An X-ray diffraction study of poly-L-arginine hydrochloride

An x-ray study has been made of polyarginine hydrochloride to investigate whether, like polylysine hydrochloride, it can undergo conformational changes merely from variations in the degree of hydration. X-ray powder and fiber photographs of specimens containing up to about five molecules of water per arginine residue show features characteristic of α-helical structures including a 5.4-Å layer line and a meridional 1.5-Å reflection. Increasing the water content from ¹/₂ to 6¹/₂ molecules per residue causes the a axis of the hexagonal unit cell to increase from 14.4 Å to 15.8 Å, with no appreciable change in the 27.0 Å c axis. Removal of the last half molecule of water results in a very diffuse α pattern, but on rehydration the sharp pattern reappears. Specimens containing five to twenty water molecules per residue show quite a different pattern, the spacing of which do not vary appreciably with hydration. This pattern includes a meridional 3.4-Å reflection, a feature commonly shown by β structures, and indeed all the reflections can be satisfactorily indexed in terms of a monoclinic unit cell with a = 9.26 Å, b = 22.05 Å, c = 6.76 Å, and γ = 108.9°. These dimensions are shown by models to be compatible with a β pleated-sheet structure.