In 2011, four U.S. stamps were issued celebrating American scientists, including Asa Gray, one of America's earliest professional botanists. Three Nobel Prize winners were also shown on a background representing their accomplishment. Melvin Calvin, chemist, studied photosynthesis and also conducted pioneering research on the use of plants as an alternative energy source. Maria Goeppert Mayer developed a theoretical model of the structure of the atomic nucleus. She became the second woman to win a Nobel Prize in Physics. Severo Ochoa, biochemist, worked in protein synthesis, and was the first to synthesize ribonucleic acid (RNA).«