= Deoxyhemoglobin
\b[<Deoxyhemoglobin>] is the unoxygenated form of <hemoglobin> whose available <haem> sites can reversibly bind <oxygen>. Releasing oxygen produces a binding-state change, rather than the ferric iron state defining <methemoglobin>. <Carboxyhemoglobin> differs again because a competing <carbon monoxide> ligand occupies a binding site. These distinctions matter when interpreting the components of an optical measurement.
In the instrument convention for functional <oxygen saturation>, HHb denotes the unoxygenated, oxygen-capable component. Its concentration appears with <oxyhemoglobin> in the denominator:
$$
s_{\mathrm O_2}=\frac{c_{\mathrm O_2\mathrm{Hb}}}{c_{\mathrm O_2\mathrm{Hb}}+c_{\mathrm{HHb}}}.
$$
The HHb label is not an instruction to treat deoxygenation as addition of a hydrogen molecule. Functional saturation also uses a different denominator from fractional oxyhemoglobin referenced to total hemoglobin.<Oxyhemoglobin by Radiometer>[FO2Hb versus oxygen saturation, definition of the functional denominator]{ref}
An original independent-site model illustrates a further counting distinction. Let each of four sites have oxygen-occupancy probability $\theta$, independently. The probability that a tetramer has exactly $m$ occupied sites is
$$
P_m=\binom4m\theta^m(1-\theta)^{4-m}.
$$
At $\theta=1/2$, half the sites are empty on average, while only $P_0=1/16$ of tetramers have all four sites empty. A haem-equivalent unoxygenated fraction therefore need not equal the fraction of wholly unliganded tetramers. Real hemoglobin binding is cooperative, so independence here is a stated teaching assumption rather than a physiological model.
<Deoxyhemoglobin> and <oxyhemoglobin> have distinguishable absorption spectra. A primary study measured the absorptivities of these and other human hemoglobin derivatives across 450–1000 nm, including fetal/adult differences relevant to multicomponent analysis.<Absorption spectra of human fetal and adult oxyhemoglobin, de-oxyhemoglobin, carboxyhemoglobin, and methemoglobin by W. G. Zijlstra and others>[Abstract]{ref} A controlled oxygenation comparison can therefore follow changing spectral contributions without inferring oxygen amount from apparent colour alone. The measured path length, concentrations and calibrated component spectra remain necessary inputs.
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