Rich and Crick solved the structure of collagen, the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content. Collagen consists of amino acids bound together to form a triple helix of elongated fibril known as a collagen helix. 
See also:
Rich, Alexander - Crick, Francis H.C. The Structure of Collagen. 1957. Offprint from "Recent Advances in Gelatin and Glue Research" (Pergamon Press: London, 1957), the Proceedings of a Conference sponsored by the British Gelatine and Glue Research Association and held at the University of Cambridge, 1-5 July 1957, pp. 20-24.