Ultrafast time resolved absorption study of ferrous, ferric and ferryl hemoglobin I-ligand complexes from Lucina pectinata

Hemoglobin I (HbI) is an invertebrate monomeric hemeprotein from Lucina pectinata that contains 142 amino acid residues. It has an unusual heme pocket structural composition, which shows an arrangement of phenylalanyl residues (Phe (29), Phe (43) and Phe (68)) that has not been observed in the globin families. The photochemistry of HbI ligand complexes will contribute to the generation of a comparative model between vertebrate and invertebrate globin families. These results will help to understand the effect of the electrostatic interaction between the heme pocket ligand moiety and its environment on a subpicosecond time scale, as well as the molecule absorption and energy dissipation. We have performed ultrafast measurements with ferrous and ferric HbI derivatives; HbI-CO, HbI-NO, HbI-N3, metHbI, and a mixture of HbI-CO and H2O2 to determine the relaxation time constants of heme-excited species and reaction intermediates previously observed in other heme proteins. Analysis of the ferrous bleaching results shows a fast component of 300 fs related to the formation of the unligated ferrous species from HbI*, and a second time constant of ∼2.4 ps which corresponds to the decay of the excited state species HbII*. The ferric bleaching transient results show a fast component of ∼600 fs and a second time constant of ∼4 ps. These can not be interpreted based on ferrous HbI analysis due to the absence of unligated ferric HbI and the difference in iron oxidation state. A band formation ∼430 nm was observed in ferric species as well as in ferrous derivatives with a formation time constant of ∼300 fs. The time decays and signals recombination remains the same to ferric complexes. Photoexcitation of ferric HbI complexes produced a photoinduced reduced species which led to the metal and ligand unbinding.