Hb I (HbI) is an invertebrate monomeric hemeprotein from Lucina pectinata that contains 142 amino acid residues. The HbI heme pocket structural compn. shows an arrangement of phenylalanyl residues (Phe (29), Phe (43) and Phe (68)) that has not been obsd. in the globin families. The photochem. of HbI ligand complexes can yield relevant information about invertebrate globins in terms of ligands rebinding and photophys. processes. We have performed ultrafast measurements to det. the extent of O2 and CO recombination subsequent to their photodissocn. from the HbI moiety. To sep. the contributions of electronic, thermal and conformational relaxation, from ligand rebinding in the spectroscopic signals, measurements of the deoxyHbI were also performed. The transient absorption signals in the Soret region of HbICO and HIO2 are composed of similar time consts. as obsd. for other heme proteins. The fast component (300 fs) is due to the formation of the unligated deoxy species from the excited heme state HbI*. The second time const. (2.2-2.5 ps) corresponds to the decay of the excited heme state HbII*. The contribution of this component to the transient signal near the Soret band max. appears to be greater in HbI complexes than in other myoglobins. Its presence in the decay of the Soret bleaching signals has been interpreted as a signature of fast geminate recombination. Thus, these results could suggest that the differences in the transient signal between HbI and Mb are influenced by the different heme-pocket structures. Photoexcitation of the HbI complexes led to the formation of a short-lived species in the 630 nm region. Formation of this species appears to be instantaneously, while it decays with a 300 fs time const. This new transient is related to an iron-to-porphyrin charge transfer state. [on SciFinder (R)]