Hemoglobin shows photoconductivity with a threshold at about 3.9 eV. A similar phenomenon is found in dyed gelatin films. In the latter case, the response arises from light absorbed by the protein rather than by the dye, and it is suggested that the energy is transferred to the dye by a nonradiative resonance process involving the aromatic amino acid residues in the protein, and that the dye functions only as a relatively easily ionizable center in the solid. The same is presumably true of the porphyrin group in hemoglobin. No evidence for the existence of bandhyphen;model structure in these proteins could be inferred from the data.
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