National Synchrotron Light Source Seminar

Biomolecular nanoparticles (BNPs), such as proteins and viruses, bound to a lipid monolayer are well suited for investigating interface-directed assembly of nanoscale building blocks. Using the air-water interface as the assembly platform, we have recently demonstrated density-driven 2D crystallization of BNPs for two types of BNP-lipid interactions, one based on electrostatic interactions (type I) and the other based on specific ligand binding (type II). For type I, the assembly of icosahedral viruses (CPMV and TYMV) on a cationic lipid monolayer was studied as a function of pH and the monolayer charge density. In-situ grazing-incidence small-angle x-ray scattering (GISAXS) measurements show that 2D crystals of these virus particles are formed above a threshold monolayer charge density and only in a narrow pH range just above the virus' isoelectric point, where the net charge on the virus is weakly negative. For CPMV, the observed 2D crystal structure is analogous to the densest lattice plane within the known 3D crystals. By contrast, for TYMV, two new forms of 2D crystal structures have been found that are distinct from any lattice planes in the previously observed 2D or 3D assemblies. The roles of particle shape and patchiness, interface-induced particle orientation, and competing anisotropic inter-particle interactions in stabilizing these structures will be discussed. For type II, the 2D assembly of the protein streptavidin (SA) on a biotin-bearing lipid monolayer was studied as a function of the surface density of biotin, a protein-binding ligand. In-situ Brewster-angle microscopy, GISAXS, and x-ray reflectivity measurements were carried out to elucidate the relationship between the adsorption, 2D phase behavior, and binding state of SA. In particular, the threshold biotin density for inducing the 2D crystallization is found to be remarkably close to the density of the ligand-binding sites in the SA crystal. Moreover, the fully bound state of SA, correspo