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 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 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 alpha -helical structures. Our results support the presence of inducible helical and turn elements, both sharing the character of DNA-binding motifs.