Vortex beams of atoms and molecules.

Angular momentum plays a central role in quantum mechanics, recurring in every length scale from the microscopic interactions of light and matter to the macroscopic behavior of superfluids. Vortex beams, carrying intrinsic orbital angular momentum (OAM), are now regularly generated with elementary particles such as photons and electrons. Thus far, the creation of a vortex beam of a nonelementary particle has never been demonstrated experimentally. We present vortex beams of atoms and molecules, formed by diffracting supersonic beams of helium atoms and dimers off transmission gratings. This method is general and could be applied to most atomic and molecular gases. Our results may open new frontiers in atomic physics, using the additional degree of freedom of OAM to probe collisions and alter fundamental interactions.

Deciphering the Mechanical Properties of Type III Secretion System EspA Protein by Single Molecule Force Spectroscopy.

Bacterial pathogens inject virulence factors into host cells during bacterial infections using type III secretion systems. In enteropathogenic Escherichia coli, this system contains an external filament, formed by a self-oligomerizing protein called E. coli secreted protein A (EspA). The EspA filament penetrates the thick viscous mucus layer to facilitate the attachment of the bacteria to the gut-epithelium. To do that, the EspA filament requires noteworthy mechanical endurance considering the mechanical shear stresses found within the intestinal tract. To date, the mechanical properties of the EspA filament and the structural and biophysical knowledge of monomeric EspA are very limited, mostly due to the strong tendency of the protein to self-oligomerize. To overcome this limitation, we employed a single molecule force spectroscopy (SMFS) technique and studied the mechanical properties of EspA. Force extension dynamic of (I91)4-EspA-(I91)4 chimera revealed two structural unfolding events occurring at low forces during EspA unfolding, thus indicating no unique mechanical stability of the monomeric protein. SMFS examination of purified monomeric EspA protein, treated by a gradually refolding protocol, exhibited similar mechanical properties as the EspA protein within the (I91)4-EspA-(I91)4 chimera. Overall, our results suggest that the mechanical integrity of the EspA filament likely originates from the interactions between EspA monomers and not from the strength of an individual monomer.