The phase behavior and aggregate morphology of mixtures of the oppositely charged surfactants cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS) are explored with cryotransmission electron microscopy, quasielastic light scattering, and surface tensiometry. Differences in the lengths of the two hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and precipitate phases), and vesicles form spontaneously over a wide range of compositions in both CTAB-rich and SOS-rich solutions. Bilayer properties of the vesicles depend on the ratio of CTAB to SOS, with CTAB-rich bilayers stiffer than SOS-rich ones. We observe two modes of microstructural transition between micelles and vesicles. The first transition, between rodlike micelles and vesicles, is first order, and so there is macroscopic phase separation. This transition occurs in CTAB-rich solutions and in SOS-rich solutions at higher surfactant concentrations. In the second transition mode, mixtures rich in SOS at low surfactant concentrations exhibit no phase separation. Instead, small micelles abruptly transform into vesicles over a narrow range of surfactant concentration. Since the vesicles that form in mixtures of oppositely charged surfactants are equilibrium microstructures, the microstructural evolution is related solely to the phase transition and is thus under thermodynamic control. This differs from experiments reported on the dissolution of metastable vesicles, such as the detergent solubilization of biological phospholipid membranes, which may be controlled by kinetics. Despite these differences, we find that the evolution in microstructure in our mixtures of oppositely charged surfactants is analogous to that reported for biological membrane solubilization.

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 P1) (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 P2). 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 P2 crystal at pH 5-6 exhibits characteristics of the pH 4 crystal and of the C222 crystal (Darst et al. Biophys. J. 1991, 59, 387) obtained at pH 7.

To obtain a general understanding of the effect of intermolecular interactions 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 interface, in the presence of the noncrystallizable impurity avidin, exhibit crystallographic and morphological changes as a function of subphase pH. Large twodimensional crystalline arrays form within minutes across a pH range from 1.5 to 11. Crystals exhibit different pH-dependent 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 crystals nucleate rapidly and form thin needle-shaped crystals consistent with a strong growth anisotropy between the two crystallographic growth directions. C222 crystals grow more isotropically and exhibit H-and X-shapes. The nucleation rates and aspect ratios of C222 crystals are also pH-dependent, both properties increasing with increasing pH. The transition 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, new C222 crystals form, but no reconfiguration of existing crystals is observed.

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