Frances Arnold named 2025 Priestley Medalist

Frances H. Arnold will receive the 2025 Priestley Medal, the American Chemical Society’s highest honor. Arnold, the Linus Pauling Professor of Chemical Engineering, Bioengineering, and Biochemistry at the California Institute of Technology, is being recognized for her contributions to the development of directed evolution. Arnold has spearheaded this method of chemical and biological design, which is now used in laboratories around the world to create enzymes that can churn out valuable molecules and accomplish chemistry that was once thought possible only with the use of purely chemical methods.

“Enzymes are by far the best chemists. The former Priestley Medal winners are pretty impressive, but nature is the best,” Arnold says. “And that’s what I want to do with the rest of my career—continue to open people’s eyes to the power of enzymes in solving chemical problems.”

Arnold has been persuasive. In addition to many academic achievements, she has founded three companies based on directed evolution: Gevo, which makes biofuel and other chemicals; Provivi, which specializes in nontoxic, pheromone-based pest control; and Aralez Bio, which produces unnatural amino acid building blocks for therapeutics.

Chaitan Khosla, a biochemist at Stanford University, says that before Arnold developed directed evolution, designing enzymes for a specific purpose was an elusive goal. “I think it’s fair to say that today, we’re in a different place when it comes to engineering enzymes to do practical, useful things,” he says. “Nobody has had a bigger impact on getting us from there to where we are today than Frances Arnold.”

“Her work has had both important fundamental consequences and really dramatic implications for how people practice the chemical sciences, both in universities and in industry,” says David Tirrell, a colleague of Arnold at Caltech. Tirrell points to her work coaxing enzymes into catalyzing reactions that they don’t ordinarily catalyze, such as making or breaking carbon-silicon bonds. “She’s really shown how it’s possible to catalyze reactions that we just don’t associate with enzyme catalysis,” he says.

In addition to her scientific accomplishments, Arnold has been a leader outside the lab, serving as cochair of the President’s Council of Advisors on Science and Technology since 2021. She has also been praised for her mentorship. Tina Boville, who was a postdoctoral fellow in Arnold’s lab and cofounded Aralez Bio with her, says in an email that Arnold is “incredibly supportive and encourages her students to take on challenging chemistry problems. . . . I am deeply grateful for her mentorship.”

The Priestley Medal adds to a long list of awards Arnold has received, including the 2011 Charles Stark Draper Prize—the highest award given by the US National Academy of Engineering—and the 2018 Nobel Prize in Chemistry.

In the past, the Priestley Medal has not come with a monetary prize. But 2025 will mark the first year that it comes with a research grant, which was made possible by the J. Lynn Fordham Fund for Major Advances in Chemistry Endowment, which was established by the late J. Lynn Fordham, a chemist and businessperson. The amount of this year’s award was not disclosed.

For her future research, Arnold says, “I’d like to show that all that chemistry that we take for granted has to be done using dirty human chemistry can be done by biology—cleanly, efficiently, and sustainably.”