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Two amphiphilic calix[4]arenes bearing four dodecyl chains at the lower rim and two amino functions (vicinal and distal) at the para-phenolic positions have been synthesized. Surface-pressure versus molecular-area isotherms reveal that Langmuir monolayers of the two regioisomers show considerably distinct self-assembly behaviors at the air–water interface. Compression isotherms, Brewster angle microscopy and synchrotron-based X-ray near-total-reflection fluorescence, X-ray reflectivity and grazing incidence X-ray diffraction reveal that the monolayers of the two diamino calix[4]arene derivatives and those of their structural analogues bearing four amino moieties in para positions exhibit significant differences in their binding properties towards HgCl2 despite the structural and functional similarity among the macrocycles.

Phosphatidylinositol-4,5-bisphosphate (PIP2) is an important signaling phospholipid in the inner leaflet of the cell membrane. Due to the high negative charge of its headgroup, PIP2 strongly interacts with cellular cations. We have used synchrotron diffraction and fluorescence techniques to determine preferential cation binding to two-dimensional PIP2 templates. The natural, highly unsaturated PIP2 is manipulated as a Langmuir monolayer on a physiological buffer containing 100 mM KCl and varying amounts of Ca2+ and Mg2+. X-ray fluorescence shows an 800% surface enhancement in K+ concentration (bound to PIP2) compared to bulk concentration. Adding physiological levels of Ca2+( 1–100 μM) results in gradual replacement of K+ by Ca2+ ions, leading to a significant change in the organization of the PIP2 model membrane, while higher concentrations (100–1000 μM) lead to three orders of magnitude increase in surface [Ca2+]. Similar experiments with Mg2+ ions also show strong ion binding to PIP2 at physiological levels (1 mM) with a lesser structural effect. For mixed solutions of Mg2+ and Ca2+ we find that Ca2+ occupies the majority of binding sites. Remarkably we find that, with both 1 mM Mg2+ and 1 mM Ca2+ in the subphase there is still a 400% surface enrichment of K+ ions at the headgroup region.

Employing synchrotron X-ray scattering, we systematically determine the absorption near-edge spectra (XANES) of iron in its ferrous (Fe(2+)) and ferric (Fe(3+)) states both as ions in aqueous solutions and as they bind to form a single layer to anionic templates that consist of carboxyl or phosphate groups at aqueous/vapor interfaces. While the XANES of bulk iron ions show that the electronic state and coordination of iron complexes in the bulk are isotropic, the interfacial bound ions show a signature of a broken inversion-symmetry environment. The XANES of Fe(2+) and Fe(3+) in the bulk possess distinct profiles however, upon binding they practically exhibit similar patterns. This indicates that both bound ions settle into a stable electronic and coordination configuration with an effective fractional valence (for example, Fe([2+ν]+), 0ν1) at charged organic templates. Such two dimensional properties may render interfacial iron, abundant in living organisms, a more efficient and versatile catalytic behavior.

We investigate two-dimensional (2D) assembly of the icosahedral turnip yellow mosaic virus (TYMV) under cationic lipid monolayers at the aqueous solution–vapor interface. The 2D crystallization of TYMV has been achieved by enhancing electrostatically induced interfacial adsorption, an approach recently demonstrated for another virus. In situ X-ray scattering reveals two close-packed 2D crystalline phases of TYMV that are distinct from the previously reported hexagonal and centered square (√2 × √2) arrays of TYMV. One of the newly observed phases arises from either a dimeric double-square (2 × 1) or tetrameric square (2 × 2) unit cell. The other is a rhombic crystal with a lattice angle of 80°. The two observed crystal phases are substantially less dense (by over 10%) than a 2D lattice of TYMV could be according to its known size and shape, indicating that local anisotropic interparticle interactions play a key role in stabilizing these crystals. TYMV's anisotropy attributes and numerical analysis of 2D arrays of virus-shaped particles are used to derive a model for the rhombic crystal in which the particle orientation is consistent with the electrostatic lipid–TYMV attraction and the interparticle contacts exhibit steric complementarity. The interplay between particle anisotropy and packing is contrasted between the rhombic crystal model and the square (√2 × √2) crystal. This study highlights how the high symmetry and subtle asphericity of icosahedral particles enrich the variety and complexity of ordered 2D structures that can be generated through self-assembly.