Induction of Secondary Structure in a COOH-terminal Peptide of Histone H1 by Interaction with the DNA

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 (D 2 O) 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 α-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 3 10 helical structure. The amide I′ band of the complexes with the DNA retains the components that were attributed to 3 10 helix and the TPKK turn. In the complexes with the DNA, the α-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.