"I can't assemble this properly."
Kana struggled with the molecular model.
Rei approached. "What are you making?"
"An amino acid. But the four bonds won't lie flat."
"Naturally," Rei answered. "Carbon bonds are in tetrahedral arrangement."
Kana looked up. "Tetrahedral?"
Milia picked up the model. "Carbon makes 4 bonds. They're arranged three-dimensionally."
"Not planar?"
"Not planar. Regular tetrahedron. 109.5-degree angles," Rei explained.
Kana rotated the model. "Why this shape?"
"Electron pair repulsion," Milia answered. "Four bonding electron pairs take the arrangement farthest from each other."
Rei supplemented. "sp3 hybridization. One s orbital and three p orbitals mix to make four equivalent orbitals."
Kana tried to draw in her notebook. "Can't draw it flat..."
"The difficulty of stereochemistry," Milia laughed. "Representing 3D in 2D."
Rei taught the notation. "Use wedge-shaped symbols. Bold wedge is front, dashed line is back."
Kana redrew. "Like this?"
"Correct. Now you can see the three-dimensional arrangement."
Milia assembled another model. "Same amino acid, but mirror image."
Kana compared the two. "Ah, they don't overlap."
"Right. Mirror-image relationship. Called enantiomers."
Rei explained. "When carbon has four different groups attached, it becomes a chiral center."
"Chiral?"
"Greek for 'hand'. Like right hand and left hand relationship."
Kana looked at her hands. "True, they're mirror images but don't overlap."
"Many amino acids are chiral. Two forms exist, L and D," Milia continued.
"What's the difference?"
"Chemical properties are the same. But spatial properties differ."
Rei gave an example. "Optical activity. The direction of rotating polarized light is opposite."
Kana became interested. "Which does biology use?"
"L-type," Milia answered immediately. "Almost all organisms use only L-amino acids."
"Why?"
"Not fully understood yet. But once chosen, needed to unify."
Rei supplemented. "Enzymes recognize spatial structure. Enzymes optimized for L-type can't recognize D-type."
"Lock and key?" Kana made a metaphor.
"Exactly. If spatial structure doesn't match, no reaction occurs."
Milia gave another example. "Same with sugars. Organisms use D-glucose."
"Opposite of amino acids?"
"Yes. Amino acids are L, sugars are D. Strange, but that's life's choice."
Kana thought. "What if it were reversed?"
"Theoretically possible," Rei answered. "But would have to reverse everything. Enzymes, receptors, all."
"Different life?"
"Mirror life. Science fiction, but chemically possible."
Milia's face became serious. "But couldn't coexist on Earth."
"Why?"
"Couldn't absorb nutrients. L-type enzymes can't break down D-type proteins."
Rei continued. "So chirality unification is important. A rule for the entire ecosystem."
Kana stared at the model. "This small difference is so important."
"Stereochemistry is the foundation of biochemistry," Milia said.
Rei added. "Same with drugs. One side has medicinal effect, the other side effects sometimes."
"Huh?"
"Thalidomide is famous. One side has sedative effect, the other teratogenic effect."
Kana was shocked. "Same molecule though?"
"Just different spatial structure, recognized as different molecules."
Milia overlapped the models. "Look similar, but different."
"Like twins. But with opposite personalities."
Rei smiled. "Good metaphor."
Kana summarized in her notebook. "Carbon's four-way promise = foundation of stereochemistry."
"And life's left-right choice," Milia added.
Outside the window, trees swayed. Looking bilaterally symmetric, each one is different. Nature loves three dimensions.