Photophysics and reactivity of heme proteins: a femtosecond absorption study of hemoglobin, myoglobin, and protoheme

Abstract

On the basis of our time-resolved absorption measurements of hemoglobin (Hb), myoglobin (Mb), and protoheme (PTH), either unligated or ligated with CO, O2, or NO, we propose a description of the photophysics of heme proteins that encompasses their photodissociation, the origin and fate of the observed short-lived transients, and the appearance of the ground-state, unligated heme proteins. Two distinct species are formed upon ligand photodissociation, which occurs in less than 50 fs. We assign these species to excited states of the unligated heme and label them (for the case of hemoglobin) as Hb*I and Hb*II. We suggest that Hb*I is already at least partially domed and has a spin state of at least S = 1. Hb*I decays in 300 fs to the ground-state unligated heme species, which we consider to be S = 2 and at least partially domed. The population of Hb*II varies with the ligand. It is more significant when the ligand is O2 or NO than when the ligand is CO. The similarities of the picosecond and femtosecond bleaching and absorption kinetics of HbCO with those of PTHCO (and of HbNO with those of PTHNO) indicate that in this time domain the importance of steric features of the protein are less important than the nature of the ligand itself in the geminate recombination process as well as in the relative amounts of the two heme excited states created. It is suggested that the quantum yield of ligand photodissociation is unity whether the ligand is O2, NO, or CO. The low yield of photodissociated heme-O2 or heme-NO compounds as measured on the microsecond time scale is thus attributed to a fast (2.5 ps) recombination of O2 or NO with Hb*II. We discuss geminate recombination measurements of cyanomet hybrid hemoglobins with NO and consider these results in terms of alpha and beta subunit heterogeneity. The first picosecond transient absorption spectra of cyanomet-CO hybrid hemoglobins are presented and are compared with the spectra of other heme compounds. The superimposability of the transient spectra on the equilibrium spectra of heme compounds that exhibit minimal or no cooperativity is noted and is compared with the case of cooperative systems where the transient spectra are distorted with respect to the equilibrium spectra. This distortion is interpreted in terms of an interaction of a domed heme with the F helix.