The Crystal Structure of the Trip

“In 2017, scientists solved the crystal structure of LSD bound to its receptor. The lid that traps the drug in place, explaining its 12-hour duration, was a protein loop nobody had anticipated.”

For most of the century since Hofmann's bicycle ride, the mechanism of LSD's action was understood in general terms, it bound to serotonin receptors, particularly 5-HT2A, but the precise structural explanation for two of its most distinctive features remained unknown: its extreme potency and its unusual duration.

LSD is active in far smaller amounts than mescaline: thousands of times less by weight for comparable effects. And where most drugs' effects follow their pharmacological half-life and clear when the drug clears, LSD's effects persist for 8-12 hours even though the drug itself metabolizes faster.

In 2017, a team led by Bryan Roth at UNC Chapel Hill published the crystal structure of LSD bound to the 5-HT2A receptor in Cell. The structure explained both mysteries at once.

On potency: LSD's molecular shape allows near-perfect complementarity with the receptor's binding pocket. The fit is so precise that even tiny amounts occupy the receptor almost completely.

On duration: a protein loop, an 'extracellular lid' formed by the receptor's second extracellular loop, closes over the LSD molecule after binding and traps it inside. The drug cannot leave until the lid opens. This explains why blocking the receptor with ketanserin before the LSD stops the effects but administering ketanserin afterward does not, the drug is locked in, and even the blocker can't reach it.

Hofmann described the intensity and duration of Bicycle Day's experience as a source of bewilderment. He'd expected the compound to metabolize more normally. The lid is why it didn't. The molecular explanation for the most famous accidental dose in pharmacological history was solved in a University of North Carolina laboratory seventy-four years after it happened.

Sources

• Wacker et al., Cell (2017)
• Hofmann, LSD: My Problem Child (1979)
• Preller & Vollenweider, Front Psychiatry (2019)