Is Biology Quantum? The Answer Is Getting Harder to Dodge
What Enzymes Know That Models Don't
A paper just published in PNAS — “What is Quantum Biology?” by Gregory Scholes and Graham Fleming — asks a question that sounds basic but isn’t. The answer has real consequences for anyone developing therapeutics.
Quantum biology studies whether quantum-mechanical effects — coherence, tunneling, entanglement — actively drive biological processes rather than merely coexist with them. That distinction is what the field is now trying to pin down rigorously.
Two examples that are hard to explain any other way
Photosynthetic light-harvesting complexes achieve near-perfect energy transfer efficiency. Classical physics cannot account for this fully. Quantum coherence — excitations existing in superposition across multiple molecular states simultaneously — may be what makes the difference. Fleming’s own lab produced key evidence here, including work on long-lived quantum coherence in photosynthetic complexes at physiological temperature.
Enzyme-catalyzed hydrogen transfer is the more immediately relevant case for drug hunters. Soybean lipoxygenase produces a kinetic isotope effect of roughly 80. Classical transition state theory predicts a maximum of about 7. The gap is too large to explain any other way: the hydrogen passes through the energy barrier rather than over it. Quantum mechanics isn’t incidental. It’s the mechanism.
What this means for computational drug design
Classical models approximate molecular interactions through force fields and statistical mechanics. They’re useful and fast. But if enzymes routinely exploit proton tunneling — and early evidence suggests many do — then classical simulations of those enzymes are working with an incomplete physical description of the active site.
This is already driving the adoption of QM/MM methods in enzyme-targeted drug design. Quantum mechanics/molecular mechanics hybrid approaches are slower and more expensive, but they capture what classical models miss: the quantum behavior of hydrogen transfer, proton-coupled electron transfer, and possibly receptor-ligand binding in certain systems. A 2025 De Gruyter review described this as a shift in how the field treats enzyme catalytic efficiency — no longer optional to model quantum effects when designing drugs against these targets.
The three open questions Scholes and Fleming pose
First: can we build experimental probes sensitive enough to detect quantum effects inside living systems — not just purified complexes under controlled lab conditions? In vivo detection remains unsolved.
Second: does biological machinery genuinely exploit quantum effects, or do they happen to occur while biology proceeds independently? “Quantum-assisted” and “quantum-driven” are not the same claim. The computational tools we need, and the drug design implications, are different depending on the answer.
Third: how does quantum coherence survive long enough to matter? Living systems are warm, wet, and noisy — conditions that should collapse quantum states almost instantly. That they apparently don’t, at least in some systems, is the puzzle that makes the whole field worth watching.
The state of the field
This is not fringe science. The experimental foundations are real — ultrafast spectroscopy, kinetic isotope measurements, cryo-EM combined with quantum chemical calculations. What’s missing is consolidation: shared definitions, agreed experimental standards, and a cleaner taxonomy of which biological systems are genuine candidates for quantum effects.
Scholes and Fleming are calling the field to that work. For anyone in drug discovery, the practical signal is this: if the quantum layer of enzyme function is real and measurable, then target engagement models built entirely on classical assumptions are incomplete. That’s worth knowing before you commit to a computational platform for a difficult enzyme target.
The paper is open access at PNAS.
Sources: Scholes & Fleming, “What is Quantum Biology?” PNAS (2026), https://www.pnas.org/doi/10.1073/pnas.2531134123 | Fleming et al., long-lived quantum coherence in photosynthetic complexes, PNAS (2010) | “The quantum revolution in enzymatic chemistry,” De Gruyter, pac-2025-0500 | “New Insight into Quantum Mechanical Hydrogen Tunneling in Enzymes,” Biochemistry ACS (2025)