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 (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 , 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 ) and random coil components virtually disappear. In TFE, the spectrum is dominated by the -helical component (1654 cm ). The band at 1662 cm 1 shifts to 1670 cm , and has been assigned to the COOH-terminal TPKK motif in a more stable turn conformation. A band at 1637 cm , 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 -helical component observed in TFE splits into two components at 1657 cm 1 and 1647 cm . Both components are inside the spectral region of -helical structures. Our results support the presence of inducible helical and turn elements, both sharing the character of DNA-binding motifs.