Nelson Leonard, a world-acclaimed chemist who combined his synthetic prowess in the laboratory with a powerful bass baritone voice that made him a featured soloist with symphony orchestras throughout the Midwest, died Monday in Pasadena, Calif. He was 90.

Leonard spent nearly half a century at the University of Illinois at Champaign-Urbana before concluding his career at the California Institute of Technology.

Beginning in the 1960s, Leonard and his students synthesized a variety of analogues of chemicals that were known to play a role in the growth and development of plants. In a fruitful collaboration over the following 20 years, these were tested by plant physiologist Folke Skoog of the University of Wisconsin.

Several of the chemicals proved to be potent stimulators of plant-cell growth, division and differentiation, and they are now widely used in horticulture to initiate the growth of intact plants, flowers and trees from tissue culture.

A decade later, Leonard met Argentine chemist Gregorio Weber, who convinced him that fluorescent variants of the nucleotide components of DNA and RNA would be useful tools for research to show where in the cell particular chemicals were located. His laboratory synthesized many fluorescent chemicals that are now widely used in research, especially a variant of adenosine triphosphate (ATP), which plays a key role in nucleotide chemistry and in energy transfer within the cell.

Fluorescent chemicals showed scientists where chemicals were in the cell. Studies of their lifetime and polarization also showed how they were attached to an enzyme or structural protein.The fluorescent molecules made it possible to study cell structures directly.

Nelson Jordan Leonard was born Sept. 1, 1916, in Newark, N.J., and grew up in Mount Vernon, N.Y. He attributed his interest in chemistry to a chemistry set he had in childhood and to an inspirational high school chemistry teacher.

Leonard received his bachelor’s degree from Lehigh University in Bethlehem, Pa., then attended Lincoln College at Oxford University on a Rhodes scholarship.War forced him to return to America, and he finished his doctorate at Columbia University in 1942 and took a job at the University of Illinois.

During the war, he worked on the synthesis of the malaria drug chloroquine, completing it in time for the drug to be widely used in the Pacific campaign.

After the war, he took a temporary job overseas with the U.S. Army examining the research publications and reports of the German chemical company I.G. Farben Industrie. He and his colleagues found several processes that were adopted by American companies, including one technique to improve the manufacture of synthetic rubber.

When Leonard returned to Illinois, he resumed his singing career. A solo performer since the age of 10, he had wide experience as a church soloist, recitalist and oratorio singer. While at Oxford, he was the bass soloist of the Lincoln College Choir. After the war, he served as a soloist with the Chicago, St. Louis and Cleveland symphonies, receiving glowing reviews. Students at Illinois often recalled passing outside his office and hearing him singing alone.

But when Leonard was elected to the National Academy of Sciences in 1955, he decided that if his peers had elected him to the prestigious position, then he had “better do something about it.”

He retired from professional music, although he continued to sing in church and in performances on campus. He remained involved, serving on the board of the Pasadena Symphony.

“He was way ahead of his time scientifically, in the sense that he collaborated with biochemists and biologists long before interdisciplinary research was fashionable,” said Steven C. Zimmerman, head of the chemistry department at Illinois. “He was one of the first people in the country to use chemistry to understand biology.”