Femtosecond photolysis of CO-ligated protoheme and hemoproteins: appearance of deoxy species with a 350-fsec time constant.

Abstract

Photolysis of [human] HbCO, [horse heart], MbCO [carboxymyoglobin] and CO-protoheme was investigated by measuring transient differential spectra and kinetics of induced absorption after excitation with a 250 fs [femtosecond] laser pulse at 307 nm. Probing was performed by a part of a continuum pulse between 395-445 nm. Photodissociation of the 3 liganded species occurred within the pulse duration. The formation of deoxy species appeared with a mean (.+-. SD) response time of 350 .+-. 50 fs. This time constant was identical for the 3 species and independent of the presence or absence of the protein structure. The formation of a transient high-spin in plane Fe(II) species which relaxes in 350 fs to a high-spin stable state with concerted kinetics of CO departure and Fe displacement is suggested. The spin transition is suspected to occur via liganded excited states which relax in part to nonreactive states with a 3.2-picosecond time constant.