Bioinorganic chemistry
Paul Ehrlich used organoarsenic (“arsenicals”) for the treatment of syphilis. This demonstrated the relevance of metals, or at least metalloids, to medicine. Then Rosenberg discovered the anti-cancer activity of cisplatin (cis-PtCl2(NH3)2). The first protein ever crystallized  was urease. This has nickel at its active site.  Vitamin B12, the cure for pernicious anemia, was shown by crystallography by Dorothy Hodgkin to have a cobalt atom in a corrin macrocycle. The Watson-Crick structure for DNA demonstrated the key structural role played by phosphate-containing polymers.