Induction of secondary structure in a COOH-terminal peptide of histone H1 by interaction with the DNA: An infrared spectroscopy study

We have studied the conformation of the peptide Ac-EPKRSVAFKKTKKEVKKVATPKK (CH-1), free in solution and bound to the DNA, by Fourier-transform infrared spectroscopy. The peptide belongs to the COOH-terminal domain of histone H1 0 (residues 99-121) and is adjacent to the central globular domain of the protein. In aqueous (D2O) solution the amide I′ is dominated by component bands at 1643 cm-1 and 1662 cm-1, which have been assigned to random coil conformations and turns, respectively. In accordance with previous NMR results, the latter component has been interpreted as arising in turn-like conformations in rapid equilibrium with unfolded states. The peptide becomes fully structured either in 90% trifluoroethanol (TFE) solution or upon interaction with the DNA. In these conditions, the contributions of turn (1662 cm-1) and random coil components virtually disappear. In TFE, the spectrum is dominated by the a-helical component (1654 cm-1). The band at 1662 cm-1 shifts to 1670 cm-1, and has been assigned to the COOH-terminal TPKK motif in a more stable turn conformation. A band at 1637 cm-1, also present in TFE, has been assigned to 310 helical structure. The amide I′ band of the complexes with the DNA retains the components that were attributed to 310 helix and the TPKK turn. In the complexes with the DNA, the a-helical component observed in TFE splits into two components at 1657 cm -1 and 1647 cm-1. Both components are inside the spectral region of α-helical structures. Our results support the presence of inducible helical and turn elements, both sharing the character of DNA-binding motifs.