One of the fundamental problems in biophysics is whether the protein medium at room temperature can be properly treated as a fluid dielectric or whether its dynamics is determined by a highly ordered molecular structure resembling the properties of crystalline and amorphous solids. Here, we measured the recombination between reduced A1 and the oxidized chlorophyll special pair P700 over a wide temperature range using preparations of photosystem I from the cyanobacterium Synechococcus sp. PCC 7002 depleted of the iron-sulfur clusters. We found that the dielectric properties of the protein matrix in early electron transfer reactions of photosystem I resemble the behavior of solids that require an implicit treatment of electron-phonon coupling even at ambient temperatures. The quantum effects of electron-phonon coupling in proteins could account for a variety of phenomena, such as the weak sensitivity of electron transfer in pigment-protein complexes to changing environmental conditions including temperature, driving force, polarity, and chemical composition.