Infra-red dichroism and chain orientation in crystalline ribonuclease

Dichroism has been observed in the infra-red spectrum of a single crystal of ribonuclease. The dichroism suggests that the crystal contains folded polypeptide chains whose direction is mainly along or near that of the c axis, in agreement with conclusions arrived at by Carlisle & Scouloudi from consideration of the X-ray diffraction pattern. A band ascribed to the N$\chembond{1,0} $H vibration of the amide groups in the side chains has been found to be dichroic, and this appears to show that the plane of the NH$_{2}$ group in the side chain amides is oriented parallel to the b axis of the crystal.