The Two Crystalline Modifications of Insulin

Abstract

PROF. E. B. MATHEWS' first examination of Abel's crystalline insulin showed the presence of two types of insulin crystals¹. One of these, the so-called prismatic or needle variety, had marked birefringence and a development of faces strongly suggestive of rhombohedral symmetry, the crystals being elongated along the trigonal axis. Crystals of this type are commonly wedge-shaped, and very small wedge-shaped crystals have also been obtained by Scott² by crystallization of insulin from acetate buffers at pH. 5·2. The more common variety of insulin crystals are those obtained first by Abel³ from phosphate buffers at pH 6·2. These are very small flat rhombohedra appearing isotropic as usually viewed along the trigonal axis, but having actually positive birefringence. X-ray examination has here shown the presence of a simple rhombohedral unit cell⁴. It has been usual in the literature to describe these two forms—prismatic and rhombohedral—as polymorphic modifications⁵.