Liquid-like and solid-like motions in proteins

Recent analyses of molecular dynamics simulations of hydrated C-phycocyanin suggest that the internal single-particle dynamics of this protein can be decomposed into four almost decoupled motion types: (1) diffusion of residues ("beads") in an effective harmonic potential, (2) corresponding vibrations in a local potential well, (3) purely rotational rigid side-chain diffusion, and (4) residue deformations, Each residue bead is represented by the corresponding C-alpha carbon atom on the main chain. The effective harmonic residue potential can be imagined as the envelope of many local wells which are separated by small energy barriers. The residue friction matrix is assumed diagonal and the individual friction constants can be related to the density of the surrounding atoms. In this article we show that our model can be applied to lysozyme in solution as well, the only difference being that the side-chain deformations are more important and seem to be strongly correlated with the side-chain rotations. Comparing the simulated coherent scattering function of C-phycocyanin to a neutron spin-echo spectrum we show that our model can also describe collective motions in proteins at the residue level. (C) 2002 Elsevier Science B.V. All rights reserved.