An X‐Ray diffraction study of poly‐L‐ornithine hydrobromide

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Abstract
An X‐ray study has been made of the synthetic polypeptide poly‐L‐ornithine hydrobromide to investigate whether, like the chemically related polypeptides poly‐L‐lysine and poly‐L‐arginine hydrochlorides, it can undergo conformational changes merely from variations in its degree of hydration. X‐ray powder and fiber photographs of specimens with from half up to about three molecules of water per ornithine residue show features that suggest a “cross‐β‐pleated‐sheet” structure. Each pleated sheet is formed from parallel chains and the sheets are piled up along the b axis. The spacings, which do not vary appreciably with hydration, can be satisfactorily indexed in terms of an orthogonal unit cell with a = 4.60 Å, b = 30.2 Å, and c = 6.64 Å. These dimensions are shown by models to be compatible with the proposed structure. Removal of the last half molecule of water results in a very diffuse pattern but on rehydration the sharp pattern reappears. Specimens containing four to nine molecules of water per residue show a quite different pattern. Reflections other than equatorial are absent in oriented diagrams except for a 5.4 Å diffuse streak across the meridian which is suggestive of an α‐helical structure. Increasing the relative humidity from 86% to about 100% causes the a axis of the hexagonal unit cell to increase from 14.7 Å to 15.3 Å. On drying, the β structure reappears once again. These conformational changes are very similar to those observed in poly‐L‐lysine hydrochloride except that the latter shows a more stable α‐helical form. This difference may be explained in terms of stabilizing hydrophobic interactions between side chains, since ornithine has a shorter side chain than lysine.