Photothermal Energy\nConversion by Heme in Cytochrome <i>c</i><sub>552</sub>:\nA Heme Protein-Based Molecular Heater

The development of efficient photothermal converters\nis essential\nfor thermal microscopy and the thermal manipulation of materials.\nPhotoexcitation of heme in proteins and its subsequent nonradiative\ntransition convert light energy into thermal energy. The converted\nenergy is released to the surroundings, resulting in heating of the\nsolvent water. Here, we examined the heat release by cytochrome <i>c</i><sub>552</sub> from Thermus thermophilus following photothermal conversion of the heme group in the protein.\nUpon irradiation with 532 nm light, the Raman bands associated with\nthe OH stretching mode of water changed in cytochrome <i>c</i><sub>552</sub> solutions but not in protein-free buffer solutions.\nThe observed spectral changes for the cytochrome <i>c</i><sub>552</sub> solutions closely resembled the temperature-dependent\nspectral changes of water and were attributed to a temperature increase\ndue to heat released by the protein. The temperature rise averaged\nover the volume irradiated with 125 mW light was 5.1 K, which exceeds\nthe temperature threshold to activate thermosensitive proteins in\ncells. We discuss the temperature rise for protein solutions based\non a thermal diffusion model and the excellent potential for applications\nof heme protein-based molecular heaters for thermal manipulation of\nliving cells.