Femtosecond near-IR absorbance study of photoexcited myoglobin: Dynamics of electronic and thermal relaxation

The dynamics of electronic and thermal relaxation of the heme in myoglobin (Mb) have been determined from subpicosecond time-resolved near-IR absorbance spectra of photoexcited Mb. A broad, short-lived absorbance feature near 12 000 cm^-1 (833 nm) was observed and assigned to an excited electronic state of the heme. The photoexcited heme relaxes to its ground electronic state with a time constant of 3.4 ± 0.4 ps, leading to an absorbance spectrum that is indicative of a "hot" heme. The ensuing thermal relaxation was characterized by probing the spectral evolution of band III, a porphyrin - iron charge-transfer transition near 13 110 cm^-1 (763 nm) that is sensitive to temperature. The temperature of the electronically relaxed heme cools exponentially with a time constant of 6.2 ± 0.5 ps. The implications of electronic and thermal relaxation to photolysis studies of liganded heme proteins are discussed.