"Look, it turned red!"
Toma shook the test tube. The solution had changed to a vivid red.
"Oxygen bound to hemoglobin," Milia explained.
"Why does the color change so much?" Kana asked with interest.
Rei answered. "Because the electronic state of the iron ion in heme changes."
"Heme?"
Milia displayed a molecular structure on her tablet. "There's an Fe²⁺ iron ion at the center of a porphyrin ring. This complex structure is heme."
"The source of the red color."
"Yes. But precisely, the color is determined by iron's oxidation state and ligands."
Rei explained in detail. "Deoxyhemoglobin without oxygen is dark red. Oxyhemoglobin with oxygen bound is vivid red."
Toma tilted the test tube. "What's happening at this moment?"
"The iron ion forms a coordinate bond with oxygen molecule O₂," Milia drew a diagram.
"Oxygen binds at the sixth coordination position of iron. On the opposite side is histidine."
Kana took notes. "Coordinate bond?"
"A bond between metal ions and ligands. Similar to covalent bonds, but both electrons come from the ligand."
Rei added. "At this time, the electron configuration of the iron ion changes. From high-spin to low-spin state."
"That's the color change?"
"Exactly. When electron configuration changes, the wavelength of absorbed light changes."
Milia got excited. "But what's even more interesting is cooperativity!"
"Cooperativity?" Toma asked.
"Hemoglobin has four subunits. When one binds oxygen, the other three also bind more easily."
Rei drew a graph. "Sigmoidal curve. Binding rate versus oxygen partial pressure."
"Why is it cooperative?" Kana questioned.
"Structural changes propagate," Milia explained. "When one subunit binds oxygen, the overall structure changes slightly. From T state to R state."
"T state?"
"Tense state. Oxygen binds less easily. R state is Relaxed, where binding occurs more easily."
Rei added. "This is the allosteric effect. Binding at distant sites influences through structural changes."
Toma pondered. "But why is such a complex mechanism necessary?"
"For efficient oxygen transport," Milia answered.
"In lungs where oxygen partial pressure is high, all four bind. In tissues where pressure is low, they can release all at once."
Kana understood. "ON-OFF is clear-cut."
"Exactly. Without cooperativity, release would only be halfway."
Rei gave a real example. "Fetal hemoglobin has higher oxygen affinity than maternal hemoglobin. It can receive oxygen efficiently."
"Evolution's wisdom," Toma murmured.
Milia gave another example. "Carbon monoxide CO has over 200 times the affinity of oxygen."
"That's why it's dangerous," Kana understood.
"Yes. Once it binds, it hardly releases. Oxygen can't bind."
Rei warned. "And when one binds, cooperativity makes others bind more easily too. A vicious cycle."
Toma held the test tube up to the light. The red liquid glowed beautifully.
"This color is the color of life."
Milia smiled. "The encounter of iron and oxygen. That single moment supports life."
"Happening hundreds of millions of times per second throughout the body," Rei added.
Kana took a deep breath. "Right now, in my lungs, heme is glowing red."
"And delivering oxygen to cells throughout the body."
Toma said quietly. "Hemoglobin is a hard worker."
The four stared at the test tube. Small molecules supporting great life.
"Science is poetry," Milia murmured.
The other three nodded. In the red glow of heme, they saw life's mystery.