Transient Thermal Expansion of a Protein in Solution after Photo-Excitation of the Chromophore: Deoxymyoglobin

The energy releasing process from a photo-excited chromophore embedded in a protein structure to the surrounding protein matrix and to the solvent is studied for deoxymyoglobin by the acoustic peak delay method with the transient grating technique. The acoustic peak delay time is usually a good indicator of the solvent heating time by the nonradiative transition after the photoexcitation and should be a positive value. However, the peak delay after the photoexcitation of deoxymyoglobin was found to be -15 ps. The observed negative peak delay was interpreted in terms of interference between two acoustic waves: an acoustic wave due to the thermal expansion of the protein and a wave of the medium. A theoretical calculation suggests that most of the energy of the heme is first transferred to the protein part and subsequently dissipates into the solvent. During this energy-releasing process, the protein experiences the transient thermal expansion and it creates an acoustic wave.