| In hemeproteins, the nature of the distal iron ligand, the distal heme environment and the heme iron electronic structure are essential factors in ligand binding dynamics. Ultrafast time resolved absorption studies were performed in HbI(Fe III)CN, HbII(Fe III)CN, HbI(Fe II)CN and HbII(Fe II)CN to evaluate the effect of distal heme environment and the iron electronic structure. The time constants for the cyanide (CN) relaxation of each compound were 3.12 ps, 3.57 ps, 5.78 ps and 5.68 ps respectively. According the results, HbI and HbII ferric CN derivatives did not photodissociate and were dominated by the relaxation of an unphotolyzed derivative through vibrational cooling. In contrast, the HbI and HbII ferrous CN complexes suggested photodissociation of the ligand with no geminate recombination and the ultrafast spectra of both species were dominated by the relaxation of the deoxy species. The results suggested also that the Phe/B10/Tyr substitution did not affect CN dynamics. However, difference in the relaxation time constants was observed between HbICN and HbIICN iron oxidation state. This was interpreted in terms of differences in the relaxation pathway of both species due to their iron electronic nature. Time resolved ultrafast absorption studies were also performed in HbICO, HbIICO and HbI mutants; HbI(Phe/B10/Tyr)CO, HbI(Phe/B10/Val)CO and HbI(Gln/E7/Val)CO. The relaxation time constants of CO derivatives were 9.19 ps, 2.87 ps, 6.23 ps, 4.29 ps and 5.80 ps for HbI, HbII, HbI(Phe/B10/Tyr), HbI(Phe/B10/Val) and HbI(Gln/E7/Val), respectively. Ultrafast results indicated that the dynamics of the HbI mutants resembled that of HbI, suggesting that they both have the similar structural conformational arrangement. The differences between HbII and the HbI(Phe/B10/Tyr) in signal relaxation support the suggestion that the orientation of these distal environment may vary in both adopting two different conformation (open and closed). Similarly, in HbI the docking site process may help to establish a barrier to the reverse rebinding process and thereby inhibits ultrafast geminate ligand rebinding in the closed conformation. |
