Large-Scale Structural Characterization of Biopolymer Systems by Small-Angle Neutron Scattering

Biopolymers are polymer structures produced by living organisms. These molecules, like synthetic polymers, are built from monomers. There are three main classes of biopolymers based on the chemical nature of themonomeric units and the structure of the biopolymer formed. Polypeptides are chains of amino acids linked together by peptide bonds. Proteins consist of one ormore polypeptide chains folded into a globular or fibrous form in a biologically functional way. Many proteins have structural or mechanical functions, such as actin in the muscle and various proteins in the cytoskeleton, which form a system of scaffolding that maintains cell shape. The building blocks of polynucleic acids are nucleotide monomers. DNA and RNA are examples of polynucleotides. Long polynucleotide chains are wound around each other in the form of a double helix. Double-helical DNA plays an essential role in gene regulation. Polysaccharides are carbohydrate structures that have diverse biological functions: they serve as either structural components (e.g., in the extracellular matrix) or energy storage molecules. They are often covalently attached to proteins to form proteoglycans. The chemical composition and the sequence of the monomeric units define the primary structure of the biopolymer. Many biopolymers (e.g., proteins) spontaneously fold into compact shapes as well as form higher order structures, which determine their biological functions and depend in a complicated way on their primary structures. The diversity in the geometry of biopolymer systems at the level of the individual molecule and molecular assemblies is remarkable. Understanding these functions and roles can help to not only elucidate the complexity of biological machinery but also form the basis for development of new therapeutic approaches. Extensive efforts have been devoted to separating the basic monomers as well as more complex structures. In many instances, biopolymer assemblies, some very similar to those observed in nature, can be reconstructed in solution (in vivo), allowing researchers to investigate the structure, dynamics, biological properties, and functions under well-controlled experimental conditions.