Molecular Arrangement in Two-Dimensional Streptavidin Crystals

We studied the molecular arrangement of two-dimensional streptavidin crystals at the air−water interface over a range of pH values. We quantified the varying amounts of coexisting P 1, P 2, and C 222 crystals in the different morphologies observed at pH 4.5−6.5. Chiral, needlelike crystals at pH 4.5 consist of P 1 crystals with frequent line defects. Larger chiral domains near pH 5 are essentially all P 1 coexisting with a small amount of P 2, whereas at slightly higher pH values (near pH 5.5), H-shaped domains contain 4 times as much P 1 coexisting with a P 2/ C 222 mixture. Morphologies intermediate to these shapes exhibit intermediate compositions. Between pH ∼6−7, crystals all display a characteristic dendritic-X morphology, but arrangement at the molecular level is quite different compared with lower pH values. Crystals are mostly P 2 in symmetry near pH 6, but at pH 7 and above, crystals have C 222 symmetry. Coexistence of P 2 and C 222 crystals occurs at intermediate pH values. We determined the orientation and arrangement of streptavidin molecules in P 1, P 2, and C 222 crystals relative to the directions exhibiting faster growth. The direction of faster growth in P 1 crystals includes both interactions between biotin-free subunits and interactions between biotin-bound subunits. In the P 2 arrangement, growth in the direction of intermolecular biotin-free subunits is preferred, whereas growth is faster along the biotin-bound direction of C 222 crystals. We developed a model of the molecular arrangement for the observed solid-phase coexistence in these crystals.