Ballistic Diffusion in Polyaromatic Hydrocarbons on Graphite.

This work presents an experimental picture of molecular ballistic diffusion on a surface, a process that is difficult to pinpoint because it generally occurs on very short length scales. By combining neutron time-of-flight data with molecular dynamics simulations and density functional theory calculations, we provide a complete description of the ballistic translations and rotations of a polyaromatic hydrocarbon (PAH) adsorbed on the basal plane of graphite. Pyrene, CH, adsorbed on graphite is a unique system, where at relative surface coverages of about 10-20% its mean free path matches the experimentally accessible time/space scale of neutron time-of-flight spectroscopy (IN6 at the Institut Laue-Langevin).

Energy landscapes of human acetylcholinesterase and its Huperzine A-inhibited counterpart.

Enzymes are animated by a hierarchy of motions occurring on time scales that span more than 15 orders of magnitude from femtoseconds (10(-15) s) to several minutes. As a consequence, an enzyme is characterized by a large number of conformations, so-called conformational substates that interconvert via molecular motions. The energy landscapes of these macromolecules are very complex, and many conformations are separated by only small energy barriers.