RESUMO
A mechanism for grain growth and formation in the interstellar medium is proposed. In this mechanism, hydrogen molecules act as moderators. The process begins when they physisorb to the surface of the grain. Then, when a collision with a heavy atom occurs, the bonding energy is carried away by the evaporation of hydrogen molecules. Estimates are made of the number of hydrogen molecules bound to the surface of a grain that would be sufficient to facilitate this mechanism at 13 K for amorphous carbon and 8 K for a silicate grain.
RESUMO
A recent theory for the long time dynamics of flexible chain molecules is applied for the first time to a peptide of biological importance, the neurotransmitter met-enkephalin. The dynamics of met-enkephalin is considerably more complicated than that of the previously studied glycine oligomers; met-enkephalin contains the interesting motions of phenyl groups and of side chains relative to the backbone, motions that are present in general flexible peptides. The theory extends the generalized Rouse (GR) model used to study the dynamics of polymers by providing a systematic procedure for including the contributions from the memory function matrices neglected in the GR theory. The new method describes the dynamics by time correlation functions instead of individual trajectories. These correlation functions are analytically expressed in terms of a set of equilibrium averages and the eigenvalues and eigenfunctions of the diffusion operator. The predictions of the theory are compared with Brownian dynamics (BD) simulations, so that both theory and simulation use identical potential functions and solvent models. The theory thus contains no adjustable parameters. Inclusion of the memory function contributions profoundly affects the dynamics. The theory produces very good agreement with the BD simulations for the global motions of met-enkephalin. It also correctly predicts the long-time relaxation rate for local motions.
