where PlcH is the protonated form of fern plastocyanin. The standard-state free energy change ∆G • will be approximated using the electronic energies for each system, neglecting entropic effects. From this, the pK a of each protein is calculated (at T = 298 K) is computed using 1KDI as a reference (pK a = 4.4 ± 0.1), 124 so the pK a for the Pro in eq. 33 is pK a = 4.4 + ∆G • RT ln (10) .