An X-ray and Crystallographic Study of Ribonuclease

A new crystalline protein of low molecular weight, ribonuclease, has been isolated and described by Kunitz (1, 2). A preliminary study of this protein has now been made with x-rays and with the polarizing microscope. The material studied was crystaUized from an ethanol-water solution. The crystals were dried in air without appreciable deterioration and the measurements here described were made with these air-dried crystals. They are long, thin needles, orthorhombic , elongated along the "c" axis. The prism faces are (110) and (1i0) and include an angle of 70 °. The other two axes bisect the angles of the cross section, "a" bisecting the obtuse angle and "b" the acute. The extinction as observed in the polarizing microscope is, of course, straight; a is along a, ~ along b, and "y along c. The crystal is positive and the optic axial angle is about 65 ° measured in air and 74 ° in glycerine. X-ray oscillation films were taken about all three crystallographic axes. These give the following values for the unit cell; a = 36.6 A, b = 40.5A, and c = 52.3 A. The space group appears to be P212z2,, 4 molecules per unit cell, each molecule without symmetry in a general position. The density of the air-dried crystals was measured to be 1.341 40.002, by floating them in a mixture of methylene chloride and carbon tetrachloride. The molecular weight has been computed from these data assuming 4 molecules per unit cell. The cell volume is 77,300 A 3 and this gives a molecular weight of 15,700 ± 300. This value is an upper limit as no correction has been made for solvent of crystallization. I