"These two look exactly the same, though?"
Kana compared two molecular models.
Rei answered. "Enantiomers. Mirror images of each other."
"Aren't they the same?"
"Can't be superimposed," Milia demonstrated with gloves. "Like left and right hands."
Toma laughed. "Hands became molecules."
"The essence of chirality," Rei explained. "With an asymmetric carbon, enantiomers exist."
"Asymmetric carbon?" Kana wrote in her notebook.
"A carbon bonded to four different groups."
Milia assembled a model. "This central carbon has all different substituents attached."
"Even if I rotate this," Toma attempted, "it doesn't overlay with the original."
"Right. Only overlays with the world beyond the mirror."
Kana put her palms together. "Palms can match though."
"But you can't superimpose a right hand onto a left hand," Rei said.
"Are chemical properties the same?"
"Mostly the same," Milia answered. "Melting point, boiling point, solubility. But..."
"But?"
"Completely different in living organisms."
Toma took out a medicine box. "Thalidomide. The tragic drug."
"I've heard of it," Kana became serious.
"One enantiomer has sedative effects, the other is teratogenic," Rei said quietly.
"Same molecule though?"
"Receptors also have three-dimensional structure," Milia explained. "A right-hand glove fits only a right hand."
Kana understood. "That's why effects differ."
"All amino acids are L-form," Rei continued. "D-form is barely used in living organisms."
"Why only one?" Toma asked.
"Mystery of life's origin," Milia answered. "The first coincidence may have decided everything."
"Coincidence?"
"The first amino acid was L-form. It replicated and became the standard."
Rei added. "Once decided, can't be changed. All proteins are left-handed."
Kana looked at the DNA model. "What about DNA?"
"Right-handed double helix," Milia answered. "Only D-form sugars too."
"Life chose one or the other."
"Symmetry breaking," Rei said philosophically. "The universe faces the same problem."
Toma started an optical activity experiment. "It rotates polarized light, right?"
"Yes. Rotates right or left."
Kana looked through the polarizer. "Even light rotates?"
"Molecular asymmetry affects light," Milia explained.
"Strange."
Rei continued. "Nearly all natural products are optically active. Synthetic products tend to be mixtures of both."
"Why?"
"Enzymes in living organisms make only one. Chemical synthesis produces both."
Toma showed a crystal. "This is pure L-form amino acid."
"How do you separate them?" Kana asked.
"Chiral chromatography," Milia answered. "Using an asymmetric stationary phase."
"Can separate enantiomers."
"With modern technology, yes. But life only makes one from the start."
Rei summarized. "Chirality is an essential characteristic of life."
"Left and right worlds," Kana murmured.
"We live in the left world," Milia said. "If there were organisms from the right world..."
"We couldn't digest their food?" Toma guessed.
"Right. All amino acids, sugars, nucleic acids would be reversed."
Kana smiled. "Alice through the looking glass."
"Chemistry's looking glass," Rei acknowledged. "Incompatible with our world."
The four fell silent. Beyond the mirror, another world exists. But they can never meet.