What is elastin; what is not.

Fibrous elastin is a biologic macromolecular construct for which there currently exists a wide disparity of descriptions. On the one hand is the view that elastin is an unambiguously random network of polypeptide chains best described functionally by analogy to rubber elasticity. On the other hand, elastin is viewed as being constructed of parallel aligned filaments that are due in large part to hydrophobic associations in an aqueous milieu and are comprised of describable, preferred conformations. One class of the conformations is elastomeric and gives rise to a proposed new mechanism of elasticity called the librational entropy mechanism of elasticity. While pertinent arguments of both perspectives are noted, this review presents the latter perspective. It begins with the century old delineation of two conditions of matter, colloids and crystalloids, making the point that biologic materials previously listed as colloidal (and as such considered to be without order) have one by one been described in terms of structures with beautiful regularities. Data on the primary structure of elastin and its cross-links are discussed as are electron microscopic studies on negatively stained fibrous elastin and coacervates of elastin peptides. It is demonstrated that conformational descriptions of repeating peptides of elastin can give rise to the filaments observed in the ultrastructural studies and to a three-component working model for a fundamental unit of elastin structure. It is argued that the dominant class of conformations in the three-component model are consistent with data on the thermodynamics of elasticity, on birefringence, and on chain mobility, which had previously been considered to be indicative only of random chains. The developing understandings of molecular conformation are shown to provide a basis with which to begin an understanding of the molecular pathology of elastin.