Solid Phase Coexistence in Chiral Domains of Two-Dimensional Streptavidin Crystals

Growth of two-dimensional streptavidin crystals at the air−water interface has been used to study protein molecular interactions at various pH values. Between pH 5 and 6, several unique crystal domain shapes are visible with fluorescence microscopy. Characterization of a chiral domain with transmission electron microscopy reveals a composition of two coexisting crystal types. The bulk crystal, one previously seen at pH 4 (space group P 1) (Hemming et al . J . Mol . Biol . 1995, 246, 308), has interspersed within it at approximately 5% aerial extent a new crystal type with unit cell parameters a = 116 Å, b = 58 Å, and γ = 107° (space group P 2). This new form exists in the chiral domains as long narrow crystals and is characterized by relatively weak and anisotropic molecular interactions. The packing arrangement for the new P 2 crystal at pH 5−6 exhibits characteristics of the pH 4 crystal and of the C 222 crystal (Darst et al . Biophys . J . 1991, 59, 387) obtained at pH 7.