SAXS Investigation of Hierarchical Structures in Biological Materials

Biological materials are distinguished by their hierarchical structures in which the organization of the basic building blocks is precisely controlled on many discrete length scales. This biophysical organization, i.e., the structure, along with the biochemical attributes, dictates their properties and function. This article is a review, and also a tutorial, that describes the use of small-angle X-ray scattering (SAXS) for determining the structures at the nanometer and sub-micron length scales in three distinct classes of scattering patterns that arise from fibrous structures, lamellae, and solutions. Fibrous structures are discussed using results from collagen, bone, hair, feathers, and silk. The use of SAXS to study the lamellar structures is illustrated using the results from myelin and membranes. SAXS in solutions is discussed by highlighting the results from multidomain proteins such as monoclonal antibodies and facile structures in intrinsically disordered proteins and protein condensates. The goal is to describe the different methods for analyzing the distinct classes of scattering patterns arising from 1- and 2-D ordered structures and from 3D structures in solutions and to illustrate how the structure imparts unique functions and properties to the biological materials. An understanding of the hierarchical structures in biology is expected to be useful in medical diagnosis and serve as a guide for fabricating functional biomaterials by mimicking these structures.