Example: Heme absorption of photon

ECE532 Biomedical Optics
Steven L. Jacques, Scott A. Prahl
Oregon Graduate Institute

For example, a 514 nm photon from an argon ion laser when absorbed by a hemoglobin molecule will transfer energy to the hemoglobin molecule:

hc/(0.514x10-1 [m]) = 3.86x10-19 [J]

Doesn't sound like much energy, does it? But consider the energy from the perspective of the absorbing chromophore. The photon is absorbed by the heme chromophore within the hemoglobin protein. The heme choromophore is roughly 1 nm is size.

After the heme chromophore absorbs one photon, the jump in energy density is roughly the photon energy divided by a nm3 volume:

(3.86x10-19 [J])/(10-7 [cm])3 = 387 [J/cm3]

For comparison, the energy density of boiling water is 418 J/cm3. So one photon carries quite a lot of energy from the perspective of a small molecule!

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