The pH of human blood is 6.8～7.8,
the pKa of (4)deoxyhomoglobinA is 8.2,
the pKa of (2)oxyhomoglobinA is 6.96,
it assumed that(1)logP₀ and (2)logP₀ or (3)logP₀ and (4)logP₀ are approximately the same value.
The yellow region in fig-5
The yellow region in fig-5 is the region of 6.8≦pH≦7.8.
and the region of ΔrG₀total = ー2.3RT[｛(3) logPー(1)logP ｝＋｛(2) logPー(4)logP ｝] = 2.3RT［｛(1) logPー(3)logP ｝＋｛(4) logPー(2)logP ｝］≦ 0
This region is within the pH value of human blood(6.8～7.8) and the sum total of Gibbs energy changes is less than 0.
Within the yellow region in fig-5 the transfer of oxygen molecules to (3)deoxymyoglobin from (2)oxyhemoglobinA is spontaneous.
The sum total of ⊿G₀ changes in that region is less than 0. ΔG₀total≦0
At that time the Gibbs energy change of solute in the aq layer ΔrG₀aqtotal is also less than 0, due to ΔG₀total = ΔrG₀aqtotal
Within the yellow region in fig-5 the reaction from (2)oxyhemoglobinA and (3)deoxymyoglobin to (4)deoxyhemoglobinA and (1)oxymyoglobin is spontaneous.
In the terminal tissues of the body which belong to the area of low pH value within the pH value range of human blood,
(1)oxymyoglobin is formed and the oxygen molecule in it is used in metabolic processes.
Within the yellow region in fig-5 the sum total of changes in Gibbs energy values based on chemical reactions involving the movement of oxgen molecules has not been taken into consideration, due to the sum total is effectively in a cancelling out relationship.
Gibbs energy values due to fluctuations in pH values
Since the variation in magnitude of solvation due to fluctuations in pH values are not the same between the solutes having different pKa values(pI values), the variations in the sum total of Gibbs energy values due to fluctuations in pH values are taken into consideration.
See fig-4. The drawing fig-4 makes it easier for you to understand the meaning mentioned above.
The reaction to form (2)oxyhemoglobinA from (4)deoxyhomoglobinA and the oxygen molecules.
The sum total of logP value change are negative in the region of pH value to the right of the point where (2)logP and (4)l.ogP intersect in fig-4.
ー {(4) logPー(2)logP} = (2)logPー(4)logP is shown in fig-6.
The inequality relation (2)logP-(4)logP <0 is established in pink region in fig-6.
The pink region in fig-6 is the range with a high pH value within the pH region of human blood.
Since the sum total of ⊿rG₀ in that region ΔrG₀total is negative(ΔrG₀total=ー2.3RT {(4) logPー(2)logP} = 2.3RT［(2)logPー(4)logP］<0 ), the changes in Gibbs energy of the solute in the aq layer（ΔrG₀aqtotal<0) is also negative.
ΔΔrG₀total is negative (ΔΔrG₀total <0 )in pink region of fig-6. Since ΔΔrG₀aqtotal at this time is also negative at different absolute values,
in the pH value region to the right of the point where (2)logP and (4)logP intersect in fig-4, the absolute value of negative values, which is the sum total of change in ΔrG₀aqtotal values, increases.
When (2)oxyhemoglobinA is formed from (4)deoxyhemoglobinA and the oxygen molecules,
Since it involves the conversion from a high-spin Fe++ complex to a low- spin Fe++ complex, the changes of reaction Gibbs energy values that are not taken the solvation into the consideration are also negative.
The reaction Gibbs energy changes considering solvaion are also negative.
Since the sum total of these negative values is negative, the reaction from (4)deoxyhemoglobinA and the oxygen molecules to form (2)oxyhemoglobinA is spontaneous one with quick process.
