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- Derivation of relationship between molar circular dichroism and molar ellipticity
This relationship is derived by defining the ellipticity of the polarization as:
where ER and EL are the magnitudes of the electric-field vectors of the right-circularly and left-circularly polarized light, respectively. When ER equals EL (when there is no difference in the absorbance of right- and left-circular polarized light), θ is 0° and the light is linearly polarized. When either ER or EL is equal to zero (when there is complete absorbance of the circular polarized light in one direction), θ is 45° and the light is circularly polarized.
Generally, the circular dichroism effect is small, so tanθ is small and can be appoximated as θ in radians. Since the intensity or irradiance, I, of light is proportional to the square of the electric-field vector, the ellipticity becomes:
The ellipticity can now be written as:
Since ΔA<<1, this expression can be approximated by expanding the exponentials in a Taylor series to first-order and then discarding terms of ΔA in comparision with unity and converting from radians to degrees:
The linear dependence of solute concentration and pathlength is removed by defining molar ellipticity as,
Then combining the last two expression with Beer's Law, molar ellipticity becomes:
Nomenclature is shown in green.
- heme function section
Hemoproteins have diverse biological functions including the transportation of diatomic gases, chemical catalysis, diatomic gas detection, and electron transfer. The heme iron serves as a source or sink of electrons during electron transfer or redox chemistry. In peroxidase reactions, the porphyrin molecule also serves as an electron source. In the transportation or detection of diatomic gases, the gas binds to the heme iron. During the detection of diatomic gases, the binding of the gas ligand to the heme iron induces conformational changes in the surrounding protein.
It has been speculated that the orginal evolutionary function of hemoproteins was electron transfer in primitive sulfur-based photosynthesis pathways in ancestral cyanobacteria before the appearance of molecular oxygen. 
Hemoproteins acheive their remarkable functional diversity by modifying the environment of the heme macrocycle within the protein matrix. For example, the ability of hemoglobin to effectively deliver oxygen to tissues is due to specific amino acid residues located near the heme molecule. Hemoglobin binds oxygen in the pulmonary vasculature, where the pH is high and the pCO2 is low, and releases it in the tissues, where the situations are reversed. This phenomenon is known as the Bohr effect. The molecular mechanism behind this effect is the steric organisation of the globin chain; a histidine residue, located adjacent to the heme group, becomes positively charged under acid circumstances, sterically releasing oxygen from the heme group.
- Hegg, Eric L.; et al. (2004). "Heme A Synthase Does Not Incorporate Molecular Oxygen into the Formyl Group of Heme A". Biochemistry. 43 (27): 8616–8624.
- Hardison, R. (1999). "The Evolution of Hemoglobin Studies: of a very ancient protein suggest that changes in gene regulation are an important part of the evolutionary story.". American Scientist. 87 (2): 126.