In recent decades, chemists have discovered how to make mirror proteins. Researchers have welded together right-handed amino acids to create mirror versions of natural proteins made by our own bodies.

Mirror proteins behave much like their natural counterparts, with one important difference: They take much longer to break down. That’s because the natural enzymes that normally degrade proteins have shapes that are adapted for attacking left-handed proteins.

They cannot grip mirror proteins and cut them into fragments. Their failure is akin to what happens if you try to twist open a lid from a jar by turning it counterclockwise, only to discover that the threads on the jar twist in the opposite direction.

Chemists are now trying to exploit mirror proteins, hoping they can be used to create long-acting drugs for diseases ranging from H.I.V. to Alzheimer’s.