Photothermal Energy Conversion by Heme in Cytochrome <i>c</i>₅₅₂: A Heme Protein-Based Molecular Heater

The development of efficient photothermal converters is essential for thermal microscopy and the thermal manipulation of materials. Photoexcitation of heme in proteins and its subsequent nonradiative transition convert light energy into thermal energy. The converted energy is released to the surroundings, resulting in heating of the solvent water. Here, we examined the heat release by cytochrome c552 from Thermus thermophilus following photothermal conversion of the heme group in the protein. Upon irradiation with 532 nm light, the Raman bands associated with the OH stretching mode of water changed in cytochrome c552 solutions but not in protein-free buffer solutions. The observed spectral changes for the cytochrome c552 solutions closely resembled the temperature-dependent spectral changes of water and were attributed to a temperature increase due to heat released by the protein. The temperature rise averaged over the volume irradiated with 125 mW light was 5.1 K, which exceeds the temperature threshold to activate thermosensitive proteins in cells. We discuss the temperature rise for protein solutions based on a thermal diffusion model and the excellent potential for applications of heme protein-based molecular heaters for thermal manipulation of living cells.