Motions in Proteins

Abstract

The chapter discusses the motions in proteins. Gelation of gelatin or precipitation of casein with mild acid are examples of reversible changes in which structural motility of the protein chains or subunits is recognized as playing a part. The sometimes reversible spreading of monolayers derived from mondisperse proteins provides some of the first evidence for the true plasticity of protein molecules and paved the way for the elegant demonstration of reversible conformational transitions. The tracing of the passage of an unfolded polypeptide chain to the compact, native structure is a primary concern of protein chemistry. One segment of polypeptide chain adapts its backbone and sidechain conformation to the structure of another segment in the process of forming the compact, functional structure of a protein was made particularly vivid by experiments on the reassociation of cleaved fragments. Overall translational and rotational motion is observed with a protein in solution, apart from any intramolecular motions. These overall motions are those that remain if the protein molecule is treated as a rigid matrix. Rotational reorientation of intramolecular components involves rotations about individual interatomic bonds. Concerted motions in proteins involve the movement of groups of side chains or segments of backbone relative to other structures. Motions of interacting ions and small molecules produce fluctuations in protein structure in addition to Brownian transfer of momentum. Time scales of various types of motions are important both for experimental recognition of the time-dependent processes and for analyzing their importance in the functional molecular mechanics. The chapter also discusses the dominant methods of observation that provide information about frequencies and amplitudes of motions in proteins.