Cassidy Humphreys and David MacMillan having a conversation in front of a wall of papers

Senior thesis: Cassidy Humphreys creates a faster, cheaper — and greener — chemical reaction

Senior Cassidy Humphreys and David MacMillan chat in the MacMillan group's coffee room. On the wall behind Cassidy are current papers from the MacMillan lab.

One way to make drugs more affordable is to make them cheaper to produce. For her senior thesis research, Cassidy Humphreys, a chemistry concentrator with a passion for medicine, took on the challenge of taking a century-old formula at the core of many modern medications — and improving it.

Cassidy Humphreys smiles at the camera

Cassidy Humphreys

“It’s a matter of taking chemistry we heavily rely on in everyday applications and trying to make it easier, cheaper, more green,” she said. “My thesis is a twist on a classic chemical reaction, taking some ‘old chemistry’ that’s in a lot of medicines. Any way that it could be made more efficient, more effective or cheaper can make a real difference.”

Humphreys is working with David MacMillan, winner of the 2021 Nobel Prize in Chemistry and Princeton’s James S. McDonnell Distinguished University Professor of Chemistry.

“Our lab uses the energy of visible light to activate molecules in ways that they couldn’t be activated before,” said MacMillan. “Cassidy has taken two molecules that are typically completely stable, and using light, she’s now been able to activate them to make these really high-energy species, controlled in such a way that they can now react with each other.

“In layman’s terms, think of two things that are completely inert, like concrete and diamonds,” he continued. “Suddenly you think, ‘Wouldn’t it be great if somehow they could merge and create a new material? That would be really valuable.’ And she’s doing exactly that, taking two chemicals that are really inert, and making them react with each other and form something special.”

With her reaction, many medicines can be made much more cheaply than before. “I think that’s inherently why we’re excited about it,” said MacMillan. “It’s fundamental. It’s novel. It’s new. And it’s also a case where people in the outside world will be able to start utilizing it pretty quickly.”

High highs, low lows

The old chemistry dates to 1906, when Fritz Ullmann and his wife Irma Goldberg published a reaction for creating a carbon-nitrogen bond — a bond so vital to modern drug production that even though the reaction depends on high temperatures, long reaction times, harsh conditions and expensive materials, the Ullmann-Goldberg method is still in use today. Over the past 116 years, chemists have looked for various ways to improve it.

Dave, and other people in the organic chemistry world, came along and said, ‘What if we used greener chemistry?’” said Humphreys. “Dave won his Nobel Prize for organocatalysis, chemistry that uses carbon, nitrogen, oxygen — all harmless things that are in the air, all around us. They’re a lot safer than classic metal compounds like copper and zinc and palladium, which older chemistry routinely used.”

Humphreys focused on a starting material called a carboxylic acid, which is much cheaper than many other alternatives and allows for a much wider range of products in the end.

“If you start with a much simpler, cheaper, more stable material, that’s going to make everybody happy,” she said. “It’s going to make the pharmaceutical companies happy, and even just the people in our lab, because we can order it off the shelf instead of laboriously making the reagents (reaction ingredients).”

The problem: carboxylic acids typically need very high reaction temperatures, at hundreds of degrees. That’s where MacMillan’s photocatalysis, or light-driven chemistry, makes all the difference. “When you’re using light, there’s no heat needed,” Humphreys said. “You just put your reaction on a blue light for hours, and then the reaction happens the same way it would happen with heat.”

She makes it sound simple, but her journey was far from straightforward.

“When something was working, it was really exciting,” she said. “And then the next week we’d find out that it wasn’t working as well as we intended. It’s definitely been hills and valleys rather than a constant incline or decline. Lots of hills and lots of valleys. High highs, low lows.”

Cassidy Humphreys and Nate Dow in the MacMillan lab

Humphreys worked closely with graduate student Nate Dow, who suggested the reagent that unlocked the highest yield she achieved for her reaction.

‘The happiest day I had in the lab’

The key metric Humphreys used to track her journey was “yield,” a calculation of the effectiveness of a reaction. Theoretically, 100% of the starting materials will turn into the reaction products, but in practice, there’s always some amount that is left in the equipment or fails to react, or that gets interfered with by another product.

In the Hollywood version of her story, her yield would start low, then gain a percentage or two with each new experiment until a triumphal final scene with a nice high yield, maybe 80 or 90%. In reality, the yield hopscotched around: 40% one day, 0% the next, 32% the day after that.

In her first attempt, her yield was a disappointing 15%. It was more than the 0% that would mean the chemicals simply didn’t react, but nowhere near high enough to be useful. She spent months working out what specific aspects of her original conditions led to the low yield. Eventually she determined that she needed a new catalyst source and starting materials.

In January, she hit a turning point when the graduate student she worked most closely with, Nate Dow, suggested a new reagent for the second step of her two-step reaction. When they tried it, the yield for that step jumped from 20-30% to 80-83%.

“It was amazing! The yields just skyrocketed!” Humphreys said. “That was probably the happiest day I had in lab: seeing how far I had come, from not even being able to do reactions by myself to a high-yielding reaction that others in the lab might keep working on after I graduate.

“Sometimes it feels a little mundane to work in a lab: You weigh things out, you spin things, you add heat, you add light, and then you get your product. But that day, the sheer joy of seeing my reaction working — it’s not just the science, it’s having ownership over a real contribution.”

When combined with the first step, these materials led to a final yield of 43%: not high enough for publication in a peer-reviewed journal, but more than enough to demonstrate that this is a reaction with real promise.

When writing up her thesis, Humphreys took her readers on her journey, walking through each new chemical she tried, explaining why she tried it, and listing the yield. “The chemistry department was amazing, telling us right off the bat that our reaction doesn’t have to be successful to make for a successful thesis,” she said. “They always framed it as, ‘Tell us a story. Tell us how you got from the beginning to the end.’ I wanted to show people that I had a lot of failures, and it was OK because I bounced back.”

Onboarding at the MacMillan lab

While some undergraduates have been in MacMillan’s lab since their first year, Humphreys was a late addition, joining remotely during spring 2021. She had been pre-med until the pandemic gave her a chance to reassess her priorities, but that left her scrambling for a thesis project. Fortunately, MacMillan had an opening in his lab group just a week after Humphreys reached out in search of an adviser.

“The stars aligned,” said Humphreys. “I met with Dave, and I was instantly in awe of him. I think a lot of people are. But he was talking about chemistry in a way that I’d never heard before. To hear somebody so excited about their work — even though I knew it would be virtual for the first six months, he definitely got me excited to just be in his lab.”

This was almost a year before MacMillan won the Nobel Prize, which he shared with Ben List of the Max-Planck Institut für Kohlenforschung, and Humphreys had had no idea what a prominent chemist he was. “I feel like it was for the best that I didn’t grasp what I was getting myself into, because I would have put too much pressure on myself.”

MacMillan also remembers their initial conversation. “I gave the speech that I give to every single person that joins my group,” he said. “‘If you’re not having fun, we’re not doing it right.’ There’s a lot of scientific work in my group, and there’s also a lot of banter and enjoyment and interaction. Science is tons of fun. That’s why we do it.”

Midway through her thesis, Humphreys was not having fun. Her yields were disappointing, and she wondered if she should abandon her project.

It’s a common scenario, MacMillan said. “I tell students, ‘Come and talk to me if you’re not having fun.’ And then we have a conversation, and a lot of the time, it’s as simple as telling them, ‘You’re doing great! Things are going really well!’ They need that recalibration of what the actual expectations are and where they are.”

In Humphreys’ case, she needed the reminder that yield isn’t everything; the fact that the reaction was working at all proved the concept was sound. “Getting proof of concept is everything,” said MacMillan. “If you can show that you can make something work, it’s great.”

Journey to confidence

Looking back over the past five years, Humphreys sees a powerful trajectory, from a high school senior in a town outside Chicago who’d barely heard of Princeton to a successful (almost) graduate of an Ivy League university.

But making that journey as a woman of color in a tough scientific discipline could be lonely, she said.

“A lot of times, I’m the only person of color or woman of color in any of the rooms I’m in,” she said. “It makes me proud to represent these backgrounds, but also, I for sure wish there were more people like me. There could never be too much representation.”

Humphreys’ identity defies easy description. She primarily identifies as Black, and additionally also as Latinx, because of her Central American roots: Both of her parents were born in Belize. But she says her “core” identifier is first generation — the first generation in her family to be born in the U.S., and the first to go to college.

“That ties back to my culture, to Belize,” she said. “I feel like ‘first generation’ really encompasses my parents’ journey, how hard they’ve worked to be here, and also how hard I’ve worked to overcome barriers to get to the place where I am today.

“My senior year, my mom didn’t really know how the college search process worked, but she came to me one day and showed me Princeton’s website, specifically their financial aid. That was the biggest pull. I was like, ‘I don’t know, wow, that’s far away, and also there’s no way I’m going to get in.’" Humphreys had always expected that she would go to nearby Northwestern and come home every weekend.

“And then my mom's like, ‘Well, if there’s no way, why not try?’”

Humphreys got the admission notification during the last class period of the high school day. “I called my parents as soon as the bell rang. I put them both on the phone, and my mom was just immediately crying. I’m standing in the hallway, people are bumping into me, I’m crying, my mom’s crying. I’d just thought it was impossible.”

When she arrived at Princeton, she found herself feeling like a small fish in a very, very big pond. It wasn’t the size of the school — her public high school had about as many students as the University — but just feeling like she was out of her depth.

Then she bonded with her neighbors in Rockefeller Hall (“Rocky”) and started going to the gym every day. She and her friend Briana Macedo, also in the Class of 2022, created a chapter of the American Physician Scientists Association (APSA), a student-led group for M.D./Ph.D. students that helped them build a wider community of like-minded people.

At the McGraw Center for Teaching and Learning, Humphreys discovered the power of having a group to help her work through problem sets. When the pandemic ended in-person instruction, she reached out to her weekly homework group and set up Zoom study groups. Her junior and senior years, she served as a residential college adviser (RCA) for Rocky, offering advice about life, love and problem sets to her advisees.

“All of these experiences over my four years here have truly changed my life,” Humphreys said. “I was this little, timid freshman who didn’t even know what the Ivy League was, taking classes I had no idea what to do in, with a lot of imposter syndrome all the time.

“And now, I think of all the things I’m involved with — working in a Nobel Prize-winning lab, creating an APSA chapter, being an RCA, contributing to so many communities on campus. If you’d told me freshman year that I would do any of it, I think I don’t think I would have believed you.

“The same thing happened with my thesis. I was just so unsure at the beginning, but I kept rolling with the punches, and eventually here I am, very happy about my research and confident in my ability to explain it. It’s taken me years, but I think I’ve made my mark on Princeton.”