FEMTOSECOND NEAR-IR ABSORBENCY STUDY OF PHOTOEXCITED MYOGLOBIN - DYNAMICS OF ELECTRONIC AND THERMAL RELAXATION

The dynamics of electronic and thermal relaxation of the heme in myogl obin (Mb) have been determined from subpicosecond time-resolved near-I R absorbance spectra of photoexcited Mb. A broad, short-lived absorban ce feature near 12 000 cm(-1) (833 nm) was observed and assigned to an excited electronic state of the heme. The photoexcited heme relaxes t o 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 spec tral evolution of band III, a porphyrin-iron charge-transfer transitio n near 13 110 cm(-1) (763 nm) that is sensitive to temperature. The te mperature 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.