INFLUENCE OF PH ON 2-DIMENSIONAL STREPTAVIDIN CRYSTALS

To obtain a general understanding of the effect of intermolecular inte ractions on the mechanisms of two-dimensional protein crystallization, we grow protein crystals and elicit a bulk molecular manipulation by changing system pH. Two-dimensional crystals of the bacterial protein streptavidin grown on a biotinylated lipid monolayer at an air-water i nterface, in the presence of the noncrystallizable impurity avidin, ex hibit crystallographic and morphological changes as a function of subp hase pH. Large two-dimensional crystalline arrays form within minutes across a pH range from 1.5 to 11. Crystals exhibit different pH-depend ent structures, lattices with P1 symmetry for 1.5 < pH < 5, P1 and P2 lattices for 5 < pH < 6, and C222 lattices for 7 < pH < 11. P1 crystal s nucleate rapidly and form thin needle-shaped crystals consistent wit h a strong growth anisotropy between the two crystallographic growth d irections. C222 crystals grow more isotropically and exhibit H- and X- shapes. The nucleation rates and aspect ratios of C222 crystals are al so pH-dependent, both properties increasing with increasing pH. The tr ansition from C222 to P1 or P2 crystals can be accomplished in minutes by lowering the system pH. The reverse transition, however, does not occur subsequent to a corresponding increase in system pH. Instead, ne w C222 crystals form, but no reconfiguration of existing crystals is o bserved.