"They connected..."
Kana gazed at an amino acid molecular model.
Rei nodded. "Peptide bond. The backbone of proteins."
"Backbone?"
Milia picked up two amino acid models. "One carboxyl group and another amino group."
"They bond?"
"Dehydration condensation," Rei explained. "One water molecule is removed, and a bond forms."
Kana wrote in her notebook. "-COOH + H-NH- → -CO-NH- + H₂O"
"Exactly. The CONH part is the peptide bond."
Milia added. "This bond is planar."
"Planar?" Kana tilted her head.
"The C-N bond has double bond character. Hard to rotate," Rei explained.
"Why?"
"Resonance structure. Electrons are delocalized."
Kana tried rotating the model. "Indeed, it's rigid."
Milia took another amino acid. "Connect these?"
"You get a peptide," Rei answered.
"Two makes a dipeptide, three makes a tripeptide."
Kana got excited. "So 100 would be?"
"Polypeptide. The basis of proteins."
Milia drew a diagram. "N-terminus and C-terminus. There's directionality."
"Directionality?"
Rei pointed. "Left end is amino group, right end is carboxyl group. Always written N→C direction."
"Why is that important?" Kana asked.
"Because it determines synthesis direction. Ribosomes also make N→C."
Milia continued. "Sequence is important. Glycine-alanine and alanine-glycine are different."
"But same components?"
"Different order. That changes function."
Rei gave an example. "Insulin has 51 amino acids. Change one in order, and it won't work."
Kana was surprised. "Just one?"
"Sequence determines everything."
Milia showed a tablet. "This is a protein database. Tens of thousands of sequences registered."
"Amazing..."
Rei explained. "From 20 types of amino acids, infinite combinations."
"20 to the 100th power?" Kana tried calculating.
"Astronomical number," Milia acknowledged.
"But organisms use only a tiny fraction."
Rei continued. "Only functional sequences selected by evolution."
Kana lifted the peptide model. "Is this bond strong?"
"Relatively strong. Unless hydrolyzed."
"Hydrolysis?"
Milia answered. "Using water to break the bond. The reverse reaction."
"Enzymes are needed," Rei added.
"Proteases. Enzymes that cut peptide bonds."
Kana asked. "Why cut them?"
"Necessary for digestion and protein degradation."
Milia explained. "Pepsin in stomach, trypsin in intestine, all proteases."
"They convert eaten protein back to amino acids," Rei said.
"Then make our own proteins again."
Kana understood. "Recycling?"
"Yes. Amino acids are precious."
Milia assembled another model. "Polypeptide chains fold."
"Why?"
"Hydrophobicity, hydrophilicity, charge... interactions work."
Rei drew a diagram. "α-helix, β-sheet. Secondary structure."
"The planarity of peptide bonds makes this possible."
Kana admired. "One bond, yet so complex..."
"But the basics are simple," Milia smiled.
"CO-NH. Just repeated."
Rei looked out the window. "The three of us in this room are made of trillions of peptide bonds."
Kana looked at her hand. "This hand too?"
"Muscle, skin, bone... all protein."
Milia said quietly. "Peptide bonds shape us."
"Can you hear the whispers of bonds?" Rei asked.
Kana closed her eyes. "CO-NH, CO-NH, CO-NH..."
"A quiet song repeated hundreds of millions of times."
Milia closed the notebook. "That's life's music."
The three gazed at the model. Small bonds create a large world.
Kana murmured. "Thank you, peptide bonds."
Rei and Milia laughed.
"Kana thanking molecules is definitely special."
On the lab bench, the amino acid models connected quietly.