionic strengths larger than or equal to 0.01. The C-peptide, on the other hand, does not appear to form aggregates of sufficient size to be detectable in velocity sedimentation studies of a few hours duration. The relative area changes which have previously been noted in the PMR spectrum of histone IV with increasing ionic strength were also observed for the N-peptide but not the C-peptide. Interpretation of these relative area changes has been made in terms of the amino acid sequence of histone IV, and an effort was made to identify that segment of the polypeptide which undergoes secondary structural change with increasing ionic strength.