Photoinduced Electron Transfer Elicits a Change in the Static Dielectric Constant of a de Novo Designed Protein

We provide a direct measure of the change in effective dielectric constant (ε_S) within a protein matrix after a photoinduced electron transfer (ET) reaction. A linked donor–bridge–acceptor molecule, PZn–Ph–NDI, consisting of a (porphinato)­Zn donor (PZn), a phenyl bridge (Ph), and a naphthalene diimide acceptor (NDI), is shown to be a “meter” to indicate protein dielectric environment. We calibrated PZn–Ph–NDI ET dynamics as a function of solvent dielectric, and computationally de novo designed a protein SCPZnI3 to bind PZn–Ph–NDI in its interior. Mapping the protein ET dynamics onto the calibrated ET catalogue shows that SCPZnI3 undergoes a switch in the effective dielectric constant following photoinduced ET, from ε_S ≈ 8 to ε_S ≈ 3.