"Won't break."
Kana pulled on the protein model.
"Of course not," Rei said. "Peptide bonds are strong."
"How strong?"
"Stronger than single bonds. They have partial double bond character."
Milia explained. "Because of resonance structures."
Kana wrote in her notebook. "Resonance?"
"Electrons are delocalized. Carbonyl electrons are shared between nitrogen and oxygen."
Rei drew a diagram. "C-N bond is intermediate between single and double bond."
"Intermediate?"
"Bond order about 1.3. Can't rotate."
Kana was surprised. "Can't rotate?"
"Peptide bonds have planarity. Six atoms are on the same plane."
Milia showed a model. "That's why they're rigid."
"Planar means rigid?"
"Rotational freedom is restricted. That determines protein shape."
Rei continued. "Also called amide bond. Form of condensed carbonyl and amine."
"Condensed?"
"Water is removed in bonding. Dehydration condensation."
Kana asked. "How is it formed?"
"At ribosome. Carboxyl group of one amino acid reacts with amino group of next."
Milia added. "Precisely, tRNA carries activated amino acids."
"Activated?"
"Aminoacyl-tRNA. High energy state."
Rei emphasized. "Peptide bond formation actually requires energy."
"Requires?"
"GTP is hydrolyzed. Reaction proceeds with that energy."
Kana calculated. "One GTP per bond?"
"At least. Precisely, ATP for amino acid activation, multiple GTPs for translation."
"High cost!"
Milia laughed. "Protein synthesis is one of most energy-consuming processes in cells."
Rei explained another property. "Peptide bonds have cis and trans."
"Isomers?"
"Yes. But mostly trans form."
"Why?"
"Steric hindrance. In cis form, side chains collide."
Kana understood. "Trans is more stable?"
"Except for proline, almost all trans."
Milia added. "Proline is exception. Because of cyclic structure."
"Cyclic?"
"Side chain returns to backbone nitrogen. Special structure."
Rei continued. "That's why proline affects protein folding."
"How does it affect?"
"Often creates beta turns. Kinks for direction change."
Kana murmured. "Each bond has meaning."
"Peptide bond sequence is primary structure," Milia said.
"Amino acid order?"
"Yes. This holds all information."
Rei emphasized. "Once primary structure is determined, higher-order structure is determined. Anfinsen's dogma."
"Dogma?"
"Principle that protein shape is determined solely by sequence."
Kana was moved. "Everything determined by peptide bond arrangement?"
"In most cases, yes," Milia acknowledged. "Though there's auxiliary help from chaperones."
Rei added. "That's why genetic information only needs to specify amino acid sequence."
"Efficient!"
Milia showed the model. "These planar bonds link hundreds, thousands."
"That's protein?"
"Yes. Chain of firm promises."
Kana stared at the model. "Promise?"
Rei said quietly. "Once bonded, won't easily break. Stable and strong."
"Bonds supporting life."
"Exactly," Milia smiled.
Kana took a deep breath. "Peptide bonds are deeper than I imagined."
"Basic chemistry, but also essence of life," Rei said.
"Firm promise," Kana murmured.
The three stared at the model. Planar bonds connecting life.