Center for Functional Nanomaterials Seminar

Biomolecular nanoparticles (BNPs), such as proteins and viruses, bound to a lipid monolayer are well suited for investigating 2D 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 specific ligand binding (type I) and the other based on electrostatic interactions (type II).
For type I, 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. Detailed in-situ x-ray scattering and optical microscopy measurements reveal that the threshold biotin density for inducing the 2D crystallization is remarkably close to the density of the ligand-binding sites in the SA crystal. Moreover, the fully bound state of SA, corresponding to two biotins per protein, is found to be achieved already below the threshold biotin density.
These results quantitatively support the longstanding, and yet hitherto untested, premise that both well-defined molecular orientation and high lateral packing density are essential to the 2D crystallization of proteins.
For type II, the 2D assembly of icosahedral viruses (CPMV and TYMV) on a cationic lipid monolayer was studied as a function of pH and the monolayer charge density. GISAXS data 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 of the previously observed 2D or 3D assemblies.