Hans Freeman is largely known for his numerous contributions to the structures and chemistry of transition metal complexes of amino acids and peptides. He is regarded as a world authority in this area and his work is widely quoted and used. This background has culminated more recently in the solution of the structure of the Copper protein plastocyanin, one of the proteins in the photosynthetic system. The structural analysis of the cupric form was a decisive experiment and it resolved some spectroscopic and redox anomalies of the entire class of "blue" copper proteins. The subsequent refinement at high resolution and the pH dependence of the structure of the reduced form have provided an important insight into the function of plastocyanin in photosynthesis.