For a reaction between an electron acceptor A^{+} and
an electron donor D, the ratio of oxidized to reduced forms of
the complex will depend on the order of the reaction. If A and
D are within a single protein, or form a complex, such that electron
transfer is by a 1^{st} order process, then:

For reaction between a redox pair in the complex, we expect the
ratio for [A.D^{+} ] to [A^{+}.D] to be equal
to K_{eq} (ie. both the A/A^{+} and D^{+}/D
ratios are equal to K_{eq})

For a collisional reaction, in which any A can reaction with any
D through diffusion, the reaction is written:

In this case, the value of the equilibrium constant is given by
the __product __of the ratios of A/A^{+} and D^{+}/D
(this reflects the entropic contribution of distribution of the
states throughout the system). The consequence is that for a given
value of K_{eq}, the ratios of A/A^{+} and D^{+}/D
are less than in the 1^{st} order case (by approximately
a square root function), and in the "magic square" representation,
the curves for the 1^{st} order process are "nearer
the corner" than for the 2^{nd} order process.

In all the Figs. for this document, the theoretical curves shown
are for the second-order case.

For E_{m} values of 450, 340, 290 and 270 for P^{+}/P,
cyt c_{2}^{+}/c_{2}, Fe_{2}S_{2}^{+}/
Fe_{2}S_{2} and cyt c_{1}^{+}/c_{1}
couples respectively, K_{eq} values are P:c_{2}
= 73; c_{2}:c_{1} = 15.4; P:Fe_{2}S_{2}
= 515; and P :c_{1} = 1124.